PROCESS FOR PREPARING CALCIUM OXIDE OR ORDINARY PORTLAND CEMENT FROM ROCKS AND MINERALS CONTAINING CALCIUM

Abstract

Aspects of the invention include a method for producing a cementitious material, comprising the steps of: first, reacting a calcium-bearing starting material with a first acid to produce a first aqueous calcium salt; second, reacting the first aqueous calcium salt with a second acid to produce a second solid calcium salt, wherein the second acid is different from the first acid and the second calcium salt is different from the first calcium salt; and heat-treating the second calcium salt to produce a first cementitious material. Preferably, but not necessarily, during the second reaction step, the reaction between the first calcium salt and the second acid regenerates the first acid.

Description

PROCESS FOR PREPARING CALCIUM OXIDE OR ORDINARY PORTLAND CEMENT FROM ROCKS AND MINERALS CONTAINING CALCIUM CROSS REFERENCE TO RELATED APPLICATIONS This application claims the benefit and priority of U.S. Provisional Application No. 62 / 886.137, filed on August 13, 2019, U.S. Provisional Application No. 62 / 913.620, filed on October 10, 2019, U.S. Provisional Application No. 62 / 932.200, filed on November 7, 2019, and U.S. Provisional Application No. 63 / 019.916, filed on May 4, 2020, each of which is incorporated herein by reference in its entirety to the extent not inconsistent with this application. BACKGROUND OF THE INVENTION The technique of producing cement materials, including Normal Portland Cement, faces considerable challenges, inefficiencies and / or drawbacks. For example, conventional cement production processes are energy-intensive, produce environmentally damaging byproducts such as CO2 and / or SO2, and utilize only a limited range of raw materials, primarily simple calcium-based materials such as calcium carbonate (limestone) or mined calcium sulfate (gypsum). This application addresses these and other challenges in the art. SUMMARY OF THE INVENTION This document provides methods for the production of cementitious materials that have any combination of the following advantages or characteristics: they are less energy intensive than previous approaches, some embodiments being net energy neutral or even producing net energy, they regenerate certain reactants, they are characterized by a net reaction free of SO2 and / or CO2, they recycle certain by-products, they do not include the production of CO2, they can use a wider range of raw material materials, including more complex materials, they generate value-added by-products and / or generate composite cementitious materials. This document describes methods for producing cementitious materials using a two-acid approach, where the materials are reacted with two different acids and / or two reaction stages with different acids. The advantages of these approaches include all or most of the advantages and features mentioned previously. For example, the disclosed two-acid approach provides the ability to digest materials containing complex calcium, including those with Ca, as well as other metal elements (including metalloids), such as Si, Al, and other species, and even form value-added byproducts from non-Ca metals, while simultaneously regenerating reactive acids.Significantly, these methods can also be CO2-free and can include the conversion of SO2 into a reactive acid, thereby eliminating or drastically reducing SO2 emissions from CaSO4-based approaches in cement manufacturing. Aspects of the invention include a method for producing a cementitious material, comprising the steps of: first, reacting a calcium-bearing starting material with a first acid to produce a first aqueous calcium salt; second, reacting the first aqueous calcium salt with a second acid to produce a second solid calcium salt; wherein the second acid is different from the first acid and the second calcium salt is different from the first calcium salt; and heat-treating one or more calcium salts to produce a first cementitious material. Preferably, but not necessarily, the one or more calcium salts are the second calcium salt. Preferably, but not necessarily, during the second reaction step, the reaction between the first calcium salt and the second acid regenerates the first acid.Preferably, but not necessarily, the methods are characterized by a net reaction free of an acid-forming gas product. Preferably, but not necessarily, the method comprises forming the second solid calcium salt characterized by a purity greater than or equal to 90% dry weight. Preferably, but not necessarily, any of the methods disclosed herein includes a first separation step after the first reaction step and before the second reaction step; the first separation step comprising separating a first aqueous fraction from a first solid fraction; wherein the first aqueous fraction comprises the first aqueous calcium salt and the first solid fraction comprises one or more solid by-products formed during the first reaction step.Preferably, but not necessarily, any of the methods disclosed herein includes a second separation step after the second reaction step and before the heat treatment step; the second separation step comprising separating a second solid fraction from a second aqueous fraction; wherein the second solid fraction comprises the second solid calcium salt and the second aqueous fraction comprises one or more aqueous byproducts formed during the second reaction step. Preferably, the solid fraction is characterized by a dry mass of which at least 90% by weight is the second calcium salt. Preferably, but not necessarily, any of the methods disclosed herein includes a second acid regeneration stage, wherein the second acid regeneration stage comprises converting one or more gas products from the heat treatment stage into the second acid. Optionally, the second acid regeneration stage is a non-electrochemical process carried out according to Formula FX1A: SO2 + V2Q2 + H2O → H2SO4 (FX1A), wherein: the SO2 in FX1A is a gas product from the heat treatment stage; the H2SO4 generated in FX1A is used as at least a fraction of the second acid during the second reaction stage.Optionally, the second acid regeneration stage is a non-electrochemical process carried out according to Formula FX1B: SO2 + H2O → H2SO3 (FX1B), wherein: the SO2 in FX1B is a gas product of the heat treatment stage; the H2SO3 generated in FX1B is used as at least a fraction of the second acid during the second reaction stage. Optionally, the second acid regeneration stage comprises (i) electrochemically oxidizing sulfur dioxide to sulfuric acid and (ii) forming hydrogen gas through a reduction reaction; and wherein the second acid regeneration stage is carried out according to Formula FX2: SO2 + 2H2O → H2SO4 + H2 (FX2); wherein: the SO2 in FX2 is a gas product of the heat treatment stage; the H2SO4 generated in FX2 is used as at least a fraction of the second acid during the second reaction stage.Optionally, the heat treatment stage involves using the energy generated from the oxidation of the hydrogen gas formed as a result of the second acid regeneration. For example, the hydrogen gas produced through this method can be used to power the electrochemical stage, such as via a fuel cell or a turbine. Optionally, the electrochemical oxidation of sulfur dioxide also involves using the energy generated as a result of the second acid regeneration stage. It should be noted that when H₂SO₄ is added to a solution containing both MgCb and CaCb, only CaSO₄ will precipitate. If H₂SO₃ is added to a solution of MgCb and CaCb, both MgSO₄ and CaSO₄ will precipitate.It should be noted that there are currently regulations that prohibit the presence of Mg in cement, so that the starting material containing calcium preferably has a low Mg content, thus minimizing the amount of precipitated Mg material. Optionally, during the second reaction stage, the reaction between the first calcium salt and the second acid regenerates the first acid according to Formula FX3: CaCb (aq.) + H2SO4 -> CaSCU (s) + 2HCl (FX3); where: the first calcium salt is CaCb; the first acid is HCl; the second acid is H2SO4; and the second calcium salt is CaSO4. The starting material containing calcium comprises Ca. The starting material containing calcium has a chemical composition comprising the element Ca. Preferably, the starting material containing calcium has a chemical composition comprising the element Ca, wherein the weight percent and / or molar percent of Ca in said starting material containing calcium is at least 0.001%, preferably at least 0.01%, more preferably at least 0.1%, more preferably at least 1%, furthermore, more preferably at least 5%, even more preferably at least 10%, and even more preferably at least 20%.Optionally, in any of the methods disclosed herein, the calcium-containing starting material has a chemical composition comprising the element Ca, wherein the weight percent and / or molar percent of Ca in said calcium-containing starting material is selected from the range of 1% to 80%, optionally from 1% to 60%, optionally from 1% to 55%, and optionally from 1% to 50%. Optionally, in any of the methods disclosed herein, the calcium-containing starting material comprises at least one multimetal oxide material having a composition comprising Ca and at least one other metal element selected from the group consisting of Al, Si, Fe, Mn, and Mg. Optionally, the composition of the at least one multimetal oxide comprises less than or equal to 55% by weight of Ca. Optionally, the composition of the at least one multimetal oxide comprises less than or equal to 60% by weight of Ca.Optionally, in any of the methods disclosed herein, the at least one multi-metal oxide material is at least one naturally occurring rock or mineral. Optionally, in any of the methods disclosed herein, the at least one naturally occurring rock or mineral comprises basalt, igneous apatites, wollastonite, anorthosite, montmorillonite, bentonite, calcium-containing feldspar, anorthite, diopside, pyroxene, pyroxenite, mafurite, kamafugite, clinopyroxene, colemanite, grossular, augite, pigeonite, marguerite, calcium serpentine, garnet, scheelite, skarn, limestone, natural gypsum, apatite, fluorapatite, or any combination thereof. Optionally, in any of the methods disclosed herein, the starting material containing calcium comprises cement, concrete, Portland cement, fly ash, slag, or any combination thereof.If the starting material containing calcium includes CaCOa, then CO2 may be generated during the process. However, in cases where CO2 is generated, it is at a high concentration and can be stored and / or used. Optionally, in any of the methods disclosed herein, the first acid comprises hydrochloric acid (HCl). Optionally, in any of the methods disclosed herein, the first acid is hydrochloric acid. Optionally, in any of the methods disclosed herein, the second acid comprises sulfuric acid (H₂SO₄) and / or sulfurous acid (H₂SO₃). Optionally, in any of the methods disclosed herein, the second acid is sulfuric acid and / or sulfurous acid. Optionally, in any of the methods disclosed herein, the second acid is sulfuric acid. Optionally, in any of the methods disclosed herein, the second acid is sulfurous acid. Optionally, in any of the methods disclosed herein, the first aqueous calcium salt is calcium chloride (CaCb).Optionally, in any of the methods disclosed herein, the second solid calcium salt is calcium sulfate (CaSO4) and / or calcium sulfite (CaSO4). Optionally, in any of the methods disclosed herein, the second solid calcium salt is calcium sulfate. Optionally, in any of the methods disclosed herein, the second solid calcium salt is calcium sulfite (CaSO4). Preferably, in any of the methods disclosed herein, the first cementitious material comprises CaO. Optionally, in any of the methods disclosed herein, the first cementitious material is calcium oxide (CaO). Optionally, in any of the methods disclosed herein, the first cementitious material is calcium oxide (CaO) or Portland cement clinker.Optionally, in any of the methods disclosed herein, the first cementitious material is Portland cement clinker. Optionally, in any of the methods disclosed herein, the acid-forming gas product is SO2 and / or CO2. Optionally, in any of the methods disclosed herein, the acid-forming gas product is SO2. Optionally, in any of the methods disclosed herein, the acid-forming gas product is CO2. Preferably, but not necessarily, in any of the methods disclosed herein, the first reaction step comprises reacting the calcium-bearing starting material with hydrochloric acid to form at least aqueous calcium chloride, aqueous aluminum chloride, and solid silica. Preferably, but not necessarily, in any of the methods disclosed herein, the first separation step comprises separating a first aqueous fraction comprising aqueous calcium chloride and aqueous aluminum chloride from a first solid fraction comprising solid silica. Preferably, but not necessarily, in any of the methods disclosed herein, the second reaction step comprises reacting at least aqueous calcium chloride, aqueous aluminum chloride, and sulfuric acid to form at least solid calcium sulfate, aqueous aluminum sulfate, and hydrochloric acid.Preferably, but not necessarily, in any of the methods disclosed herein, the heat treatment step comprises heating the calcium sulfate to form calcium oxide. Preferably, but not necessarily, in any of the methods disclosed herein, the first reaction step comprises reacting the calcium-containing starting material with hydrochloric acid to form at least aqueous calcium chloride, aqueous aluminum chloride, aqueous iron chloride, aqueous magnesium chloride, and solid silica. Preferably, but not necessarily, in any of the methods disclosed herein, the first separation step comprises separating a first aqueous fraction comprising aqueous calcium chloride and aqueous aluminum chloride from a first solid fraction comprising solid silica.Preferably, but not necessarily, in any of the methods disclosed herein, the second reaction step comprises reacting at least aqueous calcium chloride and sulfuric acid to form at least solid calcium sulfate and hydrochloric acid. Preferably, but not necessarily, in any of the methods disclosed herein, the heat treatment step comprises heating the calcium sulfate to form calcium oxide. Optionally, any of the methods disclosed herein comprises an ion-exchange step, wherein the ion-exchange step comprises exchanging one or more anions of the first calcium salt and / or the second calcium salt for one or more hydroxyl anions to form a third calcium salt. Alternatively, the ion-exchange step comprises reacting the first calcium salt and / or the second calcium salt with a chelating agent to form a calcium-chelating compound and reacting the calcium-chelating compound with a base to form the third calcium salt.Optionally, the ion exchange step comprises using an ion exchange membrane to exchange one or more anions from the first calcium salt and / or the second calcium salt for one or more hydroxyl anions to form the third calcium salt. Optionally, the one or more calcium salts from the heat treatment step are the third calcium salt. Optionally, the third calcium salt is Ca(OH)₂. Optionally, any of the methods disclosed herein comprises a chelating agent regeneration step, wherein the chelating agent regeneration step comprises producing the third calcium salt. Optionally, any of the methods disclosed herein comprises a step of forming the first cementitious material from the third calcium salt.Optionally, the step of forming the first cementitious material from the third calcium salt comprises either dehydrating the third calcium salt or directly releasing the first cementitious material from the calcium-chelating compound, optionally via a base. For example, CaO can be formed by using a chelating agent or a base to react CaCb, CaSOa, or CaSO4. The chelating agent or base can then be regenerated in a manner that releases Ca(OH)2, which can be dehydrated to CaO, or the CaO could be released directly from the chelating agent. For example, if CaSO4 is precipitated, a chelating agent, such as EDTA, can be reacted with the CaSO4 to prepare Ca EDTA. A base, such as NaOH, can then be used to directly produce Ca(OH)2 and regenerate the EDTA. Preferably, but not necessarily, any method disclosed herein comprises a step of forming a composite cementitious material; wherein: (i) the heat treatment step comprises the step of forming the composite cementitious material and the first cementitious material is the composite cementitious material or (ii) the step of forming the composite material is carried out using the first cementitious material formed during the heat treatment step. For example, where the step of forming the composite material is carried out using the first cementitious material formed during the heat treatment step, the formation of the composite material may occur simultaneously with the formation of the first cementitious material (e.g., CaO) or subsequently after the formation of the first cementitious material (e.g., CaO).Optionally, any method disclosed herein comprises a step of forming a composite cementitious material; wherein the heat treatment step comprises the step of forming the composite cementitious material and the first cementitious material is the composite cementitious material. Optionally, any method disclosed herein comprises a step of forming a composite cementitious material; wherein the step of forming the composite material is carried out using the first cementitious material formed during the heat treatment step. Optionally, in any of the methods disclosed herein, the step of forming the composite cementitious material comprises heating the second calcium salt and / or the first cementitious material together with one or more additives.Optionally, in any of the methods disclosed herein, the cementitious material formation step of the composite material comprises heating the second calcium salt together with one or more additives. Optionally, in any of the methods disclosed herein, the cementitious material formation step of the composite material comprises heating the first cementitious material together with one or more additives. Optionally, in any of the methods disclosed herein, the cementitious material formation step of the composite material is carried out simultaneously with the heat treatment step. Optionally, in any of the methods disclosed herein, after the heat treatment step. Optionally, in any of the methods disclosed herein, the cementitious material of the composite material is Portland cement clinker.Optionally, in any of the methods disclosed herein, the cementitious material of the composite material is ordinary Portland cement and / or the first cementitious material is calcium oxide. Optionally, in any of the methods disclosed herein, the first cementitious material is calcium oxide. Optionally, in any of the methods disclosed herein, the cementitious material of the composite material is ordinary Portland cement. Optionally, any method disclosed herein comprises forming one or more additives from the starting material containing calcium.Optionally, in any of the methods disclosed herein, one or more additives may be byproducts of the first reaction stage and / or formed from one or more byproducts of the first reaction stage and / or formed from one or more byproducts of the second reaction stage. Optionally, in any of the methods disclosed herein, one or more additives may be byproducts of the first reaction stage and / or formed from one or more byproducts of the first reaction stage. Optionally, in any of the methods disclosed herein, one or more additives may be byproducts of the second reaction stage and / or formed from one or more byproducts of the second reaction stage.Optionally, in any of the methods disclosed herein, one or more additives are one or more byproducts of the first reaction step. Optionally, in any of the methods disclosed herein, one or more additives are one or more byproducts of the second reaction step. Optionally, in any of the methods disclosed herein, one or more additives are one or more byproducts of the first reaction step and / or one or more byproducts of the second reaction step. Optionally, in any of the methods disclosed herein, a combined chemical composition of one or more additives comprises Al and Si. Optionally, in any of the methods disclosed herein, one or more additives are at least Al₂O₃ and SiO₂. One advantage of the methods disclosed herein is that they can form value-added byproducts. For example, instead of using simple calcium sources, such as limestone or gypsum, as starting materials, complex minerals containing Ca and Si, and optionally other metals such as Al, Mg, and / or Fe, can be used. Rather than these being unwanted elements that contaminate the cement product, for example, the methods disclosed herein can include steps to form and isolate valuable products containing these additional elements, such as Al oxides, Mg oxides, and / or Fe oxides. These steps do not incur significant additional operating costs. Preferably, but not necessarily, any method disclosed herein comprises forming and isolating silica of fume-grade silica, nanosilica, and / or microsilica from the calcium-bearing starting material. Preferably, but not necessarily, any method disclosed herein comprises forming and isolating alumina from the calcium-bearing starting material. Preferably, but not necessarily, in any method disclosed herein, the first reaction step comprises reacting the calcium-bearing starting material with hydrochloric acid to form at least aqueous aluminum chloride; wherein the method further comprises: precipitating the aluminum chloride in the presence of hydrochloric acid; and, optionally, reacting the precipitated aluminum chloride with sulfuric acid to form solid aluminum sulfate; heating the aluminum sulfate and / or the aluminum chloride to form alumina. Preferably, but not necessarily, in any method disclosed herein, the hydrochloric acid is regenerated in these steps. Preferably, but not necessarily, in any method disclosed herein, the reaction of the precipitated aluminum chloride forms hydrochloric acid.Preferably, but not necessarily, in any method disclosed herein, hydrochloric acid is regenerated in these steps. Preferably, but not necessarily, in any method disclosed herein, the second reaction step comprises the reaction step of precipitated aluminum chloride. Optionally, in any of the methods disclosed herein, the heat treatment step comprises heating the aluminum sulfate step. Optionally, in any of the methods disclosed herein, the heat treatment step comprises heating the aluminum chloride step. Optionally, any method disclosed herein comprises forming and isolating iron oxide from the starting material that carries calcium.Optionally, in any of the methods disclosed herein, the formation and isolation of iron oxide comprises: forming an aqueous solution having iron sulfate and / or aqueous iron chloride and, optionally, at least one other metal magnesium sulfate salt and / or chloride salt formed as byproducts during the second reaction step; wherein the aqueous solution is free of a calcium salt and free of an aluminum salt; drying the aqueous solution to form solid iron sulfate and / or solid iron chloride and, optionally, the at least one other metal sulfate salt; heating the solid iron sulfate and, optionally, the at least one other metal sulfate salt to form a water-insoluble iron oxide; and, optionally, dissolving the at least one other metal sulfate salt to isolate the water-insoluble iron oxide. Optionally, any method disclosed herein comprises forming and isolating iron oxide from the starting material that carries calcium.Optionally, in any of the methods disclosed herein, the formation and isolation of iron oxide comprises: forming an aqueous solution having aqueous iron sulfate or iron chloride and, optionally, at least one other metal chloride or magnesium sulfate salt formed as byproducts during the second reaction step; wherein the aqueous solution is free of a calcium salt and free of an aluminum salt; using SO2 to precipitate MgSOa; separating the aqueous iron salt from the solid magnesium salt; drying the aqueous solution to form solid iron sulfate or iron chloride and, optionally, the at least one other metal sulfate salt; heating the solid iron sulfate and, optionally, the at least one other metal sulfate salt to form a water-insoluble iron oxide; and, optionally, dissolving the at least one other metal sulfate salt to isolate the water-insoluble iron oxide. Optionally, iron chloride, iron sulfate, aluminum chloride, and / or aluminum sulfate, and / or any other iron and / or aluminum salt, are produced and marketed and / or combined with electrochemical strategies to prepare iron and aluminum metals from aluminum chloride, sulfate, and / or other salts. For example, aluminum chloride can be isolated and aluminum can be electroextracted from aluminum chloride, while co-producing chlorine gas. This chlorine gas can be reacted with hydrogen, possibly hydrogen from the cogeneration of sulfuric acid and hydrogen, to regenerate HCl. Another example is iron, which can be electroextracted from iron sulfate to regenerate sulfuric acid. The methods disclosed herein may comprise one or more acid-forming reactions. An acid-forming reaction may be a reaction to regenerate an acid that is consumed in a different reaction of the method. The acid-forming reaction may be a reaction that supplies the acid to the reaction step (first and / or second) where the acid is consumed. For example, instead of supplying an acid to a reaction step where the acid is consumed to form a calcium salt, the reactants that form that acid are supplied to the reaction step in such a way that the reaction step comprises both the formation of the acid and the respective acid-consuming reaction (or salt formation). Preferably, but not necessarily, any method disclosed herein comprises a first acid formation step, wherein: (i) the first reaction step comprises the first acid formation step and the first acid formation step occurs simultaneously with the first reaction step, or (ii) the first acid formation step is carried out separately from the first reaction step. Optionally, any method disclosed herein comprises a first acid formation step, wherein the first reaction step comprises the first acid formation step and the first acid formation step occurs simultaneously with the first reaction step. Optionally, any method disclosed herein comprises a first acid formation step, wherein the first acid formation step is carried out separately from the first reaction step. Preferably, but not necessarily, any method disclosed herein comprises a second acid formation step, wherein: (i) the second reaction step comprises the second acid formation step and the second acid formation step occurs simultaneously with the second reaction step, or (ii) the second acid formation step is carried out separately from the second reaction step. Optionally, any method disclosed herein comprises a second acid formation step, wherein the second reaction step comprises the second acid formation step and the second acid formation step occurs simultaneously with the second reaction step. Optionally, any method disclosed herein comprises a second acid formation step, wherein the second acid formation step is carried out separately from the second reaction step.Optionally, in any of the methods disclosed herein, the step of forming the second acid comprises reacting SO2 with water to form H2SO3 and / or H2SO4, where the second acid is H2SO3 and / or H2SO4. Alternatively, in any of the methods disclosed herein, the second acid is H2SO3 and / or H2SO4 and the second calcium salt is CaSO4 and / or CaSO4, respectively. Optionally, in any of the methods disclosed herein, the first acid and / or the second acid is a bulk acid. Optionally, in any of the methods disclosed herein, the first acid formation step comprises creating a pH gradient through water electrolysis; wherein the first acid is formed through water electrolysis. Optionally, in any of the methods disclosed herein, the second acid formation step comprises creating a pH gradient through water electrolysis; wherein the second acid is formed through water electrolysis. Optionally, in any of the methods disclosed herein, the second acid regeneration stage according to Formula FX1A is carried out at a temperature selected from the range of 400 °C to 1,800 °C. Alternatively, in any of the methods disclosed herein, the second acid regeneration stage according to Formula FX1A is carried out at a temperature selected from the range of 400 °C to 600 °C. Optionally, in any of the methods disclosed herein, the second acid regeneration stage according to Formula FX1A is carried out at a temperature selected from the range of 400 °C to 600 °C, is exothermic, and is carried out in the presence of a catalyst. Optionally, the catalyst comprises vanadium oxide.Optionally, in any of the methods disclosed herein, the method is characterized by a net energy selected from the range of -2 to +2 GJ per metric tonne of cementitious material produced (e.g., first cementitious material produced or cementitious composite material produced, such as OPC).Optionally, in any of the methods disclosed herein, the method is characterized by a net energy selected from the range of -10 to +10 GJ / t, optionally from -5 to +5 GJ / t, optionally from -5 to +4 GJ / t, optionally from -5 to +3 GJ / t, optionally from -5 to +2 GJ / t, optionally from -5 to +1 GJ / t, optionally from -5 to +0.5 GJ / t, optionally from -5 to +0.2 GJ / t, optionally from -5 to +0.1 GJ / t, optionally from -5 to 0 GJ / t, optionally from -2 to 1 GJ / t, optionally from -2 to 1.5 GJ / t, optionally from -2 to +1.0 GJ / t, optionally from -2 to +0.5 GJ / t, optionally from -2 to +0.3 GJ / t, optionally from -2 to +0.2 GJ / t, optionally from -2 to +0.1 GJ / t, or optionally from -2 to +0 GJ / t. Optionally, in any of the methods disclosed herein, the first reaction step is exothermic. Optionally, in any of the methods disclosed herein, the second acid regeneration step is exothermic.Optionally, in any of the methods disclosed herein, the first reaction step is carried out at a temperature of at least 50 °C. Optionally, in any of the methods disclosed herein, the first reaction step is carried out at a temperature selected from the range of 80 °C to 100 °C, preferably 90 ± 5 °C. Optionally, in any of the methods disclosed herein, the heat treatment step is carried out at a temperature selected from the range of 1100 °C to 1800 °C. Optionally, in any of the methods disclosed herein, the heat treatment step comprises heat-treating the second calcium salt in the presence of a chemical reducing agent and is carried out at a temperature selected from the range of 800 °C to 1200 °C.Optionally, the reducing agent is water, carbon (or any allotrope or combination of allotropes of carbon), hydrogen gas, methane gas, or any combination thereof. Optionally, the reducing agent is water, carbon (or any allotrope or combination of allotropes of carbon), methane gas, or any combination thereof. Optionally, the heat treatment step can be carried out according to any one or a combination of Formulas FX4A, FX4B, FX4C, and FX4D: CaSO4 + H2O → CaO + H2SO4 (FX4A); CaSO4 + 2C → CaS + 2CO2 (FX4B); CaSO4 + CH4 → CaS + CO2 + 2H2O (FX4C); CaS + 3CaSO4 → 4CaO (FX4D). Any of the methods disclosed herein may be implemented as a batch process, plug flow process, semi-continuous process, phased process, continuous process, or any combination thereof. Any stage of any method disclosed herein may be implemented as a batch process, plug flow process, semi-continuous process, phased process, continuous process, or any combination thereof. Additional aspects of the invention disclosed herein include a method for producing a cementitious material through reductive thermal decomposition, the method comprising the steps of reacting a calcium-bearing material with a chemically reducing gas to produce methane and a cementitious material. Preferably, but not necessarily, in any method for producing a cementitious material through reductive thermal decomposition, the calcium-bearing material comprises CaCO3. CaSO4, CaS, a calcium salt, or any combination thereof. Preferably, but not necessarily, in any method for producing a cementitious material by reducing thermal decomposition, the calcium-bearing material is CaCO3, CaSO4, CaS, or any combination thereof. Preferably, but not necessarily, in any method for producing a cementitious material by reducing thermal decomposition, the calcium-bearing material is CaCO3. Preferably, but not necessarily, in any method for producing a cementitious material by reducing thermal decomposition, the calcium-bearing material comprises CaCO3. Preferably, but not necessarily, in any method for producing a cementitious material by reducing thermal decomposition, the chemically reducing gas is hydrogen gas or a gas comprising hydrogen gas, such as a forming gas.Preferably, but not necessarily, in any method for producing a cementitious material through reductive thermal decomposition, the cementitious material comprises CaO. Preferably, but not necessarily, in any method for producing a cementitious material through reductive thermal decomposition, the cementitious material is CaO. Preferably, but not necessarily, in any method for producing a cementitious material through reductive thermal decomposition, the molar ratio of the calcium-bearing material reacted with the chemically reducing gas is 1:4 or 1:2. Optionally, in any method for producing a cementitious material through reductive thermal decomposition, the reaction is carried out in the presence of water.Optionally, in any method for producing a cementitious material through reductive thermal decomposition, the reaction is carried out in the absence of water. Optionally, in any method for producing a cementitious material through reductive thermal decomposition, the molar ratio of CaCO₃ is reacted with hydrogen gas in a 1:4 molar ratio during the reaction stage. Optionally, in any method for producing a cementitious material through reductive thermal decomposition, the molar ratio of CaCO₃ is reacted with hydrogen gas in a 1:2 molar ratio during the reaction stage. Generally, the reaction according to a 1:4 molar ratio has lower energy but a higher OpEx because more H₂ is needed, but a lower temperature can be used. Generally, the reaction according to a 1:2 molar ratio has higher energy but a lower OpEx.Optionally, in any method for producing a cementitious material through reductive thermal decomposition, oxygen gas, water, or a combination of oxygen gas and water is produced during the reaction step. Optionally, any method for producing a cementitious material through reductive thermal decomposition comprises a step of decomposing methane to produce hydrogen gas and one or more carbon materials. Optionally, in any method for producing a cementitious material through reductive thermal decomposition, the method does not comprise the formation of CO2. Optionally, in any method for producing a cementitious material through reductive thermal decomposition, the reaction step is characterized by a lower heating value (LHV) of 720 kJ / mol or less and a higher heating value (HHV) of 800 kJ / mol or less.Optionally, in any method for the production of a cementitious material through reducing thermal decomposition, the reaction stage is carried out at a temperature of at least 700 °C. Additional aspects of the invention disclosed herein include methods for producing a cementitious material according to a single-acid approach, wherein only one acid or only one acid reaction stage is required.In one aspect, a method for producing a cementitious material comprises the steps of: first, reacting a calcium-bearing starting material with a first acid to produce a first aqueous fraction comprising a first aqueous calcium salt and a first solid fraction comprising one or more solid byproducts; wherein: the calcium-bearing starting material has a chemical composition comprising a plurality of metal elements including at least Ca and Si; the one or more solid byproducts comprise a silicon salt; first, separating the first aqueous fraction from the first solid fraction; and treating the first calcium salt to produce a first cementitious material. Optionally, the treatment step comprises thermally treating (or thermally decomposing) the first calcium salt in the presence of water to produce the first cementitious material.Optionally, heat treatment (or thermal decomposition) of the first calcium salt regenerates the first acid. Optionally, the treatment step comprises an ion exchange step, wherein the ion exchange step comprises exchanging one or more anions of the first calcium salt for one or more hydroxyl anions to form a third calcium salt. Optionally, the ion exchange step comprises reacting the first calcium salt with a chelating agent to form a calcium-chelating compound and reacting the calcium-chelating compound with a base to form the third calcium salt. Optionally, the ion exchange step comprises reacting the first calcium salt with a base to form the third calcium salt.Optionally, the ion exchange step comprises using an ion exchange membrane to exchange one or more anions of the first calcium salt for hydroxyl anions to form the third calcium salt. Preferably, the third calcium salt is Ca(OH)₂. Optionally, the treatment step comprises thermally treating (or thermally decomposing) the third calcium salt to produce the first cementitious material. Optionally, the base is a hydroxide compound. Optionally, the first calcium salt is CaCh. Optionally, for example, the treatment step comprises thermally decomposing CaClz in the presence of air according to the formula: CaCb + O2 → CaO + Cl2 + 1 / 2 O2. Optionally, for example, the treatment step comprises thermally treating CaCh in the presence of water according to the formula: CaCh + H2O → CaO + 2HCl. Optionally, for example, the treatment step comprises ion exchange using an ion-exchange membrane to exchange Cl ions for OH ions, thereby forming Ca(OH)2. Optionally, the treatment step further comprises dehydrating the Ca(OH)2 to prepare the first cementitious material. Optionally, for example, the treatment step comprises reacting the first calcium salt with a chelating agent to form a calcium-chelating compound.Optionally, for example, the treatment step comprises reacting a base, such as NaOH, Mg(OH)₂, or MgCl(OH), with the first calcium salt, such as CaCh, to form Ca(OH)₂. Optionally, for example, the treatment step further comprises thermally decomposing Ca(OH)₂ to prepare the first cementitious material. The first cementitious material is optionally CaO. Optionally, the first acid is hydrogen chloride. Optionally, the one or more solid by-products comprise SiO₂. Optionally, the at least one multimetal oxide material is at least a natural rock or mineral. Optionally, in any of the methods disclosed herein, the first calcium salt and / or the second calcium salt is other than Ca(OH)₂ or comprises a salt other than Ca(OH)₂. Optionally, in any of the methods disclosed herein, the first reaction step is not an electrochemical step. Optionally, in any of the methods disclosed herein, the second reaction step is not an electrochemical step. Optionally, in any of the methods disclosed herein, the starting material containing calcium is other than CaCO₃ or comprises a material other than CaCO₃. Without wishing to be bound to any particular theory, this document may contain discussions regarding beliefs or understandings of the underlying principles related to the devices and methods disclosed herein. It is recognized that, regardless of the ultimate correctness of any mechanistic explanation or hypothesis, an embodiment of the invention may nevertheless be operational and useful. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1. A graph showing methane production in a tube furnace as a function of methane partial pressure and temperature. 0.3 Ipm forming gas flow rate (H2 at 5%, N2 at 95%). FIG. 2. A graph showing methane production in a tube furnace in moles of methane as a function of time. 0.3 Ipm forming gas flow rate (H2 at 5%, N2 at 95%). FIG. 3. An XPS pattern of a product obtained from the reaction of CaCOs in a reducing environment illustrating the pure CaO that appears to be produced. STATEMENTS RELATING TO CHEMICAL COMPOUNDS AND NOMENCLATURE In general, the terms and expressions used herein have recognized meanings in the art, which can be found by reference to conventional texts, journal references, and contexts familiar to those skilled in the art. The following definitions are provided to clarify their specific use in the context of the invention. The terms thermal conversion and thermal conversion refer to the conversion of a first chemical species into a second chemical species through a thermally activated or thermally driven process, which can also be called a thermochemical process. An example of a process for the thermal conversion of a chemical species is combustion, although thermal conversion processes are not necessarily limited to it. For example, the thermal conversion of sulfur to sulfur dioxide can involve burning the sulfur, such as through a sulfur burner system. Thermal oxidation of a species is a form of thermal conversion of that species. For example, the thermal conversion of sulfur to sulfur dioxide can be called the thermal oxidation of sulfur to sulfur dioxide. In some embodiments, thermal conversion can be assisted by a catalyst.In some embodiments, thermal conversion does not require a catalyst or is carried out without one. It should be noted that thermal oxidation and electrochemical oxidation are different processes, where thermal oxidation is thermally driven or activated (through heat or burning) and electrochemical oxidation is electrochemically driven (e.g., through the application or extraction of electrical energy, optionally with the aid of an electrochemical catalyst). The term "thermally treat" refers to the heat treatment or exposure to heat, preferably with excess heat at room temperature, of one or more materials (such as a calcium salt, such as CaSO4), such that the one or more materials can be thermally converted, thermally decomposed, or otherwise undergo a heat-induced chemical change into another material (such as a cementitious material, such as CaO).For example, calcium sulfate (gypsum) can be thermally converted / decomposed into calcium oxide (CaO), along with the formation of byproducts such as SO2 and oxygen. Heat treatment can also cause a plurality of materials, such as those comprising calcium, aluminum, and silicon, to be converted or otherwise form a cementitious composite material, such as Ordinary Portland Cement (OPC). The term "calcium-containing starting material" refers to one or more materials whose chemical composition includes calcium. A calcium-containing starting material may be a single material, such as a mineral whose chemical composition includes the element calcium, such as in the form of calcium cations, or as part of an ionic material, such as a multimetal oxide material. A calcium-containing starting material may also be a plurality of materials, such as one or more rocks, minerals, and / or industrially processed material, where the chemical composition of the combination of such plurality of materials includes the element calcium, such as in the form of calcium cations or as part of an ionic material, such as a multimetal oxide material.In cases where a calcium-containing starting material is a plurality of materials, any one or any combination of such plurality of materials may have a chemical composition comprising the element Ca, such that the chemical composition of the combination of such plurality of materials (which together constitute the calcium-containing starting material) includes the element Ca. Preferably, a calcium-containing starting material having a chemical composition comprising the element Ca refers to the weight percent and / or molar percent of Ca in said calcium-containing starting material being at least 0.001%, preferably at least 0.01%, preferably at least 0.1%, more preferably at least 1%, furthermore, more preferably at least 5%, still more preferably at least 10%, and still more preferably at least 20%.On the other hand, the methods disclosed in this document are compatible with a starting material that carries calcium whose chemical composition has a low weight percentage and / or molar percentage, such as less than 60%, less than 55%, less than 50%, less than 45%, less than 40% and less than 20%, at least because the Ca, together with the respective counterions, can be isolated. Generally, the material or species having a chemical composition characterized by comprising an element X (where X is an element from the Periodic Table of Elements) refers to the weight percent and / or molar percent of X in said material or species being at least 0.001%, preferably at least 0.01%, more preferably at least 0.1%, and still more preferably at least 1%. The term calcium salt refers to a salt whose chemical composition includes the element Ca, for example, in the form of Ca cations. A salt is a chemical compound comprising ionic species associated with each other, at least in part, through ionic bonds. For example, CaSCU and CaCb are calcium salts, where Ca is a cation and SO4 and Cl, respectively, are anions. A regeneration step refers to a step in a process for the production of a species or material using a product from a different step that it consumes (e.g., is converted through a chemical change into another species or material) into that species or material. For example, a reaction characterized by (A + B → C + D) consumes species A and B to form species C and D. A reaction characterized by (C + E → A + F) can be called a regeneration reaction for the regeneration of species A using a product (species C) from the reaction that consumed species A. The term solid fraction refers to solid species present in a mixture of solid(s) and liquid(s). The term liquid fraction refers to liquid species and species dissolved in liquid species in a mixture of solid(s) and liquid(s). For example, a solid fraction may contain the solid products of a chemical reaction, and a liquid fraction may contain the liquid and dissolved products of the chemical reaction. Each of the solid and liquid fractions may optionally include unreacted reactants. The liquid fraction may also include solvent(s) and ions dissolved in that solvent(s). A dry mass of one or more materials, such as a solid fraction, refers to the mass of the one or more materials that are free of water and, optionally, free of any liquid species. The term metal oxide generally refers to a material whose chemical composition includes one or more metal elements and the element oxygen (O). Optionally, a metal oxide material is an ionic material, or at least partially an ionic material, where at least a fraction of the chemical bonding is characterized as ionic. A metal element is any metal or metalloid element from the periodic table of elements. Generally, a metalloid element is selected from the group consisting of boron (B), silicon (Si), germanium (Ge), arsenic (As), selenium (Se), antimony (Sb), terephthalate (Te), potassium (Po), and atmosphere (At). The term "natural rock" or "natural mineral" refers to one or more materials that occur naturally and have been extracted from the Earth's crust. Natural rocks and minerals include, but are not limited to, basalt, igneous apatites, wollastonite, anorthosite, montmorillonite, bentonite, calcium-containing feldspar, anorthite, diopside, pyroxene, pyroxenite, mafurite, kamafugite, clinopyroxene, colemanite, grossular, augite, pigeonite, marguerite, calcium serpentine, garnet, scheelite, skarn, limestone, natural gypsum, apatite, fluorapatite, and any combination thereof. In contrast, cement, concrete, Portland cement, fly ash, and slag are not natural rocks or minerals but may be termed industrially produced materials. The term bulk acid refers to an acid or acidic solution that does not require a continuous input of energy (such as electrical energy) and / or the exchange of electrons with an electrode surface to exist and function, as required in a particular process or stage thereof. Conversely, a heterogeneous or local acidic solution, such as one of hydronium ions or protons, near an electrode and formed as a result of and substantially only during the exchange of electrons between the electrode and the solution, is not a bulk acid. For example, a bulk acid is not a heterogeneous, local, or acidic solution corresponding to a pH gradient formed at an electrode during the electrolysis of water.In certain embodiments, the term "bulk acid" refers to an acid or acid solution that exhibits thermodynamic, chemical, and / or kinetic stability over a timescale of at least 10 seconds, preferably at least 1 minute, in the absence of electrical energy input. In certain embodiments, the term "bulk acid" refers to an acid or acid solution that exhibits or is capable of exhibiting thermodynamic, chemical, and / or kinetic stability over a timescale of at least 1 second and a length scale of at least 10 cm, preferably at least 10 cm, from the surface of a bulk material, in the absence of electrical energy input. The term electrochemical cell refers to devices and / or device components that perform electrochemistry. Electrochemistry refers to the conversion of chemical energy into electrical energy or vice versa. Chemical energy can correspond to a chemical change or a chemical reaction. Therefore, electrochemistry can refer to a chemical change (e.g., a chemical reaction of one or more chemical species into one or more different species) that generates electrical energy and / or the electrical energy that is converted into or used to induce a chemical change. Electrical energy refers to electrical potential energy, corresponding to a combination of electric current and electrical potential in an electrical circuit. Electrochemical cells have two or more electrodes (e.g., positive and negative electrodes; cathode and anode) and one or more electrolytes.An electrolyte can include species that are oxidized and species that are reduced during the charging or discharging of an electrochemical cell. Reactions that occur at the electrode, such as the adsorption and desorption of a chemical species or oxidation and reduction reactions, contribute to the charge transfer processes in the electrochemical cell. Electrochemical cells include, but are not limited to, electrolytic cells, such as electrolyzers, and fuel cells. Electrochemical oxidation can occur at the positive electrode, for example, and electrochemical reduction can occur at the negative electrode, for example. Electrochemical oxidation refers to a chemical oxidation reaction accompanied by a transfer of electrical energy (e.g., the input of electrical energy that drives the oxidation reaction) that occurs within an electrochemical cell.Similarly, electrochemical reduction refers to a chemical reduction reaction accompanied by a transfer of electrical energy that occurs within an electrochemical cell. A chemical species electrochemically oxidized during charging, for example, can be electrochemically reduced during discharging, and vice versa. The term electrochemically can describe a reaction, process, or a stage thereof, in which chemical energy is converted into electrical energy or electrical energy is converted into chemical energy. For example, a product can be formed electrochemically when electrical energy is supplied to facilitate the chemical conversion of one or more reactants into the product. The term non-electrochemical refers to a reaction or process that does not involve electrochemistry and / or does not require electrochemistry to occur. A reaction step refers to a stage of the process where a chemical reaction occurs, characterized by one or more chemical species undergoing a chemical change (such as through a chemical reaction with each other) into one or more other chemical species. The term elemental sulfur refers to any one or combination of the allotropes of sulfur, such as, but not limited to, S₁, Se₂, Se₃, S₁₂ and Si₃, and including crystalline, polycrystalline and / or amorphous sulfur. RHE refers to the reference electrode commonly called the reversible hydrogen electrode. SCE refers to the reference electrode commonly called the saturated calomel electrode. The term "initial operating hours" refers to the time during which the cell is operational, starting from the initial operation or startup of the cell. The time during which the cell or system is not operational (i.e., no electrochemical reduction or oxidation occurs, or no electrical energy input or output) is not included in the determination of initial operating hours. In some embodiments, the term aqueous refers to a solution where the solvent is water, such that other solution species, or solutes, are substantially solvated by water. In some embodiments, the term aqueous may generally refer to a solution comprising water. Optionally, but not necessarily, an aqueous solution or aqueous solvent includes 5% by volume or less of non-aqueous solvent and / or solute species. The term "agricultural water modification" refers to changing or adding something, such as a solute, to agricultural water. For example, acidifying agricultural water by adding sulfuric acid, such as a solution containing sulfuric acid, to the water. Agricultural water refers to water used for an agricultural purpose, such as irrigation. The term "soil modification" refers to changing or adding something to the soil. For example, acidifying the soil by adding sulfuric acid, such as a solution containing sulfuric acid, to the soil. The term cement refers to hydraulic, non-hydraulic, or both hydraulic and non-hydraulic cementitious materials. An example of cement is, but is not limited to, Portland cement. Generally, cement is a binding material that, for example, can be mixed with fine aggregate particles (such as to produce masonry mortar) or with sand and gravel (to produce concrete). According to certain embodiments, cement comprises calcium oxide. Cement may optionally further comprise one or more materials including, but not limited to, certain silicates, S₁₂O₂, certain oxides, Fe₂O₃, certain aluminates, Al₂O₃, belite, alite, tricalcium alumina, and brownmillerite. A cementitious material refers to a material that is or can be a constituent of cement. Preferably, a cementitious material has a chemical composition comprising Ca or CaO. For example, CaO is a cementitious material.For example, a cementitious material is a cementitious material. A cementitious composite material may include a plurality of materials, including at least one cementitious material and, optionally, one or more additives. Example cementitious composite materials are, but are not limited to, Portland cement clinker and Portland cement, such as Ordinary Portland Cement (OPC). The term "substantially" refers to a property or condition that is within 20%, optionally within 10%, optionally within 5%, optionally within 1%, or optionally equivalent to a reference property or condition. The expression "substantially equal," "substantially equivalent," or "substantially unchanged," when used in conjunction with a reference value describing a property or condition, refers to a value or condition that is within 20%, optionally within 10%, optionally within 5%, optionally within 1%, optionally within 0.1%, or optionally equivalent to the given reference value or condition. For example, a voltage that is substantially 500 mV (or substantially equivalent to 500 mV) is within 20%, optionally within 10%, optionally within 5%, optionally within 1%, or optionally equal to 500 mV.The term "substantially greater," when used with a reference value or condition describing a property or condition, refers to a value that is at least 2%, optionally at least 5%, optionally at least 10%, or optionally at least 20% greater than the provided reference value or condition. For example, a voltage is substantially greater than 500 mV if the voltage is at least 20% greater than, optionally at least 10% greater than, optionally at least 5% greater than, or optionally at least 1% greater than 500 mV. The term "substantially less," when used with a reference value or condition describing a property or condition, refers to a value or condition that is at least 2%, optionally at least 5%, optionally at least 10%, or optionally at least 20% less than the provided reference value or condition.For example, a voltage is substantially less than 500 mV if the voltage is at least 20% less than, optionally at least 10% less than, optionally at least 5% less than, or optionally at least 1% less than 500 mV. In addition, U.S. Patent Publication No. 2019 / 0376191 (Finke; U.S. Application No. 16 / 415,275) is incorporated herein by reference to the extent that it is not inconsistent with this application, which may contain additional useful terms and expressions, descriptions, and embodiments. In one embodiment, a composition or compound of the invention, such as an alloy or alloy precursor, is substantially isolated or purified. In one embodiment, an isolated or purified compound is at least partially isolated or substantially purified, as understood in the art. In one embodiment, a substantially purified composition, compound, or formulation of the invention has a chemical purity of 95%, optionally 99% in some applications, optionally 99.9% in some applications, optionally 99.99% in some applications, and optionally 99.999% in some applications. DETAILED DESCRIPTION OF THE INVENTION In the following description, numerous specific details of the devices, device components, and methods of the present invention are set forth in order to provide a detailed explanation of the precise nature of the invention. However, it will be evident to those skilled in the art that the invention can be implemented without these specific details. The invention can be further understood by means of the following non-limiting examples. Example 1: A process for preparing calcium oxide or ordinary Portland cement from calcium-bearing rocks and minerals Conventional cement is prepared by the thermal decomposition of CaCOa into CaO, followed by mixing it with other materials, including Al₂Si₂O₅(OH)₄, Fe₂O₃, and CaSO₄. The thermal decomposition occurs at approximately 900 °C, and the final production of OPC takes place at approximately 1,450 °C. Most of the energy required and CO₂ emissions for cement preparation come from the thermal decomposition of limestone. CaCOs —> CaO + CO2 ΔΗ = 178 (non-spontaneous) (FX5). Conventional cement requires 2.7 to 6 GJ / ton of OPC and produces 0.7 to 1.3 tons of CO2 per ton of OPC (Ordinary Portland Cement). The ordinary cement process emits a lot of CO2 and consumes a lot of energy. This disclosure includes a process for producing CaO from any calcium-bearing rock or mineral. In nature, acid (e.g., H₂CO₃ or H₂SO₄) weathers calcium-bearing minerals to typically produce CaCO₃ or CaSO₄. The general trend of weathering is as follows: H2CO3 + CaAI2S¡208 + H2O -> CaCO3+ AI2S¡2O5(OH)4 (FX6); either H2SO4 + CaAl2SI20e + H2O—► CaSO4 + Al2SI2Os(OH)4 (FX7). In nature, these acids are very dilute and, typically, this weathering occurs over long periods of time (from weeks to decades). Weathering can occur with any mineral or rock containing calcium. Common examples include wollastonite, anorthosite, calcium bentonite, montmorillonite, plagioclase, and basalt. All calcium-bearing rocks are susceptible, including all mafic and ultramafic rocks. The methods disclosed herein may use an acid (e.g., H₂SO₄, HF, HCl, and H₂CO₃) or a combination of acids plus a calcium-bearing rock or mineral (e.g., anorthosite, montmorillonite, and wollastonite) to produce a calcium salt (e.g., CaSO₄, CaF₂, CaCb, and CaCO₃). This salt may then be hydrated or thermally decomposed to produce CaO. It may also be possible to achieve the correct ratios of starting materials to thermally decompose the calcium salt and byproducts into a cementitious material including ordinary Portland cement or calcium sulfoaluminate cement.The strength, concentration, or quality of the acid and the particle size of the mined calcium-bearing rock can change the kinetics of the removal of calcium salts from the calcium-bearing starting material, and different acid concentrations and crushed rock sizes may be optimal for different versions of this process. The advantages of the processes described in this document may include that they can be CO2-free and energy-neutral. For example: H2SO4 + CaAI2SI2O8+ H2CX CaSO4 + AI2SI2O5(OH)4 (FX8); CaSO4 —> CaO + SO2 + 702 (FX9); SO2 + H2O + 702 — H2SO4 (FX10); Net product: CaAl2S12O8 + 2H2O -+ CaO + Al2S12O5(OH)4 (FX11); ΔH = ~0 This process could also be used to prepare clean hydrogen if electrochemical cogeneration of H2 and H2SO4 were used: H2SO4 + CaALShOe + H2O^ CaSO4+ Al2Si2O5(OH)4 (FX12); CaSO4-> CaO + SO2+ / 2O2 (FX13); SO2 + 2H2O — H2 + H2SO4 (FX14); Net product: CaAI2Si2O8 + 2H2O —» CaO + AI2S¡2O5(OH)4 +1ΛΟ2+ H2(FX15); ΔΗ = 50 (slightly increasing) Example 2: Reductive thermal decomposition of limestone to prepare lime or cement Lime is used directly as a basic chemical and as the main constituent of cement, which is the most widely used man-made material on the planet. Lime is currently produced through the thermal decomposition of limestone in an air atmosphere (FX16). CaCO3-+ CO2+ CaO (FX16); The heat of decomposition of this reaction is 178 kJ / mol. The present invention includes a process for producing cement from limestone through reductive thermal decomposition with hydrogen. The first stage in the process can follow the following reactions: CaCO3+ 4H2CH4 + 2H2O (FX17A); either CaCO3+ 2H2CH4 + O2(FX17B); The water content of the reaction gas influences whether the reaction proceeds according to FX17A, FX17B, or both. CaCO3 can react with H2 to prepare CaO + CH4 + O2 or CaO + CH4 + 2H2O. If H2O is formed, 4 H2 are consumed. If O2 is formed, only 2 H2 are consumed. This reaction can be driven to consume only 2 H2 if a water atmosphere is present, for example. The reaction can be stopped at the same stage, or the second stage can be the pyrolysis of methane to regenerate hydrogen, or any chemical reaction involving methane: ChU —> 2H2+ C (FX18); One benefit of this reaction is that solid carbon can be produced instead of CO2, thus preventing atmospheric pollution. Another benefit of the FX17A reaction is its lower energy requirement compared to traditional thermal decomposition of limestone (13.1 kJ / mol). A benefit of the FX17B reaction is that 100% of the required hydrogen can be regenerated from methane pyrolysis. In certain embodiments, reaction FX17A occurs under reducing conditions above 700 °C, for example, in H2, an H2 / N2 atmosphere, or any other combination. Reaction FX17B can occur above 700 °C with H2 in a water atmosphere. For example, 2.011 g of CaCOa powder were placed in a tube furnace and heated at 7 °C per minute. For example, the forming gas was flowed at 0.3 liters per minute (Lpm). For example, a gas analyzer was attached to the back of the furnace to measure the methane concentration. The data are shown in Figures 1 and 2. The XPS test consists of determining that the resulting thermal decomposition produced >99% lime (Figure 3). By integrating these curves, it is determined that ~100% decarbonization was achieved. Example 3: Production of vesicle and cement materials Example Aspect 1: The production of ordinary Portland cement (OPC) from any calcium-containing starting material without the net production of acid-forming gases (e.g., SO2 and CO2). Examples of calcium-containing starting materials include: basalt, igneous apatites, wollastonite, slag, ash dust, anorthosite, montmorillonite, bentonite, calcium-containing feldspar, anorthite, diopside, pyroxene, pyroxenite, mafurite, kamafugite, clinopyroxene, colemanite, grossular, augite, pigeonite, daisy, calcium serpentine, garnet, scheelite, OPC, concrete, any rock that has some Ca or CaO by mass, especially rocks with >5%, >10%, or >15% CaO, any skarn, limestone, gypsum, apatite, or fluorapatite. In certain embodiments, this is accomplished by first producing >90% pure synthetic gypsum from the aforementioned calcium-containing rocks (details in claim 2), then thermally decomposing this gypsum to prepare CaO, and subsequently mixing it with appropriate ratios of other materials to form OPC. The SO2 produced is then converted to sulfuric acid (via the contact process or via a sulfur depolarization electrolyzer), which can then be recycled to prepare synthetic gypsum. The general chemistry is as follows: 1. CaAl2SI20e + H2SO4 -> CaSO4 (at >90% by weight in dry pure) + Al2O3 + SiO2 (FX19); 2. CaSO4+ heat -> CaO + 1 / 202 + SO2 (FX20); 3. CaO + XAI2O3 + yS¡02-> OPC (FX21); 4. SO2 + I / 2O2 + H2O-> H2SO4 (FX22); Today, ordinary Portland cement (OPC) is produced industrially exclusively from limestone (primarily CaCO3). OPC production involves, first, the production of CaO through the thermal decomposition of CaCO3 (e.g., CaCO3 + heat → CaO + CO2), and then heating the CaO with silica and alumina to form OPC, which is approximately 60% CaO by mass. CO2 production from cement manufacturing accounts for more than 5% of global CO2 emissions. In addition to limestone, OPC can also be prepared from gypsum (CaSO4). Mined gypsum (CaSO4) can be used in some methods for OPC production. In this process, CaSO4 is thermally decomposed to produce CaO (e.g., CaSO4 + heat → CaO + 1 / 2 O2 + SO2). This process can also be achieved with carbothermic or hydrothermal reduction, in which case CaS is produced by reacting CaSO4 with a reducing agent (e.g., coal), and then the CaS is cothermally decomposed with CaSO4 to prepare CaO. This process is known as the Mueller-Kuehne process. Neither of these processes is currently commercially viable because SO2 cannot be released into the atmosphere, and the global demand for SO2 is much lower than the demand for OPC. OPC can be produced from phosphogypsum (CaSO4 produced by reacting phosphate rock with H2SO4 to prepare phosphoric acid and gypsum). The fertilizer industry produces CaSO4 waste by reacting sulfuric acid with phosphate rock (mainly Ca5(PO4)3OH) to prepare phosphoric acid. This synthetic gypsum can be thermally decomposed to prepare CaO and then OPC, as in the process described above. The methods disclosed in this document drastically expand the range of starting materials from which OPC can be prepared, compared to conventional methods. Example 2: Production of CaSO4 at >90% purity from any calcium-containing rock. For example, first, HCl is reacted with the rock to dissolve the calcium chloride. The resulting SiO2 at >90% dry weight purity and other byproducts are separated by precipitation. Next, the dissolved solution is reacted with sulfuric acid, which selectively precipitates out CaSO4, as this is the only sulfate salt among the common sulfate salts (MgSO4, Al2(SO4)3, and Fe2(SO4)s) that is insoluble in water. This also regenerates the HCl. The sample chemistry is as follows: 1. CaAI2S¡208+ 8HCI -> CaCI200 + 2AICI3 (aq.) + S1O2(S) (FX23); 2. The solid and aqueous fractions are separated (FX24); 3. CaCl2 (aq) + 2AICI3(aq) + 4H2SO4 -> CaSO4(s) + Al2(SO4)3(aq) + 8HCI (FX25); The methods disclosed in this document include the production of CaSO4 at >90% dry weight purity, which consists of making the process less expensive, less complicated, and more controllable in terms of the material ratios required for the accurate production of OPC. Calcium sulfate can also be produced at 90% dry weight as a byproduct of the reaction of sulfuric acid with limestone (CaCO3) and phosphate rock (Ca5(PO4)3OH or Cas(PO4)3F). The products of these reactions are soluble in water (HF, H2PO4), liquids (H2O), or gases (CO2). Advantageously, the methods disclosed herein can produce high-purity synthetic gypsum from any rock, even if the by-products are not soluble in sulfuric acid. Example 4: Generation of valuable co-products Example 3: The production of alumina from any calcium-containing rock. This can be done by first leaching with HCl and then saturating the leaching solution with HCl, causing the high concentration of HCl to precipitate AlCl3. The AlCl3 can then be mixed with H2SO4 to prepare Al2(SO4)3 and regenerate the HCl. The Al2(SO4)3 can be thermally decomposed to prepare Al2O3 and SO2 in order to regenerate sulfuric acid. The example chemistry is as follows: 1. CaAhS^Oe + 8HCI -> CaCb (aq.) + 2AICl3(ac) + S¡O2(s) (FX26); 2. AlCbfac ) + HCI (ac.) -> AICh (s) + HCI (ac.) (FX27); 3. 2AICl3(s) + H2SO4 -> AI2(SO4)3 (FX28); 4. Al2(SO4)3 + heat -> 3SO2 + AI2O3 + 3 / 202 (FX29); 5. SO2 + 1 / 202 + H2O -> H2SO4 (FX30); Example aspect 4: the production of iron oxide from any rock containing calcium. Once the Al, Ca, and Si are removed through the process described above, only aqueous iron sulfate and magnesium sulfate remain in solution. If the water evaporates and the salts are heated to 500–700 °C, the iron sulfate will decompose into insoluble iron oxide, and the remaining magnesium sulfate can be dissolved in water, leaving only iron oxide. Example aspect 5: the production of complementary cementitious materials, including silica fume, from calcium-containing rocks. An additional benefit of the present process is that, because it dissolves everything except silica, the particle size of everything is very small, and therefore synthetic silica fume can be prepared. The production of value-added co-products is a significant advantage of the methods disclosed herein. An unexpected additional benefit of the leaching stage(s), corresponding to the first reaction stage, or the reaction of a calcium-bearing starting material with a first acid, is that they can produce numerous co-products, including Al₂O₃, SiO₂, silica of silica fume quality, FeO₃, and MgO. These products can also be highly pure due to the use of chemical separation. The use of concentrated HCl to precipitate aluminum had been employed to prepare AlCl₃ from aluminum-bearing rocks, but not to prepare Al₂(SO₄)₃, as disclosed herein, according to certain embodiments, which has the benefit of greater efficiencies in thermal decomposition and regeneration of valuable HCl. The methods disclosed in this document include the benefits of expanding the starting materials that are capable of preparing these products and, in many cases, achieving better process efficiencies, purities, and product qualities than conventional processes. STATEMENTS RELATING TO INCORPORATION BY REFERENCE AND VARIATIONS All references throughout this application, for example, patent documents, including patents filed or granted or equivalent; publications of patent applications; and documents from the non-patent bibliography or other source material; are incorporated herein by reference in their entirety, as if they were individually incorporated by reference, to the extent that each reference is not at least partially inconsistent with the disclosure in this application (for example, a reference that is partially inconsistent is incorporated by reference, with the exception of the partially inconsistent part of the reference). The terms and expressions employed herein are used for descriptive purposes and not for limitation. There is no intention in using such terms and expressions to exclude any equivalent of the features shown and described, or any part thereof. It is acknowledged that various modifications are possible within the scope of the claimed invention. Therefore, it should be understood that, although the present invention has been specifically disclosed by means of preferred embodiments, exemplary embodiments, and optional features, those skilled in the art may resort to modifications and variations of the concepts disclosed herein, and such modifications and variations are deemed to be within the scope of the present invention as defined in the appended claims.The specific embodiments provided herein are examples of useful embodiments of the present invention, and it will be evident to a person skilled in the art that the present invention can be carried out using a large number of variations of the devices, device components, and method steps set forth herein. As will be evident to a person skilled in the art, the methods and devices useful for the present methods may include a large number of optional composition and processing elements and steps. As used herein and in the appended claims, the singular forms a, an, and the include plural references, unless the context clearly dictates otherwise. Thus, for example, a reference to an excipient includes a plurality of such cells and equivalents thereof known to those skilled in the art. Likewise, the expressions a, one, or more, and at least one may be used interchangeably herein. It should also be noted that the expressions comprising, including, and having may be used interchangeably. The expression in any one of claims XX-YY (where XX and YY refer to claim numbers) is intended to provide for a multiple dependent claim in the alternative form and, in some embodiments, is interchangeable with the expression as in any one of claims XX-YY. When a group of substituents is disclosed herein, it is understood that all individual members of that group and all subgroups, including any isomers, enantiomers, and diastereomers of the group members, are disclosed separately. When a Markush group or other grouping is used herein, all individual members of the group and all possible combinations and subcombinations of the group are intended to be included individually in the disclosure. When a compound is described herein in such a way that a particular isomer, enantiomer, or diastereomer of the compound is not specified, for example, in a formula or chemical name, that description is intended to include every isomer and enantiomer of the compound described individually or in any combination.Additionally, unless otherwise specified, all isotopic variants of the compounds disclosed herein are intended to be covered by the disclosure. For example, it is understood that any one or more hydrogens in a disclosed molecule may be replaced with deuterium or tritium. Isotopic variants of a molecule are generally useful as standards in assays for the molecule and in chemical and biological investigations related to the molecule or its use. Methods for the preparation of such isotopic variants are known in the art. The specific compound names are intended as examples, since it is known that a person skilled in the art may name the same compounds differently. Certain molecules disclosed herein may contain one or more ionizable groups [groups from which a proton can be removed (e.g., -COOH) or added (e.g., amines) or which can be quaternized (e.g., amines)]. All possible ionic forms of such molecules and their salts are intended to be included individually in the disclosure herein. With respect to the salts of the compounds herein, a person skilled in the art may select from a wide variety of available counterions those suitable for preparing salts of the present invention for a given application. In specific applications, the selection of a given anion or cation for the preparation of a salt may result in an increase or decrease in the solubility of that salt. Each device, system, formulation, composition, combination of components, or method, or step thereof, described or exemplified herein may be used to implement the invention, unless otherwise indicated. When a range is given in the descriptive specification, for example, a temperature range, a time range, or a composition or concentration range, all intermediate ranges and subranges, as well as individual values ​​within those ranges, are intended to be included in the disclosure. Any subrange or individual value within a range or subrange included in the description herein may be excluded from the claims herein. All patents and publications mentioned in the specification are indicative of the levels of expertise of those skilled in the art to which the invention pertains. The references cited herein are incorporated herein in their entirety to indicate the state of the art as of their publication or filing date, and it is intended that this information may be used herein, if necessary, to exclude specific embodiments found in the prior art. For example, when claiming a material composition, it should be understood that compounds known and available in the art prior to the applicant's invention, including compounds for which enabling disclosure is provided in the references cited herein, are not intended to be included in the material composition of the claims herein.As used herein, the expression "comprising" is synonymous with "including," "containing," or "characterized by," and is inclusive or open-ended and does not exclude additional elements or method steps not mentioned. As used herein, the expression "consisting of" excludes any element, step, or ingredient not specified in the element of the claim. As used herein, the expression "consisting essentially of" does not exclude materials or steps that do not materially affect the basic and novel features of the claim. In each instance, any of the expressions "comprising," "consisting essentially of," and "consisting of" may be replaced by any of the other two expressions.The invention described in an illustrative manner herein can be properly implemented in the absence of any element or elements, limitation or limitations not specifically disclosed herein. A person skilled in the art will appreciate that the starting materials, biological materials, reagents, synthetic methods, purification methods, analytical methods, testing methods, and biological methods other than those specifically exemplified can be employed in the practice of the invention without resorting to excessive experimentation. All functional equivalents known in the art of any such materials and methods are intended to be included in the present invention. The terms and expressions employed are used for descriptive purposes and not for limitations, and there is no intention in their use to exclude any equivalent of the features shown and described, or parts thereof, but rather it is acknowledged that various modifications are possible within the scope of the claimed invention.Therefore, it should be understood that, although the present invention has been specifically disclosed by means of preferred embodiments and optional features, those skilled in the art may resort to modification and variation of the concepts disclosed herein, and such modifications and variations are deemed to be within the scope of the present invention, as defined in the appended claims.

Claims

1. A method for producing a cementitious material, the method comprising the steps of: firstly, reacting a starting material containing calcium with a first acid to produce a first aqueous calcium salt; secondly, reacting the first aqueous calcium salt with a second acid to produce a second solid calcium salt; wherein the second acid is different from the first acid and the second calcium salt is different from the first calcium salt; and heat-treating one or more calcium salts to produce a first cementitious material.

2. The method of claim 1, wherein the one or more calcium salts are the second calcium salt.

3. The method of claim 1 or 2, wherein, during the second reaction stage, the reaction between the first calcium salt and the second acid regenerates the first acid.

4. The method of any one of claims 1-3, characterized by a net reaction free of an acid-forming gas product.

5. The method of any one of claims 1-4, comprising forming the second solid calcium salt characterized by a purity greater than or equal to 90% by weight purity.

6. The method of any one of claims 1-5, comprising a first separation step after the first reaction step and before the second reaction step; the first separation step comprising separating a first aqueous fraction from a first solid fraction; wherein the first aqueous fraction comprises the first aqueous calcium salt and the first solid fraction comprises one or more solid by-products formed during the first reaction step.

7. The method of any one of claims 1-6, comprising a second separation step after the second reaction step and before the heat treatment step; the second separation step comprising separating a second solid fraction from a second aqueous fraction; wherein the second solid fraction comprises the second solid calcium salt and the second aqueous fraction comprises one or more aqueous by-products formed during the second reaction step.

8. The method of claim 7, wherein the solid fraction is characterized by a dry mass of which at least 90% by dry weight is the second calcium salt.

9. The method of any one of claims 1-8, comprising a second acid regeneration stage; wherein the second acid regeneration stage comprises converting one or more gas products from the heat treatment stage into the second acid.

10. The method of claim 9, wherein the second acid regeneration step is a non-electrochemical process carried out according to Formula FX1A: SO2 + ½O2 + H2O H2SO4 (FX1A); wherein: the SO2 in FX1A is a gas product from the heat treatment step; the H2SO4 generated in FX1A is used as at least a fraction of the second acid during the second reaction step.

11. The method of claim 9, wherein the second acid regeneration step is a non-electrochemical process carried out according to Formula FX1B: SO2 + H2O H2SO3 (FX1B); wherein: the SO2 in FX1B is a gas product from the heat treatment step; the H2SO3 generated in FX1B is used as at least a fraction of the second acid during the second reaction step.

12. The method of claim 9, wherein the second acid regeneration step comprises (i) electrochemically oxidizing sulfur dioxide to sulfuric acid and (ii) forming hydrogen gas through a reduction reaction; and wherein the second acid regeneration step is carried out according to Formula FX2: SO2 + 2H2O -> H2SO4 + H2 (FX2); wherein: the SO2 in FX2 is a gas product from the heat treatment step; the H2SO4 generated in FX2 is used as at least a fraction of the second acid during the second reaction step.

13. The method of claim 12, wherein the heat treatment step comprises the use of energy generated from the oxidation of hydrogen gas formed as a result of the second acid regeneration.

14. The method of claim 12, wherein the electrochemical oxidation of sulfur dioxide comprises using the energy generated as a result of the second acid regeneration stage.

15. The method of any of claims 1-14, wherein, during the second reaction step, the reaction between the first calcium salt and the second acid regenerates the first acid according to Formula FX3: CaCl2(aq) + H2SO4 -> CaSO4(s) + 2HCl (FX3); wherein: the first calcium salt is CaCl2; the first acid is HCl; the second acid is H2SO4; and the second calcium salt is CaSO4.

16. The method of any one of claims 1-15, wherein the starting material containing calcium has at least 1% by dry weight of Ca.

17. The method of any one of claims 1-16, wherein the calcium-bearing starting material comprises at least one multi-metal oxide material having a composition comprising Ca and at least one other metal element selected from the group consisting of Al, Si, Fe, Mn and Mg.

18. The method of claim 17, wherein the composition of the at least one multi-metal oxide comprises less than or equal to 55% by dry weight of Ca.

19. The method of any one of claims 1-18, wherein the starting material containing calcium comprises at least one rock or natural mineral.

20. The method of any one of claims 17-18, wherein the at least one multi-metal oxide material is at least a natural rock or mineral.

21. The method of any one of claims 19-20, wherein the at least one rock or natural mineral comprises basalt, igneous apatites, wollastonite, anorthosite, montmorillonite, bentonite, calcium-containing feldspar, anorthite, diopside, pyroxene, pyroxenite, mafurite, kamafugite, clinopyroxene, colemanite, grossular, augite, pigeonite, margarita, calcium serpentine, garnet, scheelite, skarn, limestone, natural gypsum, apatite, fluorapatite, or any combination thereof.

22. The method of any one of claims 1-21, wherein the starting material containing calcium comprises cement, concrete, Portland cement, ash dust, slag, or any combination thereof.

23. The method of any one of claims 1-22, wherein the first acid is hydrochloric acid.

24. The method of any one of claims 1-23, wherein the second acid is sulfuric acid and / or sulfurous acid.

25. The method of any one of claims 1-24, wherein the first aqueous calcium salt is calcium chloride.

26. The method of any one of claims 1-25, wherein the second solid calcium salt is calcium sulfate and / or calcium sulfite.

27. The method of any one of claims 1-26, wherein the first cement material is calcium oxide.

28. The method of any one of claims 1-27, wherein the acid-forming gas product is SO2 and / or CO2.

29. The method of any one of claims 1-28, wherein the first reaction step comprises reacting the starting material containing calcium with hydrochloric acid to form at least aqueous calcium chloride, aqueous aluminum chloride, and solid silica.

30. The method of claim 29, wherein the first separation step comprises separating a first aqueous fraction comprising aqueous calcium chloride and aqueous aluminum chloride from a first solid fraction comprising solid silica.

31. The method of claim 30, wherein the second reaction step comprises reacting at least aqueous calcium chloride, aqueous aluminum chloride and sulfuric acid to form at least solid calcium sulfate, aqueous aluminum sulfate and hydrochloric acid.

32. The method of claim 31, wherein the heat treatment step comprises heating the calcium sulfate to form calcium oxide.

33. The method of any one of claims 1-32, comprising a step of forming a composite cement material; wherein: (i) the heat treatment step comprises the step of forming the composite cement material and the first cement material is the composite cement material or (ii) the composite material formation step is carried out using the first cement material formed during the heat treatment step.

34. The method of claim 33, wherein the cementitious material formation step of the composite material comprises heating the second calcium salt and / or the first cementitious material together with one or more additives.

35. The method of any one of claims 33-34, wherein the cementitious composite material formation step is carried out simultaneously with the heat treatment step.

36. The method of any one of claims 33-35, wherein the cementitious composite material formation step is performed subsequently after the heat treatment step.

37. The method of any one of claims 33-36, wherein the cementitious material of the composite material is ordinary Portland cement or Portland cement clinker and / or the first cementitious material is calcium oxide.

38. The method of any one of claims 34-37, comprising forming one or more additives from the starting material containing calcium.

39. The method of any one of claims 34-38, wherein the one or more additives are one or more by-products of the first reaction step and / or are formed from one or more by-products of the first reaction step and / or are one or more by-products of the second reaction step and / or are formed from one or more by-products of the second reaction step.

40. The method of any one of claims 34-39, wherein a combined composition of the one or more additives comprises Al and Si.

41. The method of any one of claims 34-40, wherein the one or more additives are at least Al2O3 and SiO2.

42. The method of any one of claims 1-41, comprising forming and isolating silica of fume-grade silica, nanosilica and / or microsilica from calcium-bearing starting material.

43. The method of any one of claims 1-42, comprising forming and isolating alumina from the starting material containing calcium.

44. The method of claim 43, wherein the first reaction step comprises reacting the starting material containing calcium with hydrochloric acid to form at least aqueous aluminum chloride; wherein the method further comprises: precipitating the aluminum chloride in the presence of hydrochloric acid; reacting the precipitated aluminum chloride with sulfuric acid to form solid aluminum sulfate; heating the aluminum sulfate and / or the aluminum chloride to form alumina.

45. The method of claim 44, wherein the reaction of the precipitated aluminum chloride forms hydrochloric acid.

46. ​​The method of claim 44 or 45, wherein the second reaction step comprises the reaction step of precipitated aluminum chloride.

47. The method of any one of claims 44-46, wherein the heat treatment step comprises heating the aluminum sulfate and / or aluminum chloride step.

48. The method of any one of claims 1-47, comprising forming and isolating iron oxide from the starting material containing calcium.

49. The method of claim 48, wherein the step of forming and isolating the iron oxide comprises: forming an aqueous solution having iron sulfate and / or aqueous iron chloride and, optionally, at least one other metal magnesium sulfate salt and / or chloride salt formed as by-products during the second reaction step; wherein the aqueous solution is free from a calcium salt and free from an aluminum salt; drying the aqueous solution to form solid iron sulfate and / or solid iron chloride and, optionally, the at least one other metal sulfate salt; heating the solid iron sulfate and, optionally, the at least one other metal sulfate salt to form a water-insoluble iron oxide; and optionally, dissolving the at least one other metal sulfate salt to isolate the water-insoluble iron oxide.

50. The method of any one of claims 1-49, further comprising a first acid formation step; wherein: (i) the first reaction step comprises the first acid formation step and the first acid formation step occurs simultaneously with the first reaction step or (ii) the first acid formation step is carried out separately from the first reaction step.

51. The method of any one of claims 1-50, further comprising a second acid formation step; wherein: (i) the second reaction step comprises the second acid formation step and the second acid formation step occurs simultaneously with the second reaction step or (ii) the second acid formation step is carried out separately from the second reaction step.

52. The method of claim 51, wherein the step of forming the second acid comprises reacting SO2 with water to form H2SO3 and / or H2SO4, wherein the second acid is H2SO3 and / or H2SO4.

53. The method of claim 52, wherein the second acid is H2SO3 and / or H2SO4 and wherein the second calcium salt is CaSOs and / or CaSO4, respectively.

54. The method of any one of claims 1-53, wherein the first acid and / or the second acid is a bulk acid.

55. The method of any one of claims 1-53, wherein the step of forming the first acid comprises forming a pH gradient through the electrolysis of water; wherein the first acid is formed through the electrolysis of water.

56. The method of any one of claims 1-53, wherein the step of forming the second acid comprises forming a pH gradient through the electrolysis of water; wherein the second acid is formed through the electrolysis of water.

57. The method of claim 10, wherein the second acid regeneration stage according to Formula FX1A is carried out at a temperature selected from the range of 400 °C to 1,800 °C, is exothermic and is carried out in the presence of a catalyst.

58. The method of any one of claims 1-57, wherein the method is characterized by a net energy selected from the range of -2 to +2 GJ per metric tonne of cement material produced.

59. The method of any one of claims 1-58, wherein the first reaction step is exothermic.

60. The method of any one of claims 1-59, wherein the second acid regeneration stage is exothermic.

61. The method of any one of claims 1-60, wherein the first reaction step is carried out at a temperature of at least 50 °C.

62. The method of any one of claims 1-61, wherein the heat treatment step is carried out at a temperature selected from the range of 1,100 °C to 1,800 °C.

63. The method of any one of claims 1-61, wherein the heat treatment step is carried out in the presence of a chemical reducing agent and is performed at a temperature selected from the range of 800 °C to 1,200 °C.

64. The method of any one of claims 1-62, further comprising an ion exchange step; wherein the ion exchange step comprises exchanging one or more anions of the first calcium salt and / or the second calcium salt for one or more hydroxyl anions to form a third calcium salt.

65. The method of claim 64, wherein the ion exchange step comprises reacting the first calcium salt and / or the second calcium salt with a chelating agent to form a calcium-chelating compound and reacting the calcium-chelating compound with a base to form the third calcium salt.

66. The method of claim 64 or 65, wherein the ion exchange step comprises reacting the first calcium salt and / or the second calcium salt with a base to form the third calcium salt.

67. The method of any one of claims 64-66, wherein the ion exchange step comprises using an ion exchange membrane to perform the exchange of one or more anions of the first calcium salt and / or the second calcium salt for one or more hydroxyl anions to form the third calcium salt.

68. The method of any one of claims 64-67, wherein the one or more calcium salts of the heat treatment step are the third calcium salt.

69. The method of any one of claims 64-68, wherein the third calcium salt is Ca(OH)2.

70. A method for producing a cementitious material through reductive thermal decomposition, the method comprising the steps of: reacting a calcium-bearing material with a chemically reducing gas to produce methane and a cementitious material.

71. The method of claim 70, wherein the calcium-bearing material comprises CaCOa, CaSO4, CaS, a calcium salt, or any combination thereof.

72. The method of claim 70, wherein the calcium-bearing material is CaCOs, CaSO4, CaS or any combination thereof.

73. The method of any one of claims 70-72, wherein the chemically reducing gas is hydrogen gas or a gas comprising hydrogen gas.

74. The method of any one of claims 70-73, wherein the cement material is CaO or the cement material comprises CaO.

75. The method of any one of claims 70-74, wherein the molar ratio of the calcium-bearing material that is reacted with the chemically reducing gas is 1:4 or 1:

2.

76. The method of any one of claims 70-75, wherein the reaction is carried out in the presence of water.

77. The method of any one of claims 70-75, wherein the reaction is carried out in the absence of water.

78. The method of any one of claims 70-77, wherein oxygen gas, water, or a combination of oxygen gas and water is produced during the reaction step.

79. The method of any one of claims 70-78, further comprising a step of decomposing methane to produce hydrogen gas and one or more carbon materials.

80. The method of any one of claims 70-79, wherein the method does not comprise the formation of CO2.

81. The method of any one of claims 70-80, wherein the reaction step is characterized by a lower heating value (LHV) of 720 kJ / mol or less and a higher heating value (HHV) of 800 kJ / mol or less.

82. The method of any one of claims 70-81, wherein the reaction step is carried out at a temperature of at least 700 °C.

83. A method for producing a cementitious material, the method comprising the steps of: firstly, reacting a calcium-bearing starting material with a first acid to produce a first aqueous fraction comprising a first aqueous calcium salt and a first solid fraction comprising one or more solid by-products; wherein: the calcium-bearing starting material has a chemical composition comprising a plurality of metal elements including at least Ca and Si; the one or more solid by-products comprise a silicon salt; firstly, separating the first aqueous fraction from the first solid fraction; and treating the first calcium salt to produce a first cementitious material.

84. The method of claim 83, wherein the treatment step comprises thermally treating the first calcium salt in the presence of water to produce the first cement material.

85. The method of claim 84, wherein the heat treatment of the first calcium salt regenerates the first acid.

86. The method of any one of claims 83-85, wherein the treatment step comprises an ion exchange step; wherein the ion exchange step comprises exchanging one or more anions of the first calcium salt for one or more hydroxyl anions to form a third calcium salt.

87. The method of claim 86, wherein the ion exchange step comprises reacting the first calcium salt with a chelating agent to form a calcium-chelating compound and reacting the calcium-chelating compound with a base to form the third calcium salt.

88. The method of claim 86 or 87, wherein the ion exchange step comprises reacting the first calcium salt with a base to form the third calcium salt.

89. The method of any one of claims 86-88, wherein the ion exchange step comprises using an ion exchange membrane to effect the exchange of one or more anions of the first calcium salt for hydroxyl anions to form the third calcium salt.

90. The method of any one of claims 86-89, wherein the third calcium salt is Ca(OH)2.

91. The method of any one of claims 86-90, wherein the treatment step comprises heat-treating the third calcium salt to produce the first cement material.

92. The method of any one of claims 83-91, wherein the first acid is hydrogen chloride.

93. The method of any one of claims 83-92, wherein the one or more solid by-products comprise S¡02.

94. The method of any one of claims 83-93, wherein the starting material containing calcium comprises at least one rock or natural mineral.

95. The method of any one of claims 49-69, wherein the step of forming and isolating the iron oxide further comprises: using SO2 to precipitate MgSOs; separating the aqueous iron salt from the solid magnesium salt; drying the aqueous solution to form solid iron sulfate or chloride and, optionally, the at least one other metal sulfate salt; heating the solid iron sulfate and, optionally, the at least one other metal sulfate salt to form a water-insoluble iron oxide; and, optionally, dissolving the at least one other metal sulfate salt to isolate the water-insoluble iron oxide.

96. The method of any one of claims 1-69, wherein the first reaction step comprises reacting the starting material containing calcium with hydrochloric acid to form at least aqueous calcium chloride, aqueous aluminum chloride, aqueous iron chloride, aqueous magnesium chloride, and solid silica.

97. The method of claim 96, wherein the first separation step comprises separating a first aqueous fraction comprising aqueous calcium chloride and aqueous aluminum chloride from a first solid fraction comprising solid silica.

98. The method of any one of claims 96-97, wherein the second reaction step comprises reacting at least aqueous calcium chloride and sulfuric acid to form at least solid calcium sulfate, solid calcium sulfate and hydrochloric acid.

99. The method of any one of claims 96-98, wherein the heat treatment step comprises heating the calcium sulfate to form calcium oxide.

100. The method of any one of claims 1-69 and 95-99, comprising forming and isolating aluminum chloride and forming aluminum metal by electroextraction of aluminum chloride.

101. The method of claim 100, wherein the formation and isolation of aluminum chloride comprises the co-formation of chlorine gas; the method further comprising reacting the chlorine gas with hydrogen to regenerate hydrochloric acid.

102. The method of any one of claims 1-69 and 95-99, comprising forming and isolating iron sulfate and forming iron metal through electroextraction of iron sulfate.

103. The method of claim 102, further comprising regenerating sulfuric acid.