A method of making a calcium and magnesium carbonate-based mixture

Abstract

A method of making a calcium and magnesium carbonate-based mixture comprises the steps of: in a first precipitation step adding a carbonate-containing solution to a solution containing calcium and magnesium cations to precipitate calcium carbonates; in a second precipitation step adding further carbonate-containing solution to the solution containing remaining calcium and magnesium cations without removing the precipitated calcium carbonates formed in the first precipitation step to precipitate magnesium carbonates and thus form the calcium and magnesium carbonate-based mixture. The calcium and magnesium carbonate-based mixture may be dried and hardened for use as a construction material.

Description

A METHOD OF MAKING A CALCIUM AND MAGNES IUM CARBONATE-BASED MIXTURE

[0001] The present invention relates to a method of making a calcium and magnesium carbonate-based mixture , and in particular a method of making a cementitious material comprising calcium and magnesium carbonates .

[0002] Nesquehonite is a common synthetic mineral which can be described as an intermediate phase between amorphous magnesium carbonates and "hydromagnesites" (a term often used to refer to both dypingite and hydromagnesite itself , due to their similarities ) . Typical precipitation conditions for nesquehonite are near ambient temperature and pressure , alkaline pH and a supply of Mg2+and COa2-ions . Precipitation of nesquehonite is thus a potential means for carbon capture and utilisation if CO2 from industrial sources or from air is used to produce the carbonate ions . Nesquehonite is often associated with other hydrated Mg2+carbonate minerals , such as barringtonite , lansfordite , dypingite and hydromagnesite , as well as minerals of other compositions , such as monohydrocalcite , in the instances where the reacted brine compositions include substantial concentrations of calcium ions .

[0003] It is known that upon heating, nesquehonite gradually loses its structural water, which affects its crystallographic properties (visible in X-ray diffraction) while preserving its morphological habit . The partially dehydrated mineral exhibits an X-ray diffraction pattern which has not been assigned a mineralogical name and hence does not appear in any of the crystallographic databases , usually being referred to as MgCO3. xH2O, depending on the remaining amount of structural water . There have been some studies on nesquehonite regeneration from the describedpartial dehydration product , and the dehydration rehydration cycles have been found to aid in the development of the compressive strengths of the nesquehonite-based materials . For example , WO 2015 / 198050 describes a method for producing an activated nesquehonite which comprises heating the nesquehonite .

[0004] Climate change arising from the increase of greenhouse gases in the atmosphere is well documented and presents a highly undesirable threat to numerous ecosystems at both local and planetary scales . Greenhouse gases such as carbon dioxide are predominantly released into the atmosphere during combustion of carbon-based fuel s such as coal , oil and natural gas , e . g . , in power plants , as well as during industrial processes such as calcination used in cement production . Although carbon dioxide is naturally captured from the atmosphere through biological processes such as photosynthesis by land and marine plant-life , the concentration of carbon dioxide in the atmosphere continues to rise . Carbon dioxide is also responsible for the acidification of the oceans posing a threat to marine ecosystems .

[0005] The present invention seeks to provide a method of making a material comprising a calcium and magnesium carbonate-based mixture through carbon capture for use as a building material . Thus , the present invention seeks to contribute towards the decarbonation of the building materials sector by introducing a new material which is , for example , suitable for the construction of interior walls and ceilings .

[0006] According to the present invention in a first aspect there is thus provided a method of making a calciumand magnesium carbonate-based mixture , which comprises the steps of : in a first precipitation step adding a carbonate- containing solution to a solution containing calcium and magnesium cations to precipitate calcium carbonates ; in a second precipitation step adding further carbonate-containing solution to the solution containing the remaining calcium and magnesium cations without removing the precipitated calcium carbonates formed in the first precipitation step to precipitate magnesium carbonates and thus form the calcium and magnesium carbonate-based mixture .

[0007] The present invention thus provides a method comprising a multi-step precipitation of carbonates , which in a first precipitation step involves removing calcium ions from an aqueous solution through precipitation as calcium carbonates , and in a second precipitation step involves growth of magnesium carbonates preferably onto the pre-existing calcium carbonates , resulting in clusters of calcium and magnesium carbonate-based mixtures having unique morphologies . Thus , the method does not involve an intermediate step of solid separation between the first and second precipitation steps , and instead involves the sequential precipitation of a secondary precipitate (magnesium carbonates ) onto the surfaces of a primary precipitate ( calcium carbonates ) . This approach contrasts with the usual method of synthetic nesquehonite production whereby calcium carbonates are removed to avoid implications on precipitation and strength development . The method may be used to capture CO2 as carbonates the production process by securely storing the CO2 within the structure of the calcium and magnesium carbonate precipitates . The total mass of CO2 which may be held within the precipitates is approximately 32 wt% for pure nesquehonite , 37 . 5wt% for pure monohydrocalcite , and 44wt%for pure calcite. In addition, the method of the present invention may use desalination reject brines, or other magnesium ion-containing industrial brines, such as from the oil and gas industry, which would otherwise be released into the ocean, causing harm to the aquatic life and general environment, as the basis for the solution containing calcium and magnesium cations.

[0008] The method of the present invention preferably further comprises an additional step of precipitate processing to form a cementitious material, and more preferably the further step of molding and curing the cementitious material to form a construction material.

[0009] Thus, the method of the first aspect of the present invention comprises a first precipitation step of adding a carbonate-containing solution to a solution containing calcium and magnesium cations to precipitate calcium carbonates. Preferably, substantially only calcium carbonates precipitate from the solution in the first precipitation step, i.e. the magnesium cations substantially remain in solution.

[0010] The carbonate-containing solution may be obtained from different sources and by different methods.

[0011] Thus, for example, the carbonate-containing solution may be obtained by dissolving carbon dioxide in an alkaline aqueous solution. The carbonate-containing solution obtained in this way will contain both carbonate (COa2-) and bicarbonate ions (HCOa-) .

[0012] The carbon dioxide may be a waste gas from an industrial plant e.g. a power plant, a chemical processing plant, a cement factory, an oil refinery or the like. Forexample, a waste gas from a coal combustor typically comprises around 15% carbon dioxide, and effluent gases from cement kilns typically comprise around 30% carbon dioxide.

[0013] The waste gas may be passed through a carbon dioxide scrubber to separate the carbon dioxide from other components of the waste gas. Any oxygen and nitrogen in the waste gas may be vented to the atmosphere. The waste gas may be passed through a particulate matter filter to remove particulates and / or a thermal heat exchanger to recover waste heat for use in other steps, such as precipitate processing. The waste gas may be passed through a water recovery unit for recovering water from the waste gas. Recovering water from the waste gas helps reduce dilution of the calcium and magnesium cations.

[0014] The alkali for forming the alkaline aqueous solution may be sodium hydroxide, although other alkalis such as potassium or calcium hydroxide may be used. With alkali metal alkalis such as sodium hydroxide, the molar ratio of metal hydroxide to carbon dioxide required to form the pure carbonate, for example sodium carbonate, Na2COs, is 2:1. However, both carbonate and bicarbonate ions are formed during formation of the carbonate-containing solution (e.g. NaHCOa) , which reduces this molar ratio. The molar ratio of the alkali to the carbon dioxide may thus be between 1:1 and 2:1, for example from 1.2 to 1.5.

[0015] The alkali may comprise Clinker Kiln Dust (CKD) , which is a waste material produced in the manufacture of Portland cement that is often disposed of as a low hazard material in landfill. Its composition is variable but typically comprises alkali-metal sulphates and oxides such as sodium hydroxide and potassium hydroxide which can be dissolved to yield hydroxide anions. An alkaline aqueous solution formed by theaddition of CKD may be filtered to remove any suspended particulates . CKD may be used in combination with sodium hydroxide or another alkali .

[0016] The alkali may comprise fly ash, also known as Pulverised Fuel Ash ( PFA) , which is a coal combustion product , the bulk of which is invariably disposed in landfill . Its composition is variable but typically contains both amorphous and crystalline silicon dioxide , aluminium oxide and calcium oxide , which can be dissolved to yield hydroxide anions . An alkaline aqueous solution formed by the addition of PFA may be filtered to remove any suspended particulates . PFA may be used in combination with sodium hydroxide or another alkali such as CKD .

[0017] The alkali may comprise Lime Sludge (LS ) , which is a waste material from wastewater remediation and purification . It is a semi-solid slurry produced as sewage sludge from the application of lime (CaO) as a biocide in wastewater treatment processes or as a settled suspension obtained from conventional drinking water treatment and numerous other industrial processes . I f the correct quicklime , hydrated lime or liquid lime dose is applied, the treated sludge will be elevated to pH 12 for at least 72 hours . LS will increase calcium cation availability and can be used in combination with sodium hydroxide and CKD or another alkali .

[0018] The alkali may comprise ammonia . Ammonia dissociates into ammonium ions (NH4+) and hydroxide ions upon dissolution in water . The molar ratio of the ammonia to the carbon dioxide in this case is preferably around 1 . Ammonia is a preferred alkali since it can be recycled ( or regenerated) and re-used .

[0019] The alkali may be added to water to form an alkalineaqueous solution (e.g. having a pH of from 9 to 11) prior to dissolution of the carbon dioxide to form the alkaline carbonate-containing solution. The carbon dioxide may be bubbled through or sprayed onto the alkaline aqueous solution. Alternatively, the carbon dioxide may react with the alkaline aqueous solution in a packed, contra-flow absorption column. In other embodiments, the alkaline aqueous solution may be sprayed into / across the flow path of a carbon dioxide-rich gas stream.

[0020] Carbon dioxide reacts with hydroxide anions to form carbonate ions as follows:

[0021] CO2(g) + 2OH“ (aq) -> CO32’ (aq) + H2O (1)

[0022] Bicarbonate ions may also be formed as follows:

[0023] CO2(g) + CO32- (aq) + H2O (l)-> 2HCO3- (aq)

[0024] Where the alkali is or includes sodium hydroxide the solution will contain sodium carbonate and / or sodium bicarbonate .

[0025] In these embodiments, the carbon dioxide is preferably dissolved in the alkaline aqueous solution at a temperature of between 0-30 °C. More preferably, the carbon dioxide is dissolved in the alkaline aqueous solution at a temperature of 20-30°C to ensure effective mixing, avoid freezing, and manage other operational factors.

[0026] Alternatively, the carbonate-containing solution may be obtained by the dissolution of industry grade sodium carbonate, for example as produced in the Solvay process in which salt (sodium chloride) is reacted with limestone(calcium carbonate) and coking coal in the presence of ammonia to produce synthetic soda ash (sodium carbonate) .

[0027] Another source of carbonate-containing solution for ue in the method of the present invention is waste streams from hydrometallurgical refining processes. These waste streams usually contain high levels of carbonates due to the alkalinity of the raw materials, for example in bauxite, zinc, and nickel processing.

[0028] The solution containing calcium and magnesium ions may be an artificial or natural brine which contains a sufficient supply of Ca2+and Mg2+ions. The calcium and magnesium ion concentrations in the solution dictate the amount of COa2-which is supplied. In the first precipitation step, the carbonate-containing solution is added to the calcium and magnesium cation containing solution, for example the artificial or natural brine, to precipitate calcium carbonates. The calcium carbonates may comprise monohydrates, anhydrous polymorphs and their mixtures. The carbonate ions are preferably provided at an equimolar ratio to the calcium ions .

[0029] The first and second precipitation steps are preferably performed at or near ambient conditions, for example at temperatures between 0°C and 50°C, preferably 20-30 °C, and at atmospheric pressure. The first and second precipitation steps are performed at alkaline pH, for example a pH in the range of approximately 9-11.

[0030] The first precipitation step is preferably carried out under dynamic conditions, for example with constant stirring for homogeneity of the aqueous system. Vigorous agitation is preferred (for example stirring at 200-400 rpm) . The first precipitation step may be performed, for example, for 1-3hours, although either longer or shorter periods may be used as appropriate.

[0031] In the second precipitation step, further carbonate- containing solution, preferably at an equimolar ratio to magnesium ions in the aqueous solution, or an excess of carbonate ions, is added to precipitate magnesium carbonates. As described herein, the method does not require the separation of the primary precipitate (calcium carbonates) prior to starting the second precipitation step.

[0032] The second precipitation step may be performed, for example, for 1-3 hours, preferably with gentle agitation compared to the first precipitation step (for example, stirring at 25 - 100 rpm, e.g. 50 rpm) . Slower stirring has been found to be more conducive to the formation of larger needles, which is desirable for a high strength product. The crystals of the magnesium carbonates may grow on the surfaces of calcium carbonates, and / or around them, forming clusters.

[0033] The product resulting from the first and second precipitation steps preferably comprises a mixture of calcium carbonates and nesquehonite with a variety of morphological habits. For example, the product may comprise elongate hexagonal prisms of nesquehonite (e.g., up to 150 pm in length) and significantly smaller calcium carbonate grains of elliptical, round, or square shapes (depending on the polymorphic selection) , for example up to 5 pm in diameter.

[0034] As noted herein, the method of the present invention preferably further comprises an additional step of precipitate processing to form a cementitious material.

[0035] In this connection, the method of the present invention preferably produces a calcium and magnesium carbonate-basedmixture product which is self- or intrinsically-cementitious . These terms indicate that to achieve structural coherence and form a hardened material , the product of the method of the present invention does not require the inclusion of cement clinker or other materials with poz zolanic activity . Instead, it requires only activation, preferably thermal activation, and mixinq with water . Prior art maqnesium and calcium carbonates produced after a carbon capture and utili zation (CCU) process are often added to cement as a supplementary material to reduce clinker content , but their CO2 content in the final product remains low, e . q . below 10 % , necessitatinq the addition of clinker to impart cementitious properties . Alternatively, curinq MqO cements in CO2 atmospheres can develop strenqth, but the production of MqO from brines demands siqnificant thermal enerqy or emits larqe amounts of CO2 when extracted from maqnesite minerals . In contrast , the method of the present invention eliminates the need for clinker or other cementitious materials , substantial thermal enerqy, or hiqh temperatures . It follows a procedure akin to qypsum plaster production, facilitatinq its inteqration into existinq qypsum plaster plants .

[0036] Thus , the precipitate product obtained from the first and second precipitation steps is preferably dried at temperatures below 35 ° C until dry (preferably, the moisture content falls below 1 % ) . The dried product preferably does not contain any NaCl . Durinq the dryinq process the material may be spread out in a thin layer, and dryinq is preferably carried out over a period which prevents phase transitions from occurrinq, for example less than 48 hours . Preferably, phases other than calcium carbonates and nesquehonite are absent , i . e . , a preferred product substantially consists of calcium carbonates and nesquehonite . I f the dried product is to be stored for lonq periods , then moisture should be removedto avoid conversion to dypingite.

[0037] The dried product may then be hardened. Hardening is preferably carried out using one of two different methodologies: (A) partial dehydration of the raw material, and (B) thermally accelerated phase transition of compressed slurries .

[0038] (A) Dehydration-rehydration based hardening comprises heating the dried product to dehydrate it. For example, the dried product may be heated at a temperature of 110 - 130°C, for example for up to 4 hours, to allow partial dehydration of nesquehonite and monohydrocalcite to occur. The dehydration conditions are preferably selected so that other carbonate phases, such as calcite, remain inert.

[0039] The heating may be carried out, for example, in a fan assisted oven or a furnace. Preferably, there are no more than 2 molecules of structural water removed from nesquehonite to allow for its reconstitution.

[0040] Pastes may be prepared from the dehydrated product by mixing it with water. The pastes are preferably made with a liquid to solid ratio of 0.5 to 0.8, more preferably 0.75. The pastes may be moulded into shapes, and the moulded paste cured at, for example ambient conditions (e.g. room temperature 25°C (± 5°C) , and atmospheric pressure) .

[0041] As part of the hardening process, phase conversions (e.g., nesquehonite - dypingite) may result in morphological alterations, creating a porous matrix which gives strength to the hardened product.

[0042] A summary of preferred conditions for the first and second precipitation steps, heating, and dehydration-rehydration hardening steps, yielding products with a 7 day compressive strength of 12 MPa or more, is shown in Table 1 below :

[0043] (B) Thermally induced phase conversion-based hardening comprises mixing the dried product with water, moulding into shapes, and then gently heating at temperatures of for example 70 - 90°C to accelerate phase transformations (e.g. nesquehonite - hydromagnesites, monohydrocalcite anhydrous calcium carbonate) . The conversion of mineral phases is associated with morphological change which plays a key role in the hardening mechanism.

[0044] A summary of preferred conditions for the first and second precipitation steps, heating and thermally induced phase conversion-based hardening steps, yielding products with a compressive strength higher than 11 MPa after 7 days, is shown in Table 2 below:

[0045] The hardened carbonate products may be used to form products which are useful in interior wall / ceiling construction and similar applications, for example an alternative to gypsum, which is one of the most common minerals used in modern construction practices.

[0046] The present invention also provides in a second aspect a calcium and magnesium carbonate-based mixture, dried, or dried and hardened product thereof made according to the method of the present invention. The calcium and magnesium carbonate-based mixture, dried, or dried and hardened product may be used as a construction material.

[0047] The present invention will now be described in detail with reference to the accompanying drawing, in which:

[0048] Figure 1 shows SEM images of expected cluster morphologies: left - large nesquehonite needles encompassing smaller calcium carbonate grains; right - nesquehonite needles covered with calcium carbonate grains.

[0049] ExampleA preferred embodiment of the method of the present invention will now be described.

[0050] Carbonate-containing solution formationA carbonate-containing solution was formed by bubbling carbon dioxide gas through a sodium hydroxide solution at ambient temperature to form carbonate anions. The molar ratio of the alkali to the carbon dioxide was from 1.2 to 1.5.

[0051] PrecipitationIn a preferred embodiment the raw material (a powder containing nesquehonite and calcium carbonates) , can be precipitated from artificial and natural brines, containing a sufficient supply of Mg2+and Ca2+ions.

[0052] In general, an equimolar concentration of Ca2++Mg2+: COa2-is required to optimise the carbon capture of the process and metal ion removal from the solution.

[0053] In a preferred embodiment the first precipitation step reaction takes place at or near ambient conditions, at temperatures above 0 °C and lower than 50 °C, ideally at 20- 30 °C and at atmospheric pressure. The reaction starts and remains at alkaline pH, in the range of approx. 9-11.

[0054] The calcium and magnesium carbonate-based product mixture is precipitated in two steps, referred to as the (1st) "calcium" and the (2nd) "magnesium" steps, to reflect the dominant precipitating ions. The chemical analysis (ion concentrations) of the solution dictates the amount of COa2-which is supplied to the system. In the first instance, the calcium and magnesium cation containing solution is mixed with COa2-solution to precipitate calcium carbonates (usually monohydrate, however, anhydrous polymorphs and their mixturesare possible ) ; the carbonate ions should preferably therefore be supplied at an equimolar ratio (Ca2+: COa2-) .

[0055] In the preferred embodiment , the first precipitation reaction is carried out under dynamic conditions , where constant stirring is employed to ensure homogeneity of the aqueous system . Vigorous stirring is preferred ( 200 -400 rpm) : the agitation can affect both the polymorphic selection and the particle si ze . Longer durations of reaction are viable , but the optimum is 1 -3 hours .

[0056] In the second precipitation step of the preferred embodiment , the solution is replenished with a further addition of carbonate ions , in this case at an equimolar ratio required to precipitate the magnesium-phase (Mg2+: COa2-) . The process does not require the separation of the primary precipitate ( calcium carbonates ) prior to starting the second stage .

[0057] The second precipitation reaction may take 1 -3 hours , and gentle agitation ( 50 rpm) will help to ensure large needle formation required for a higher strength product . The nesquehonite crystals may grow on the surfaces of calcium carbonates , and / or around them, forming clusters . The final product comprises a mixture of calcium carbonates and nesquehonite with a variety of morphological habits , but generally elongate hexagonal prisms (nesquehonite up to 150 pm in length) and significantly smaller calcium carbonate grains of elliptical , round, or square shapes (depending on the polymorphic selection) , up to 5 pm in diameter .

[0058] Scanning electron microscope pictures depicting two of the most typical morphological habits of coexisting phases are shown in Figure 1 . The left-hand image shows largenesquehonite needles encompassing smaller calcium carbonate grains . The right-hand image shows nesquehonite needles covered with calcium carbonate grains .

[0059] Raw powder processingThe raw material containing both calcium and magnesium phases is gently dried at temperatures below 35 ° C, until dry (moisture content below 1 % ) . The solid should preferably contain less than lwt% NaCl . During the drying process the material is spread out in a thin layer over less than 48 hours to prevent any phase transitions . X-ray diffraction should confirm the absence of dypingite , hydromagnesite or other phases (except for calcium carbonates and nesquehonite ) . I f the raw material is required to be stored for longer periods prior to thermal activation, any moisture content should be removed to avoid conversion to dypingite , and the material should preferably be sealed and stored in a fridge at or below 5 ° C .

[0060] Hardening is preferably carried out using one of two different methodologies : (A) partial dehydration of the raw material and (B) thermally accelerated phase transition of compressed raw material slurries .

[0061] (A) Dehydration - rehydration based hardeningThe dry raw material is heated at temperatures of 110 - 130 ° C, for up to 4 hours . During the heat treatment , partial dehydration of nesquehonite and monohydrocalcite will occur ; other carbonate phases , such as calcite will remain inert at the selected condition range . The heating is carried out in a fan assisted oven or a furnace . There should be no more than 2 molecules of structural water removed from nesquehonite to allow for its reconstitution .

[0062] Pastes are prepared by mixing water and thermally processed raw material. The pastes are made with a liquid to solid ratio of 0.5 to 0.8, preferably 0.75. Once moulded into shapes, the product is cured at ambient conditions (room temperature 25°C (+ 5) , atmospheric pressures, and low relative humidity (preferably 50% or less) .

[0063] As part of the hardening process, phase conversions (e.g. nesquehonite - dypingite) will result in morphological alternations, creating a porous matrix which gives strength to the hardened product. Preferred conditions for dehydration - rehydration based hardening are set out in Table 1 above.

[0064] (B) Thermally induced phase conversion-based hardening The raw material containing calcium carbonate and nesquehonite mixtures is gently dried at temperatures lower than 35 °C to avoid phase transition. Mixed with water, the raw material is moulded into shapes and then gently heated at temperatures of 70 - 90°C to accelerate phase transformations (nesquehonite - hydromagnesites, monohydrocalcite - anhydrous calcium carbonate) . The conversion of mineral phases is associated with morphological change which plays a key role in the hardening mechanism.

[0065] Preferred conditions for thermally induced phase conversion-based hardening are set out in Table 2 above.

[0066] It will be appreciated that the specific embodiments described herein are for illustrative purposes only, and that further modifications and variations of the embodiments are possible without departing from the scope of the present invention as defined by the appended claims.

Claims

CLAIMS1 . A method of making a calcium and magnesium carbonate- based mixture , which comprises the steps of : in a first precipitation step adding a carbonate- containing solution to a solution containing calcium and magnesium cations to precipitate calcium carbonates ; in a second precipitation step adding further carbonate-containing solution to the solution containing remaining calcium and magnesium cations without removing the precipitated calcium carbonates formed in the first precipitation step to precipitate magnesium carbonates and thus form the calcium and magnesium carbonate-based mixture .2 . A method according to claim 1 wherein substantially only calcium carbonates precipitate from the solution in the first precipitation step .3 . A method according to claim 1 or 2 wherein the carbonate containing solution is formed by dissolving carbon dioxide in an alkaline aqueous solution .4 . A method according to claim 3 wherein the carbon dioxide is derived from an effluent gas from an industrial plant .5 . A method according to claim 3 or 4 wherein the alkaline aqueous solution is formed from an alkali selected from sodium hydroxide , potassium hydroxide , calcium hydroxide , Clinker Kiln Dust (CKD) , Lime Sludge (LS ) and ammonia .6 . A method according to any preceding claim wherein the carbonate-containing solution is obtained by the dissolution of sodium carbonate .7 . A method according to claim 6 wherein the sodium carbonate is produced by the Solvay process .8 . A method according to any preceding claim wherein the carbonate-containing solution is obtained from a waste stream from a hydrometallurgical refining process .9 . A method according to any preceding claim wherein the magnesium carbonates grow onto the pre-existing calcium carbonates , resulting in clusters of calcium and magnesium carbonate-based mixtures .10 . A method according to any preceding claim wherein in the first precipitation step a substantially equimolar amount of carbonate ions relative to calcium cations is added, and in the second precipitation step a substantially equimolar or excess of carbonate ions relative to magnesium ions is added .11 . A method according to any preceding claim wherein the alkaline aqueous solution comprises a desalination rej ect brine or other industrial waste brine .12 . A method according to any preceding claim wherein the first and / or second precipitation steps are performed at a temperature between 0 ° C and 50 ° C and at atmospheric pressure .13 . A method according to claim 12 wherein the first and / or second precipitation steps are performed at a temperature of 20 -30 ° C .14 . A method according to any preceding claim wherein the first and second precipitation steps are performed at a pH in the range of 9- 11 .15 . A method according to any preceding claim wherein theprecipitated calcium carbonates comprise monohydrate , and optionally anhydrous polymorphs and their mixtures .16 . A method according to any preceding claim wherein in the first precipitation step the carbonate ions are provided at a substantially equimolar ratio to the calcium ions .17 . A method according to any preceding claim wherein the first precipitation step is carried out with stirring of the aqueous system .18 . A method according to claim 17 wherein in the f irst precipitation step the aqueous system is stirred at 200 -400 rpm .19 . A method according to any preceding claim wherein the first precipitation step is performed for 1 -3 hours .20 . A method according to any preceding claim wherein the second precipitation step is performed for 1 -3 hours .21 . A method according to any preceding claim wherein the first and second precipitation steps are performed with stirring of the aqueous system, and the stirring of the second precipitation step is performed at a lower rpm than the stirring of the first precipitation step .22 . A method according to claim 21 wherein in the second precipitation step the aqueous system is stirred at 25 - 100 rpm .23 . A method according to any preceding claim wherein the magnesium carbonates grow on the surfaces of calcium carbonates , and / or around them, forming clusters .24 . A method according to any preceding claim wherein the calcium and magnesium carbonate-based mixture comprises amixture of calcium carbonates and nesquehonite .25 . A method according to any preceding claim wherein the calcium and magnesium carbonate-based mixture comprises elongate hexagonal prisms of nesquehonite and calcium carbonate grains .26 . A method according to claim 25 wherein the elongate hexagonal prisms of nesquehonite are up to 150 pm in length and the calcium carbonate grains are up to 5 pm in diameter .27 . A method according to any preceding claim which further comprises processing of the calcium and magnesium carbonate- based mixture to form a cementitious material .28 . A method according to claim 27 which does not require the addition of further cementitious materials .29 . A method according to claim 27 or 28 wherein the precipitate product obtained from the first and second precipitation steps is dried at a temperature below 35 ° C, to obtain a moisture content below 1 % .30 . A method according to claim 29 wherein the dried product is hardened by partial dehydration and rehydration .31 . A method according to claim 30 wherein the dried product is dehydrated by heating at a temperature of 110 - 130 ° C for up to 4 hours .32 . A method according to any one of claims 29 to 31 wherein no more than 2 molecules of structural water is removed from nesquehonite .33 . A method according to any one of claims 29 to 32 wherein a paste is prepared from the dehydrated product by mixing with water .

34. A method according to claim 33 wherein the paste is made with a liquid to solid ratio of 0.5 to 0.8.

35. A method according to claim 33 or 34 wherein the paste is moulded and cured at 25°C (+ 5°C) , and atmospheric pressure .

36. A method according to claim 29 wherein the dried product is mixed with water, moulded and heated at 70 - 90°C.

37. A method according to any one of claims 30 to 36 wherein the hardened carbonate products are for use as construction materials .

38. A method according to claim 37 wherein the construction materials include interior wall and ceiling construction materials .

39. A calcium and magnesium carbonate-based mixture, dried, or dried and hardened product thereof made according to a method of any preceding claim.

40. A calcium and magnesium carbonate-based mixture, dried, or dried and hardened product according to claim 39 for use as a construction material.

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