Method for obtaining a pulverulent hydraulic binder

Abstract

The invention relates to a method for obtaining a pulverulent hydraulic binder, comprising a step of hydrating biomass ash, said hydration step comprising bringing said biomass ash into contact with liquid water, the amount of water brought into contact being from 1 to 5% by weight relative to the weight of biomass ash, or said hydration step comprising bringing said biomass ash into contact with gaseous water.

Description

Process for obtaining a powdered hydraulic binder

[0001] The invention relates to the field of construction materials. It relates more particularly to the production of hydraulic binders, as well as to the hydraulic binders obtained and the dry mortar compositions comprising said hydraulic binders.

[0002] Dry mortar compositions are powdered mixtures comprising a hydraulic binder and aggregates. After mixing ("stirring") with water, a paste (wet or fresh mortar) is obtained, which can then be shaped and hardened to form a hardened mortar. Such mortars can have various applications: facade coatings, tile adhesives, flooring products such as screeds, jointing mortars, masonry mortars, and many others.

[0003] There is a need to reduce the carbon footprint of construction materials, and in particular mortar mixes. The hydraulic binder in these mixes is often cement, especially Portland cement, the production of which releases large quantities of CO2 into the atmosphere (between 800 and 1000 kg of CO2 per ton of cement produced). This is due, on the one hand, to limestone decarbonation reactions, and on the other hand, to the high temperatures (around 1450°C) required to produce hydraulic phases. To address this problem, various solutions have been proposed, some of which involve at least partially replacing cement with industrial by-products, such as slag, fly ash, or calcined clays. More recently, the use of biomass ash has been suggested.

[0004] Biomass ash is ash formed by the combustion of biomass, used for example to produce electricity, steam, and / or heat. Biomass can come from sources such as agricultural products, forestry products, household and municipal waste, the paper or pulp industry, and / or crops grown for energy production.

[0005] However, the use of biomass ash is not without its drawbacks. In particular, the hydration of certain types of ash can lead to excessive swelling, which prevents their use or at least significantly reduces the maximum amount of ash that can be used in mortar mixes.

[0006] The present invention aims to solve this problem by proposing a process for treating biomass ash to obtain a hydraulic binder exhibiting reduced swelling, or even no swelling, during its hydration.

[0007] To this end, the invention relates to a process for obtaining a powdery hydraulic binder comprising a step of hydrating biomass ash, said hydration step comprising bringing said biomass ash into contact with water in liquid form, the quantity of water brought into contact being 1 to 5% by weight relative to the weight of biomass ash, or said hydration step comprising bringing said biomass ash into contact with water in gaseous form.

[0008] In this document, the starting material of the process will be referred to as "biomass ash," and the final product, in powder form, resulting from the hydration of the biomass ash and, where applicable, additional steps, will be called "hydraulic binder." The final product is in powder form and retains its hydraulic properties, allowing it to be used as a hydraulic binder, for example, in the production of mortar. The inventors have demonstrated that contact with small quantities of liquid water or with gaseous water resolves the swelling problem without affecting the hydraulic properties of the biomass ash. Furthermore, the process is simpler than washing with large quantities of water, which reduces the hydraulic properties and requires costly water removal treatment, or even grinding.It is known, for example from application WO2021193668, to wash biomass ash to reduce its chlorine and heavy metal content. Such washing, however, requires large quantities of water, first to form a slurry, then to wash said slurry, and also requires subsequent dewatering steps, for example by means of a filter press, and grinding.

[0009] In this presentation, the following terms are synonymous: free lime, which is a phase with the chemical formula CaO, is also called quicklime or calcium oxide; slaked lime is also called calcium hydroxide or Ca(OH)2; calcium carbonate is also called calcite or CaCO3.

[0010] The process according to the invention is particularly useful for biomass ash containing free lime (CaO). The free lime content in the biomass ash is preferably at least 5% by weight, even at least 10%, for example from 6 to 18% or from 7 to 15% by weight. As explained in more detail later in the text, the hydration step reduces the amount of free lime by transforming at least a portion of it, preferably at least 50%, and even at least 60% or at least 70%, or even at least 80% or 90%, and even all of it, into slaked lime and / or calcium carbonate.

[0011] The process is also particularly useful for biomass ash containing metallic aluminum (Al), which exhibits the greatest swelling. Metallic aluminum is present primarily in the form of fine particles. Without being bound by any scientific theory, it would appear that the dissolution of aluminum during the hydration of the hydraulic binder could lead to the generation of hydrogen gas. The metallic aluminum content in biomass ash typically ranges from 0.1 to 5.0% by weight, specifically from 0.5 to 4.0%, and even from 1.0 to 3.0%.

[0012] The most pronounced swelling was observed when the biomass ash contained both free lime and metallic aluminum. It appears that the presence of free lime creates conditions favorable to the dissolution of metallic aluminum.

[0013] Biomass ash is preferably ash obtained by burning waste (or residues, or by-products) from the paper or pulp industry. For example, this waste can be generated during the production of paper or board from recycled paper or board. This waste can be recovered, for instance, during the pulping stage, in which water and chemicals such as hydrogen peroxide, sodium hydroxide, and sodium silicate are added to shredded paper to separate the cellulose fibers, and / or during screening stages. This type of biomass ash may contain small amounts of metallic aluminum, for example, from barrier layers of cardboard packaging or from labels, caps, or lids used with cardboard packaging.

[0014] Combustion is achieved, for example, by fluidized bed combustion, particularly by bubbling fluidized bed combustion. Other combustion processes include fixed bed combustion and circulating fluidized bed combustion. The combustion temperature is preferably at least 700°C, or even at least 800°C. Regardless of the combustion process, the biomass ash used is preferably recovered from the flue gases (for example, by filtration), as opposed to ash collected at the bottom of the boiler, which frequently contains undesirable materials such as heavy metals.

[0015] Biomass ash preferably has a chemical composition by weight comprising 5-35%, preferably 9-20%, SiO2, 3-20%, preferably 5-15%, Al2O3, 0-5%, preferably 0.5-3%, Fe2O3, and 30-70%, in particular 40-65%, CaO. This is a chemical, not a mineralogical, composition: the quantity of calcium, expressed here as "CaO," does not imply the mineral forms in which the calcium is present. It is preferably present partly as free lime, as previously indicated, but also, preferably, incorporated into hydraulic phases.

[0016] Biomass ash preferably comprises at least 10% by weight of hydraulic phases, more particularly at least 15%, and even at least 20%, relative to the total weight of biomass ash. This content is advantageously at least 25%, or even at least 30%. It is preferably at most 60%, and even at most 50%. The hydraulic phases are preferably selected from calcium silicates, calcium aluminates, free lime, and mixtures thereof. The calcium silicates and calcium aluminates preferably comprise one or more of the following phases: C2S, C3S, C3A, C4AF, C2A2S, CA, C12A7, and CA2. Biomass ash preferably comprises at least 10% by weight of C2S phases, at least 5% by weight of C2A2S phases and at least 1% by weight of C3A phases, relative to the total weight of biomass ash.The C3S phase content is preferably at most 1% by weight, or even zero, because it prevents the formation of aluminate phases during hydration.

[0017] Biomass ash can also contain amorphous phases, including CaO, Al2O3, and SiO2, in amounts sufficient to dissolve these elements. The quantity of amorphous phase is, for example, 10 to 30% by weight.

[0018] Biomass ash can contain additional reactive phases, such as carbonates (notably calcite), sulfates (e.g., calcium sulfate), or aluminosilicates. The calcite content, for example, ranges from 10 to 40% by weight relative to the total weight of biomass ash.

[0019] In terms of particle size, the D50 (based on volume distribution) of biomass ash is preferably 8 to 50 µm, particularly 10 to 30 µm, for example, approximately 15 µm. The maximum size (Dmax) is preferably less than or equal to 200 µm. These dimensions are generally measured by laser particle size analysis.

[0020] In order to avoid unduly negatively impacting the reactivity of the resulting hydraulic binder, the process according to the invention preferably reduces the total weight content of hydraulic phases in biomass ash, other than free lime, by no more than 30%, in particular by no more than 20%, or even by no more than 10%, relative to its initial content. This percentage of relative reduction is calculated using the following formula: (Hi-Hf) / Hi, where Hi is the total initial weight content (before hydration) of hydraulic phases (excluding free lime) and Hf is the total final weight content (after hydration) of hydraulic phases (again, excluding free lime).

[0021] When biomass ash contains free lime, the process according to the invention preferably reduces the free lime content by weight by at least 50%, in particular by at least 60%, and even by at least 70%, or even by at least 80% or 90%, relative to its initial content. In other words, the ash is brought into contact with water in such a way that the weight content is reduced by the percentages indicated above. This percentage of relative reduction is calculated by the following formula: (Ci-Cf) / Ci, where Ci is the initial total weight content (before hydration) of free lime and Cf is the final total weight content (after hydration) of free lime.

[0022] It has been shown that it is possible to selectively hydrate free lime without excessively hydrating other hydraulic phases. Given the much stronger affinity of free lime for water, the latter will tend to react first with the free lime, before potentially hydrating other hydraulic phases. This is also possible by manipulating the conditions of contact between the biomass ash and the water, particularly the amount of water added and / or the contact time. For example, a longer contact time is likely to reduce the content of hydraulic phases other than free lime more significantly. The same is true when the amount of water added is greater.

[0023] During the hydration stage, biomass ash is brought into contact with water in liquid or gaseous form. The hydration stage can be a continuous or batch process. If necessary, the hydration stage can be followed by a drying stage.

[0024] The amount of water with which the biomass ash is mixed is preferably adjusted according to the free lime content in the ash. The amount of water mixed is 1 to 5%, specifically 2 to 4% by weight relative to the weight of biomass ash.

[0025] In a first embodiment, the biomass ash is brought into contact with water in liquid form. For example, liquid water can be sprayed onto a bed of biomass ash, possibly moving on a conveyor, for example, a belt conveyor. Alternatively, liquid water can be added to the biomass ash in a mixer, for example, a screw conveyor or a rotary drum.

[0026] In a second embodiment, biomass ash is brought into contact with water in gaseous form. The ash can, for example, be brought into contact with a stream of humid air or water vapor. During contact, the ash can be conveyed. Alternatively, the contact can involve the formation of a fluidized bed. According to yet another alternative, water vapor can be injected into a mass of ash. For example, a mass of ash can be contained by a gas-permeable jacket while water vapor is injected from below into the ash mass. The injection can be carried out by a plurality of homogeneously distributed nozzles.

[0027] Preferably, the process includes, after or during the hydration step, a carbonation step by contacting the biomass ash with carbon dioxide. The carbonation step allows at least partial conversion of the slaked lime into calcium carbonate (or calcite).

[0028] The carbonation step, in combination with the hydration step, significantly reduces, or even eliminates, swelling, particularly in the case of ash containing both free lime and metallic aluminum. It appears that the carbonation of the lime prevents the conditions under which binder hydration can lead to hydrogen generation. It is possible that this neutralization of the free lime initially present in the ash slows the dissolution of metallic aluminum, with hydrogen generation then occurring later, once the mortar has sufficiently hardened.

[0029] According to a first embodiment, the carbonation step is carried out simultaneously with the hydration step. For example, the ashes can be brought into contact with humid air, which therefore contains both water in gaseous form and carbon dioxide.

[0030] According to a second embodiment, the carbonation step is carried out after the hydration step. The carbonation step can be achieved by contact with air. Air storage may suffice if the storage conditions allow for sufficiently intimate contact, for example, by using thin bedding. Alternatively, air or any other gas containing carbon dioxide can be injected into the ash after it has undergone the hydration step.

[0031] Preferably, the carbonation step reduces the slaked lime (or calcium hydroxide) content by weight by at least 30%, in particular by at least 40%, and even by at least 50% or 60%, or even by at least 70% or 80%, or at least 90%, compared to its initial content. This percentage of relative reduction is calculated using the following formula: (Ei-Ef) / Ei, where Ei is the initial total weight content (before hydration) of slaked lime and Ef is the final total weight content (after hydration) of slaked lime.

[0032] The invention also relates to a powdered hydraulic binder obtained, or capable of being obtained, by the process according to the invention.

[0033] The resulting powdered hydraulic binder preferably comprises at least 8% by weight of calcium silicate and / or calcium aluminate hydraulic phases, in particular hydraulic phases selected from C2S, C3A, and C2A2S. This content is preferably at least 10%, or even at least 15%, for example, between 20 and 50%, or between 25 and 40% by weight. The free lime content is preferably no more than 5%, in particular no more than 3% by weight.

[0034] The D50, based on the volume distribution of the hydraulic binder obtained, is preferably 8 to 50 µm, in particular 10 to 30 µm.

[0035] Another object of the invention is also a dry mortar composition comprising such a powdery hydraulic binder as well as aggregates.

[0036] The hydraulic binder according to the invention can be mixed with other hydraulic binders and / or pozzolanic materials, in particular selected from Portland cement, aluminous cements, sulfoaluminous cements, lime (hydraulic or aerial), calcium sulfate sources, slags (in particular crushed granulated blast furnace slags), fly ash, silica fume, calcined shale, natural or calcined pozzolans or calcined clays.

[0037] Aggregates may include fillers, which are finely ground inert mineral materials, generally of the limestone or siliceous type. Preferably, aggregates comprise sands, particularly siliceous sands, and / or fillers, particularly limestone or dolomitic fillers. Aggregates may also include lightweight aggregates selected from perlite, vermiculite, expanded glass beads, expanded polystyrene beads, cenospheres, expanded silicates, aerogels, and mixtures thereof.

[0038] The dry mortar composition may also include one or more additives, selected from rheological agents, plasticizers or superplasticizers, water-retaining agents, air-entraining agents, thickening agents, biocidal preservatives, dispersing agents, pigments, setting or hardening accelerators and / or retarders, polymeric resins, and antifoaming agents. The total content of additives and admixtures preferably varies between 0.001 and 5% by weight relative to the total weight of the dry composition.

[0039] The dry mortar composition preferably includes an expansion inhibitor, in particular selected from lithium salts (especially lithium nitrate and sodium sulfate), nitrates (especially sodium or potassium nitrate), and carboxylic acids (especially oxalic acid and citric acid). The total content of the expansion inhibitor is preferably between 0.5 and 3.0%, in particular between 1.0 and 2.5% by weight of hydraulic binder.

[0040] The composition of mortar can be used in particular to obtain floor products, facade coatings, tile adhesives, jointing mortars, repair mortars, masonry mortars or even mortars for additive manufacturing.

[0041] Examples

[0042] The following examples illustrate the invention in a non-limiting manner.

[0043] The various examples use biomass ash from the combustion of waste from the paper and cardboard industry, the main chemical components of which are 11% SiO2, 11% Al2O3, 1% Fe2O3, 57% CaO, and 2% MgO. In terms of mineralogical composition, the ash contains 50% calcite by weight, 6% free lime, 5% Ca(OH)2, 19% larnite (C2S), 6% gehlenite (C2A2S), 2% C3A, 2% metallic aluminum, and 19% amorphous phase. The median diameter D50 was 14 µm.

[0044] In a comparative example, a mortar-adhesive composition containing these biomass ashes was mixed with water, and the resulting wet mortar was poured into a container. The mortar-adhesive comprised 20% by weight of biomass ash, 3% aluminous slag, 0.9% Portland cement CEM I 42.5, 0.4% hemihydrate, 0.5% sodium sulfate, 0.1% lime, a cellulose ether, and 48.7% sand. Significant swelling was observed.

[0045] In the first series of examples, a 5 mm thick ash bed was maintained at a temperature of 20°C and a relative humidity of 70%. After approximately 2 hours, about half of the free lime had been transformed into slaked lime, without any significant change in the content of other hydraulic phases. After 24 hours, almost all of the free lime had been transformed into slaked lime, while the amount of the C2S phase was reduced by only 20%.

[0046] In a second series of examples, a 5 mm thick bed of ash was placed in a closed chamber at 20°C and 100% relative humidity. The treatment transformed 45% of the free lime into slaked lime after 4 hours, and 60% after 24 hours, without significant variation in the C2S and C3A phase contents.

[0047] In a third series of examples, a 5 mm thick ash bed was maintained at a temperature of 20°C and a relative humidity of 70%, under an atmosphere containing 10% carbon dioxide. After 6 hours of treatment, the free lime content was reduced by 80%, with no change in the C3A phase content, but with an approximately 30% change in the C2S phase content. With ash treated in this way, no swelling was observed in the case of a mortar-adhesive of the same composition as that of the comparative example.

[0048] In a fourth series of examples, biomass ash was mixed with liquid water at a ratio of 2.7% by weight, then spread in a thin layer and exposed to air for 24 hours. With the ash treated in this way, no swelling was observed in the case of a mortar-adhesive of the same composition as that of the comparative example. In this case, thermogravimetric analysis showed that mixing with water followed by exposure to air converted a large portion of the free lime and slaked lime into calcite. Conversely, in the absence of exposure to air (storage in a closed container after the hydration stage), moderate swelling was observed. In this case, the hydrated, but not carbonated, ash contained large quantities of slaked lime.

Claims

A process for obtaining a powdery hydraulic binder comprising a step of hydrating biomass ash, said hydration step comprising bringing said biomass ash into contact with water in liquid form, the quantity of water brought into contact being 1 to 5% by weight relative to the weight of biomass ash, or said hydration step comprising bringing said biomass ash into contact with water in gaseous form. A process according to claim 1, wherein the biomass ash comprises free lime (CaO). A process according to claim 2, wherein the free lime content in the biomass ash is at least 5% by weight, in particular between 6 and 18%. A process according to any one of the preceding claims, wherein the biomass ash comprises metallic aluminum. A process according to claim 4, wherein the metallic aluminum content in the biomass ash is between 0.1 and 5.0% by weight. A process according to any one of the preceding claims, wherein the biomass ash has a chemical composition by weight comprising 5-35% SiO2, 3-20% Al2O3, 0-5% Fe2O3 and 30-70% CaO. A process according to any one of the preceding claims, wherein the process reduces the total weight content of hydraulic phases of biomass ash, other than free lime, by not more than 30%, in particular by not more than 10%, relative to its initial content. A process according to any one of claims 2 to 7, wherein the process reduces the weight content of free lime by at least 50%, relative to its initial content. A process according to any one of the preceding claims, comprising, after or during the hydration step, a carbonation step by contacting the biomass ash with carbon dioxide. A process according to claim 9, wherein the carbonation step is carried out simultaneously with the hydration step. A process according to claim 9, wherein the carbonation step is carried out after the hydration step. A process according to any one of the preceding claims, wherein the resulting powdered hydraulic binder comprises at least 8% by weight of calcium silicate and / or calcium aluminate hydraulic phases, in particular hydraulic phases selected from C2S, C3A and C2A2S. Powdered hydraulic binder obtained by the process according to one of the preceding claims. Hydraulic binder according to claim 13, of which D50, based on the volume distribution, is between 8 and 50 µm, in particular between 10 and 30 µm. Dry mortar composition comprising a powdery hydraulic binder according to one of claims 12 or 13, as well as aggregates.