Carbon sequestration type lithium slag composite admixture low-carbon concrete and preparation method thereof

Abstract

The application provides a carbon sequestration type lithium residue composite admixture low-carbon concrete and a preparation method thereof, and belongs to the technical field of green low-carbon concrete. The raw materials of the concrete include low-C3A high-Berlitz Portland cement, a sulphoaluminate reaction component, a lithium residue composite admixture, calcined magnesium-aluminum hydrotalcite, ettringite seeds, fine aggregate, coarse aggregate, water and a water reducing agent; wherein the lithium residue composite admixture is obtained by cross-linking and fixing low-methoxyl pectin and a soluble calcium salt on the surface of lithium residue powder, and then mixing calcium aluminate material, carbide slag micro-powder and performing high-humidity pre-reaction and CO2 mineralization treatment. The lithium residue composite admixture, the early reaction product of low-C3A high-Berlitz Portland cement, the sulphoaluminate reaction component, ettringite seeds and calcined magnesium-aluminum hydrotalcite, ettringite nucleation guidance and the remaining sulphate auxiliary buffer synergistic system improve the problems of high water absorption of lithium residue, insufficient early activity and difficult control of sulphate reaction.

Description

Technical Field

[0001] This invention relates to the field of green and low-carbon concrete technology, specifically to a low-carbon concrete with carbon-fixing lithium slag composite admixture and its preparation method. Background Technology

[0002] With the rapid development of new energy vehicles and energy storage industries, the demand for lithium salt products such as lithium carbonate and lithium hydroxide is growing rapidly, and the production scale of lithium extraction using lepidolite as raw material continues to expand. During the lithium extraction process, lepidolite undergoes roasting, leaching, purification, and solid-liquid separation, generating a large amount of solid lithium extraction residue (lithium slag). Currently, this type of lithium slag is mainly disposed of through stockpiling, landfilling, and sintering into building materials, resulting in a low overall utilization level. Furthermore, long-term stockpiling poses potential environmental risks such as occupying land resources, dust generation, and the migration of soluble salts. Processing lithium slag into auxiliary cementitious materials and using it in cement concrete can not only reduce the consumption of conventional cementitious materials but also promote the resource utilization of lithium slag solid waste, demonstrating significant environmental benefits and engineering application value.

[0003] Lithium extraction residue from lepidolite typically consists mainly of silicon and aluminum components, along with certain amounts of calcium, sulfur, potassium, and residual lithium. When sulfur-containing processes such as sulfate roasting, acid roasting, or sulfuric acid leaching are used, the resulting residue will also contain residual sulfur-containing components such as gypsum, anhydrous calcium sulfate, and alkali metal sulfates, with SO3 content often higher than that of conventional mineral admixtures. Appropriate amounts of sulfate can react with calcium and aluminum components to form ettringite, promoting early hardening of the paste; however, when sulfate release is mismatched with the supply of calcium and aluminum components, it can easily lead to abnormal setting time, accelerated slump loss, localized concentrated formation of ettringite, and decreased volumetric stability. On the other hand, lithium slag particles often exhibit irregular, porous, or lamellar stacked morphologies, exhibiting strong adsorption of mixing water and water-reducing agents. High admixture dosages can easily increase the water demand of concrete, raise paste viscosity, and cause fluctuations in workability over time. In addition, some of the silica-alumina components in lithium slag exist in crystalline forms such as quartz, feldspar and residual mica. Although the low-crystallinity silica-alumina phase has the potential for pozzolanic reaction, its early dissolution and reaction rate at room temperature are slow, resulting in insufficient early strength performance of concrete.

[0004] Existing technologies typically employ mechanical grinding, heat treatment, strong alkali or salt activation, and compounding with materials such as lime and steel slag to improve lithium slag performance. Mechanical grinding increases the reaction surface area of ​​lithium slag, but excessive grinding increases energy consumption and exacerbates fine particle agglomeration, water absorption, and adsorption of water-reducing agents. While strong alkali activation promotes the dissolution of the silica-alumina phase, it may lead to increased alkali content, excessively rapid solidification, increased shrinkage, and decreased adaptability to additives. Directly mixing calcium-rich materials with lithium slag can supplement the calcium source required for the reaction, but the different powders are mainly randomly distributed, making it difficult to match the reaction sites of the calcium and aluminum sources with the surface active components of the lithium slag. Mineralizing lithium slag with calcium-rich materials using carbon dioxide can generate calcium carbonate and fix some carbon dioxide. If the mineralization product forms a continuous, excessively thick, dense layer on the lithium slag surface, it reduces surface water absorption and increases carbonate content, while also hindering the migration of water and alkaline ions into the lithium slag interior, making it difficult to simultaneously achieve surface modification and mid-to-late-stage activity.

[0005] Therefore, it is necessary to develop a carbon-fixing composite admixture technology that takes into account the characteristics of high sulfate content, porous and highly absorbent structure, and low early-stage activity of lithium mica extraction residue. This technology would combine surface physical modification with the retention of internal activity to achieve directional bonding between lithium slag and calcium and aluminum components, while ensuring early-stage stable utilization of sulfate and enhancing the later-stage activity of lithium slag. Simultaneously, considering the sulfate reaction characteristics of lithium slag, the overall cementitious system and aggregate materials should be synergistically designed to achieve a comprehensive improvement in concrete workability, early and later-stage strength, volume stability, and durability, thereby promoting the high-volume, high-performance, and low-carbon resource utilization of lithium slag. Summary of the Invention

[0006] To address the shortcomings of existing technologies, this invention provides a carbon-fixed lithium slag composite admixture for low-carbon concrete and its preparation method. This invention utilizes the silicon, aluminum, and sulfate components inherent in lithium mica extraction residue, synergistically combining them with calcium carbide slag powder and calcium aluminate materials. Through processes such as cross-linking fixation, high-humidity pre-reaction, and CO2 mineralization, a calcium-aluminum-silicon reaction interface and a discontinuous mineralization reaction layer are formed on the exterior of the lithium slag particles, resulting in a carbon-fixed lithium slag composite admixture. Simultaneously, low-C3A high-belite silicate cement is used in the concrete, and sulfoaluminate reaction components, ettringite seed crystals, and calcined magnesium aluminum hydrotalcite are introduced to form a synergistic system of early reactant supply, ettringite nucleation guidance, and residual sulfate auxiliary buffering, improving problems such as high water absorption, insufficient early activity, and difficulty in controlling sulfate reactions in lithium slag.

[0007] To achieve the above objectives, the specific technical solution of the present invention is as follows:

[0008] This invention provides a carbon-fixing lithium slag composite admixture for low-carbon concrete, comprising the following raw materials in parts by weight: 210-240 parts of low-C3A high-belite silicate cement, 20-40 parts of sulfoaluminate reaction component, 110-140 parts of lithium slag composite admixture, 1.5-3 parts of calcined magnesium aluminum hydrotalcite, 1.5-2.5 parts of ettringite seed crystals, 800-840 parts of fine aggregate, 1020-1040 parts of coarse aggregate, 155-165 parts of water, and 7-10 parts of water-reducing agent;

[0009] The preparation method of the lithium slag composite admixture is as follows:

[0010] An aqueous solution of low-methoxyl pectin and an aqueous solution of soluble calcium salt were sequentially added to lithium slag powder and mixed evenly to obtain mixture I.

[0011] Mixture I, calcium carbide slag powder, and calcium aluminate are mixed evenly to obtain mixture II;

[0012] Mixture II was pre-reacted in an environment with a temperature of 35-50 ℃ and a relative humidity of not less than 95% for 4-6 h to obtain mixture III; during the pre-reaction process, the material was intermittently turned over to control the material to remain in a loose state after the pre-reaction, without obvious hardening and agglomeration;

[0013] Mixture III was mineralized for 1-2 hours at a temperature of 30-45 ℃, a relative humidity of 55%-70%, and a CO2 volume fraction of 20%-50%. After drying, low-speed mechanical deagglomeration, and sieving, lithium slag composite admixture was obtained. During the mineralization process, the material was continuously turned over to ensure uniform contact with CO2.

[0014] The lithium slag composite admixture consists of a lithium slag active core, a calcium-aluminum-silicon reaction interface, and a discontinuous mineralization reaction layer; wherein, the low-methoxyl pectin molecules are rich in carboxyl groups, which, together with the Ca provided by the soluble calcium salt, form a high-carbon dioxide content. 2+ A spatial cross-linked network is formed, confining the calcium carbide slag powder and calcium aluminate material around the lithium slag particles, thereby improving surface loading stability. Simultaneously, this cross-linked structure restricts migratable calcium ions to the vicinity of the lithium slag particles, allowing subsequent pre-reaction and mineralization products to preferentially form on the lithium slag surface.

[0015] During the high-humidity (relative humidity not less than 95%) pre-reaction stage, carbide slag provides Ca. 2+ and OH -This promotes the limited dissolution of the low-crystallinity silica-alumina phase on the surface of lithium slag; the calcium aluminate material provides active calcium-alumina components, which together with the silicon, aluminum and sulfate ions released from the surface of lithium slag form a low-crystallinity calcium-alumina-silicon reaction interface. This reaction interface is not a continuous and dense coating layer formed by complete hydration, but an active transition region composed of low-crystallinity calcium-alumina-silicon precursor, unreacted calcium and calcium aluminate components. Ion migration channels are retained in the region, and the reaction can continue after water is added to the concrete.

[0016] During controlled mineralization, Ca(OH)₂ located on the outside of the composite particles preferentially reacts with CO₂ to form calcium carbonate clusters. As the reaction proceeds, the formed calcium carbonate clusters and the liquid film on the particle surface restrict the inward diffusion of CO₂, thus retaining some Ca(OH)₂, residual calcium aluminate components, and low-crystallinity reaction precursors in the region near the lithium slag core. By controlling the net mineralization carbon fixation and the degree of carbonization of the calcium components, complete carbonization of the carbide slag and the formation of a continuous, excessively thick carbonate inert film on the lithium slag surface can be avoided.

[0017] In this invention, the particle size of carbide slag powder and calcium aluminate is smaller than that of lithium slag powder. Under the fixation effect of pectin-calcium crosslinking, the two fine particles preferentially adhere to the surface of lithium slag particles and shallow open pores, which can avoid random distribution and spatial separation when the three powders are directly mixed. By loading carbide slag powder and calcium aluminate onto the surface of lithium slag, performing high-humidity pre-reaction, and controlled mineralization, the problem of high water absorption of lithium slag can also be synergistically improved. The smaller particle size of carbide slag powder and calcium aluminate fills part of the open pores on the surface of lithium slag. The low crystallinity reaction products formed by the high-humidity pre-reaction further reduce the high-energy adsorption sites on the surface, while the discontinuous calcium carbonate crystal clusters have a local covering and filling effect on the remaining open pores, reducing the capillary water absorption rate of lithium slag in the early stage of concrete mixing and the ineffective adsorption of water-reducing agents. Since the mineralization products do not form a continuous dense film layer, moisture and OH- - and Ca 2+ It can still enter the active core of lithium mica slag through inter-cluster channels in the middle and late stages, so that the inhibition of surface water absorption and subsequent reaction activity can be taken into account.

[0018] Furthermore, in the preparation process of the lithium slag composite admixture, the mass ratio of solid raw materials lithium slag powder, carbide slag micro powder, and calcium aluminate material is (76-84):(10-16):(5-9); the amount of low-methoxyl pectin used is 0.08%-0.12% of the total mass of solid raw materials (lithium slag powder, carbide slag micro powder, and calcium aluminate material); and the amount of soluble calcium salt used is 0.20%-0.40% of the total mass of solid raw materials.

[0019] Furthermore, an aqueous solution of low-methoxyl pectin and an aqueous solution of soluble calcium salt are sequentially added to the lithium slag powder via spraying.

[0020] Furthermore, the aqueous solution of the low-methoxyl pectin has a low-methoxyl pectin mass concentration of 1.5%-2.0%; the aqueous solution of the soluble calcium salt has a soluble calcium salt mass concentration of 8%-10%.

[0021] Furthermore, the degree of esterification of the low-methoxyl pectin is ≤ 50%, preferably 30%-40%.

[0022] Furthermore, the soluble calcium salt is a soluble organic calcium salt, preferably calcium acetate. The soluble organic calcium salt is used to provide Ca. 2+ This induces cross-linking of low-methoxyl pectin and improves the load stability of calcium carbide slag powder and calcium aluminate materials around lithium slag particles.

[0023] Furthermore, the Blaine specific surface area of ​​the lithium slag powder is 380-450 m². 2 / kg, median particle size D 50 The particle size is 12-18 μm, and it is obtained by drying and grinding lithium extraction residue from lepidolite. The lithium extraction residue from lepidolite is a solid residue produced after lithium extraction processes such as sulfur-containing roasting, acid roasting, or sulfuric acid leaching, using lepidolite ore as the main raw material. By mass percentage, the lithium extraction residue from lepidolite contains 45%-60% SiO2, 18%-28% Al2O3, 5%-10% CaO, and 8%-14% SO3.

[0024] Furthermore, the Ca(OH)2 mass content of the calcium carbide slag powder is ≥ 70%, and the median particle size D is... 50 It is 2-4 μm in size and is a solid residue mainly composed of calcium hydroxide produced during the hydrolysis of calcium carbide to produce acetylene.

[0025] Furthermore, in the lithium slag composite admixture, the apparent carbonization conversion rate of Ca(OH)2 in the calcium carbide slag powder is 45%-70%, and the apparent carbonization conversion rate is calculated based on the change in Ca(OH)2 content in the composite particles before and after mineralization treatment; the mineralization products are mainly distributed discontinuously on the outside of the composite particles in the form of fine calcium carbonate crystal clusters, without forming a continuous dense coating film.

[0026] Furthermore, the Al2O3 mass content of the calcium aluminate material is 55%-70%, and the median particle size D is... 50 The particle size is 2-4 μm, and it is selected from calcium aluminate clinker powder, high alumina cement clinker powder or a combination thereof.

[0027] Furthermore, based on the total dry weight of lithium slag powder, calcium carbide slag powder, and calcium aluminate material before mineralization treatment, the net mineralized carbon fixation content of the lithium slag composite admixture is 1.8%-3.5%. The net mineralized carbon fixation content refers to the percentage of the CO2 mass converted from the inorganic carbon content in the lithium slag composite admixture after mineralization treatment, after deducting the CO2 mass converted from the original inorganic carbon content of the solid raw materials before mineralization treatment, relative to the total dry weight of the solid raw materials before mineralization treatment.

[0028] Specifically, the preparation method of the lithium slag composite admixture is as follows:

[0029] S1: Place lithium slag powder in a high-speed mixer. Under the condition of 600-1000 r / min, spray a low-methoxyl pectin aqueous solution with a mass concentration of 1.5%-2.0% into the lithium slag powder in the form of droplets. After spraying for 5-8 min, continue stirring for 3-6 min. Then spray in an 8%-10% soluble calcium salt aqueous solution to obtain mixture I.

[0030] S2: Add the calcium carbide slag powder and calcium aluminate material sequentially into a high-speed mixer and mix for 8-12 minutes at a speed of 1000-1600 r / min to obtain mixture II. Depending on the actual amount of water introduced by the low-methoxy pectin aqueous solution and the soluble calcium salt aqueous solution, add water via atomization if necessary, so that the mass of process water in mixture II is 9%-14% of the total mass of the solid raw materials (lithium slag powder, calcium carbide slag powder, and calcium aluminate material). The process water includes the water introduced by the low-methoxy pectin aqueous solution and the soluble calcium salt aqueous solution, as well as the atomized water added as needed.

[0031] S3: Mixture II is pre-reacted in an environment with a temperature of 35-50 ℃ and a relative humidity of not less than 95% for 4-6 hours to obtain mixture III; during the pre-reaction process, the material is intermittently turned over to control the material to remain in a loose state after the pre-reaction, without obvious hardening and agglomeration;

[0032] S4: Place mixture III in a drum-type mineralization device and mineralize it for 1-2 hours at a temperature of 30-45 ℃, a relative humidity of 55%-70%, and a CO2 volume fraction of 20%-50%. During the mineralization process, continuously turn the material to ensure uniform contact with CO2. After mineralization, dry it at a temperature of 50-70 ℃ until the moisture content is ≤ 1.0%. Then, perform low-speed mechanical deagglomeration and sieving to obtain lithium slag composite admixture. The low-speed mechanical deagglomeration is used to break up the loose agglomerates formed during the drying process, eliminating the need for further grinding of the powder.

[0033] Furthermore, in the silicate cement clinker used in the low-C3A high-belite silicate cement, the C3A mass content is ≤5%, and the C2S mass content is ≥40%. The lower C3A content can reduce the degree of rapid reaction between the cement aluminate phase and lithium slag sulfate in the initial stage of mixing, reduce the slump loss caused by excessively rapid slurry setting and competitive adsorption by water-reducing agents; the belite component continues to hydrate in the middle and later stages, forming hydrated calcium silicate gel and providing Ca(OH)2, which provides a calcium source and alkaline environment for the pozzolanic reaction of the active core of lithium slag.

[0034] Furthermore, the anhydrous calcium sulfoaluminate mineral content in the sulfoaluminate reaction component is 50%-70%, and it is selected from sulfoaluminate cement clinker powder, clinker powder containing anhydrous calcium sulfoaluminate, or a combination thereof. High-belite silicate cement with low C3A content is used to reduce the early disordered reaction between the aluminate phase of the base cement and the sulfate in the lithium slag; the sulfoaluminate reaction component serves as an independently measured source of active calcium and aluminum, reacting with the sulfate ions released from the lithium slag in the early stages of hydration, and forming relatively dispersed ettringite under the action of ettringite seed crystals to compensate for the insufficient early strength formation of the lithium slag system.

[0035] Furthermore, the ettringite seed crystal is a powder containing the ettringite crystal phase, with the ettringite crystal phase content not less than 80%, D 50 The size is 0.5-1.5 μm. Etnacite seed crystals can provide dispersed heterogeneous nucleation sites for the reaction of calcium, aluminum and sulfate, reduce the nucleation energy barrier for ettringite precipitation, and reduce the phenomenon that ettringite grows only in a few sulfoaluminate clinker particles or around the local sulfur-rich areas of lithium slag. This is conducive to the formation of a more uniform, fine and continuous early hydration structure.

[0036] Furthermore, the specific surface area of ​​the calcined magnesium aluminum hydrotalcite is 50-100 m². 2 / g, which is obtained by calcining magnesium-aluminum layered double hydroxide at 400-600 °C. In the magnesium-aluminum layered double hydroxide, the molar ratio of magnesium to aluminum (Mg / Al) is (2.5-3.5):1. Calcined magnesium-aluminum hydrotalcite can absorb water and undergo structural reconstruction in concrete pore solutions, and can also absorb residual SO4 that has not yet participated in the sulfoaluminate reaction. 2- It plays a supplementary adsorption and buffering role, reducing the possibility of sulfate migration and local enrichment in the middle and late stages.

[0037] Furthermore, the fine aggregate is low-mud limestone manufactured sand with a fineness modulus of 2.6-2.9, a stone powder content of 6%-10%, a methylene blue value not exceeding 1.0 g / kg, and a mud content not exceeding 1.0%. The appropriate amount of limestone micropowder adhering to the surface of the low-mud limestone manufactured sand can play a role in micro-filling and heterogeneous nucleation in the mortar interface zone, and together with the calcium carbonate crystal clusters on the exterior of the lithium slag composite admixture, provides a carbonate nucleation substrate of different scales.

[0038] Furthermore, the coarse aggregate is continuously graded limestone crushed stone with a particle size of 5-25 mm, and its water absorption rate is not higher than 1.5% and its crushing index is not higher than 12%.

[0039] Furthermore, the water-reducing agent is a polycarboxylate high-performance water-reducing agent with a solid content of 20%-25% and a water reduction rate of ≥ 25%.

[0040] The present invention also provides a method for preparing low-carbon concrete with the above-mentioned carbon-fixing lithium slag composite admixture, comprising the following steps:

[0041] M1: Add fine aggregate, coarse aggregate, low C3A high belite silicate cement, sulfoaluminate reaction components, lithium slag composite admixture and calcined magnesium aluminum hydrotalcite to the mixer and dry mix for 30-60 seconds to ensure that all solid components are mixed evenly.

[0042] M2: Add 65%-70% of the total amount of water and 65%-75% of the total amount of water-reducing agent, stir for 60-90 seconds to fully wet the cementitious materials and aggregates and form a continuous slurry;

[0043] M3: Pre-disperse ettringite seed crystals in 15%-20% of the total amount of water to form an ettringite seed crystal dispersion;

[0044] M4: Add the seed crystal dispersion to the concrete mixture obtained in step M2 and stir for 45-60 s; then add the remaining water and the remaining water-reducing agent, and continue stirring for 60-120 s to obtain carbon-fixing lithium slag composite admixture low-carbon concrete.

[0045] This invention adds ettringite seed crystals after the initial formation of the cement paste, which reduces agglomeration and ineffective adsorption caused by prolonged direct contact between the seed crystals and dry powder, ensuring uniform dispersion of the seed crystals in the paste. The water-reducing agent is added in stages, compensating for the adsorption of sulfoaluminate reactants, lithium slag composite admixtures, and calcined magnesium aluminum hydrotalcite during the initial mixing stage. Combined with the reduction of open pores on the surface of the lithium slag composite admixture, this improves the initial fluidity and slump retention of the concrete.

[0046] The mechanism of the technical solution of this invention is as follows:

[0047] This invention achieves uniform loading of calcium carbide slag powder and calcium aluminate onto the surface of lithium slag particles through pectin-calcium crosslinking fixation, high-humidity pre-reaction, and single-stage controlled mineralization. During the high-humidity pre-reaction, the alkaline environment provided by the calcium carbide slag promotes the dissolution of the silica-alumina components on the lithium slag surface, forming a calcium-aluminum-silicon reaction interface with the calcium and aluminum components. Subsequent mineralization transforms the Ca(OH)2 on the outer surface of the particles into dispersed calcium carbonate clusters, while retaining some active calcium components and ion migration channels. This reduces the open porosity on the lithium slag surface, lowers early water absorption and adsorption of water-reducing agents, and provides nucleation sites and a reaction basis for subsequent hydration reactions.

[0048] After the concrete is mixed with water, the calcium-aluminum-silicon reaction interface continues to hydrate, and calcium carbonate crystal clusters promote the nucleation of silicate and aluminate hydration products. The sulfoaluminate reaction components provide active calcium and aluminum components, which, under the guidance of ettringite seed crystals, react with sulfate ions released from lithium slag to form dispersed and fine ettringite, filling early pores and improving the stability of the slurry structure; calcined magnesium-aluminum hydrotalcite assists in the adsorption and buffering of remaining sulfate ions, reducing their local enrichment and the risk of subsequent disordered reactions.

[0049] As the cement ages, the low-C3A high-belite silicate cement continues to hydrate, providing Ca(OH)2 and an alkaline environment to the lithium slag core. This promotes the pozzolanic reaction of the low-crystallinity silica-alumina phase, generating hydrated calcium silicate gel and aluminum-containing hydrated calcium silicate gel. This further fills the pores and strengthens the interface, thus forming a strength development process that connects early ettringite support, mid-term interface hydration, and late-term continuous reaction of lithium slag.

[0050] Compared with the prior art, the advantages of the present invention are:

[0051] (1) This invention addresses the problems of porous, highly absorbent, and insufficient early-stage reactivity of lithium mica extraction residue particles. It utilizes low-methoxyl pectin and soluble calcium salts to pre-form a surface cross-linking network, then directionally fixes calcium carbide slag powder and calcium aluminate materials onto the surface of the lithium slag particles and their shallow open pores. Further, through high-humidity pre-reaction and controlled CO2 mineralization, a calcium-aluminum-silicon reaction interface and a discontinuous mineralization reaction layer are formed on the outside of the lithium slag particles, filling some of the open pores and effectively reducing early capillary water absorption and ineffective adsorption of water-reducing agents. Simultaneously, the external calcium carbonate crystal clusters provide heterogeneous nucleation sites, while the retained active calcium components and ion migration channels facilitate the continuous reaction of the lithium slag in the middle and later stages, thus balancing concrete workability, early reaction conditions, and later-stage activity.

[0052] (2) In view of the problem that the sulfate content in lithium extraction residue of lithium mica is high and the release process is difficult to control stably, the present invention uses low C3A high belite silicate cement to reduce the abnormal setting and slump loss caused by the rapid reaction of the highly active aluminate phase in the base cement with the lithium slag sulfate in the early stage of mixing; at the same time, sulfoaluminate reaction components, ettringite seed crystals and calcined magnesium aluminum hydrotalcite are introduced to construct a synergistic regulation system of calcium aluminum component supply, ettringite dispersion and nucleation and residual sulfate auxiliary buffering, which promotes the uniform generation of ettringite, reduces local concentrated precipitation and disordered sulfate reaction, and significantly improves the early structure formation, strength development, volume stability and paste density of concrete.

[0053] (3) This invention uses lithium extraction residue from lepidolite and industrial solid waste such as carbide slag in synergistic application in the preparation of composite admixtures, and converts some calcium components into calcium carbonate through CO2 mineralization, thereby achieving synergistic utilization of solid waste resources and carbon dioxide fixation. By reducing the amount of conventional mineral admixtures such as fly ash and slag powder, as well as some cement clinker, the consumption of natural resources and traditional cementitious materials is reduced. At the same time, the sources and reaction relationships of calcium, aluminum, silicon and sulfate in the cementitious system are made clearer, which is conducive to raw material control and concrete performance stability. It has good potential for high-value utilization and low-carbon application of solid waste. Detailed Implementation

[0054] To enable those skilled in the art to clearly and completely understand the technical solution of the present invention, the present invention will be further described in detail below with reference to embodiments. Obviously, the embodiments described herein are only for explaining the present invention and are not intended to limit the scope of the present invention. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0055] This invention provides a carbon-fixing lithium slag composite admixture for low-carbon concrete, comprising the following raw materials in parts by weight: 210-240 parts of low-C3A high-belite silicate cement, 20-40 parts of sulfoaluminate reaction component, 110-140 parts of lithium slag composite admixture, 1.5-3 parts of calcined magnesium aluminum hydrotalcite, 1.5-2.5 parts of ettringite seed crystals, 800-840 parts of fine aggregate, 1020-1040 parts of coarse aggregate, 155-165 parts of water, and 7-10 parts of water-reducing agent;

[0056] The preparation method of the lithium slag composite admixture is as follows:

[0057] An aqueous solution of low-methoxyl pectin and an aqueous solution of soluble calcium salt were sequentially added to lithium slag powder and mixed evenly to obtain mixture I.

[0058] Mixture I, calcium carbide slag powder, and calcium aluminate are mixed evenly to obtain mixture II;

[0059] Mixture II was pre-reacted in an environment with a temperature of 35-50 ℃ and a relative humidity of not less than 95% for 4-6 h to obtain mixture III; during the pre-reaction process, the material was intermittently turned over to control the material to remain in a loose state after the pre-reaction, without obvious hardening and agglomeration;

[0060] Mixture III was mineralized for 1-2 hours at a temperature of 30-45 ℃, a relative humidity of 55%-70%, and a CO2 volume fraction of 20%-50%. After drying, low-speed mechanical deagglomeration, and sieving, lithium slag composite admixture was obtained. During the mineralization process, the material was continuously turned over to ensure uniform contact with CO2.

[0061] In some examples, during the preparation of the lithium slag composite admixture, the mass ratio of the solid raw materials lithium slag powder, carbide slag powder, and calcium aluminate is (76-84):(10-16):(5-9); the amount of low-methoxyl pectin is 0.08%-0.12% of the total mass of the solid raw materials (lithium slag powder, carbide slag powder, and calcium aluminate); and the amount of soluble calcium salt is 0.20%-0.40% of the total mass of the solid raw materials.

[0062] In some examples, an aqueous solution of low-methoxyl pectin and an aqueous solution of soluble calcium salt are sequentially added to lithium slag powder via spraying.

[0063] In some examples, the aqueous solution of the low-methoxyl pectin has a mass concentration of 1.5%-2.0%; the aqueous solution of the soluble calcium salt has a mass concentration of 8%-10%.

[0064] In some examples, the degree of esterification of the low-methoxyl pectin is ≤ 50%.

[0065] In some examples, the soluble calcium salt is a soluble organic calcium salt, such as calcium acetate.

[0066] In some examples, the Blaine specific surface area of ​​the lithium slag powder is 380-450 m². 2 / kg, median particle size D 50 The lithium extraction residue, with a particle size of 12-18 μm, is obtained by drying and grinding lithium extraction residue from lepidolite. This residue is a solid residue produced after processing lepidolite ore as the main raw material through lithium extraction processes such as sulfur-containing roasting, acid roasting, or sulfuric acid leaching. By mass percentage, the residue contains 45%-60% SiO2, 18%-28% Al2O3, 5%-10% CaO, and 8%-14% SO3.

[0067] In some examples, the Ca(OH)₂ mass content of the carbide slag powder is ≥ 70%, and the median particle size D is...50 It is 2-4 μm in size and is a solid residue mainly composed of calcium hydroxide produced during the hydrolysis of calcium carbide to produce acetylene.

[0068] In some examples, the apparent carbonization conversion rate of Ca(OH)2 in the lithium slag composite admixture is 45%-70%, and the apparent carbonization conversion rate is calculated based on the change in Ca(OH)2 content in the composite particles before and after mineralization treatment; the mineralization products are mainly distributed discontinuously on the outside of the composite particles in the form of fine calcium carbonate crystal clusters, without forming a continuous dense coating film.

[0069] In some examples, the Al2O3 mass content of the calcium aluminate material is 55%-70%, and the median particle size D is... 50 The particle size is 2-4 μm, and it is selected from calcium aluminate clinker powder, high alumina cement clinker powder or a combination thereof.

[0070] In some examples, based on the total dry weight of lithium slag powder, calcium carbide slag powder, and calcium aluminate material before mineralization treatment, the net mineralized carbon fixation of the lithium slag composite admixture is 1.8%-3.5%. The net mineralized carbon fixation refers to the percentage of CO2 mass converted from inorganic carbon content in the mineralized lithium slag composite admixture, after deducting the CO2 mass converted from the original inorganic carbon content of the solid raw materials before mineralization treatment, relative to the total dry weight of the solid raw materials before mineralization treatment.

[0071] In some examples, the silicate cement clinker used in the low C3A high belite silicate cement has a C3A mass content of ≤5% and a C2S mass content of ≥40%.

[0072] In some examples, the mass content of anhydrous calcium sulfoaluminate mineral in the sulfoaluminate reaction component is 50%-70%, which is selected from sulfoaluminate cement clinker powder, clinker powder containing anhydrous calcium sulfoaluminate, or a combination thereof.

[0073] In some examples, the ettringite seed crystal is a powder containing the ettringite crystalline phase, with the ettringite crystalline phase content not less than 80%. 50 The thickness is 0.5-1.5 μm.

[0074] In some examples, the specific surface area of ​​the calcined magnesium aluminum hydrotalcite is 50-100 m². 2 / g, which is obtained by calcining a magnesium-aluminum layered double hydroxide at 400-600 °C. In the magnesium-aluminum layered double hydroxide, the molar ratio of magnesium to aluminum, Mg / Al, is (2.5-3.5):1.

[0075] In some examples, the fine aggregate is low-mud limestone manufactured sand with a fineness modulus of 2.6-2.9, a stone powder content of 6%-10%, a methylene blue value of not more than 1.0 g / kg, and a mud content of not more than 1.0%.

[0076] In some examples, the coarse aggregate is continuously graded limestone crushed stone with a particle size of 5-25 mm, a water absorption rate of not more than 1.5%, and a crushing index of not more than 12%.

[0077] In some examples, the water-reducing agent is a polycarboxylate high-performance water-reducing agent with a solid content of 20%-25% and a water reduction rate of ≥25%.

[0078] In some examples, the method for preparing low-carbon concrete with carbon-fixing lithium slag composite admixture includes the following steps:

[0079] M1: Add fine aggregate, coarse aggregate, low C3A high belite silicate cement, sulfoaluminate reaction components, lithium slag composite admixture and calcined magnesium aluminum hydrotalcite to the mixer and dry mix for 30-60 seconds to ensure that all solid components are mixed evenly.

[0080] M2: Add 65%-70% of the total amount of water and 65%-75% of the total amount of water-reducing agent, stir for 60-90 seconds to fully wet the cementitious materials and aggregates and form a continuous slurry;

[0081] M3: Pre-disperse ettringite seed crystals in 15%-20% of the total amount of water to form an ettringite seed crystal dispersion;

[0082] M4: Add the seed crystal dispersion to the concrete mixture obtained in step M2 and stir for 45-60 s; then add the remaining water and the remaining water-reducing agent, and continue stirring for 60-120 s to obtain carbon-fixing lithium slag composite admixture low-carbon concrete.

[0083] Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Unless otherwise specified, the raw materials, methods and equipment used in this invention are conventional raw materials, methods and equipment in the art.

[0084] In the following specific implementation cases, the specifications and sources of the raw materials used are shown in Table 1 below.

[0085] Table 1: Specifications and sources of raw materials used in the examples

[0086]

[0087] Example 1

[0088] This embodiment provides a carbon-fixing lithium slag composite admixture for low-carbon concrete, comprising the following raw materials in parts by weight: 225 parts of low-C3A high-belite silicate cement, 30 parts of sulfoaluminate reaction component, 125 parts of lithium slag composite admixture, 2.2 parts of calcined magnesium aluminum hydrotalcite, 2.0 parts of ettringite seed crystals, 820 parts of low-mud limestone manufactured sand, 1030 parts of limestone crushed stone, 160 parts of water, and 8.5 parts of polycarboxylate superplasticizer.

[0089] The solid raw materials of the lithium slag composite admixture include 80 parts of lithium slag powder, 13 parts of calcium carbide slag powder, and 7 parts of calcium aluminate material; the amount of low-methoxyl pectin is 0.10% of the total mass of the solid raw materials, the amount of calcium acetate is 0.30% of the total mass of the solid raw materials, and the amount of process water is 12% of the total mass of the solid raw materials (the process water includes water introduced from the low-methoxyl pectin aqueous solution and the soluble calcium salt aqueous solution, as well as atomized water added as needed).

[0090] The preparation method of the lithium slag composite admixture is as follows:

[0091] S1: Lithium slag powder is placed in a high-speed mixing device. Under the condition of 800 r / min, a 2.0% low-methoxyl pectin aqueous solution is sprayed into the lithium slag powder in the form of droplets over 6 min. After spraying, stirring is continued for 4 min. Then, a 10% calcium acetate aqueous solution is sprayed in. Calcium carbide slag powder and calcium aluminate powder are added in sequence, and the remaining atomized water is added. Stirring is continued for 10 min at a speed of 1300 r / min to obtain surface-loaded lithium slag composite particles.

[0092] S2: Place the composite particles obtained in step S1 in an environment with a temperature of 42 ℃ and a relative humidity of not less than 95% for pre-reaction for 5 h. During this period, turn the material over once every 30 min to keep the pre-reacted material loose and without obvious hardening and clumping.

[0093] S3: The pre-reacted composite particles are placed in a drum-type mineralization device and mineralized for 90 min under the conditions of drum speed of 12 r / min, temperature of 38 ℃, relative humidity of 62% and CO2 volume fraction of 35%. After the mineralization treatment, the particles are dried at 60 ℃ until the moisture content is not higher than 1.0%, and then mechanically depolymerized at a low speed of 250 r / min for 4 min and passed through a 0.30 mm sieve to obtain lithium slag composite admixture.

[0094] The preparation method of the carbon-fixing lithium slag composite admixture low-carbon concrete is as follows:

[0095] M1: Add fine aggregate, coarse aggregate, low C3A high belite silicate cement, sulfoaluminate reaction components, lithium slag composite admixture, and calcined magnesium aluminum hydrotalcite to a forced concrete mixer and dry mix for 45 seconds.

[0096] M2: Add 68% of the total amount of water and 70% of the total amount of water-reducing agent, stir for 75 seconds to fully wet all materials and form a continuous slurry.

[0097] M3: Pre-disperse ettringite seed crystals in water with a total volume of 18% to form a seed crystal dispersion;

[0098] M4: Add the seed crystal dispersion to the mixture obtained in step M2 and stir for 50 s; then add the remaining water and the remaining water-reducing agent, and continue stirring for 90 s to obtain carbon-fixed lithium slag composite admixture low-carbon concrete.

[0099] Example 2

[0100] This embodiment provides a carbon-fixing lithium slag composite admixture low-carbon concrete, comprising the following raw materials in parts by weight: 210 parts of low C3A high belite silicate cement, 40 parts of sulfoaluminate reaction component, 140 parts of lithium slag composite admixture, 3.0 parts of calcined magnesium aluminum hydrotalcite, 2.5 parts of ettringite seed crystals, 800 parts of low mud limestone manufactured sand, 1040 parts of limestone crushed stone, 165 parts of water, and 7 parts of polycarboxylate superplasticizer.

[0101] The solid raw materials of the lithium slag composite admixture include 76 parts of lithium slag powder, 16 parts of calcium carbide slag powder and 8 parts of calcium aluminate; the amount of low methoxy pectin is 0.12% of the total mass of the solid raw materials, the amount of calcium acetate is 0.40% of the total mass of the solid raw materials, and the amount of process water is 14% of the total mass of the solid raw materials.

[0102] The preparation method of the lithium slag composite admixture is basically the same as that in Example 1, except that: in step S1, the mass concentration of the low methoxy pectin aqueous solution is 1.5% and the mass concentration of the calcium acetate aqueous solution is 8%; in the high-humidity pre-reaction in step S2, the temperature is 50 ℃ and the time is 6 h; in the mineralization process of step S3, the temperature is 45 ℃, the relative humidity is 70%, the CO2 volume fraction is 50%, and the time is 120 min.

[0103] The preparation method of the carbon-fixing lithium slag composite admixture low-carbon concrete is the same as in Example 1.

[0104] Example 3

[0105] This embodiment provides a carbon-fixing lithium slag composite admixture low-carbon concrete, comprising the following raw materials in parts by weight: 240 parts of low C3A high belite silicate cement, 20 parts of sulfoaluminate reaction component, 110 parts of lithium slag composite admixture, 1.5 parts of calcined magnesium aluminum hydrotalcite, 1.5 parts of ettringite seed crystal, 840 parts of low mud limestone manufactured sand, 1020 parts of limestone crushed stone, 155 parts of water, and 10 parts of polycarboxylate superplasticizer.

[0106] The solid raw materials of the lithium slag composite admixture include 84 parts of lithium slag powder, 10 parts of calcium carbide slag powder and 6 parts of calcium aluminate; the amount of low methoxy pectin is 0.08% of the total mass of the solid raw materials, the amount of calcium acetate is 0.20% of the total mass of the solid raw materials, and the amount of process water is 9% of the total mass of the solid raw materials.

[0107] The preparation method of the lithium slag composite admixture is basically the same as that in Example 1, except that: in step S1, the mass concentration of the low methoxy pectin aqueous solution is 2.0% and the mass concentration of the calcium acetate aqueous solution is 10%; in the high-humidity pre-reaction in step S2, the temperature is 35 ℃ and the time is 4 h; in the mineralization process of step S3, the temperature is 30 ℃, the relative humidity is 55%, the CO2 volume fraction is 20%, and the time is 60 min.

[0108] The preparation method of the carbon-fixing lithium slag composite admixture low-carbon concrete is the same as in Example 1.

[0109] Example 4

[0110] This embodiment provides a carbon-fixing lithium slag composite admixture low-carbon concrete, comprising the following raw materials in parts by weight: 220 parts of low C3A high belite silicate cement, 35 parts of sulfoaluminate reaction component, 130 parts of lithium slag composite admixture, 2.5 parts of calcined magnesium aluminum hydrotalcite, 2.2 parts of ettringite seed crystals, 810 parts of low mud limestone manufactured sand, 1025 parts of limestone crushed stone, 157 parts of water, and 9 parts of polycarboxylate superplasticizer.

[0111] The solid raw materials of the lithium slag composite admixture include 78 parts of lithium slag powder, 14 parts of calcium carbide slag powder and 8 parts of calcium aluminate material; the amount of low methoxy pectin is 0.10% of the total mass of solid raw materials, the amount of calcium acetate is 0.35% of the total mass of solid raw materials, and the amount of process water is 13% of the total mass of solid raw materials.

[0112] The preparation method of the lithium slag composite admixture is basically the same as that in Example 1, except that: in step S1, the mass concentration of the low methoxy pectin aqueous solution is 1.8% and the mass concentration of the calcium acetate aqueous solution is 9%; in the high-humidity pre-reaction in step S2, the temperature is 45 ℃ and the time is 5 h; in the mineralization process of step S3, the temperature is 40 ℃, the relative humidity is 65%, the CO2 volume fraction is 40%, and the time is 100 min.

[0113] The preparation method of the carbon-fixing lithium slag composite admixture low-carbon concrete is the same as in Example 1.

[0114] Comparative Example 1

[0115] The concrete provided in this comparative example differs from that in Example 1 in that it uses an equal mass of unmodified lithium slag powder instead of the lithium slag composite admixture in Example 1. Specifically, the raw materials for the concrete in this comparative example are: 225 parts low-C3A high-belite silicate cement, 30 parts sulfoaluminate reactive component, 125 parts lithium slag powder, 2.2 parts calcined magnesium aluminum hydrotalcite, 2.0 parts ettringite seed crystals, 820 parts low-mud limestone manufactured sand, 1030 parts limestone crushed stone, 160 parts water, and 8.5 parts polycarboxylate superplasticizer.

[0116] This comparative example illustrates the impact of directly applying lithium mica extraction residue powder to concrete on the workability and strength development of concrete.

[0117] Comparative Example 2

[0118] The concrete provided in this comparative example differs from that in Example 1 in that: low-methoxyl pectin and calcium acetate are not added during the preparation of the lithium slag composite admixture in this comparative example, and high-moisture pre-reaction and CO2 mineralization treatment are not performed. That is, the preparation method of the lithium slag composite admixture in this comparative example is as follows:

[0119] 80 parts of lithium slag powder, 13 parts of calcium carbide slag powder and 7 parts of calcium aluminate were directly dry-mixed to obtain lithium slag composite admixture.

[0120] This comparative example is used to illustrate the difference between simple physical compounding and the composite modification process of the present invention.

[0121] Comparative Example 3

[0122] The concrete provided in this comparative example differs from that in Example 1 in that, in the preparation process of the lithium slag composite admixture, low-methoxyl pectin and calcium acetate are not added; instead, the same mass of process water is added in the form of atomized water. That is, the preparation method of the lithium slag composite admixture in this comparative example is as follows:

[0123] S1: Place lithium slag powder in a high-speed mixing device and spray atomized water at a speed of 800 r / min (the amount of atomized water is the same as the total mass of the low methoxy pectin aqueous solution and the calcium acetate aqueous solution); then add carbide slag micro powder and calcium aluminate material micro powder in sequence, and add the remaining atomized water. Continue stirring for 10 min at a speed of 1300 r / min to obtain surface-loaded lithium slag composite particles.

[0124] S2: Place the composite particles obtained in step S1 in an environment with a temperature of 42 ℃ and a relative humidity of not less than 95% for pre-reaction for 5 h. During this period, turn the material over once every 30 min to keep the pre-reacted material loose and without obvious hardening and clumping.

[0125] S3: The pre-reacted composite particles are placed in a drum-type mineralization device and mineralized for 90 min under the conditions of drum speed of 12 r / min, temperature of 38 ℃, relative humidity of 62% and CO2 volume fraction of 35%. After the mineralization treatment, the particles are dried at 60 ℃ until the moisture content is not higher than 1.0%, and then mechanically depolymerized at a low speed of 250 r / min for 4 min and passed through a 0.30 mm sieve to obtain lithium slag composite admixture.

[0126] This comparative example illustrates the effect of low-methoxyl pectin-calcium crosslinking on the surface load stability of carbide slag powder and calcium aluminate materials.

[0127] Comparative Example 4

[0128] The concrete provided in this comparative example differs from that in Example 1 in that high-moisture pre-reaction is not performed during the preparation of the lithium slag composite admixture in this comparative example. That is, the preparation method of the lithium slag composite admixture in this comparative example is as follows:

[0129] S1: Lithium slag powder is placed in a high-speed mixing device. Under the condition of 800 r / min, a 2.0% low-methoxyl pectin aqueous solution is sprayed into the lithium slag powder in the form of droplets over 6 min. After spraying, stirring is continued for 4 min. Then, a 10% calcium acetate aqueous solution is sprayed in, followed by the addition of calcium carbide slag powder and calcium aluminate material powder, and the remaining atomized water is added. Stirring is continued for 10 min at a speed of 1300 r / min to obtain surface-loaded lithium slag composite particles.

[0130] S2: The composite particles obtained in step S1 are placed in a drum-type mineralization device and mineralized for 90 min under the conditions of drum speed of 12 r / min, temperature of 38 ℃, relative humidity of 62% and CO2 volume fraction of 35%. After the mineralization treatment, the particles are dried at 60 ℃ until the moisture content is not higher than 1.0%, and then mechanically depolymerized at a low speed of 250 r / min for 4 min and passed through a 0.30 mm sieve to obtain lithium slag composite admixture.

[0131] This comparative example is used to illustrate the role of high-humidity pre-reaction in forming the calcium-aluminum-silicon reaction interface.

[0132] Comparative Example 5

[0133] The concrete provided in this comparative example differs from that in Example 1 in that CO2 mineralization treatment is not performed during the preparation of the lithium slag composite admixture in this comparative example. That is, the preparation method of the lithium slag composite admixture in this comparative example is as follows:

[0134] S1: Lithium slag powder is placed in a high-speed mixing device. Under the condition of 800 r / min, a 2.0% low-methoxyl pectin aqueous solution is sprayed into the lithium slag powder in the form of droplets over 6 min. After spraying, stirring is continued for 4 min. Then, a 10% calcium acetate aqueous solution is sprayed in, followed by the addition of calcium carbide slag powder and calcium aluminate material powder, and the remaining atomized water is added. Stirring is continued for 10 min at a speed of 1300 r / min to obtain surface-loaded lithium slag composite particles.

[0135] S2: Place the composite particles obtained in step S1 in an environment with a temperature of 42 ℃ and a relative humidity of not less than 95% for pre-reaction for 5 h. During this period, turn the material over once every 30 min to keep the pre-reacted material loose and without obvious hardening and clumping.

[0136] S3: The pre-reacted composite particles are dried at 60 ℃ until the moisture content is no more than 1.0%, then mechanically depolymerized at a low speed of 250 r / min for 4 min and passed through a 0.30 mm sieve to obtain lithium slag composite admixture.

[0137] This comparative example illustrates the role of CO2 mineralization in reducing water absorption of powders and providing nucleation sites for calcium carbonate.

[0138] Comparative Example 6

[0139] The concrete provided in this comparative example differs from that in Example 1 in that: during the preparation of the lithium slag composite admixture in this comparative example, a mineralization treatment was performed for 180 min at a temperature of 45 ℃, a relative humidity of 65%, and a CO2 volume fraction of 80%, resulting in an apparent carbonation conversion rate of approximately 86% for Ca(OH)2 in the calcium carbide slag. This resulted in a more continuous carbonate coating layer appearing on the surface of the obtained lithium slag composite admixture powder. Therefore, the preparation method of the lithium slag composite admixture in this comparative example is as follows:

[0140] S1: Lithium slag powder is placed in a high-speed mixing device. Under the condition of 800 r / min, a 2.0% low-methoxyl pectin aqueous solution is sprayed into the lithium slag powder in the form of droplets over 6 min. After spraying, stirring is continued for 4 min. Then, a 10% calcium acetate aqueous solution is sprayed in, followed by the addition of calcium carbide slag powder and calcium aluminate material powder, and the remaining atomized water is added. Stirring is continued for 10 min at a speed of 1300 r / min to obtain surface-loaded lithium slag composite particles.

[0141] S2: Place the composite particles obtained in step S1 in an environment with a temperature of 42 ℃ and a relative humidity of not less than 95% for pre-reaction for 5 h. During this period, turn the material over once every 30 min to keep the pre-reacted material loose and without obvious hardening and clumping.

[0142] S3: The pre-reacted composite particles are placed in a drum-type mineralization device and mineralized for 180 min under the conditions of drum speed of 12 r / min, temperature of 45 ℃, relative humidity of 65% and CO2 volume fraction of 80%. After the mineralization treatment, the particles are dried at 60 ℃ until the moisture content is not higher than 1.0%, and then mechanically depolymerized at a low speed of 250 r / min for 4 min and passed through a 0.30 mm sieve to obtain lithium slag composite admixture.

[0143] This comparative example illustrates the effect of overmineralization on the reactivity of lithium slag in the later stages of reaction.

[0144] Comparative Example 7

[0145] The concrete provided in this comparative example differs from that in Example 1 in that it uses an equal mass of P·O 42.5 ordinary Portland cement (clinker C3A content of 8.0% and C2S content of 19.2%) instead of low C3A high belite Portland cement. Specifically, the raw materials for this comparative example concrete are: 225 parts P·O 42.5 ordinary Portland cement, 30 parts sulfoaluminate reactive component, 125 parts lithium slag composite admixture, 2.2 parts calcined magnesium aluminum hydrotalcite, 2.0 parts ettringite seed crystals, 820 parts low-mud limestone manufactured sand, 1030 parts limestone crushed stone, 160 parts water, and 8.5 parts polycarboxylate superplasticizer.

[0146] This comparative example illustrates the effect of the aluminate phase content of the base cement on the early reaction and workability of lithium slag sulfate.

[0147] Comparative Example 8

[0148] The concrete provided in this comparative example differs from that in Example 1 in that an equal mass of low-C3A high-belite silicate cement is used to replace the sulfoaluminate reactive component, thus maintaining the total amount of cementitious materials without the presence of the sulfoaluminate reactive component. Specifically, the raw materials for the concrete in this comparative example are: 255 parts low-C3A high-belite silicate cement, 125 parts lithium slag composite admixture, 2.2 parts calcined magnesium aluminum hydrotalcite, 2.0 parts ettringite seed crystals, 820 parts low-mud limestone manufactured sand, 1030 parts limestone crushed stone, 160 parts water, and 8.5 parts polycarboxylate superplasticizer.

[0149] This comparative example illustrates the role of sulfoaluminate reaction components in the early structural formation of high-dosage lithium slag systems.

[0150] Comparative Example 9

[0151] The concrete provided in this comparative example differs from that in Example 1 in that it uses an equal mass of low-C3A high-belite silicate cement to replace ettringite seed crystals, thus maintaining the total amount of cementitious materials without the presence of ettringite seed crystals. Specifically, the raw materials for the concrete in this comparative example are: 227 parts low-C3A high-belite silicate cement, 30 parts sulfoaluminate reactive component, 125 parts lithium slag composite admixture, 2.2 parts calcined magnesium aluminum hydrotalcite, 820 parts low-mud limestone manufactured sand, 1030 parts limestone crushed stone, 160 parts water, and 8.5 parts polycarboxylate superplasticizer.

[0152] This comparative example is used to illustrate the role of ettringite seed crystals in the early dispersion and nucleation of ettringite.

[0153] Comparative Example 10

[0154] The concrete provided in this comparative example differs from that in Example 1 in that it uses an equal mass of low-C3A high-belite silicate cement instead of calcined magnesium aluminum hydrotalcite, thus maintaining the total amount of cementitious materials unchanged even without calcined magnesium aluminum hydrotalcite. Specifically, the raw materials for the concrete in this comparative example are: 227.2 parts of low-C3A high-belite silicate cement, 30 parts of sulfoaluminate reactive component, 125 parts of lithium slag composite admixture, 2.0 parts of ettringite seed crystals, 820 parts of low-mud limestone manufactured sand, 1030 parts of limestone crushed stone, 160 parts of water, and 8.5 parts of polycarboxylate superplasticizer.

[0155] This comparative example illustrates the auxiliary adsorption and buffering effect of calcined magnesium aluminum hydrotalcite on residual sulfate ions.

[0156] Experimental Example 1: Performance Testing of Lithium Slag Composite Admixture

[0157] This experiment tested the performance of lithium slag powder, lithium slag composite admixtures obtained in Examples 1-4 and Comparative Examples 2-6. The Blaine surface area was determined using the air permeability method. The 30-minute water absorption rate was determined by weighing: 100 g of dry powder was weighed, 200 g of water was added, and stirring was performed for 1 minute, starting the timer from the addition of water; after 30 minutes, the mixture was vacuum filtered for 5 minutes at -0.08 MPa, and the mass of the resulting wet powder was weighed. The 30-minute water absorption rate was calculated as the percentage increase in mass of the wet powder relative to the original dry powder mass. The water demand ratio was calculated as the ratio of the water used to achieve the same fluidity as the reference mortar to the water used in the reference mortar; the activity index was calculated as the ratio of the compressive strength of the test mortar to the compressive strength of the reference mortar at the same age; during the water demand ratio and activity index tests, the test mortar was prepared by replacing 30% of the reference cement with an equal mass of the powder to be tested, and the reference mortar did not contain the powder to be tested. The net mineralized carbon fixation was calculated using the total inorganic carbon determination results, after deducting the original inorganic carbon in the raw material before mineralization. The apparent carbonization conversion rate of Ca(OH)2 was calculated based on the change in Ca(OH)2 content obtained from thermogravimetric analysis before and after mineralization. The test results are shown in Table 2.

[0158] Table 2: Performance Test Results of Lithium Slag Powder and Lithium Slag Composite Admixture

[0159]

[0160] Table 2 shows that the 30-minute water absorption rate of lithium slag powder is 18.6%, the water demand ratio is 115%, and the activity indices at 7 days and 28 days are 63% and 84%, respectively, indicating that untreated lithium slag powder has problems with rapid water absorption and insufficient early reaction. The lithium slag composite admixtures prepared in Examples 1-4 of this invention have a 30-minute water absorption rate reduced to 8.4%-9.6%, a water demand ratio reduced to 100%-103%, an activity index increased to 79%-83% at 7 days, and an activity index increased to 95%-98% at 28 days. This demonstrates that surface loading, high-humidity pre-reaction, and controlled mineralization treatment can significantly reduce the early water absorption of lithium slag powder while effectively preserving and improving the reaction conditions of the lithium slag powder.

[0161] Compared to Example 1, Comparative Example 2, which simply mixed three inorganic powders directly, showed significantly lower water absorption, water demand ratio, and activity index in its lithium slag composite admixture. This indicates that the effect of the present invention is not simply due to the addition of calcium carbide slag and calcium aluminate. Comparative Example 3, which did not include low-methoxyl pectin and calcium acetate in its preparation process, resulted in the calcium carbide slag and calcium carbonate products easily agglomerating independently between powders during mineralization, thus its improvement in water absorption and activity was weaker than that of Example 1. Comparative Example 4, which did not undergo high-humidity pre-reaction in its preparation process, although it could reduce some water absorption through mineralization, lacked a pre-formed calcium-aluminum-silicon reaction interface, and its activity index at 7 days and 28 days was lower than that of Example 1. Comparative Example 5, which did not undergo CO2 mineralization in its preparation process, had a higher activity index, but its water absorption and water demand ratio were significantly higher than those of Example 1. This indicates that high-humidity pre-reaction cannot replace the covering and filling effect of the calcium carbonate crystal clusters formed by mineralization on open pores. After overmineralization of the lithium slag composite admixture in proportion 6, the water absorption rate further decreased, but the activity index dropped to 87% after 28 days. This indicates that continuous and excessively thick carbonate cover will hinder the entry of water and alkaline ions into the lithium slag core, which is not conducive to the mid-to-late stage volcanic ash reaction.

[0162] The net mineralized carbon fixation of Examples 1-4 is 1.9%-3.4%. Based on its actual usage in concrete, each cubic meter of concrete can directly mineralize and fix approximately 2.1-4.8 kg of CO2, indicating that the present invention can achieve a certain amount of CO2 fixation while improving the performance of lithium slag powder.

[0163] Experimental Example 2: Degree of Mineralization Reaction and Interfacial Reactivity Test

[0164] To further verify the effects of CO2 mineralization on the surface modification and internal activity of lithium slag composite admixtures, this experiment conducted thermogravimetric analysis (TGA), early water absorption, polycarboxylate superplasticizer adsorption, and alkaline leaching tests on lithium slag powder, lithium slag composite admixtures obtained in Examples 1-4, and Comparative Examples 2-6. For TGA, the temperature was increased from 30 °C to 900 °C under a nitrogen atmosphere at a rate of 10 °C / min. The mass fractions of Ca(OH)₂ and CaCO₃ in the powder were calculated based on the weight loss at the corresponding temperature range and corrected using a blank sample before mineralization. The water absorption rate at 5 min was determined by weighing. The polycarboxylate superplasticizer adsorption rate was tested by adding the powder to a polycarboxylate superplasticizer solution with an initial concentration of 1.0 g / L, controlling the liquid-to-solid ratio at 10:1, stirring for 10 min, centrifuging and filtering, and measuring the organic carbon concentration in the supernatant to calculate the adsorption rate. The alkaline dissolution test was conducted as follows: 5 g of powder was added to 100 mL of 0.1 mol / L NaOH solution, stirred for 24 h at 20±2 ℃, filtered, and the concentrations of Si and Al in the filtrate were measured. The test results are shown in Table 3.

[0165] Table 3: Results of mineralization reaction and interfacial reactivity tests of lithium slag powder and lithium slag composite admixtures

[0166]

[0167] As shown in Table 3, the mass fraction of CaCO3 in the lithium slag composite admixtures prepared in Examples 1-4 of this invention is 8.2%-11.4%, while retaining 3.0%-5.1% Ca(OH)2. This indicates that the mineralization treatment of this invention does not completely carbonize the external calcium components, but rather retains active calcium components that can continue to participate in the hydration reaction while generating calcium carbonate. The 5-minute water absorption rate and polycarboxylate superplasticizer adsorption rate of the lithium slag composite admixtures in Examples 1-4 are significantly lower than those of pure lithium slag powder, indicating that the composite treatment of this invention can reduce the loss of free water in the initial mixing stage and the ineffective adsorption of the superplasticizer.

[0168] Compared to Example 1, the lithium slag composite admixture in Comparative Example 2 was obtained by direct compounding of inorganic powders. It had a higher Ca(OH)2 content and a lower CaCO3 content, resulting in a still relatively high early water absorption rate and water-reducing agent adsorption rate. The lithium slag composite admixture in Comparative Example 3 lacked pectin-calcium cross-linking fixation. Although its mineralization degree was similar to Example 1, its water absorption rate and water-reducing agent adsorption rate were significantly increased, indicating that the lack of pectin-calcium cross-linking fixation reduces the effective modification of the lithium slag particle surface by the mineralization products. The lithium slag composite admixture in Comparative Example 4 did not undergo high-humidity pre-reaction, and its Si and Al leaching concentrations were lower than in Example 1, indicating that high-humidity pre-reaction is beneficial for improving the subsequent reaction conditions between the lithium slag surface and the calcium and aluminum components. The lithium slag composite admixture in Comparative Example 5 did not undergo CO2 mineralization treatment, retaining a higher amount of Ca(OH)2 and Si and Al leaching, but its water absorption rate and water-reducing agent adsorption rate were significantly higher than in Example 1, indicating that surface loading and high-humidity pre-reaction alone cannot sufficiently improve the early water absorption and adsorption problems of lithium slag. The lithium slag composite admixture in proportion 6 was subjected to overmineralization treatment, which reduced the Ca(OH)2 content to 0.9% and the CaCO3 content to 14.6%. Its early water absorption and water-reducing agent adsorption were further reduced, but the Si and Al dissolution was significantly reduced, indicating that overmineralization would weaken the contact and dissolution of alkaline media with the active components inside the lithium slag.

[0169] The above results corroborate the changes in water requirement ratio and activity index in Table 2, indicating that the mineralization degree defined by the present invention can take into account both surface modification and mid-to-late stage reaction activity.

[0170] Experimental Example 3: Testing of Concrete Workability, Setting Time, and Mechanical Properties

[0171] According to the test method for concrete mixture performance, this test example tested the initial slump, initial spread, 60-minute slump loss, 120-minute slump loss, initial setting time, and bleeding rate of the concrete in Examples 1-4 and Comparative Examples 1-10. The concrete specimens in each group were cured under standard curing conditions for 3 days, 7 days, 28 days, and 56 days, and their compressive strength was tested. The test results are shown in Table 4.

[0172] Table 4: Test Results of Concrete Workability, Setting Time, and Compressive Strength

[0173]

[0174] As shown in Table 4, the initial slump of the concrete prepared in Examples 1-4 of this invention is 215-225 mm, the slump loss after 120 min is 26-35 mm, and the bleeding rate is not higher than 0.4%. The compressive strengths at 3 days, 7 days, 28 days, and 56 days are 21.8-23.4 MPa, 34.0-35.5 MPa, 47.9-49.2 MPa, and 55.6-56.5 MPa, respectively, indicating that the concrete of this invention has good workability, mixture stability, and continuous strength development ability.

[0175] Compared to Example 1, the concrete in Comparative Example 1, made with unmodified lithium slag powder, had an initial slump of only 170 mm, a slump loss of 86 mm at 120 min, and a 28-day compressive strength of 36.9 MPa. This indicates that directly incorporating lithium slag powder into concrete, with its high water absorption and high adsorption characteristics, simultaneously affects both the workability and strength development of the concrete. Comparative Examples 2-5, in the preparation of lithium slag composite admixtures, lacked cross-linking fixation, high-humidity pre-reaction, or CO2 mineralization treatment steps beyond direct loading, resulting in concrete workability and strength at all ages lower than in Example 1. This demonstrates the synergistic effect of the various process steps in the lithium slag composite admixture. The lithium slag composite admixture in Comparative Example 6 underwent excessive mineralization treatment. While the workability of the resulting concrete was similar to that of Example 1, the 28-day and 56-day compressive strengths decreased to 43.2 MPa and 48.2 MPa, respectively. This indicates that while excessive carbonization can reduce early water absorption, it inhibits subsequent reactions in the lithium slag core. Comparative Example 7, using ordinary Portland cement with a high C3A content, had a shortened initial setting time of 365 min, an increased slump loss of 62 mm at 120 min, and lower mid-to-late-stage strength than Example 1. Comparative Example 8, lacking sulfoaluminate reactants, had an extended initial setting time of 565 min, with significant decreases in 3-day and 7-day strength. Comparative Example 9, lacking ettringite seed crystals, showed reduced strength at all ages. Comparative Example 10, lacking calcined magnesium aluminum hydrotalcite, exhibited minimal changes in early workability and strength, but a decrease in 56-day strength. These results indicate that the synergistic effect between low-C3A high-belite Portland cement, sulfoaluminate reactants, ettringite seed crystals, and calcined magnesium aluminum hydrotalcite is beneficial for balancing the workability, setting process, and strength development of the concrete system.

[0176] Test Example 4: Concrete Volume Stability and Durability Test

[0177] Following the test methods for long-term performance and durability of concrete, this test investigated the 90-day drying shrinkage, 28-day electrical flux, 56-day chloride ion migration coefficient, and water permeability resistance of the concrete in Examples 1-4 and Comparative Examples 1-10. Water permeability resistance was evaluated using the water penetration height method. The test results are shown in Table 5.

[0178] Table 5: Test Results of Concrete Volume Stability and Durability

[0179]

[0180] As shown in Table 5, the 90-day drying shrinkage of the concrete prepared in Examples 1-4 of this invention is 398 × 10⁻⁶. -6 -435×10 -6 The electrical flux was 900-1020 C after 28 days, and the chloride ion migration coefficient was 4.0 × 10⁻⁶ after 56 days. -12 -4.7×10 -12 m 2 The average water seepage height is 21-26 mm, indicating that the concrete of this invention has good volume stability, impermeability and resistance to chloride ion intrusion.

[0181] Although the shrinkage and permeability indices of the concrete in Comparative Examples 1-5 showed some improvement, they were still weaker than those in Example 1, indicating that the surface structure of the lithium slag composite admixture not only affects the fresh performance of the concrete but also the formation of subsequent hydration products and the pore structure. In Comparative Example 6, after excessive mineralization of the lithium slag composite admixture, although the water absorption rate of the lithium slag composite admixture powder was low, the hindered later reaction of the lithium slag led to a significant increase in the electrical flux, chloride ion migration coefficient, and permeability height of the concrete compared to Example 1. The concrete in Comparative Example 7, using ordinary Portland cement with a high C3A content, exhibited higher drying shrinkage, electrical flux, chloride ion migration coefficient, and permeability height than Example 1, indicating that the disordered reaction between the early aluminate phase and lithium slag sulfate was detrimental to the stability of the paste structure and the refinement of pores. The concrete in Comparative Examples 8 and 9 lacked sulfoaluminate reaction components and ettringite seed crystals, respectively, resulting in insufficient early structure formation and a corresponding decrease in durability. The concrete in Comparative Example 10, which did not contain calcined magnesium aluminum hydrotalcite, showed an increased chloride ion migration coefficient and water penetration height, indicating that the structural reconstruction and sulfate-assisted adsorption of calcined magnesium aluminum hydrotalcite are beneficial to improving the pore structure and durability of the slurry.

[0182] Experimental Example 5: Test of Sulfate Concentration Changes in Pore Solution

[0183] To further evaluate the reaction and migration processes of sulfate ions in different cementitious systems, corresponding neat cement paste samples were prepared according to the composition of the cementitious materials and the corresponding water-cement ratios in Example 1 and Comparative Examples 7-10. After curing under standard conditions for 1 day, 7 days, and 28 days, the pore solution was extracted using a high-pressure pore solution press. After filtration through a 0.45 μm aqueous filter membrane, the SO4 content in the pore solution was determined by ion chromatography. 2- The concentration was determined. Three parallel samples were set up for each group, and the average value of the test results was taken. The results are shown in Table 6.

[0184] Table 6: Results of sulfate concentration test in pore solution

[0185]

[0186] As shown in Table 6, the pore solution SO4 of the paste sample corresponding to Example 1 2- The concentration decreased from 1280 mg / L on day 1 to 160 mg / L on day 28, indicating that sulfate ions in the system could participate in the early reaction under relatively uniform conditions and receive auxiliary buffering in the later stages. Comparative Example 7 used ordinary Portland cement with a high C3A content, and the corresponding neat paste sample showed a SO4 concentration of 1 mg / L on day 1. 2- The concentration of sulfate was significantly reduced, while the residual concentrations at 7 days and 28 days were still higher than in Example 1, indicating that an excessively rapid early aluminate reaction is not conducive to the uniform and continuous utilization of sulfate. Comparative Example 8 did not include any sulfoaluminate reaction components, and its SO4 concentration at all ages was significantly lower. 2- The concentrations remained at high levels; the neat pulp sample corresponding to Comparative Example 9 did not contain ettringite seeds, and its sulfate consumption rate was between that of Example 1 and Comparative Example 8; the neat pulp sample corresponding to Comparative Example 10 did not contain calcined magnesium aluminum hydrotalcite, and its 28-day pore solution SO4 2- The concentration remains high, indicating that calcined magnesium aluminum hydrotalcite has an auxiliary adsorption and buffering effect on the remaining sulfate ions that did not participate in the reaction in time.

[0187] The above detailed embodiments describe the implementation of the present invention; however, the present invention is not limited to the specific details described in the above embodiments. Within the scope of the claims and technical concept of the present invention, various simple modifications and changes can be made to the technical solution of the present invention, and these simple modifications all fall within the protection scope of the present invention.

Claims

1. A low-carbon concrete with a carbon-fixing lithium slag composite admixture, characterized in that, The raw materials include the following parts by weight: 210-240 parts of low C3A high belite silicate cement, 20-40 parts of sulfoaluminate reaction components, 110-140 parts of lithium slag composite admixture, 1.5-3 parts of calcined magnesium aluminum hydrotalcite, 1.5-2.5 parts of ettringite seed crystals, 800-840 parts of fine aggregate, 1020-1040 parts of coarse aggregate, 155-165 parts of water, and 7-10 parts of water-reducing agent; The preparation method of the lithium slag composite admixture is as follows: An aqueous solution of low-methoxyl pectin and an aqueous solution of soluble calcium salt were sequentially added to lithium slag powder and mixed evenly to obtain mixture I. Mixture I, calcium carbide slag powder, and calcium aluminate are mixed evenly to obtain mixture II; Mixture II was pre-reacted in an environment with a temperature of 35-50 ℃ and a relative humidity of not less than 95% to obtain mixture III; Mixture III was mineralized at a temperature of 30-45 ℃, a relative humidity of 55%-70%, and a CO2 volume fraction of 20%-50%. After drying, low-speed mechanical deagglomeration, and sieving, lithium slag composite admixture was obtained.

2. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 1, characterized in that, The mass ratio of lithium slag powder, carbide slag powder, and calcium aluminate is (76-84):(10-16):(5-9); the amount of low-methoxyl pectin is 0.08%-0.12% of the total mass of lithium slag powder, carbide slag powder, and calcium aluminate; and the amount of soluble calcium salt is 0.20%-0.40% of the total mass of lithium slag powder, carbide slag powder, and calcium aluminate.

3. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 2, characterized in that, The aqueous solution of the low-methoxyl pectin has a low-methoxyl pectin mass concentration of 1.5%-2.0%; the aqueous solution of the soluble calcium salt has a soluble calcium salt mass concentration of 8%-10%.

4. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 3, characterized in that, The degree of esterification of the low-methoxyl pectin is ≤ 50%.

5. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 3, characterized in that, The soluble calcium salt is a soluble organic calcium salt.

6. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 2, characterized in that, The calcium aluminate material is selected from calcium aluminate clinker powder, high-alumina cement clinker powder, or a combination thereof.

7. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 1, characterized in that, The sulfoaluminate reaction component is selected from sulfoaluminate cement clinker powder, clinker powder containing anhydrous calcium sulfoaluminate, or a combination thereof.

8. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 1, characterized in that, The ettringite seed crystal is a powder containing the ettringite crystal phase, and the ettringite crystal phase content is not less than 80%.

9. The low-carbon concrete with carbon-fixing lithium slag composite admixture according to claim 1, characterized in that, The calcined magnesium aluminum hydrotalcite is obtained by calcining magnesium aluminum layered double hydroxide at 400-600 ℃.

10. The method for preparing low-carbon concrete with carbon-fixing lithium slag composite admixture according to any one of claims 1-9, characterized in that, Includes the following steps: Fine aggregate, coarse aggregate, low C3A high belite silicate cement, sulfoaluminate reaction components, lithium slag composite admixture and calcined magnesium aluminum hydrotalcite are mixed evenly to obtain a dry mixture. Mix the dry mixture with 65%-70% of the total amount of water and 65%-75% of the total amount of water-reducing agent until homogeneous to form a continuous slurry; Erythrite seed crystals are pre-dispersed in 15%-20% of the total amount of water to form an erythrite seed crystal dispersion. After the seed crystal dispersion is mixed evenly with the continuous slurry, the remaining water and the remaining water-reducing agent are added and mixed evenly to obtain carbon-fixed lithium slag composite admixture low-carbon concrete.

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