Ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction and preparation method thereof

Abstract

The invention relates to the field of ecological concrete, in particular to ecological concrete based on synergistic alkali reduction of ardealite-sulphoaluminate cement and a preparation method of the ecological concrete. The ecological concrete comprises the following raw materials in parts by weight: 50-60 parts of sulphoaluminate cement; 20 to 30 parts of ardealite or modified ardealite; 0.5-1.5 parts of a carbon quantum dot composite water-retaining agent, 25-35 parts of graded aggregate and 0.2-0.5 part of a foaming agent; the water-binder ratio is 0.35-0.45. Meanwhile, the invention provides a corresponding preparation method. According to the ecological concrete and the preparation method thereof, waste utilization of ardealite is realized, the performance of the ecological concrete is optimized, and the survival rate of plants is increased.

Description

Technical Field

[0001] The present invention relates to the field of ecological concrete, and in particular to an ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction and a preparation method thereof. Background Art

[0002] Traditional concrete uses Portland cement as its primary binder. The alkaline products generated during hydration cause the hardened slurry to have a pH as high as 12-13, severely inhibiting plant root growth and making it difficult to meet the vegetation compatibility requirements of projects such as ecological slope protection and sponge cities. The development of low-alkali, highly environmentally adaptable eco-concrete has become a key focus in the building materials industry.

[0003] Phosphogypsum is an industrial solid waste generated during the production of wet-process phosphoric acid. Its current comprehensive utilization rate is very low, and its long-term storage causes groundwater pollution and wastes land resources. Existing technologies attempt to use phosphogypsum as a cement admixture or retarder, but its soluble phosphorus and fluorine impurities slow the hydration of Portland cement and reduce its early strength. Furthermore, its high sulfur content makes it incompatible with traditional cement systems, resulting in reduced workability and durability of concrete, significantly limiting its large-scale application.

[0004] Sulphoaluminate cement is considered a potential binder for eco-concrete due to its low alkalinity, early strength, rapid hardening, and strong erosion resistance. For example, Patent Publication No. CN115403342B discloses a low-carbon, bioremediation concrete and its preparation method. This concrete features low carbon emissions, a low native pH, simple curing conditions, low curing costs, and strong carbon dioxide carbonization absorption capacity, making it environmentally friendly. Patent Publication No. CN116120030B provides a solid waste-recycled bioremediation concrete binder, which enhances water retention through modified phosphogypsum and a water-absorbing resin.

[0005] These technologies increase water absorption by using common water-absorbing resins such as sodium polyacrylate. However, these passive water absorption and release mechanisms cannot dynamically adjust to ambient light conditions. Daytime evaporation causes a sudden drop in soil moisture, leading to water loss under high temperatures and strong sunlight, necessitating frequent water replenishment. Excessive water release at night can cause root rot, especially in the early stages of plant growth, hindering germination and root growth, and affecting plant survival rates. Therefore, further optimizing the utilization of phosphogypsum waste and improving the performance of eco-concrete remain pressing challenges. Summary of the Invention

[0006] In view of the above-mentioned deficiencies in the prior art, the present invention proposes an ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction and a preparation method thereof, which realizes the waste utilization of phosphogypsum and improves the performance of ecological concrete.

[0007] The technical solutions of the present invention are as follows:

[0008] In one aspect, the present invention provides an ecological concrete based on phosphogypsum-sulfoaluminate cement synergistic alkali reduction, comprising the following raw materials in weight fractions: 50-60 parts of sulfoaluminate cement; 20-30 parts of phosphogypsum or modified phosphogypsum; 0.5-1.5 parts of carbon quantum dot composite water retaining agent, 25-35 parts of graded aggregate, 0.2-0.5 parts of foaming agent; water-binder ratio: 0.35-0.45;

[0009] The modified phosphogypsum is nano-SiO2 coated phosphogypsum;

[0010] The preparation method of the carbon quantum dot composite water-retaining agent is as follows:

[0011] (1) Waste biomass was carbonized under nitrogen, acid-washed and purified, and then hydrothermally synthesized carbon quantum dots (CQDs);

[0012] (2) dissolving monomers of acrylic acid and N-isopropylacrylamide in water, adding CQDs and a crosslinking agent, N,N'-methylenebisacrylamide; reacting at 60-80°C for 2-3.5 hours under nitrogen protection to obtain a carbon quantum dot composite water-retaining agent;

[0013] The molar ratio of acrylic acid to N-isopropylacrylamide is 3-5:1, the mass ratio of acrylic acid to water is 1:6-1:10; CQDs account for 1-2% of the total mass; and N,N'-methylenebisacrylamide accounts for 0.1-0.5% of the mass of acrylic acid.

[0014] Phosphogypsum, a byproduct of wet-process phosphoric acid production, contains small amounts of free acids (such as phosphoric acid and sulfuric acid) and soluble impurities (such as P and F). Its acidic components can neutralize alkaline substances (such as Ca(OH)2) produced by the hydration of sulfoaluminate cement, lowering the pH of the system and inhibiting the negative effects of alkaline environments on plant and microbial growth. The amount of Ca(OH)2 generated in the hydration products of sulfoaluminate cement is significantly lower than that of ordinary Portland cement, resulting in a lower initial pH, providing a more suitable microenvironment for plant growth in eco-concrete.

[0015] SO4 in phosphogypsum 2- Reacts with the aluminum in sulphoaluminate cement to promote the formation of ettringite, further consuming Ca 2+ , reducing the liquid alkalinity. Phosphorus and fluorine ions in phosphogypsum can be chemically adsorbed or encapsulated in hydration products, reducing their potential harm to the environment.

[0016] Carbon quantum dots (CQDs) possess photothermal conversion capabilities, absorbing light energy and converting it into heat. Under high temperature / strong light conditions, the heat generated by CQDs induces hydrophobic contraction in the thermosensitive resin (N-isopropylacrylamide), increasing its crosslinking density and thus reducing water absorption and locking in moisture. At low temperatures / weak light, the resin regains its hydrophilicity and slowly releases water. The copolymer of acrylic acid and N-isopropylacrylamide is temperature-responsive, with its crosslinking density changing dynamically with temperature. Combined with the photothermal effect of CQDs, this enables intelligent control of "locking in moisture during the day and releasing it at night."

[0017] A further improvement of this solution is that the preparation method of the modified phosphogypsum is as follows:

[0018] (1) The phosphogypsum is washed with water and then dried, crushed to a D50 of 5-20 μm, and the crushed phosphogypsum is soaked in a dilute sulfuric acid solution and washed with water until neutral, and then dried;

[0019] (2) Add phosphogypsum, nano-SiO2 and dispersant into ethanol solution and disperse them evenly.

[0020] (3) ball milling modification to form a uniform coating layer;

[0021] (4) drying and then heat treating to obtain modified phosphogypsum;

[0022] The weight fractions of the raw materials are: 100 parts of phosphogypsum, 3-5 parts of nano-SiO2; the dispersant is 0.5-1% of the volume fraction of ethanol, and the solid-liquid ratio is 1:3-5.

[0023] Preferably, the preparation method of the modified phosphogypsum is as follows:

[0024] (1) The phosphogypsum is washed with water and then dried to a moisture content of <0.5%, crushed to a D50 of 5-20 μm, and the crushed phosphogypsum is immersed in a 3% dilute sulfuric acid solution at room temperature for 20-30 minutes, washed with water until neutral, and then dried;

[0025] (2) adding phosphogypsum, nano-SiO2, and dispersant KH-570 to an ethanol solution and ultrasonically treating for 15-30 minutes, wherein the nano-SiO2 particle size is 10-20 nm;

[0026] (3) Ball milling modification: Use a ball mill at a speed of 300-400 rpm for 2-3 hours to form a uniform coating layer;

[0027] (4) After vacuum drying at 50-60°C, heat treatment is performed at 150-180°C for 15-30 minutes, and the mixture is passed through a 200-mesh sieve to obtain modified phosphogypsum. The specific surface area of the modified phosphogypsum is ≥800m 2 / g.

[0028] In a further improvement of this solution, the graded aggregate comprises the following raw materials in weight fractions: 60-70 parts of coarse aggregate; 20-30 parts of fine aggregate; and 0.5-1 part of microporous additive.

[0029] Preferably, the coarse aggregate is crushed stone or / and ceramsite with a particle size of 5 to 10 mm, the fine aggregate is machine-made sand or / and iron tailings sand with a particle size of 0.5 to 1 mm; and the microporous additive is expanded perlite powder with a particle size of 0.1 to 0.3 mm.

[0030] In a further improvement of this solution, the foaming agent is 30% concentration H2O2 or aluminum powder with a particle size of 10 to 20 μm or a protein-based foaming agent.

[0031] In a further improvement of this solution, the waste biomass is one or more of rice husks or corn stalks.

[0032] In a further improvement of this solution, the carbon quantum dot composite water-retaining agent product is crushed and sieved to have a particle size of 45-75 μm.

[0033] According to a further improvement of the present invention, the sulphoaluminate cement comprises the following raw materials in weight fractions: 40-60 parts of bauxite, 25-40 parts of limestone, 10-15 parts of dihydrate gypsum, and 1-5 parts of iron ore.

[0034] On the other hand, the present invention provides a method for preparing the above-mentioned ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction, comprising the following steps:

[0035] (1) dry-mix sulphoaluminate cement, modified phosphogypsum or phosphogypsum, and graded aggregate for 3-10 minutes until uniform;

[0036] (2) Add the carbon quantum dot composite water retaining agent and continue mixing for 2-10 minutes;

[0037] (3) Add clean water according to the water-binder ratio and stir to form a slurry; add a foaming agent and stir quickly to foam;

[0038] (4) After injection molding, let it stand and cure after the pores are stable.

[0039] The beneficial effects of the present invention are:

[0040] (1) The present invention mixes sulfoaluminate cement and phosphogypsum to prepare ecological concrete, using phosphogypsum as a raw material to address the land occupation and pollution problems caused by its long-term storage. Through synergistic effects, the pH value of the concrete pore fluid is controlled within the range of 8.5-9.5, which is significantly lower than that of traditional concrete (pH>12). This is conducive to plant root growth and microbial activity, and is suitable for scenarios such as ecological slope protection and vegetation concrete.

[0041] (2) Ordinary water-absorbing resins passively absorb and release water, and are unable to dynamically adjust according to the lighting environment conditions, resulting in water loss under high temperature and strong light, and excessive water release at night. The present invention prepares a photothermal responsive water-absorbing resin by compounding carbon quantum dots (CQDs) with water-absorbing resin, and utilizes the photothermal effect of CQDs to achieve intelligent regulation of "high temperature water locking during the day and low temperature water release at night". During strong light during the day: CQDs absorb light energy and convert it into heat energy, the cross-linking density inside the resin increases, the water absorption rate decreases, and the water is locked in to prevent evaporation; at night under weak light: the heat energy is released, the cross-linking density decreases, the high water absorption rate is restored, and the water is released slowly.

[0042] (3) By optimizing aggregate grading and introducing a foaming agent, a hierarchical pore structure of "macropores for penetration - medium pores for water retention - micropores for fertilizer storage" is formed, taking into account the permeability, water retention, and root anchoring requirements. Chemical foaming generates closed pores for strength compensation, while physical grading ensures through pores, resolving the "high permeability - low strength" contradiction.

[0043] (4) To address the problem of rapid sulfate release in the early stage, intense hydration reaction, and high early strength but concentrated hydration heat, which can lead to thermal damage to seeds, we also modified the phosphogypsum by loading nano-silica onto the surface of the phosphogypsum through a mechanochemical method to form a "core-shell" structure: achieving a gradient effect of "early slow release and later sustained release". The core layer: phosphogypsum provides sulfate and calcium sources, and the shell layer: the nano-SiO2 network controls the ion diffusion rate. BRIEF DESCRIPTION OF THE DRAWINGS

[0044] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments or the description of the prior art. Obviously, for ordinary technicians in this field, other drawings can be obtained based on these drawings without any creative work.

[0045] Figure 1 The figures are scanning electron microscope images of Example 3 (left) and Example 4 (right). DETAILED DESCRIPTION

[0046] In order to enable those skilled in the art to better understand the technical solutions of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described below in conjunction with the accompanying drawings of the embodiments of the present invention. Obviously, the described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts should fall within the scope of protection of the present invention.

[0047] Example 1: Synthesis of Carbon Quantum Dots (CQDs) from Waste Biomass

[0048] (1) The rice husks were repeatedly rinsed with deionized water to remove surface sand and impurities, dried at 60°C for 24 hours, and crushed to a particle size of ≤1 mm;

[0049] (2) The crushed rice husks were placed in a corundum crucible and placed in a tube furnace. The temperature was raised to 300°C at 5°C / min and kept at this temperature for 30 minutes to perform pre-carbonization to remove volatiles. The temperature was then raised to 500°C at 5°C / min and kept at this temperature for 2 hours, while nitrogen was introduced to prevent oxidation. The crucible was then naturally cooled to room temperature to obtain a black carbonized product.

[0050] (3) The black carbonized product was mixed with an acid solution at a ratio of 1:10 (mass to volume ratio), wherein the acid solution was concentrated hydrochloric acid (HCl, 36-38%), and magnetically stirred at 80°C for 6 hours to remove ash and metal ions; the precipitate was collected by centrifugation and washed with deionized water until the filtrate pH was ≈ 7 to remove unreacted particles, thereby obtaining an acid-washed carbon material (denoted as AC).

[0051] (4) Hydrothermal synthesis of CQDs: AC was mixed with deionized water and ultrasonically dispersed to form a uniform suspension. The mixture was transferred to a hydrothermal reactor, heated to 180°C, and kept at this temperature for 6 hours. The mixture was then cooled naturally to room temperature. The mixture was filtered through a 0.22 μm filter membrane to remove large particles. The filtrate was the crude CQD solution.

[0052] (5) The crude CQDs solution was placed in a dialysis bag to remove unreacted organic matter and small molecule impurities. The solution was concentrated using a rotary evaporator and freeze-dried to obtain a solid CQDs powder.

[0053] Example 2: Preparation of acrylic acid-acrylamide composite water-retaining agent

[0054] (1) Weigh the monomers to a molar ratio of acrylic acid (AA) to N-isopropylacrylamide (NIPAM) of 3-5:1, and a mass ratio of acrylic acid to water of 1:6–1:10. Add CQDs (1–2 wt.%, based on the total system mass) and ultrasonically disperse for 15 min. Add the crosslinker N,N'-methylenebisacrylamide (MBA, 0.1–0.5% by mass of the acrylic acid) and stir for 10 min.

[0055] (2) Nitrogen is introduced into the reaction system for 30 minutes to eliminate oxygen interference, prevent oxygen inhibition, and ensure free radical chain growth. The temperature is raised to 60-80°C and the reaction is kept at this temperature for 2-3.5 hours to form a gel-like product.

[0056] (3) The product was cut into pieces and mechanically crushed into particles, 45–75 μm particles were screened using a standard sieve, and oven-dried to constant weight to obtain a carbon quantum dot composite water-retaining agent.

[0057] An orthogonal experimental design was designed based on the preparation process of acrylic acid-acrylamide composite water-retaining agent to analyze the effects of various raw material factors. The heating temperature was 60℃ and the constant temperature reaction time was 3 hours.

[0058] Select L9(34) orthogonal table, the specific factor levels are as follows Table 1:

[0059] Table 1

[0060]

[0061] Test indicators and evaluation methods: Light water retention: water retention rate under light (decline in water absorption rate); dark water release: ability to recover water absorption rate in darkness; intelligent control efficiency: difference in water absorption rate between light and darkness (reflecting dynamic response capability).

[0062] The steps are as follows:

[0063] Take 1.0 g of dry constant weight carbon quantum dot composite water-retaining agent particles, soak them in deionized water until swelling equilibrium, filter them and weigh the wet weight (W0).

[0064] 1. Light lock water test:

[0065] Spread the swollen sample evenly in a Petri dish and place it under light to simulate the photothermal effect of CQDs. Remove the sample every hour, blot the surface moisture with filter paper, and weigh it (W4) until the difference between two consecutive weighings is less than 5%. Calculate the water retention rate: Water retention rate = W4 / W0 * 100%.

[0066] 2. Dark water release test

[0067] The sample after the light water lock test (W4) was placed in the dark. The sample was re-immersed in deionized water and weighed every 2 hours (W 25 ) until it is saturated with water (the difference between two consecutive weighings is less than 5%).

[0068] Calculation of water absorption recovery: Recovery = (W 25 -W4) / (W0-W4)×100%

[0069] 33. Intelligent control efficiency test (dynamic difference of photothermal response)

[0070] The water-retaining agent particles were placed in alternating cycles of light and darkness for three cycles. The water retention rate (light) and water absorption rate (darkness) after each cycle were recorded. Control efficiency = (water retention rate - water absorption rate recovery) / water absorption rate recovery. See Table 2 for experimental results.

[0071] Table 2 Orthogonal test results:

[0072]

[0073] Through experiments, we found that:

[0074] 1. AA / NIPAM molar ratio (A): Acrylic acid (AA) provides carboxylic acid groups (-COOH), while N-isopropylacrylamide (NIPAM) is responsible for photosensitivity. A high AA ratio (5:1) increases carboxylic acid groups and improves water absorption, but may also loosen the crosslinking network and reduce high-temperature water retention. A low AA ratio (3:1) reduces water absorption due to insufficient hydrophilic groups.

[0075] 2. CQDs Addition (B): Carbon quantum dots (CQDs) dynamically adjust the resin crosslink density through the photothermal effect. Low CQDs (1%): Insufficient photothermal conversion efficiency, small crosslink density change, and weak intelligent control capabilities.

[0076] 3. MBA crosslinker addition (C) Mechanism: MBA determines the initial crosslink density, which affects the sensitivity of intelligent control. High MBA initial crosslink density limits dynamic adjustment space and reduces intelligent response capabilities.

[0077] 4. Monomer / water mass ratio (D): The concentration of the reaction system affects the degree of polymer chain extension and network uniformity. High concentration (1:6): The cross-linked network is dense and has strong water retention at high temperatures, but poor water release recovery at low temperatures. Low concentration (1:10): The network is loose and has high water absorption, but insufficient mechanical strength.

[0078] Best combination: A4:1+B2%+C0.1%+D1:8

[0079] (1) Acrylic acid (AA) and N-isopropylacrylamide were weighed at a molar ratio of 4:1, and a mass ratio of acrylic acid to 1:8. CQDs (2 wt.%, based on the total system mass) were added and ultrasonically dispersed for 15 minutes. The crosslinker N,N'-methylenebisacrylamide (MBA, 0.1% by weight of the acrylic acid) was added and stirred for 10 minutes.

[0080] Next, we conducted experimental analysis of different particle sizes. Larger sizes, however, slow water absorption due to the internal cross-linking network, but offer high water retention stability. However, this also leads to stress concentration, accelerating crack growth. If the particles are too small (less than 45 μm), they tend to agglomerate, forming weak zones and reducing the compressive strength of the concrete. Therefore, a particle size of 45–75 μm was selected.

[0081] Example 3: Preparation of modified phosphogypsum

[0082] (1) After washing with water, the phosphogypsum is dried to a moisture content of <0.5%, and crushed to D50 = 5-20 μm. The crushed phosphogypsum is immersed in a 3% dilute sulfuric acid solution at room temperature for 30 minutes, washed with water until neutral, and then dried; Activate Ca on the surface of the phosphogypsum 2+ , enhance the binding sites with SiO2;

[0083] (2) Adding silane coupling agent KH-570 to the ethanol solution, directly mixing it with phosphogypsum and nano-SiO2, and ultrasonically treating it for 15-30 minutes. The nano-SiO2 particle size is 10-20 nm;

[0084] The weight fractions of the raw materials are: 100 parts of phosphogypsum, 4 parts of nano-SiO2; KH-570 is 0.7% of the volume fraction of ethanol, and the solid-liquid ratio is 1:4;

[0085] (3) Ball milling modification: Using a ball mill at 400 rpm for 3 hours, the coupling agent grafting and SiO2 coating were achieved simultaneously through mechanochemical action. KH-570 contains methacryloyloxy groups, which are more likely to react with phosphogypsum / SiO2 under the frictional heat of ball milling.

[0086] (4) After vacuum drying at 55°C, heat treatment was performed at 160°C for 20 minutes and passed through a 200-mesh sieve to obtain nano-modified phosphogypsum. The specific surface area of the nano-modified phosphogypsum was 850 m 2 / g.

[0087] Example 4:

[0088] The difference from Example 3 is that it is not treated with 3% dilute sulfuric acid solution. Figure 1 The left and right sides are scanning electron microscope images of Examples 3 and 4, respectively. It can be seen that the surface of the phosphogypsum that has not been treated with dilute sulfuric acid solution is mainly a coupling agent, and the bonding between the phosphogypsum and nano-SiO2 is weak.

[0089] Example 5:

[0090] The preparation method of ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction comprises the following steps:

[0091] (1) Dry mix sulphoaluminate cement, modified phosphogypsum and graded aggregate for 5 minutes until uniform;

[0092] (2) Add carbon quantum dot composite water retention agent and continue mixing for 5 minutes;

[0093] (3) Add clean water at a water-to-binder ratio of 0.4 and stir to form a slurry; add a protein-based foaming agent and stir rapidly to foam;

[0094] (4) After injection molding, let it stand and cure after the pores are stable.

[0095] The sulphoaluminate cement is commercially available from Shanshui Cement, and its ingredients include the following raw materials in weight fractions: 50 parts of bauxite, 30 parts of limestone, 15 parts of dihydrate gypsum, and 5 parts of iron ore.

[0096] The carbon quantum dot composite water-retaining agent was obtained by the optimal method of Example 2.

[0097] The modified phosphogypsum was obtained by the optimal method of Example 3.

[0098] The graded aggregate includes coarse aggregate: 5-10 mm crushed stone, accounting for 69%, providing main pores and water-permeable main channels; fine aggregate: 0.5-1 mm machine-made sand / iron tailings sand, accounting for 30%, filling the gaps between the coarse aggregates to form medium-pore water-retention and fertilizer storage; microporous additive: 0.91% expanded perlite powder, particle size 0.1-0.3 mm, absorbing nutrients.

[0099] The proportions of the raw materials are as follows: 55 parts of sulphoaluminate cement; 25 parts of modified phosphogypsum; 1 part of carbon quantum dot composite water-retaining agent, 30 parts of graded aggregate, and 0.4 parts of foaming agent.

[0100] Example 6:

[0101] The difference between the control group and Example 5 is:

[0102] Control group A, no phosphogypsum was added, and other components were the same as in Example 5;

[0103] Control group B, sulphoaluminate cement was replaced with ordinary phosphate cement (P·O 42.5R);

[0104] Control group C, using unmodified phosphogypsum;

[0105] Control group D, no water retaining agent;

[0106] Control group E, using ordinary water-retaining agent;

[0107] Control group F, single aggregate (no gradation optimization) + no foaming agent;

[0108] Control group G, graded material + no foaming agent.

[0109] Experimental Design: Plant Growth Verification of Phosphogypsum-Sulphoaluminate Cement-Based Eco-Concrete:

[0110] Test block preparation: Concrete test blocks (size: 10×10×5 cm) were cast according to the optimized group formula, with 10 replicates in each group.

[0111] Seeding rate: Evenly sow 20 ryegrass seeds per test plot (seed pretreatment: soak for 24 hours). Soil covering: Cover the surface with 1 cm thick nutrient soil (simulating natural soil covering conditions).

[0112] The pH value was measured on the initial 1-7 days, 14 / 28 / 60 days, and the germination rate and root penetration depth were measured on the 60th day. The experimental results are shown in Table 3:

[0113] Table 3

[0114]

[0115]

[0116] Comparison of Controls A and B with Example 5 shows that the pH of the ordinary Portland cement in Control B is as high as 12-13, far exceeding the plant tolerance range, severely inhibiting seed germination and resulting in an extremely low germination rate. Although Control A is better than Comparative Example 2, the alkalinity of pure sulfoaluminate cement still inhibits plant growth.

[0117] Comparing Control C with Example 5, we can see that while ordinary phosphogypsum neutralizes alkalinity, its release fluctuates. Early sulfate release is rapid, and early hydration heat causes heat damage to some seeds, impacting germination. Therefore, modifying phosphogypsum during the initial germination phase is crucial to minimize the impact later in the process.

[0118] As shown in Controls D and E and Example 5, rapid water loss without a water-retaining agent leads to seed dehydration, while insufficient water retention restricts root development. Conventional water-absorbing resins passively retain water and improve water supply, but this can lead to daytime water shortages, while excessive water release at night can cause root rot and easily lead to root hypoxia. Example 5 dynamically retains water and optimizes the rhizosphere microenvironment. This is particularly effective in arid and semi-arid regions with large diurnal temperature differences and significant fluctuations in light intensity.

[0119] From the control groups F, G and Example 5, it can be seen that the single aggregate has too high a proportion of macropores, resulting in poor water retention, and the pores are connected but lack micropores for fertilizer storage.

[0120] Without a foaming agent, the pore structure is simple (only graded aggregate), which reduces water retention and restricts root extension. Graded aggregate (large, medium, and micropores) combined with a foaming agent to close the pores improves water permeability and water retention.

[0121] Example 7:

[0122] The difference from Example 5 is that the proportions of the raw materials are as follows: 50 parts of sulphoaluminate cement; 20 parts of 6-modified phosphogypsum; 0.5 parts of carbon quantum dot composite water-retaining agent, 25 parts of graded aggregate, 0.2 parts of foaming agent, water-binder ratio: 0.35, germination rate 93%.

[0123] Example 8:

[0124] The difference from Example 5 is that the proportions of the raw materials are as follows: 60 parts of sulphoaluminate cement; 30 parts of modified phosphogypsum; 1.5 parts of carbon quantum dot composite water retaining agent, 35 parts of graded aggregate, 0.5 parts of foaming agent, water-binder ratio: 0.45, germination rate 91%.

Claims

1. Ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction, characterized in that: The invention comprises the following raw materials in weight fractions: 50-60 parts of sulphoaluminate cement; 20-30 parts of phosphogypsum or modified phosphogypsum; 0.5-1.5 parts of carbon quantum dot composite water retaining agent; 25-35 parts of graded aggregate; 0.2-0.5 parts of foaming agent; water-binder ratio: 0.35-0.45; The modified phosphogypsum is nano-SiO2 coated phosphogypsum; The preparation method of the carbon quantum dot composite water-retaining agent is as follows: (1) Waste biomass was carbonized under nitrogen, acid-washed and purified, and then hydrothermally synthesized into carbon quantum dots (CQDs); (2) dissolving monomers of acrylic acid and N-isopropylacrylamide in water, adding CQDs and a crosslinking agent, N,N'-methylenebisacrylamide; reacting at 60-80°C for 2-3.5 hours under nitrogen protection to obtain a carbon quantum dot composite water-retaining agent; The molar ratio of acrylic acid to N-isopropylacrylamide is 3-5:1, the mass ratio of acrylic acid to water is 1:6-1:10; CQDs account for 1-2% of the total mass; and N,N'-methylenebisacrylamide accounts for 0.1-0.5% of the mass of acrylic acid.

2. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 1, characterized in that: The preparation method of the modified phosphogypsum is as follows: (1) The phosphogypsum is washed with water and then dried, crushed to a D50 of 5-20 μm, and the crushed phosphogypsum is soaked in a dilute sulfuric acid solution and washed with water until neutral, and then dried; (2) adding phosphogypsum, nano-SiO2 and dispersant into ethanol solution and dispersing them evenly; (3) ball milling modification to form a uniform coating layer; (4) drying and then heat treating to obtain modified phosphogypsum; The weight fractions of the raw materials are: 100 parts of phosphogypsum, 3-5 parts of nano-SiO2; the dispersant is 0.5-1% of the volume fraction of ethanol, and the solid-liquid ratio is 1:3-5.

3. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 2, characterized in that: The preparation method of the modified phosphogypsum is as follows: (1) The phosphogypsum is washed with water and then dried to a moisture content of <0.5%, crushed to a D50 of 5-20 μm, and the crushed phosphogypsum is immersed in a 3% dilute sulfuric acid solution at room temperature for 20-30 minutes, washed with water until neutral, and then dried; (2) adding phosphogypsum, nano-SiO2, and dispersant KH-570 to an ethanol solution and ultrasonically treating for 15-30 minutes, wherein the nano-SiO2 particle size is 10-20 nm; (3) Ball milling modification: Use a ball mill at a speed of 300-400 rpm for 2-3 hours to form a uniform coating layer; (4) After vacuum drying at 50-60°C, heat treatment is performed at 150-180°C for 15-30 minutes, and the mixture is passed through a 200-mesh sieve to obtain modified phosphogypsum. The specific surface area of the modified phosphogypsum is ≥800m 2 / g.

4. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 1, characterized in that: The graded aggregate comprises the following raw materials in weight fractions: 60-70 parts of coarse aggregate; 20-30 parts of fine aggregate; and 0.5-1 part of microporous additive.

5. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 4, characterized in that: The coarse aggregate is crushed stone or / and ceramsite with a particle size of 5 to 10 mm, the fine aggregate is machine-made sand or / and iron tailings sand with a particle size of 0.5 to 1 mm; the microporous additive is expanded perlite powder with a particle size of 0.1 to 0.3 mm.

6. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 1, characterized in that: The foaming agent is 30% concentration H2O2 or aluminum powder with a particle size of 10 to 20 μm or a protein-based foaming agent.

7. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 1, characterized in that: The waste biomass is one or more of rice husks and corn stalks.

8. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 1, characterized in that: The carbon quantum dot composite water-retaining agent product is crushed and sieved to have a particle size of 45-75 μm.

9. The ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to claim 1, characterized in that: The sulphoaluminate cement comprises the following raw materials in weight fractions: 40-60 parts of bauxite, 25-40 parts of limestone, 10-15 parts of dihydrate gypsum, and 1-5 parts of iron ore.

10. The method for preparing ecological concrete based on phosphogypsum-sulphoaluminate cement synergistic alkali reduction according to any one of claims 1 to 9, characterized in that: The following steps are involved: (1) dry-mix sulphoaluminate cement, modified phosphogypsum or phosphogypsum, and graded aggregate for 3-10 minutes until uniform; (2) Add the carbon quantum dot composite water retaining agent and continue mixing for 2-10 minutes; (3) Add clean water according to the water-binder ratio and stir to form a slurry; add a foaming agent and stir quickly to foam; (4) After injection molding, let it stand and cure after the pores are stable.

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