A method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment

Abstract

The present application relates to a method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment, comprising the following steps: S1: crystal form regulation pretreatment of gypsum raw materials, converting it from calcium sulfate dihydrate to hexagonal columnar calcium sulfate hemihydrate; S2: mixing the pretreated gypsum obtained in S1 with slag and steel slag in proportion, and adding an appropriate amount of water to prepare a cementitious material slurry; S3: obtaining a solid waste-based cementitious material after forming and curing. By crystal form regulation pretreatment of gypsum, it is converted from gypsum dihydrate to hexagonal columnar gypsum hemihydrate, the internal structure of the particles is loose, the number of cracks increases, and it is easy to crack into small particles during the hydration process, greatly increasing the active site and improving the early hydration reaction rate of the solid waste-based cementitious material. Not only does it achieve high value-added utilization of solid waste, but also significantly reduces the carbon emissions of cementitious materials, in line with the development direction of green building materials under the "double carbon" target.

Description

Technical Field

[0001] This invention relates to the fields of solid waste resource utilization and cementitious materials technology, and in particular to a method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment. Background Technology

[0002] Solid waste-based cementitious materials, as a green alternative to cement, have significant advantages such as low carbon footprint, energy saving, and high solid waste utilization. They achieve their cementing properties through the synergistic effect of various industrial solid wastes and can be widely used in construction, roads, mine backfilling, and other fields. However, existing solid waste-based cementitious materials generally suffer from technical bottlenecks such as low early strength and long setting time, severely limiting their engineering applicability.

[0003] Solid waste-based cementitious materials, as a low-carbon alternative to cement, have become a core direction for the building materials industry to achieve its "dual-carbon" goals due to their advantage of "treating waste with waste" (large-scale utilization of industrial solid wastes such as slag, steel slag, and desulfurized gypsum). Through synergistic hydration reactions between various solid wastes (such as the reaction of the silica-alumina components of slag with the calcium-sulfur components of steel slag and gypsum to generate ettringite, hydrated calcium silicate gel, and other strength products), they can replace cement in multiple engineering fields. However, existing solid waste-based cementitious materials generally suffer from low early strength (compressive strength at 3 days is often below 18 MPa) and long setting time (initial setting time often exceeds 200 minutes) due to slow hydration reaction rates, severely limiting their application in scenarios requiring high construction efficiency and early performance, such as shotcrete, road repair, and precast components.

[0004] The main existing technical means to improve the above problems and their limitations are as follows: First, existing research on physical grinding to refine raw materials attempts to increase the contact area between the raw materials and water, thereby accelerating the hydration reaction, by grinding slag, steel slag, or gypsum to a higher specific surface area (e.g., increasing the specific surface area of ​​slag to over 500 m² / kg). However, this method has significant drawbacks: on the one hand, ultrafine grinding requires a large amount of electrical energy (energy consumption increases by about 15% for every 100 m² / kg increase in specific surface area), significantly increasing preparation costs; on the other hand, excessively fine particles are prone to agglomeration, which reduces the fluidity of the slurry, affects its workability, and has a limited effect on improving early strength (usually no more than 20%).

[0005] Secondly, some technologies involving the addition of chemical admixtures attempt to introduce admixtures such as chlorides, nitrates, or polycarboxylate superplasticizers to improve performance by catalyzing hydration reactions or enhancing slurry dispersibility. However, the alkaline environment (pH 12-13) of solid waste-based cementitious materials differs significantly from traditional cement systems, resulting in poor compatibility of most cement admixtures. For example, chloride admixtures may introduce harmful ions, causing subsequent steel reinforcement corrosion; polycarboxylate superplasticizers are easily over-consumed by adsorption sites on the slag surface, failing to achieve water-reducing effects and potentially leaving unreacted components that reduce the durability of the cementitious material.

[0006] Third, optimizing the external curing environment can be achieved by increasing the curing temperature (e.g., using steam curing at 40-60℃), increasing relative humidity (e.g., mulching + constant humidity curing), or using autoclaving (100-120℃, 0.1-0.2MPa) to accelerate the formation of hydration products. However, these methods rely on specialized curing equipment (e.g., steam generators, autoclaves), resulting in high construction costs and poor flexibility. For on-site projects such as roads and tunnels, large-scale constant temperature and humidity curing is difficult to achieve. At the same time, high-temperature curing may lead to uneven formation of hydration products, which may reduce long-term strength (the 28-day strength may decrease by 5%-10%).

[0007] In summary, existing technologies mostly focus on "intervening in performance during the hydration reaction process through external means," neglecting to address the "regulation of the reactivity of the raw materials themselves." This results in limited performance improvement and is accompanied by problems such as high cost, poor compatibility, and construction limitations. This invention proposes a forward-looking approach: by "pre-treating" gypsum before the reaction, its crystal structure is altered, thereby enhancing the reactivity of the raw material from the source. This achieves synergistic optimization of early strength and setting time in solid waste-based cementitious materials without relying on additives or complex curing conditions. This approach breaks through the limitations of existing technologies in "process control," shifting the core of performance optimization from "external intervention" to "inherent modification of raw materials," providing a completely new technical path for the high performance of solid waste-based cementitious materials. Developing a preparation method that can simultaneously improve the early strength and shorten the setting time of solid waste-based cementitious materials without relying on additives and through the regulation of gypsum's own crystal structure has significant engineering value and environmental implications. Summary of the Invention

[0008] This invention aims to solve the above-mentioned problems. In a first aspect, it provides a method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment, comprising the following steps: S1: Pre-treatment of gypsum raw materials to regulate crystal form, transforming them from dihydrate calcium sulfate to hexagonal columnar hemihydrate calcium sulfate; S2: Mix the pretreated gypsum obtained in S1 with slag and steel slag in a certain proportion, and add an appropriate amount of water to prepare a cementitious slurry; S3: Solid waste-based cementitious material is obtained after molding and curing.

[0009] Based on the above technical solution, the crystal form control pretreatment in step S1 includes the following steps: S11: Raw material selection: Select flue gas desulfurization gypsum with calcium sulfate dihydrate content ≥95%, attached water ≤10%, and d90 ≤200μm; S12: High-pressure hydrothermal synthesis: The reaction is carried out at a constant temperature of 120~160℃ and 0.1~0.2 MPa for 60~150 minutes; more preferably, the constant temperature is 140~150℃ and the reaction time is 120~140 minutes.

[0010] S13: Drying treatment: Dry at 90~100℃ until the moisture content is ≤0.5%; S14: Quality Inspection: Ensure that the product is mainly hemihydrate gypsum, with residual dihydrate gypsum ≤5%.

[0011] Furthermore, during the high-pressure hydrothermal synthesis process, the pH value of the reaction system is controlled to be 6-8.

[0012] Based on the above technical solution, the composition of the cementitious material by mass percentage is as follows: slag, 70%-80%; steel slag, 10%-20%; pretreated gypsum, 5%-15%; and water-cement ratio of 0.30-0.35.

[0013] Furthermore, the slag is S95 grade granulated blast furnace slag powder with a specific surface area of ​​450–500 m². 2 / kg.

[0014] Furthermore, the steel slag is hot-quenched converter steel slag with a specific surface area of ​​500–550 m². 2 / kg.

[0015] Furthermore, the specific surface area of ​​the pretreated gypsum is 400–450 m². 2 / kg.

[0016] Secondly, based on the same invention, the present invention also provides a solid waste-based cementitious material prepared by the method described in the first aspect.

[0017] Furthermore, the solid waste-based cementitious material has a 3-day compressive strength ≥20MPa and an initial setting time ≤150 minutes.

[0018] Thirdly, the present invention also provides a gypsum crystal form control pretreatment method, including the high-pressure hydrothermal synthesis step described in the first aspect, wherein the reaction temperature is 140-150℃, the reaction time is 120-140 minutes, and the pH value of the reaction system is 6-8.

[0019] The present invention has the following beneficial effects: 1. This invention pre-treats gypsum by regulating its crystal structure, transforming it from dihydrate gypsum into hexagonal columnar hemihydrate gypsum. This results in a looser internal structure with increased cracks, facilitating pyrolysis into smaller particles during hydration and significantly increasing the number of reactive sites. Consequently, it dramatically improves the early hydration rate of the solid waste-based cementitious material. Experimental results show that the 3-day compressive strength can reach a maximum of 24.45 MPa, an increase of over 60% compared to the untreated system, demonstrating significant early strength development.

[0020] 2. The gypsum with regulated crystal form exhibits stronger reactivity and a faster dissolution rate, accelerating the formation of hydration products such as ettringite, thereby significantly shortening the initial and final setting times of the cementitious material. Experimental data shows that the shortest initial setting time is only 35 minutes, more than 85% shorter than the conventional system, which is beneficial for improving construction efficiency and suitable for fast-hardening and fast-setting engineering scenarios.

[0021] 3. This invention improves performance by modifying the raw materials themselves, without the need to add chemical additives such as polycarboxylate superplasticizers. This avoids the problems of poor compatibility, durability risks and additional material costs associated with additives in high-alkali environments, and achieves a green and low-cost high-performance path.

[0022] 4. Compared with existing technologies such as physical grinding to refine raw materials, the gypsum pretreatment process adopted in this invention has an energy consumption of only 18 kW·h / t, which is less than 40% of the energy consumption of physical grinding. Moreover, the process flow is standardized, the equipment is highly versatile, and no complicated curing conditions are required, which has good prospects for industrial application.

[0023] 5. This invention makes full use of industrial solid wastes such as flue gas desulfurization gypsum, slag, and steel slag, and improves their synergistic hydration efficiency through crystal form regulation. This not only realizes the high added value utilization of solid waste, but also significantly reduces the carbon emissions of cementitious materials, which is in line with the green building materials development direction under the "dual carbon" target.

[0024] 6. By adjusting the pretreatment process parameters (such as temperature, time, pH value) and raw material ratio, the strength development and setting behavior of cementitious materials can be flexibly controlled. It is suitable for various engineering scenarios such as road repair, shotcrete, and precast components, and has good adaptability and scalability. Attached Figure Description

[0025] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only one embodiment of the present invention. For those skilled in the art, other embodiments can be derived from the provided drawings without creative effort.

[0026] Figure 1 Flowchart of gypsum crystal pretreatment; Figure 2 Electron micrograph of gypsum after crystallization treatment; Figure 3 Comparison of ordinary gypsum (dihydrate gypsum) and crystalline pretreated gypsum (hemihydrate gypsum); Figure 4 Comparison of electron micrographs of ordinary gypsum and crystalline pretreated gypsum; Figure 5 XRD test results of ordinary gypsum and crystalline pretreated gypsum; Figure 6 The effects of isothermal time and temperature on the pretreatment reaction. Detailed Implementation

[0027] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, not all of them. The following embodiments are only for illustrative purposes and are not intended to limit the scope of the present invention in any way. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention. Unless otherwise specified, the experimental methods used in the present invention are conventional methods. Unless otherwise specified, the materials and reagents used in the present invention are commercially available. Furthermore, other terms used in the present invention, unless otherwise specified, generally have the meanings commonly understood by those skilled in the art.

[0028] Example 1: Pretreatment process for regulating gypsum crystal form This embodiment provides a complete process for preparing crystal-modified gypsum, which is one of the core technical aspects of this invention. The process consists of four standardized steps: raw material screening → high-pressure hydrothermal synthesis → drying and dehydration → quality inspection. The equipment selection, process parameters, and operating procedures for each step have been precisely quantified and verified to ensure the stability, reproducibility, and industrial-scale feasibility of the process. The pretreatment process diagram is shown below. Figure 1 As shown.

[0029] 1. Raw material screening Raw flue gas desulfurization gypsum meeting or exceeding the Class II quality requirements of the national standard GB / T 37785-2019 "Flue Gas Desulfurization Gypsum" should be selected as the pretreatment raw material. The raw material should meet the following specific technical specifications: Chemical composition: The mass content of calcium sulfate dihydrate (CaSO4·2H2O) is not less than 95%; Moisture control: The attached water content shall not exceed 10%; Particle size requirement: The particle size distribution of the powder must meet the following requirements. 90 ≤ 200μm (confirmed by using a laser particle size analyzer, such as the Malvern Mastersizer 3000).

[0030] The raw material screening stage is fundamental to ensuring the efficiency of subsequent conversion reactions and the purity of products.

[0031] 2. High-pressure hydrothermal synthesis treatment This step aims to induce a crystal transformation from dihydrate gypsum to hemihydrate gypsum under hydrothermal conditions. Through hydrothermal synthesis in a high-pressure reactor, the original flue gas desulfurization gypsum is transformed from an irregular granular dihydrate to a hexagonal columnar hemihydrate. This crystal form possesses a unique cracking and dissolution mechanism, readily breaking down into smaller particles during hydration, thus increasing the number of reactive sites.

[0032] Equipment configuration: The equipment adopts an intermittent high-pressure reactor, which should have a heating capacity of 20–300℃ and integrate a PID precise temperature control system, a mechanical stirring device and a pressure safety valve.

[0033] The process steps are as follows: S1 Loading: Prepare the qualified gypsum from step 1 into a slurry with a solid concentration of 10%-20% and load it into the reactor. The loading amount should be controlled to be 60%-70% of the effective volume of the reactor.

[0034] S2 Heating: After sealing the reactor, the temperature is programmed to rise to the target temperature range of 140–150℃ at a constant rate of 5℃ / min.

[0035] S3 Isothermal Reaction: After reaching the target temperature, maintain the system temperature within ±2℃ of the set value, and simultaneously turn on the stirrer at a speed of 200 r / min. Continue the reaction under these conditions for 120–140 minutes. During the holding period, system pressure fluctuations should be controlled within ±0.05 MPa.

[0036] S4 Cooling and Unloading: After the reaction is complete, slowly open the vent valve and control the cooling rate to not exceed 2℃ / min. After the pressure inside the reactor returns to atmospheric pressure, open the reactor lid and quickly transfer the reaction product to the next process to prevent the product from becoming damp due to moisture absorption.

[0037] 3. Drying treatment Materials treated with high-pressure hydrothermal heat contain a certain amount of free moisture and need to be dried to obtain dry and stable hemihydrate gypsum products.

[0038] Equipment configuration: Use a hot air circulating drying oven with a temperature control accuracy of ±1℃. Its working temperature range should cover 20–200℃, and it should have a forced air circulation function to ensure uniform heat distribution.

[0039] Process parameters and operation: S1: Spreading material: Spread the wet material evenly on the drying tray, with a material layer thickness not exceeding 3 cm.

[0040] S2: Drying: Set the drying temperature to 90–100℃, the hot air velocity to 0.5–1.0 m / s, and continue drying for 2–3 hours until the moisture content of the material drops below 0.5%.

[0041] S3: Process Management: During the drying process, the material is turned over manually or mechanically every hour to ensure uniform heating and avoid local overheating that could lead to clumping or secondary phase change.

[0042] S4: Post-processing: After drying, allow the material to cool naturally to room temperature in a dry environment, then pack it into a polyethylene (PE) sealed bag to strictly isolate it from air and prevent the product from absorbing moisture.

[0043] 4. Quality Inspection Strict quality inspections are conducted on the dried finished products to ensure that their phase and morphology meet the requirements for crystal form control.

[0044] Crystallization water analysis: The content of adhering water and crystallization water in the product was determined using a MAX5000XL moisture analyzer. The adhering water test program was set at a temperature of 80℃. When the mass loss rate was less than 0.001%, the adhering water test program was stopped and automatically switched to the crystallization water test program. The crystallization water test program was set at a temperature of 230℃. When the mass loss rate was less than 0.001%, the crystallization water test program was stopped. Phase analysis: X-ray diffraction was used to identify the phase composition of the samples. The XRD pattern of qualified crystal-controlled gypsum should match the main characteristic peaks of the standard card (PDF#84-0439) for calcium sulfate hemihydrate (CaSO4·0.5H2O), and the area ratio of the characteristic peaks of residual gypsum dihydrate (CaSO4·2H2O) in the raw material should not exceed 5%.

[0045] Microscopic morphology observation: The microscopic morphology of the product is observed using a scanning electron microscope. Successful crystal form control should transform the originally irregular, multi-faceted dihydrate gypsum particles into hemihydrate gypsum with regular morphology, smooth surface, and a typical hexagonal prismatic crystal structure.

[0046] like Figure 2 The electron microscope image shows numerous small, smooth particles of various shapes, including cubic, hexagonal plates, and hexagonal prisms. These particles are widely distributed and abundant. EDS analysis revealed a Ca:S atomic ratio of 1:1, confirming them as gypsum particles. Cracks appear at the ends and sides of the gypsum particles, indicating further fragmentation behavior.

[0047] Through the above four standardized processes, highly reactive crystal-modified gypsum can be prepared stably and efficiently, providing a key raw material guarantee for the subsequent preparation of high-performance solid waste-based cementitious materials.

[0048] Example 1: The hemihydrate gypsum obtained from the pretreatment process is shown in the figure. Figure 3 As shown in the right figure, Figure 3 The left image shows ordinary dihydrate gypsum. Figure 4 The images show scanning electron microscope (SEM) images of two types of gypsum. The morphology of the products was analyzed using a Hitachi SU8010 field emission SEM system at 20 kV. It can be seen that the original gypsum had an irregular morphology, which transformed into a regular hexagonal prism after pre-treatment. Figure 5 The images show the XRD test results for two types of gypsum. XRD analysis reveals a change in the water of crystallization in the pretreated gypsum. It changed from dihydrate gypsum to hemihydrate gypsum.

[0049] Example 2: Effects of isothermal time and temperature on pretreatment reaction in high-pressure hydrothermal synthesis The theoretical water of crystallization content of gypsum dihydrate is 20.93%, and that of gypsum hemihydrate is 6.2%. One quick way to determine the extent of a reaction is to measure the water of crystallization content of the reaction products. The lower the water of crystallization content, the higher the degree of dehydration; conversely, the higher the water of crystallization content, the lower the degree of dehydration. Therefore, the degree of reaction can be quickly determined using the water of crystallization index.

[0050] The grade, or purity, of industrial by-product gypsum varies, resulting in different crystal water contents after complete conversion into hemihydrate gypsum. Considering that most actual impurities are non-volatile substances, the crystal water content (CW) of hemihydrate gypsum and the purity of dihydrate gypsum are obtained from the reaction equation. x The relationship is as follows:

[0051] Based on the purity of the desulfurized gypsum raw materials used in the experiment, the crystal water content of each hemihydrate gypsum product was calculated using formula (1), as shown in Table 1.

[0052] Table 1: Hemihydrate gypsum crystal water content (%) of the raw materials used in the experiment

[0053] When the water of crystallization in the hemihydrate gypsum product prepared from desulfurized gypsum is exactly 5.84%, it can be considered that the dihydrate gypsum has been completely converted into hemihydrate gypsum. If the water of crystallization content is higher than 5.84%, then some dihydrate gypsum has not been completely converted into hemihydrate gypsum. If the water of crystallization is lower than 5.84%, it indicates that some hemihydrate gypsum has been further dehydrated and converted into anhydrous gypsum.

[0054] The experimental results show that, at the same constant temperature, the longer the isothermal time, the lower the water of crystallization content; and at the same isothermal time, the higher the constant temperature, the lower the water of crystallization content. At a constant temperature of 120℃, the water of crystallization content was relatively high. After 150 minutes of isothermal treatment, the water of crystallization decreased to approximately 9.5%, indicating that the dehydration reaction was incomplete. Further extending the isothermal time could allow the dehydration reaction to continue; however, considering production efficiency, excessively long dehydration times are not conducive to improving production efficiency. Therefore, this experiment does not recommend using 120℃ for isothermal treatment. At a constant temperature of 130℃ for 150 minutes, the water of crystallization content was still relatively high, exceeding the theoretical value of 5.84%. At a constant temperature of 160℃, the water of crystallization content was below 5%, indicating the formation of anhydrous gypsum. At constant temperatures of 140℃ for 60 minutes and 150℃ for 60 minutes, the water of crystallization content was approximately 6%.

[0055] The phase transition process from dihydrate gypsum to hemihydrate gypsum involves a synergistic effect of temperature and time. At lower temperatures, the reaction requires a longer time to complete. Higher temperatures can accelerate the phase transition, but the excessive formation of crystal nuclei at high temperatures leads to insufficient crystal development and subsequent dehydration to anhydrous gypsum, resulting in inconsistent product quality. Therefore, taking into account production efficiency, energy consumption and product performance, the constant temperature can be in the range of 140~150℃ and the time range of 120-140 minutes. Subsequent experimental studies will use this temperature and time range.

[0056] Example 3: Effect of pH on the reaction Hemihydrate gypsum is formed by recrystallization under aqueous conditions. CaSO4·2H2O exists in the aqueous medium as Ca... 2+ SO4 2- H + OH - Plasma and molecules, pH value affects the solubility of each ion and changes the equilibrium direction of ionic equations.

[0057] The pH values ​​were selected for the experiment at 4, 6, 8, and 10. Other experimental conditions were: constant temperature of 150℃ and constant temperature time of 120 min. The pH value of the material was adjusted to the required experimental value using dilute H2SO4 solution or KOH solution. The experimental results are shown in Table 1. The aspect ratio was determined by measuring the ratio of the crystal length to the diameter in the crystal image.

[0058] Table 1

[0059] For industrial production equipment, acidic materials are highly corrosive. Prolonged operation in an acidic environment necessitates the use of higher-quality steel, increasing equipment investment and potentially shortening its lifespan. Conversely, neutral materials are more widely applicable in building materials applications, thus requiring a pH level close to neutral.

[0060] Taking into account the growth characteristics of hemihydrate gypsum in this process, the lifespan of production equipment, and the development and application of downstream products, the pH value of the reaction slurry fed into the reactor is controlled at around 7.

[0061] Example 4: Preparation and performance characterization of solid waste-based gel materials This embodiment elaborates on the composition, preparation process, and performance evaluation method of high-performance solid waste-based cementitious materials based on gypsum crystal form regulation, aiming to provide a complete technical solution that is quantifiable and reproducible.

[0062] 4.1 Composition of Gel Material A high-performance solid waste-based cementitious material based on gypsum crystal form regulation is characterized in that its composition, by mass percentage, includes 70%~80% slag, 10%~20% steel slag, 5%~15% crystal form-regulating gypsum and an appropriate amount of water, with a water-cement ratio of 0.30~0.35. The slag is S95 grade granulated blast furnace slag powder that meets the requirements of GB / T 18064-2017 "Granulated blast furnace slag powder for use in cement, mortar and concrete", with a specific surface area of ​​450~500 m² / kg. The steel slag is hot-quenched converter steel slag that meets the requirements of YB / T 022-2008 "Steel Slag for Cement" and has a specific surface area of ​​500~550 m² / kg; The crystal-modified gypsum described in Example 1 is calcium sulfate dihydrate (CaSO4) that has undergone high-pressure steam treatment. 2H2O) is transformed into hexagonal columnar calcium sulfate hemihydrate (CaSO4). The flue gas desulfurization gypsum (0.5H2O) has a specific surface area of ​​400~450 m² / kg.

[0063] 4.2 Preparation method The preparation method of the above-mentioned high-performance solid waste-based cementitious material is characterized by comprising the following steps: 1. Raw material weighing and premixing: Accurately weigh the specified amounts of slag, steel slag, and crystal-modifying gypsum according to the established ratio. Place the weighed dry materials into a mortar mixer and premix for 30 seconds until uniformly mixed.

[0064] 2. Add water in stages and stir: Initial mixing: Add approximately 70% (by weight) of the total mixing water, start the mixer, and mix at medium speed for 2 minutes to initially wet and disperse the dry materials.

[0065] Secondary mixing: Add the remaining approximately 30% of the mixing water and continue mixing for 1 minute until a homogeneous, fluid cementitious material slurry is obtained.

[0066] 3. Vibration molding Quickly pour the well-mixed slurry into a standard mold (e.g., 40 mm × 40 mm × 160 mm or equivalent). Place the mold on a concrete vibrating table, set the vibration frequency to 50 Hz and the amplitude to 0.5 mm, and vibrate for 30 to 60 seconds, or until no obvious air bubbles overflow from the surface of the slurry, ensuring the specimen is compacted.

[0067] 4. Standard maintenance Curing in mold: Immediately after molding, the mold is placed in a standard curing chamber with a temperature of 20 ± 1℃ and a relative humidity of not less than 95% and left to cure for 24 hours.

[0068] Post-demolding curing: After curing for 24 hours, carefully demold the specimen and continue to cure it under the same temperature and humidity conditions (20±1℃, RH ≥ 95%) until the specified test age (e.g., 3 days, 7 days, 28 days, etc.).

[0069] 4.3 Performance Evaluation Indicators and Test Methods 1. Compressive strength The test was conducted according to the national standard GB / T 17671-1999, "Test Method for Strength of Cement Mortar (ISO Method)". Compressive strength is a core indicator for evaluating the mechanical properties of cementitious materials, and its value increases with age. By comparing the compressive strength of different groups of cementitious materials (e.g., those using ordinary gypsum and those using crystal-modified gypsum) at various ages, the hydration reactivity of the raw materials (especially gypsum) and the promoting effect of crystal modification on strength development can be directly reflected.

[0070] 2. Setting time The tests were conducted according to the national standard GB / T 1346-2024, "Test Methods for Standard Consistency Water Requirement, Setting Time and Soundness of Cement". Setting time characterizes the rate at which a cementitious paste transitions from a plastic to a rigid state, specifically including: Initial setting time: The time required for the paste to begin to lose its plasticity.

[0071] Final setting time: The time required for the paste to completely lose its plasticity and initially acquire a certain structural strength. By measuring the setting time, the early hydration reaction rate of cementitious materials with different formulations can be effectively evaluated, especially the contribution of gypsum crystal form regulation to accelerating the setting process.

[0072] The material formulation, standardized preparation process, and rigorous performance evaluation system provided in this embodiment can be used to systematically and scientifically verify the effectiveness of gypsum crystal form regulation pretreatment in improving the overall performance of solid waste-based cementitious materials.

[0073] Experimental Example 1: Screening of the ratio of raw flue gas desulfurization gypsum to crystal-modified gypsum This Experiment Example 1 aims to systematically compare the effects of different crystal forms on the mechanical properties and setting behavior of cementitious materials, verify its optimization effect on early strength and setting time, and determine the optimal synergistic ratio.

[0074] Raw material parameters: Slag: S95 grade, specific surface area 486 m² / kg; Steel slag: Hot-quenched converter steel slag, specific surface area 512 m² / kg; Original flue gas desulfurization gypsum (A): CaSO4 2H2O content ≥95%, specific surface area 441 m² / kg; crystal form controlled gypsum (B).

[0075] The proportions (by weight) are shown in Table 2: Table 2

[0076] Preparation process: Mix the dry materials for 2 minutes, add water and stir for 4 minutes; pour into a 30mm×30mm×50mm triple steel mold and vibrate to form; cure at 20±1℃ and 95% relative humidity for 24 hours, then demold and continue curing until the specified age; Performance test results: The compressive strength test results are shown in Table 3 (MPa): Table 3

[0077] The results of the setting time measurement are shown in Table 4 (min): Table 4

[0078] As can be seen from the comparison, the present invention, through the regulation of gypsum crystal form, increases the strength of cementitious materials by 60.64% at 3 days, 26.01% at 7 days, and 40.26% at 28 days, while shortening the initial setting time by 87% and the final setting time by 84%, demonstrating a significant performance improvement effect.

[0079] The significance of crystal form regulation lies in altering the gypsum dissolution pathway. The conversion of dihydrate gypsum cells to hemihydrate gypsum cells is a process of volume reduction. Defects exist between grains, resulting in a loose internal structure and relatively weak binding forces, leading to decomposition into independent hemihydrate gypsum grains. The presence of more small gypsum particles means the reaction has more active sites, and smaller particles have a faster reaction rate. Gypsum serves as the active center for hydration reactions. AlO4 dissolved from slag under alkaline conditions... 5– Able to react with SO4 in gypsum 2– and Ca 2+ In the formation of ettringite, gypsum is key. Flue gas desulfurization gypsum with regulated crystal structure breaks down into more small gypsum particles, which then freely diffuse and distribute in the solution. This accelerates the hydration process, shortens setting time, and improves early strength.

[0080] Comparative Example 1: Comparison of physical grinding and refining technologies for raw materials This comparative example aims to systematically compare the core process of this invention (gypsum crystal form regulation) with the commonly used physical grinding and refining methods for raw materials in the prior art, highlighting the technical advantages of the present invention in terms of both performance improvement and energy consumption cost.

[0081] 1. Comparative Experiment Design Raw material processing: No gypsum crystal form control is performed. Steel slag and mineral slag are further ground and refined using a ball mill (model: SM500*500), reducing the specific surface area of ​​the steel slag from 512 m² to... 2 / kg increased to 650 m 2 / kg, slag surface area increased from 486 m² 2 / kg increased to 650 m 2 / kg. The gypsum is used directly as raw flue gas desulfurization gypsum (CaSO4·2H2O) without any pretreatment.

[0082] Material proportions: 70% slag, 20% steel slag, 10% raw gypsum, with a water-cement ratio fixed at 0.32. This proportion is consistent with the basic proportion (Group A) in the embodiments of this invention to ensure comparability.

[0083] Preparation and curing process: The stirring, molding and curing conditions of the sample are exactly the same as those of the above-mentioned process of the present invention, so as to eliminate the interference of process differences on performance.

[0084] 2. Energy consumption calculation: The energy consumption of physical grinding and refining processes is significant, as calculated below: Energy consumption for steel slag grinding: specific surface area from 512 m² 2 / kg increased to 600 m 2 / kg, the increase in unit energy consumption is 55kW·h / t.

[0085] Energy consumption for slag grinding: specific surface area from 486 m² 2 / kg increased to 550 m 2 / kg, the increase in unit energy consumption is 52kW·h / t.

[0086] Total additional grinding energy consumption: Based on the weighted calculation of the proportion, the total energy consumption = 55 kW·h / t × 20% + 52 kW·h / t × 70% = 52.4 kW·h / t.

[0087] 3. Performance Test Results: Tested according to a unified standard, the performance of the cementitious material obtained in this comparative example is as follows: Compressive strength: 18.7 MPa at 3 days; 35.2 MPa at 7 days; 46.5 MPa at 28 days.

[0088] Initial setting time: 220 min; final setting time: 390 min.

[0089] 4. Comparative Analysis: The additional energy consumption for raw material grinding in Comparative Example 1 was 52.4 kW. h / t, while the energy consumption of the core gypsum pretreatment process of this invention in Experimental Example 1 is calculated to be approximately 18kW. h / t. The energy consumption of physical secretion is 2.9 times that of this invention.

[0090] Early strength: The 3-day compressive strength of the comparative example was 18.7 MPa, which was 23.6% lower than that of Group B of the present invention (24.45 MPa).

[0091] Setting time: The initial setting time of the comparative example was 220 minutes, which was 528% longer than that of Group B of the present invention (35 minutes).

[0092] While physical grinding of raw materials can improve performance to some extent, it suffers from significant drawbacks of "high energy consumption and low efficiency." Its energy consumption is far higher than that of this invention, and its effectiveness in improving early strength and shortening setting time—two key indicators—is far inferior to the gypsum crystal form control technology of this invention. This further demonstrates that a crystal form control strategy that addresses the inherent reactivity of raw materials is significantly superior in terms of techno-economic efficiency compared to grinding that relies solely on external mechanical energy input.

[0093] Comparative Example 2: Comparison of technologies for adding chemical admixtures This comparative example aims to systematically compare the core process of this invention (gypsum crystal form regulation) with existing methods that improve performance by adding chemical additives, and further verify the technical and economic advantages of this invention from the two dimensions of performance improvement effect and material cost.

[0094] 1. Comparative Experiment Design To simulate and compare technical approaches that rely on chemical admixtures, the following experimental scheme was adopted in this comparative example: Raw material processing: No gypsum crystal form control is performed. The gypsum used is raw flue gas desulfurization gypsum (CaSO4·2H2O) without any pretreatment.

[0095] Material ratio: 70% slag, 20% steel slag, 10% raw gypsum, with a fixed water-cement ratio of 0.32. An additional 2% (based on the total mass of cementitious materials) of polycarboxylate superplasticizer (model: PCA-Ⅰ, solid content 40%) is added.

[0096] Preparation and curing process: The stirring, molding and curing conditions of the sample are exactly the same as the standard process of the present invention, so as to eliminate the interference of process differences on performance.

[0097] 2. Cost Calculation Adding chemical admixtures will directly lead to a significant increase in material costs. Taking the preparation of one cubic meter of concrete (density calculated at 2400 kg / m³) as an example, the specific cost calculation is as follows: Water-reducing agent unit price: calculated based on the market price of 4,000 yuan / ton (i.e., 4 yuan / kg).

[0098] Cost of water-reducing agent per cubic meter of concrete: The dosage of water-reducing agent is 2% of the total mass of cementitious materials. Cementitious materials usually account for a high proportion in concrete. Here, we use a simplified model to calculate its maximum impact based on the total weight of concrete. Cost = 2% × 2400 kg / m³ × 4 yuan / kg = 192 yuan / m³.

[0099] This section represents the additional pure material cost relative to the basic formulation of this invention (without any additives).

[0100] 3. Performance Test Results According to a unified standard test, the properties of the cementitious material obtained in this comparative example are as follows: Compressive strength: 3 days: 19.5 MPa; 7 days: 36.8 MPa; 28 days: 48.2 MPa.

[0101] Setting time: Initial setting time: 130 minutes; Final setting time: 350 minutes.

[0102] 4. Comparative Analysis and Conclusions The performance data of Comparative Example 2 (with added water-reducing agent) were compared with the data of Group B (the optimal group) in Example 4 of this invention, which used only crystal-modified gypsum: (1) Performance comparison: Early strength: The 3-day compressive strength of the comparative example was 19.5 MPa, which was 20.2% lower than that of Group B of the present invention (24.45 MPa).

[0103] Setting time: The initial setting time of the comparative example was 130 minutes, which was 271% longer than that of Group B of the present invention (35 minutes).

[0104] Although the performance of the added water-reducing agent is improved compared to the untreated original gypsum system (such as Group A in Comparative Example 1 or Example 4), it is still far from reaching the level achieved by the present invention through raw material modification in terms of the two key indicators of early strength and setting speed.

[0105] (2) Cost comparison: In order to achieve the above limited performance improvement, this scheme introduces high admixture costs, with an additional cost of about 192 yuan per cubic meter of concrete.

[0106] In summary, the technical approach relying on chemical admixtures such as polycarboxylate superplasticizers suffers from drawbacks of "high cost and limited efficiency improvement." Not only can it not match the crystal form control technology of this invention in terms of early performance, but it also significantly increases material costs and may pose compatibility and long-term durability risks in solid waste-based high-alkali systems. In contrast, this invention achieves "high-performance improvement without admixture cost investment" by controlling the gypsum crystal form at the source, demonstrating significant technical and economic advantages.

[0107] It is understood that those skilled in the art can make equivalent substitutions or modifications to the technical solutions and concepts of this invention, and all such substitutions or modifications should fall within the protection scope of the appended claims.

Claims

1. A method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment, characterized in that, Includes the following steps: S1: Pre-treatment of gypsum raw materials to regulate crystal form, transforming them from dihydrate calcium sulfate to hexagonal columnar hemihydrate calcium sulfate; S2: Mix the pretreated gypsum obtained in S1 with slag and steel slag in a certain proportion, and add an appropriate amount of water to prepare a cementitious slurry; S3: Solid waste-based cementitious material is obtained after molding and curing.

2. The method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment according to claim 1, characterized in that, The crystal form regulation pretreatment in step S1 includes the following steps: S11: Raw material selection: Select flue gas desulfurization gypsum with calcium sulfate dihydrate content ≥95%, attached water ≤10%, and d90 ≤200μm; S12: High-pressure hydrothermal synthesis: constant temperature reaction at 120 ~ 160℃ and 0.1 ~ 0.2 MPa for 60 ~ 150 minutes; S13: Drying treatment: Dry at 90~100℃ until the moisture content is ≤0.5%; S14: Quality Inspection: Ensure that the product is mainly hemihydrate gypsum, with residual dihydrate gypsum ≤5%.

3. The method for improving the performance of solid waste-based cementitious materials based on gypsum pretreatment according to claim 2, characterized in that, During the high-pressure hydrothermal synthesis process, the pH value of the reaction system is controlled to be 6-8.

4. The method according to any one of claims 1-3, characterized in that, The composition of the cementitious material by mass percentage is as follows: slag, 70%-80%; steel slag, 10%-20%; pretreated gypsum, 5%-15%; water-cement ratio of 0.30-0.

35.

5. The method according to claim 4, characterized in that, The slag is S95 grade granulated blast furnace slag powder with a specific surface area of ​​450–500 m². 2 / kg.

6. The method according to claim 4, characterized in that, The steel slag is hot-quenched converter steel slag with a specific surface area of ​​500–550 m². 2 / kg.

7. The method according to claim 4, characterized in that, The specific surface area of ​​the pretreated gypsum is 400–450 m². 2 / kg.

8. A solid waste-based cementitious material, characterized in that, Prepared by the method described in any one of claims 1-7.

9. A solid waste-based cementitious material according to claim 8, characterized in that, Its 3-day compressive strength is ≥20MPa, and its initial setting time is ≤150 minutes.

10. A pretreatment method for regulating gypsum crystal form, characterized in that, It includes the high-pressure hydrothermal synthesis step as described in claim 2, and the reaction temperature is 140-150℃, the reaction time is 120-140 minutes, and the pH value of the reaction system is 6-8.

Images