Double-solid-waste co-doped basic magnesium sulfate cement and preparation method thereof

Abstract

The invention discloses double solid waste co-doped basic magnesium sulfate cement and a preparation method thereof, and belongs to the technical field of building materials. The method comprises the following steps: firstly, uniformly dry-mixing two solid wastes of gasification slag and CFB ash with magnesium oxide according to different proportions according to 50-60% of the mass of the magnesium oxide to obtain composite powder, then uniformly mixing a magnesium sulfate solution with the composite powder, and curing in air for a fixed time to obtain the double-solid-waste co-doped basic magnesium sulfate cement. The 28-day compressive strength of the double-solid-waste co-doped basic magnesium sulfate cement prepared by the invention can reach 72.6 MPa, and the softening coefficient of the double-solid-waste co-doped basic magnesium sulfate cement after being soaked in water for 28 days reaches 0.83, which are respectively improved by 14.7% and 107.5% compared with that of soda ash type magnesium sulfate cement. According to the invention, the material cost can be reduced on the basis of improving the performance of the basic magnesium sulfate cement system, and the recycling and large-scale utilization of solid wastes can be promoted.

Description

Technical Field

[0001] The invention belongs to the technical field of building materials, and particularly relates to a double-solid-waste composite basic magnesium sulfate cement and a preparation method thereof. Background Art

[0002] Ordinary Portland cement is currently the most widely used building cementitious material, but its production process consumes vast quantities of raw materials and energy, emitting significant amounts of carbon dioxide, making it a significant contributor to global warming. Therefore, there is an urgent need for a low-carbon cement that can offset cement demand while reducing carbon emissions. In recent years, basic magnesium sulfate cement (BMSC), a new air-hardening cementitious material, has become a key alternative to Portland cement due to its rapid setting, early strength, and environmentally friendly properties. However, the interlaced growth of needle-shaped crystals of the 5·1·7 phase (5Mg(OH)2·MgSO4·7H2O, referred to as the 5·1·7 phase) creates numerous capillaries, allowing water to easily penetrate the cement and rehydrate unreacted magnesium oxide, resulting in reduced water resistance. Furthermore, the retarding effect of admixtures prolongs the setting time of basic magnesium sulfate cement, resulting in low construction efficiency. Furthermore, the high cost of the raw material magnesium oxide contributes to its high cost and poor market competitiveness.

[0003] Fly ash contains a large amount of silicon aluminum oxides and has a certain volcanic ash activity. It is often used as a solid waste admixture to improve the microstructure and water resistance of magnesium cement and reduce its cost. However, under the weak alkaline conditions of magnesium cement, the volcanic ash activity of single doped fly ash is difficult to be stimulated. It can only play the role of physical micro-aggregate and fill the gaps in cement. Although it improves water resistance, it is not good for mechanical properties. When Wu et al. doped fly ash into BMS cement prepared with industrial waste sulfuric acid, they found that fly ash did not participate in the hydration reaction and reduced SO4 2-The collision probability with the MgO hydration shell hinders the formation of the 5·1·7 phase, resulting in a decrease in the mechanical properties of BMS cement. Patent CN105800979A discloses a water-resistant magnesium phosphate cement and its use method. This invention incorporates fly ash (25-35%) and steel slag powder (15-20%) into magnesium phosphate cement, utilizing their combined gelling effect to improve the product's impermeability. The strength loss rate after 28 days of immersion in water is approximately 15%. Patent CN113880548A discloses a concrete mixture using magnesium oxychloride cement to solidify coal gasification slag and its preparation method. This invention performs particle grading of coal gasification slag (50-85 parts) and fly ash (1-30 parts) of different particle sizes, and combines the synergistic gelling effect of fly ash and magnesium oxychloride cement, which not only improves the density of cement, but also improves the structural stability of 518 phase and the strength of concrete (28d compressive strength is 45MPa, which is 136.8% higher than that of silicate cement system), and improves water resistance (softening coefficient is 0.87, which is 10.1% higher than that of silicate system).

[0004] Circulating fluidized bed fly ash (CFB ash) is a solid waste generated during the combustion process of circulating fluidized bed coal-fired boilers. The free calcium oxide (f-CaO) and calcium sulfate in CFB fly ash readily hydrate or react after cement hardens, forming Ca(OH)2 and ettringite (AFt), which causes expansion and cracking, making it unsuitable for use in ordinary Portland cement. However, the f-CaO in CFB ash reacts slowly in the weakly alkaline environment of basic magnesium sulfate cement, preventing significant volume deformation. To date, no research has been reported on the combined addition and modification of CFB ash and fumed slag in basic magnesium sulfate cement. This present invention combines these two solid wastes and utilizes their synergistic gelling effect with magnesium cement to develop a low-cost basic magnesium sulfate cement with high mechanical properties and excellent water resistance. Summary of the Invention

[0005] In light of this, the present invention aims to provide a dual-solid-waste composite-doped basic magnesium sulfate cement and its preparation method. This invention utilizes circulating fluidized bed fly ash (CFB ash) and coal gasification slag as mineral admixtures to prepare a dual-solid-waste composite-doped basic magnesium sulfate cement. Due to the synergistic gelling effect of coal gasification slag and CFB ash, the mechanical properties and water resistance of the magnesium cement can be significantly improved, while also reducing its cost.

[0006] To achieve the above object, the technical solution of the present invention is as follows: A method for preparing double-solid waste composite-doped basic magnesium sulfate cement comprises the following steps: Step 1: Mix CFB ash, gasified slag and magnesium oxide to obtain composite powder; Step 2: dissolving magnesium sulfate heptahydrate and an admixture in water to obtain a magnesium sulfate solution containing the admixture; Step 3: adding the composite powder obtained in step 1 to the magnesium sulfate solution containing the admixture obtained in step 2, and stirring evenly to obtain a fumed slag-CFB ash composite mixed with basic magnesium sulfate cement slurry; Step 4: pouring the cement slurry obtained in step 3 into a mold, demoulding after curing, and obtaining double-solid waste composite basic magnesium sulfate cement.

[0007] Furthermore, the particle size of the gasified slag in step 1 is D50=11.29~13.42 μm, and the specific surface area is 509.3~578.5 m 2 / kg; the particle size of CFB ash is D50 = 21.21~21.74 μm, and the specific surface area is 265.8~384.5 m 2 / kg.

[0008] Furthermore, the magnesium oxide in step 1 is light-burned magnesium oxide, wherein the content of active magnesium oxide is 60-63 wt%.

[0009] Furthermore, in step 1, the total mass of the CFB ash and the fumed slag is 50-60% of the mass of the magnesium oxide, wherein the mixing ratio of the fumed slag to the CFB ash is 0.25-4.0.

[0010] Furthermore, in step 2, the molar ratio of magnesium sulfate heptahydrate to activated magnesium oxide is 1:5-5.5.

[0011] Furthermore, the admixture in step 2 is citric acid, and its mass is 0.7-1.2% of the active magnesium oxide.

[0012] Furthermore, the water-cement ratio of the cement slurry prepared in step 3 is 0.40-0.75.

[0013] Furthermore, the stirring speed in step 3 is 750-850 r / min.

[0014] Furthermore, the curing temperature in step 4 is 15-30° C., and the curing time is 24 h.

[0015] A double-solid waste composite basic magnesium sulfate cement prepared by the preparation method as described above.

[0016] Compared with the prior art, the present invention has the following beneficial effects: 1. In the dual-solid waste blended basic magnesium sulfate cement prepared by this invention, fumed slag is rich in active SiO2 and Al2O3. Its incorporation into basic magnesium sulfate cement facilitates hydration reactions and simultaneously stimulates the pozzolanic activity of CFB ash. The two interact synergistically, generating cementitious phases such as CMSH and MASH, which tightly fill the spaces between the needle-shaped crystals of the 5·1·7 phase of magnesium cement. This not only improves the material's density and mechanical properties, but also prevents water molecules from penetrating the material, reduces secondary hydration of inactive MgO, and enhances water resistance. Furthermore, although the particle sizes of fumed slag and CFB ash are similar (D50: 11.29-13.42 μm and 21.21-21.74 μm, respectively), both falling within the ultrafine powder category, fumed slag can still partially fill the micropores of CFB ash. During the blending and doping process, a particle size distribution is formed, further improving the material's pore structure and density, thereby enhancing its compressive strength.

[0017] 2. The dual-solid waste composite basic magnesium sulfate cement prepared by the present invention has excellent mechanical properties and significantly improved water resistance. When the total content of gasified slag and CFB ash is 50-60% of the mass of magnesium oxide, and the mass ratio of the two is m(GS):m(FA)=3:2, the 28-day compressive strength can reach 72.6 MPa, and the softening coefficient after 28 days of soaking in water reaches 0.83, which are improvements of 14.7% and 107.5%, respectively, compared with pure basic magnesium sulfate cement. The present invention can not only reduce material costs while improving the performance of the basic magnesium sulfate cement system, but also promote the resource utilization and large-scale utilization of solid waste. BRIEF DESCRIPTION OF THE DRAWINGS

[0018] Figure 1 The XRD patterns of the cement test blocks of Comparative Examples 1-3 and Examples 1-4 after hydration for 28 days are shown.

[0019] Figure 2 This is the SEM image of the cement specimen of Comparative Example 1 after hydration for 28 days.

[0020] Figure 3 This is the SEM image of the cement specimen of Comparative Example 2 after hydration for 28 days.

[0021] Figure 4 This is the SEM image of the cement specimen of Comparative Example 3 after hydration for 28 days.

[0022] Figure 5 This is the SEM image of the cement specimen of Example 1 after hydration for 28 days.

[0023] Figure 6 This is the SEM image of the cement specimen of Example 2 after hydration for 28 days.

[0024] Figure 7 This is the SEM image of the cement specimen of Example 3 after hydration for 28 days.

[0025] Figure 8 This is the SEM image of the cement specimen of Example 4 after hydration for 28 days.

[0026] Figure 9 The compressive strength and 28-day softening coefficient results of the cements of Comparative Examples 1-3 and Examples 1-4 at different curing ages are shown. DETAILED DESCRIPTION

[0027] The present invention is further described below. This embodiment is implemented based on the technical solution of the present invention, and provides a detailed implementation method and specific operation process, but the protection scope of the present invention is not limited to the following embodiment.

[0028] The particle size of the gasified slag involved in the present invention is D50 = 11.29 ~ 13.42 μm, and the specific surface area is 509.3 ~ 578.5 m 2 / kg; the particle size of CFB ash is D50 = 21.21~21.74 μm, and the specific surface area is 265.8~384.5 m 2 / kg; magnesium oxide is light-burned magnesium oxide, wherein the content of active magnesium oxide is 60~63wt%.

[0029] Comparative Example 1: Preparation of Basic Magnesium Sulfate Cement (BMSC): (1) Weigh 667 g of magnesium sulfate heptahydrate and 325 g of water to prepare a magnesium sulfate solution, then add 6 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (2) Weigh 923 g of magnesium oxide and add it to the magnesium sulfate solution obtained in step (1), and stir at high speed (750-850 r / min) to obtain basic magnesium sulfate cement slurry (water-cement ratio of 0.75); (3) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The mold was cured in air at room temperature for 24 h and then demoulded. The mold was then cured until the test age to obtain basic magnesium sulfate cement.

[0030] The XRD characterization results of the basic magnesium sulfate cement prepared in this comparative example after curing for 28 days are shown in FIG. Figure 1 , SEM results are shown in Figure 2 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0031] Comparative Example 2: Preparation of Fumed Slag-Basic Magnesium Sulfate (50% GS-BMSC) Cement: (1) Weigh 859 g of magnesium oxide and 430 g of fumed slag and mix them for 5 minutes to obtain a composite powder; (2) Weigh 602 g of magnesium sulfate heptahydrate and 340 g of water to prepare a magnesium sulfate solution, then add 5.41 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (3) adding the composite powder obtained in step (1) to the magnesium sulfate solution obtained in step (2), and stirring at high speed (750-850 r / min) to obtain a gasified slag-basic magnesium sulfate cement slurry (water-cement ratio of 0.40); (4) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The slag was placed in a mold and cured in air at room temperature for 24 h before demoulding. The slag was then cured until the test age to obtain fumed slag-basic magnesium sulfate cement.

[0032] The XRD characterization results of the fumed slag-basic magnesium sulfate cement prepared in this comparative example after curing for 28 days are shown in Figure 1 , SEM results are shown in Figure 3 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0033] Comparative Example 3: Preparation of CFB ash-basic magnesium sulfate cement (50% CFBFA-BMSC): (1) Weigh 1000 g of magnesium oxide and 500 g of CFB ash and mix them for 5 min to obtain composite powder; (2) Weigh 700 g of magnesium sulfate heptahydrate and 562 g of water to prepare a magnesium sulfate solution, then add 6.3 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (3) adding the composite powder obtained in step (1) to the magnesium sulfate solution obtained in step (2), and stirring at high speed (750-850 r / min) to obtain CFB ash-basic magnesium sulfate cement slurry (water-cement ratio of 0.50); (4) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The mold was cured in air at room temperature for 24 h and then demoulded. The mold was then cured until the test age to obtain CFB ash-basic magnesium sulfate cement.

[0034] The XRD characterization results of the CFB ash-basic magnesium sulfate cement prepared in this comparative example after curing for 28 days are shown in FIG. Figure 1 , SEM results are shown in Figure 4 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0035] Example 1: Preparation of double solid waste mixed with basic magnesium sulfate cement (G1F4): (1) Preparation of dual solid waste mixed basic magnesium sulfate cement: 776 g of magnesium oxide, 310 g of CFB ash, and 77 g of fumed slag were weighed and mixed for 5 min to obtain composite powder (the mixing ratio of fumed slag to CFB ash was 0.25); (2) Weigh 543 g of magnesium sulfate heptahydrate and 338 g of water to prepare a magnesium sulfate solution, then add 4.88 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (3) adding the composite powder obtained in step (1) to the magnesium sulfate solution obtained in step (2), and stirring at high speed (750-850 r / min) to obtain a gasified slag-CFB ash composite-doped basic magnesium sulfate cement slurry (water-cement ratio of 0.43); (4) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The mold was cured in air at room temperature for 24 h before demoulding, and the cement was cured until the test age to obtain the double solid waste mixed basic magnesium sulfate cement.

[0036] The XRD characterization results of the double solid waste mixed basic magnesium sulfate cement prepared in this example after curing for 28 days are shown in Figure 2. Figure 1 , SEM results are shown in Figure 5 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0037] Example 2: Preparation of double solid waste mixed with basic magnesium sulfate cement (G2F3): (1) Preparation of dual solid waste mixed basic magnesium sulfate cement: 776 g of magnesium oxide, 232 g of CFB ash, and 155 g of fumed slag were weighed and mixed for 5 min to obtain composite powder (the mixing ratio of fumed slag to CFB ash was 0.67); (2) Weigh 543 g of magnesium sulfate heptahydrate and 338 g of water to prepare a magnesium sulfate solution, then add 4.88 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (3) adding the composite powder obtained in step (1) to the magnesium sulfate solution obtained in step (2), and stirring at high speed (750-850 r / min) to obtain a gasified slag-CFB ash composite-doped basic magnesium sulfate cement slurry (water-cement ratio of 0.43); (4) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The mold was cured in air at room temperature for 24 h before demoulding, and the cement was cured until the test age to obtain the double solid waste mixed basic magnesium sulfate cement.

[0038] The XRD characterization results of the double solid waste mixed basic magnesium sulfate cement prepared in this example after curing for 28 days are shown in Figure 2. Figure 1 , SEM results are shown in Figure 6 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0039] Example 3: Preparation of double solid waste mixed with basic magnesium sulfate cement (G3F2): (1) Preparation of dual solid waste mixed basic magnesium sulfate cement: 776 g of magnesium oxide, 155 g of CFB ash, and 232 g of fumed slag were weighed and mixed for 5 min to obtain composite powder (the mixing ratio of fumed slag to CFB ash was 1.5); (2) Weigh 543 g of magnesium sulfate heptahydrate and 338 g of water to prepare a magnesium sulfate solution, then add 4.88 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (3) adding the composite powder obtained in step (1) to the magnesium sulfate solution obtained in step (2), and stirring at high speed (750-850 r / min) to obtain a gasified slag-CFB ash composite-doped basic magnesium sulfate cement slurry (water-cement ratio of 0.43); (4) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The mold was cured in air at room temperature for 24 h before demoulding, and the cement was cured until the test age to obtain the double solid waste mixed basic magnesium sulfate cement.

[0040] The XRD characterization results of the double solid waste mixed basic magnesium sulfate cement prepared in this example after curing for 28 days are shown in Figure 2. Figure 1 , SEM results are shown in Figure 7 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0041] Example 4: Preparation of double solid waste mixed with basic magnesium sulfate cement (G4F1): (1) Preparation of dual solid waste mixed basic magnesium sulfate cement: 776 g of magnesium oxide, 77 g of CFB ash, and 310 g of fumed slag were weighed and mixed for 5 min to obtain composite powder (the mixing ratio of fumed slag to CFB ash was 4.0); (2) Weigh 543 g of magnesium sulfate heptahydrate and 338 g of water to prepare a magnesium sulfate solution, then add 4.88 g of citric acid (1% of the mass of the active magnesium oxide) and stir to dissolve to obtain a magnesium sulfate solution containing citric acid; (3) adding the composite powder obtained in step (1) to the magnesium sulfate solution obtained in step (2), and stirring at high speed (750-850 r / min) to obtain a gasified slag-CFB ash composite-doped basic magnesium sulfate cement slurry (water-cement ratio of 0.43); (4) Pour the cement paste obtained in step (3) into a 40×40×40 mm 3 The mold was cured in air at room temperature for 24 h before demoulding, and the cement was cured until the test age to obtain the double solid waste mixed basic magnesium sulfate cement.

[0042] The XRD characterization results of the double solid waste mixed basic magnesium sulfate cement prepared in this example after curing for 28 days are shown in Figure 2. Figure 1 , SEM results are shown in Figure 8 The compressive strength and 28-day softening coefficient of different curing ages are shown in Figure 9 .

[0043] Figure 1 The following are XRD patterns of cement blocks prepared in Comparative Examples 1-3 and Examples 1-4 after 28 days of hydration. Analysis of the figure reveals that the sample incorporating 50% CFB ash and fumed slag still exhibits a distinct characteristic peak of the 5·1·7 phase, but this peak decreases as the magnesium oxide ratio decreases. As the fumed slag ratio increases, the content of active silicon and aluminum components increases, the system's reactivity increases, and it reacts with active Mg(OH)2 to form a hydrated calcium magnesium silicate cementitious phase. The peak of the Mg(OH)2 phase decreases after both single and combined additions of the two solid wastes. This is because the CFB ash and fumed slag particles encapsulate unreacted MgO, hindering its reaction with water to form Mg(OH)2, resulting in a weakening of its diffraction peak. Furthermore, as the proportion of CFB ash in the combined system increases, the resulting high alkaline environment is not conducive to the growth of the 517 phase. Therefore, the peak of the 517 phase decreases as its ratio increases.

[0044] Figures 2 to 8 The SEM images of the cement blocks prepared in Comparative Examples 1-3 and Examples 1-4 after hydration for 28 days are shown in the figure. As can be seen from the comparison of the two solid wastes, the generated cementitious material encapsulates the 5·1·7 phase, and the development of the 5·1·7 phase whiskers is better than that of pure basic magnesium sulfate cement and basic magnesium sulfate cement with solid waste alone, which is consistent with the Figure 9 The compressive strength and softening coefficient of cement blocks prepared in Comparative Examples 1-3 and Examples 1-4 after 28 days of hydration are consistent. This indicates that the combined addition of magnesium and magnesium cement creates a synergistic gelling effect. The resulting gelling material is interspersed within the 5·1·7 phase structure, improving the material's compressive strength and water resistance.

[0045] The above description is only for better explanation of the embodiments of the present invention and is not intended to limit the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention are intended to be within the scope of the present invention.

Claims

1. A method for preparing double solid waste mixed with basic magnesium sulfate cement, characterized in that: The following steps are involved: Step 1: Mix CFB ash, gasified slag and magnesium oxide to obtain composite powder; Step 2: dissolving magnesium sulfate heptahydrate and an admixture in water to obtain a magnesium sulfate solution containing the admixture; Step 3: adding the composite powder obtained in step 1 to the magnesium sulfate solution containing the admixture obtained in step 2, and stirring evenly to obtain a fumed slag-CFB ash composite mixed with basic magnesium sulfate cement slurry; Step 4: pouring the cement slurry obtained in step 3 into a mold, demoulding after curing, and obtaining double-solid waste composite basic magnesium sulfate cement.

2. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 1, characterized in that: The particle size of the gasified slag in step 1 is D50 = 11.29 ~ 13.42 μm, and the specific surface area is 509.3 ~ 578.5 m 2 / kg; the particle size of CFB ash is D50 = 21.21~21.74 μm, and the specific surface area is 265.8~384.5 m 2 / kg.

3. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 1, characterized in that: The magnesium oxide in step 1 is light-burned magnesium oxide, wherein the content of active magnesium oxide is 60-63 wt %.

4. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 3, characterized in that: In step 1, the total mass of CFB ash and fumed slag is 50-60% of the mass of magnesium oxide, wherein the mixing ratio of fumed slag to CFB ash is 0.25-4.

0.

5. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 3, characterized in that: In step 2, the molar ratio of magnesium sulfate heptahydrate to activated magnesium oxide is 1:5-5.

5.

6. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 1, characterized in that: The admixture in step 2 is citric acid, and its mass is 0.7-1.2% of the active magnesium oxide.

7. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 1, characterized in that: The water-cement ratio of the cement slurry prepared in step 3 is 0.40-0.

75.

8. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 1, characterized in that: The stirring speed in step 3 is 750-850 r / min.

9. The method for preparing double solid waste mixed basic magnesium sulfate cement according to claim 1, characterized in that: The curing temperature in step 4 is 15-30° C., and the curing time is 24 h.

10. A double solid waste mixed basic magnesium sulfate cement prepared by the preparation method according to claims 1 to 9.

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