Autoclaved aerated concrete block prepared from phosphogypsum and method

Abstract

The application provides a steam pressure aerated concrete block prepared from phosphogypsum and a method, and belongs to the technical field of resource utilization of bulk solid waste. The method directly uses dry desulfurization phosphogypsum generated in flue gas purification of smelting silicon industry to replace natural gypsum in full amount, and prepares high-performance steam pressure aerated concrete blocks, and provides an industrial production method. The method realizes the consumption of industrial solid waste in bulk, guarantees the mechanical properties, size stability and long-term durability of the final product, thereby promoting the steam pressure aerated concrete industry to complete the resource utilization task of solid waste in high quality, and creating significant environmental, economic and social benefits.

Description

Technical Field

[0001] This invention belongs to the field of bulk solid waste resource utilization technology, specifically relating to an autoclaved aerated concrete block and method for preparing it using phosphogypsum. Background Technology

[0002] Dry desulfurization phosphogypsum is a large-scale industrial by-product gypsum generated during flue gas desulfurization processes in metallurgy, chemical industry, and other sectors. Unlike natural gypsum, its composition is complex and variable, often containing unreacted calcium-based desulfurizing agents (such as Ca(OH)2, CaCO3) and small amounts of impurities, resulting in unstable physical properties. For a long time, large stockpiles of phosphogypsum have not only encroached on land, but the leaching of its dust and soluble salts has also posed a pollution threat to surrounding soil and water bodies. Although its use in building material production is an ideal disposal method, its unique physicochemical properties (such as containing soluble phosphorus and fluorine, or exhibiting significant differences in reactivity as type II anhydrite) present technical bottlenecks when applied to precision building material systems such as autoclaved aerated concrete. These bottlenecks include inhibiting lime digestion, affecting slurry stability, and causing fluctuations in product performance, thus restricting its large-scale and safe application. Summary of the Invention

[0003] This invention provides a method for preparing autoclaved aerated concrete (AAC) blocks using phosphogypsum. It directly utilizes dry-process desulfurized phosphogypsum produced from flue gas purification in the silicon smelting industry, completely replacing natural gypsum to prepare high-performance AAC blocks, and provides an industrial-scale production method. This method offers stable processing and produces products with excellent dimensional stability.

[0004] To achieve the above objectives, the present invention adopts the following technical solution: an autoclaved aerated concrete block prepared using phosphogypsum, wherein the block is prepared from dry materials and auxiliary materials, the dry materials comprising the following components by weight percentage: 10%~20% ordinary cement; 5%~15% quicklime; 1%~10% phosphogypsum; 55%~70% quartz sand, quartz tailings sand, or other sand with SiO2 content greater than 70%; the remainder being waste generated during the autoclaved aerated concrete block production process; the auxiliary material is aluminum powder or aluminum powder paste; wherein the phosphogypsum is dry desulfurized phosphogypsum powder with a Ca(OH)2 content greater than 30%, a CaSO4 content greater than 40%, and a moisture content less than 1.5%.

[0005] Preferably, the ordinary cement is silicate cement with a C3A content greater than 5% and a CaO content greater than 50%.

[0006] Preferably, the quicklime has a slaking time of 5-10 minutes, a slaking temperature of 85-100℃, an effective CaO content of more than 70%, and a fineness that meets the requirement of less than 15% residue on a 150-mesh sieve.

[0007] This invention provides a method for preparing the above-mentioned autoclaved aerated concrete blocks, comprising the following steps: (1) Raw material pretreatment: The quartz tailings sand is ground to a sieve residue of less than 25% on a 0.08mm square hole sieve and prepared into a suspension slurry with a moisture content of less than 40%; (2) Premixing: Mix the raw slurry, cement, phosphogypsum and water obtained in step (1) at a water-to-material ratio of 0.54~0.64 and stir for 60~120 seconds to obtain premixed slurry; (3) Activation and gas generation: Add quicklime to the premixed slurry, continue stirring until the slurry diffusion reaches 20~26cm and the temperature is raised to 40~60℃, then add aluminum powder or aluminum powder paste and stir for 20~30 seconds, and then pour; (4) Curing: The cast green body is cured for 1.5 to 3 hours in an environment with a temperature of 20 to 60°C and a humidity of 50% to 60%; (5) Cutting and autoclaving: After the curing is completed, the blank is demolded and cut, and then sent into the autoclave for autoclaving. (6) Autoclaving process: First, evacuate for 30 to 60 minutes to achieve a vacuum of -0.04 MPa to -0.08 MPa in the autoclave; then, increase the pressure and temperature to 0.70 MPa to 1.25 MPa and 183°C to 210°C within 1.5 to 3 hours; maintain constant pressure and temperature for 5 to 10 hours; finally, degas and reduce the pressure and temperature within 1.5 to 2 hours. (7) Curing after removal from the autoclave: After removal from the autoclave, the product is inspected and then cured naturally for 5 days to obtain the finished product.

[0008] Preferably, in step (2), the molar ratio of sulfate to activated alumina in the slurry system is controlled between 2.6 and 3.2 by controlling the amount of phosphogypsum; the activated alumina is derived from tricalcium aluminate in cement.

[0009] Preferably, the molar ratio of sulfate to activated alumina is controlled between 2.8 and 3.0.

[0010] Preferably, the precise dosage of the phosphogypsum is determined by calculating using the following formula: The total molar amount of activated alumina, M_Al (mol), is approximately equal to the cement dosage, M_c (kg), and the mass percentage of C3A in the cement, W_C3A (%), × 0.037. The total required amount of sulfate moles M_s (mol) = M_Al × R, where R is the target sulfur-aluminum molar ratio. The dosage of phosphogypsum, m_s (kg), is calculated as follows: (M_s × 0.080) / (mass percentage of SO3 in phosphogypsum, W_SO3 / 100) × K; Wherein, K is the empirical correction coefficient, with a value ranging from 0.9 to 1.1.

[0011] Preferably, the premixing and stirring in step (2) causes the sulfate ions generated from the dissolution of phosphogypsum to preferentially react with tricalcium aluminate in the cement to generate ettringite crystal nuclei.

[0012] Compared with traditional technologies, the advantages of this invention lie in its use of industrial solid waste "dry-process desulfurized phosphogypsum" as a composite modifier, instead of natural gypsum, to produce autoclaved aerated concrete (AAC) blocks that meet national standards (B04, B05, B06, and B07). This invention enables large-scale, stable production of AAC blocks by fully or equally replacing natural gypsum with dry-process desulfurized phosphogypsum. While achieving large-scale disposal of industrial solid waste, it also ensures the mechanical properties, dimensional stability, and long-term durability of the final product, thereby promoting the high-quality completion of solid waste resource utilization in the AAC industry and creating significant environmental, economic, and social benefits. Detailed Implementation

[0013] To enable those skilled in the art to better understand and implement the present invention, the technical means and effects of achieving the intended purpose of the invention are described in detail below with reference to specific embodiments. These embodiments are for illustrative purposes only.

[0014] This invention discloses an autoclaved aerated concrete (AAC) block utilizing phosphogypsum. The AAC block comprises B04, B05, B06, and B07 grade AAC blocks, made from dry materials and auxiliary materials. The dry materials consist of the following components by weight percentage: 10%–20% ordinary cement, 5%–15% quicklime, 1%–10% phosphogypsum, 55%–70% quartz sand or quartz tailings sand or other sand with a SiO2 content greater than 70%, with the remainder being waste material, specifically bread bits generated during the AAC block production process. The auxiliary material is aluminum powder / paste.

[0015] Specifically, the cement mentioned includes any type of cement with a C3A content greater than 5% and a CaO content greater than 50%.

[0016] Specifically, the quicklime mentioned includes any brand of medium-speed quicklime with a slaking time of 5-10 min, a slaking temperature of 85-100℃, an effective CaO content greater than 70%, a fineness greater than 150 mesh, and a sieve residue of less than 15%.

[0017] Specifically, the phosphogypsum includes dry desulfurization phosphogypsum powder with a Ca(OH)2 content greater than 30%, a CaSO4 content greater than 40%, a CaCO3 content less than 15%, and a moisture content less than 1.5%.

[0018] Specifically, the quartz tailings sand includes solid waste tailings or natural sand from quartz mining with a SiO2 content greater than 70%, an organic matter content less than 2.5%, and a mud content less than 5%.

[0019] The method for preparing autoclaved aerated concrete blocks using the above-mentioned dry desulfurized phosphogypsum includes the following steps: (1) Grind the quartz tailings sand to a size of less than 25% on a 0.08 square mesh sieve to prepare a suspension slurry with a water content of less than 40%; (2) Mix the raw slurry, cement, dry desulfurized phosphogypsum, and water (water-to-material ratio 0.54~0.64) for 60s~120s; (3) Add quicklime and continue stirring (control the slurry diffusion to 20cm~26cm) and heat to the design temperature (40℃~60℃). Then add aluminum powder (paste) and continue stirring for 20s~30s before pouring. (4) After pouring, place it in a curing room with a temperature of 20℃~60℃ and a humidity of 50%~60% for 1.5~3 hours to allow it to thicken; (5) After static curing, demolding and cutting are carried out to obtain the blanks of the required specifications and dimensions. Then the blanks are transferred to the autoclave for steam curing. (6) Pre-evacuate the vessel for 30 to 60 minutes to maintain the vacuum level inside the vessel at -0.04 MPa to -0.08 MPa; (7) Within 1.5~3h, the intake pressure and temperature are increased to 0.70MPa~1.25MPa and 183℃~210℃. After maintaining constant pressure and temperature for 5h~10h, the exhaust temperature and pressure are reduced within 1.5h~2h. After exiting the reactor, the product is inspected, packaged and stored in the warehouse. It is then naturally cured for 5 days to obtain the finished product.

[0020] The reaction mechanism of sulfur-aluminum equilibrium is as follows: In autoclaved aerated concrete (AAC) systems employing dry-process desulfurized phosphogypsum, the reaction between sulfates and active aluminates is the core chemical process for achieving early structural stability and final performance optimization. Its mechanism can be divided into two distinct stages: Phase 1: Static Curing Period – Formation of the Etuff Structural Framework This stage occurs in the static environment (30~60℃) after slurry pouring. The sulfate ions (SO42-) provided by the dissolution of dry desulfurized phosphogypsum... 2- The precipitate reacts rapidly with tricalcium aluminate (3CaO·Al2O3, abbreviated as C3A) in silicate cement to form needle-like ettringite crystals. This reaction is the fundamental source of the green body's early cutting strength.

[0021] The main chemical reaction formulas are as follows: C3A+3CaSO4+32H2O→3CaO·Al2O3·3CaSO4·32H2O Key point: The Ca(OH)2 component in the dry desulfurized phosphogypsum increases the alkalinity of the slurry, further promoting the dissolution of C3A and the above-mentioned reactions, enabling the ettringite network to form more quickly and fully.

[0022] Phase Two: Autoclaving Period – Transformation of Eundum and Formation of Tobermorite In a high-temperature, high-pressure autoclave (183℃~210℃, 0.70MPa-1.25 MPa), ettringite is a thermodynamically unstable phase. It will decompose, and its decomposition products (such as Al) 3+ SO4 2- It reacts with a large amount of siliceous materials (SiO2) and calcareous materials (Ca(OH)2) in the system to generate the main crystalline phase that ultimately gives the product its strength—tobermorite.

[0023] The calculation method for the sulfur-aluminum balance is as follows: The core objective of sulfur-aluminum balance is to ensure the formation of an appropriate amount of ettringite during the static resting period to stabilize the green body by precisely controlling the initial ratio of sulfate to aluminum phase, while ensuring that it can be completely and smoothly transformed into stable tobermorite during the autoclaving period, thus avoiding the formation of harmful phases such as residual gypsum (CaSO4) or monosulfate calcium sulfoaluminate (AFm) due to excessive sulfate or aluminum phase.

[0024] To achieve optimized control of the above mechanism, this invention proposes the following quantitative calculation method, the core of which is to control the molar ratio (S / A ratio) of sulfate (calculated as SO3) to activated alumina (Al2O3).

[0025] Step 1: Determine the total molar amount of the active aluminum phase (based on Al2O3). The active aluminum phase mainly originates from C3A in cement, while the aluminum phase in other raw materials is considered inactive during the static resting period.

[0026] Let M be the amount of cement used. C (kg), the mass percentage of C3A in the cement used is W C3A(%)。 The formula for calculating the total molar amount of active Al2O3 is: M Al (mol)=M C ×(W C3A / 100)×P Where P is a conversion factor. Given that the mass percentage of Al₂O₃ (molecular weight approximately 10⁻²) in C₃A (molecular weight approximately 270) is approximately 0.377, therefore P≈1000 / 10²×0.377≈3.70. That is: MAl ≈Mc×W C3A ×0.037 (W in the formula) C3A Substitute the percentage value, for example, substitute 7% into 7). Step 2: Set the target sulfur-aluminum molar ratio (S / A) Based on extensive experimental verification, controlling the S / A molar ratio (n(SO3) / n(Al2O3)) between 2.6 and 3.2, preferably between 2.8 and 3.0, achieves the best balance between processability and final performance. This ratio is denoted as R.

[0027] Step 3: Calculate the total molar amount M of sulfate (as SO3) required. S M S (mol) = M Al ×R Step 4: Calculate the precise dosage m of dry desulfurization phosphogypsum. g Let the mass percentage of SO3 in the target dry desulfurization phosphogypsum be W. SO3 (%) (obtained from the test report). The molar mass of SO3 is approximately 80 g / mol.

[0028] The theoretically required mass of dry desulfurized phosphogypsum is: m g (kg) = (M S ×0.080) / (W SO3 / 100) Considering the reaction efficiency of raw materials and process fluctuations, an empirical correction coefficient K (0.9-1.1) is introduced, and the final calculation formula is: m g (kg) = (M S ×0.080) / (W SO3 / 100)×K The key processes for achieving sulfur-aluminum balance are as follows: To ensure that the above theoretical calculations can be accurately realized in industrial production, a matching "stepwise feeding and stirring" process must be adopted to control the reaction sequence.

[0029] (1) Premixing and preferential reaction stage: The raw slurry, cement, dry desulfurized phosphogypsum, and water (water-to-material ratio 0.54~0.64) are mixed and stirred for 60s~120s. The core purpose of this stage is to force SO4 to react in an environment free from quicklime interference. 2- It preferentially and fully reacts with C3A to generate ettringite crystal nuclei, pre-constructs an early strength framework, and "consumes" most of the sulfate ions.

[0030] (2) Lime addition and final activation stage: All quicklime is added to the premixed slurry and stirring continues. At this time, since a large amount of sulfate has been consumed by coordination, its inhibitory effect on quicklime is greatly weakened, and quicklime can be rapidly digested and violently exothermic, which efficiently drives the thickening of the slurry and the gas generation of aluminum powder.

[0031] (3) Process synergy: This step-by-step process is closely integrated with the sulfur-aluminum balance calculation model, realizing the unity of "precise stoichiometric design" and "precise control of the reaction process". It ensures that the calculated sulfate is used to the maximum extent to form a beneficial structural framework (ettringite), while freeing up the heat source function of quicklime, thereby simultaneously optimizing the performance of the green body and the final product.

[0032] The following is a preferred embodiment of the present invention: In this embodiment, A5.0B07 grade autoclaved aerated concrete blocks were designed and prepared according to the method described in this invention.

[0033] (1) Formula (based on the percentage of dry material weight in the design) Quartz tailings sand: 66.6%, P·O42.5 cement: 18.0%, quicklime: 6.0%, dry desulfurized phosphogypsum: 4.0%, recycled waste slurry (dry basis): 5.4%, aluminum powder paste: 0.032% (approximately 0.320 kg / m³ of finished product). During production, the water-to-material ratio is controlled at 0.56. Following the aforementioned preparation steps, the material is poured, cured, cut, and autoclaved. After 7 hours of constant temperature autoclaving at 187℃ and 0.80 MPa, it undergoes inspection upon exiting the autoclave, is packaged and stored, and then naturally cured for 5 days to obtain the finished product.

[0034] (2) Product performance test results According to GB / T11969-2008 "Test Methods for Performance of Autoclaved Aerated Concrete", the dry density of autoclaved aerated concrete blocks in Example 1 was 698 kg / m³. 3 The cubic compressive strength is 6.6 MPa, the drying shrinkage value is 0.29 mm / m, and the thermal conductivity is 0.11 W / (m·K). The key durability index, the drying shrinkage value of 0.29 mm / m, is significantly better than the requirements of the national standard GB / T 11968-2020 for superior grade products (0.50 mm / m); the thermal conductivity is 0.11 W / (m·K), which is significantly better than the requirements of the national standard GB / T11968-2020 for superior grade products (≤0.18 W / (m·K), indicating good thermal insulation performance.

[0035] The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent changes and alterations made to the above embodiments based on the technical essence of the present invention without departing from the scope of the present invention shall still fall within the scope of the present invention.

Claims

1. An autoclaved aerated concrete block prepared using phosphogypsum, characterized in that, The blocks are prepared from dry materials and auxiliary materials. The dry materials include the following components by weight percentage: 10%~20% ordinary cement; 5%~15% quicklime; 1%~10% phosphogypsum; 55%~70% quartz sand, quartz tailings sand, or other sand with a SiO2 content greater than 70%; the balance is waste generated during the production process of autoclaved aerated concrete blocks. The auxiliary materials are aluminum powder or aluminum powder paste. The phosphogypsum is dry desulfurized phosphogypsum powder with a Ca(OH)2 content greater than 30%, a CaSO4 content greater than 40%, and a moisture content less than 1.5%.

2. The autoclaved aerated concrete block according to claim 1, characterized in that, The ordinary cement is silicate cement with a C3A content greater than 5% and a CaO content greater than 50%.

3. The autoclaved aerated concrete block according to claim 1, characterized in that, The quicklime has a slaking time of 5-10 minutes, a slaking temperature of 85-100℃, an effective CaO content of more than 70%, and a fineness that meets the requirement of less than 15% residue on a 150-mesh sieve.

4. A method for preparing autoclaved aerated concrete blocks as described in any one of claims 1-3, characterized in that, Includes the following steps: (1) Raw material pretreatment: The quartz tailings sand is ground to a sieve residue of less than 25% on a 0.08mm square hole sieve and prepared into a suspension slurry with a moisture content of less than 40%; (2) Premixing: Mix the raw slurry, cement, phosphogypsum and water obtained in step (1) at a water-to-material ratio of 0.54~0.64 and stir for 60~120 seconds to obtain premixed slurry; (3) Activation and gas generation: Add quicklime to the premixed slurry, continue stirring until the slurry diffusion reaches 20~26cm and the temperature is raised to 40~60℃, then add aluminum powder or aluminum powder paste and stir for 20~30 seconds, and then pour; (4) Curing: The cast green body is cured for 1.5 to 3 hours in an environment with a temperature of 20 to 60°C and a humidity of 50% to 60%; (5) Cutting and autoclaving: After the curing is completed, the blank is demolded and cut, and then sent into the autoclave for autoclaving. (6) Autoclaving process: First, evacuate for 30 to 60 minutes to achieve a vacuum of -0.04 MPa to -0.08 MPa in the autoclave; then, increase the pressure and temperature to 0.70 MPa to 1.25 MPa and 183°C to 210°C within 1.5 to 3 hours; maintain constant pressure and temperature for 5 to 10 hours; finally, degas and reduce the pressure and temperature within 1.5 to 2 hours. (7) Curing after removal from the autoclave: After removal from the autoclave, the product is inspected and then cured naturally for 5 days to obtain the finished product.

5. The method according to claim 4, characterized in that, In step (2), the molar ratio of sulfate to activated alumina in the slurry system is controlled between 2.6 and 3.2 by controlling the amount of phosphogypsum; the activated alumina is derived from tricalcium aluminate in cement.

6. The method according to claim 5, characterized in that, The molar ratio of sulfate to activated alumina is controlled between 2.8 and 3.

0.

7. The method according to claim 5, characterized in that, The precise dosage of the phosphogypsum is determined by the following formula: The total molar amount of activated alumina, M_Al (mol), is approximately equal to the cement dosage, M_c (kg), and the mass percentage of C3A in the cement, W_C3A (%), × 0.

037. The total required amount of sulfate moles M_s (mol) = M_Al × R, where R is the target sulfur-aluminum molar ratio. The dosage of phosphogypsum, m_s (kg), is calculated as follows: (M_s × 0.080) / (mass percentage of SO3 in phosphogypsum, W_SO3 / 100) × K; Wherein, K is the empirical correction coefficient, with a value ranging from 0.9 to 1.

1.

8. The method according to claim 4, characterized in that, The premixing and stirring described in step (2) allows the sulfate ions generated from the dissolution of phosphogypsum to preferentially react with tricalcium aluminate in the cement to form ettringite crystal nuclei.