Phosphogypsum-based hydraulic material based on microwave chemical compounding and preparation and application thereof

Abstract

The application provides a preparation method of a phosphogypsum-based hydraulic material based on microwave chemical compounding, comprising the following steps: S1, removing impurities and drying phosphogypsum; S2, adding coke powder, sodium sulfate and calcined alunite, uniformly mixing, and microwave irradiating the mixture; S4, adding silicate cement clinker, steel slag powder, mineral powder and fly ash, uniformly mixing, and then crushing to 380-480 meshes; S5, adding calcium aluminate, quicklime and quick-dissolving solid water glass, mixing and grinding, and the phosphogypsum-based hydraulic material is obtained. The phosphogypsum-based hydraulic material has high strength.

Description

Technical Field

[0001] This invention relates to a phosphogypsum-based hydraulic material based on microwave chemical composites, its preparation and application, belonging to the field of building materials technology. Background Technology

[0002] Phosphogypsum is a waste product generated during the production of ammonium phosphate. It contains residual organic and inorganic phosphorus, making it acidic. Under current production processes, approximately four tons of phosphogypsum are generated for every ton of ammonium phosphate produced. Currently, the utilization rate of large quantities of industrial solid waste such as phosphogypsum and phosphate slag is low, leading to their large-scale accumulation and storage, increasingly occupying land resources. This not only threatens environmental safety but also easily triggers landslides, mudslides, and other safety accidents, causing serious casualties and property damage. If not properly managed, it can also seep out of the stockpiles with rainwater, causing pollution to surrounding soil, groundwater, and rivers. If phosphogypsum can be improved and transformed into a building material for roadbed and base course mixtures, it would not only solve the environmental problems associated with phosphogypsum stockpiling but also significantly reduce the demand for cement and aggregates in engineering projects, possessing significant environmental and socio-economic benefits.

[0003] Phosphogypsum contains over 95% CaSO4·2H2O. Theoretically, phosphogypsum can be used as a raw material for the production of cement, sulfuric acid, and building gypsum. To produce cement and sulfuric acid from phosphogypsum, it is generally necessary to first dehydrate it into anhydrous gypsum, then add additives such as clay, sandstone, iron oxide, and alumina to bring the chemical composition of the cement clinker to the range required for cement production. Then, it is reduced and decomposed using coke, reacting with the additives to form cement clinker. The silica, alumina, and iron oxide in the additives can lower the decomposition temperature of calcium sulfate and accelerate its decomposition rate, while simultaneously undergoing mineralization reactions to produce sulfuric acid and cement. This method has practical significance for the utilization of sulfur resources. Currently, researchers have conducted extensive research on this topic, but many problems remain. The main issue is that the produced cement has insufficient strength, and it has not yet been approved by the state for use in the building materials industry. Furthermore, the high cost and poor economic benefits of phosphogypsum are significant challenges.

[0004] Phosphogypsum can be used as a retarder in cement production, replacing natural gypsum. However, impurities such as soluble phosphorus and organic matter in phosphogypsum can negatively impact cement strength and it is also corrosive. Therefore, modification treatments such as water washing, lime neutralization, and heat treatment are necessary. However, when used as a cement retarder, the dosage is low, resulting in minimal consumption.

[0005] Phosphogypsum can also be used to prepare cementitious materials. Phosphogypsum can be calcined at high temperature to remove all the water of crystallization to obtain anhydrous gypsum. By adding various sulfates as coagulants, high-strength anhydrous gypsum cementitious active materials can be prepared. These materials have a certain degree of water stability, but they cannot meet the actual needs of engineering projects.

[0006] After processing, phosphogypsum can be combined with cement, lime, mineral powder, and other materials to prepare high-strength cementitious materials. These materials have low energy consumption and can utilize large amounts of industrial waste. However, gypsum-based cementitious materials are air-hardening cementitious materials. Although they have characteristics such as rapid strength development and light weight, compared with silicate cement, gypsum cementitious materials have lower strength and poorer water stability, which limits their widespread application in building materials.

[0007] Typically, phosphogypsum contains 20%–30% free water and about 20% water of crystallization. It has high viscosity, making it difficult to transport. Drying and calcination can remove the moisture. Calcination of phosphogypsum at around 800℃ converts eutectic phosphorus into inert, stable, insoluble pyrophosphate, while organic matter evaporates and is removed. Type II anhydrous gypsum prepared by neutralization with lime and calcination at 800℃ has properties comparable to anhydrous gypsum made from natural gypsum of the same grade. Since conventional pretreatment processes are insufficient to remove eutectic phosphorus, this process is the only effective way to eliminate its influence. However, the calcination process has relatively high production costs and energy consumption, resulting in poor environmental and economic benefits. Summary of the Invention

[0008] This invention provides a phosphogypsum-based hydraulic material based on microwave chemical composites, its preparation and application, which can effectively solve the above-mentioned problems.

[0009] This invention is implemented as follows:

[0010] A method for preparing a phosphogypsum-based hydraulic material based on microwave chemical composites includes the following steps:

[0011] S1, remove impurities and dry the phosphogypsum;

[0012] S2, add coke powder, sodium sulfate and calcined alum stone, mix evenly, and then microwave irradiate the mixture;

[0013] S4, add silicate cement clinker, steel slag powder, mineral powder and fly ash, mix evenly, and then crush to 380 mesh to 480 mesh;

[0014] S5, add calcium aluminate, quicklime and fast-dissolving solid water glass, mix and grind to obtain the phosphogypsum-based hydraulic material.

[0015] In some embodiments, the preparation method of the microwave chemical composite phosphogypsum-based hydraulic material comprises the following raw materials in terms of mass fraction: phosphogypsum 400-800 parts, coke powder 1-5 parts, sodium sulfate 5-10 parts, calcined alunite 2-8 parts, silicate cement clinker 50-100 parts, steel slag powder 100-300 parts, mineral powder 100-300 parts, fly ash 80-160 parts, calcium aluminate 10-20 parts, quicklime 5-10 parts, and fast-dissolving solid water glass 1-5 parts.

[0016] In some embodiments, the preparation method of the microwave chemical composite phosphogypsum-based hydraulic material comprises the following raw materials in terms of mass fraction: 500-700 parts phosphogypsum, 2-4 parts coke powder, 6-8 parts sodium sulfate, 3-7 parts calcined alunite, 60-90 parts silicate cement clinker, 150-250 parts steel slag powder, 150-250 parts mineral powder, 100-150 parts fly ash, 12-15 parts calcium aluminate, 6-8 parts quicklime, and 2-4 parts fast-dissolving solid water glass.

[0017] In some embodiments, step S1, the impurity removal step, is as follows: phosphogypsum and water are fed into a flotation device in a suitable ratio, then stirred, allowed to stand, and suspended matter on the liquid surface is removed, followed by solid-liquid separation using a separation device.

[0018] In some embodiments, the microwave irradiation has a frequency of 300MHz to 945GHz, a temperature of 140 to 150°C, and a duration of 20 to 40 minutes.

[0019] In some embodiments, the coke powder is finely ground metallurgical coking coal with a carbon content >90% and a fineness of 280 mesh to 400 mesh.

[0020] In some embodiments, the steel slag powder is steel slag that has been finely ground from waste slag produced during steelmaking in steel plants after hot quenching, and has a specific surface area of ​​400–800 m². 2 / kg, with a mesh size of 380-600 mesh.

[0021] A phosphogypsum-based hydraulic material prepared by the above method.

[0022] Application of the above-mentioned phosphogypsum-based hydraulic material in the preparation of building materials.

[0023] In some embodiments, the building material is cement, concrete, or mortar.

[0024] The beneficial effects of this invention are:

[0025] This invention employs a microwave-physical-chemical composite excitation method to excite soluble P2O5 and Fe in phosphogypsum. 2+ The harmful components are stabilized and reduced to crystals to become harmless and stable materials. The phosphorus is then encapsulated by a gelling agent to prepare a gypsum-based hydraulic material. This material has the characteristics of high activation activity, stable solidification and hardening in water, high strength after hardening, good volume stability, and excellent water stability.

[0026] The preparation process of the phosphogypsum-based hydraulic material of the present invention is simple and inexpensive. Attached Figure Description

[0027] To more clearly illustrate the technical solutions of the embodiments of the present invention, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of the present invention and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained from these drawings without creative effort.

[0028] Figure 1 A flowchart illustrating the preparation process of phosphogypsum-based hydraulic materials provided in this embodiment of the invention.

[0029] Figure 2 The XRD mineral composition spectrum of the phosphogypsum-based hydraulic material provided in Example 1 of this invention.

[0030] Figure 3 The XRD mineral composition spectrum of the phosphogypsum-based hydraulic material provided in Comparative Example 1 of this invention is shown. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, 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. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Therefore, the following detailed description of the embodiments of the present invention provided in the accompanying drawings is not intended to limit the scope of the claimed invention, but merely to represent selected embodiments of the invention. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0032] This invention provides a method for preparing a phosphogypsum-based hydraulic material based on microwave chemical composites, comprising the following steps:

[0033] S1, the phosphogypsum is cleaned and dried.

[0034] S2, add coke powder, sodium sulfate and calcined alum stone, mix evenly, and then microwave irradiate the mixture; microwave irradiation can heat and dehydrate to activate the phosphogypsum, causing the dihydrate gypsum in the phosphogypsum to decrystallize into anhydrous and hemihydrate gypsum, forming an air-hardening cementitious material mainly composed of β-type anhydrous gypsum, with the following specific composition: anhydrous gypsum: 67.30%, hemihydrate gypsum: 12.63%, dihydrate gypsum: 2.61%.

[0035] S3, add silicate cement clinker, steel slag powder, mineral powder and fly ash, mix evenly, and then crush to 380 mesh to 480 mesh.

[0036] S4, add calcium aluminate, quicklime and fast-dissolving solid water glass, mix and grind to obtain the phosphogypsum-based hydraulic material.

[0037] The phosphogypsum is an ultrafine acidic waste produced by phosphate chemical industry. Its main component is CaSO4·2H2O, and it also contains P2O5, MgO, Fe2O3, Al2O3 and CaO. Among them, the content of P2O5 is greater than that of MgO, and the content of Fe2O3 is greater than that of Al2O3. It provides calcium ions and sulfate ions for cementing materials.

[0038] The coke powder is finely ground metallurgical coking coal with a carbon content >90%, with a fineness of 280-400 mesh, and is used as a microwave absorbing heating material.

[0039] The sodium sulfate used is commercially available sodium sulfate (Na₂SO₄). The addition of sodium sulfate can accelerate the formation of supersaturation in anhydrous gypsum and reduce its crystallization activation energy, thus accelerating crystallization and significantly improving the hydration rate. Therefore, the addition of sodium sulfate can significantly improve the early strength of hardened anhydrous gypsum.

[0040] The calcined alunite mentioned is commercially available and is a natural mineral mainly composed of potassium aluminum sulfate double salts. It is produced through calcination, and the desired active ingredient is aluminum oxide (aluminum oxide). Higher sulfur trioxide content results in better water activation. Under the activation of alkali and sulfate, alunite forms ettringite, which effectively develops the strength of cement stone. After calcination, dehydration, and activation, alunite exhibits high solubility and the ability to react with cement hydrates. It can rapidly interact with gypsum to form ettringite, thereby further accelerating and deepening the hydration and hardening of cement.

[0041] The silicate cement clinker is an unprocessed and finely ground finished product produced by a cement plant; it provides active activating substances for cementitious materials.

[0042] The steel slag powder is steel slag that has been finely ground from waste slag produced during steelmaking in steel plants after hot quenching, and has a specific surface area of ​​400-800 m². 2 / kg, mesh size 380-600 mesh; main components are as follows: Al2O3 28-40%, SiO2

[0043] 3-10%, CaO 36-43%, Fe2O3 1-3%, SO3 8-15%, C4A3 55-75%, C2S 15-30%, C4AF 3-6%. It provides potential active raw materials for cementitious materials, mainly consuming excess sulfate.

[0044] The ore powder mentioned is blast furnace slag, a waste product generated during the ironmaking process, whose main components are iron oxide and silicon oxide. The iron oxide content is approximately 50%–60%, and the silicon oxide content is approximately 30%–40%. In addition, the slag also contains small amounts of impurities such as aluminum oxide, calcium oxide, and magnesium oxide. After being ground to a finer mesh (above 400 mesh), the finely ground slag contains more than 30% active calcium, silicon, aluminum, and other inorganic substances, with an activity index greater than 95%. Silicon oxide in the blast furnace slag is an active inorganic compound that provides the system with active silicon exhibiting a pozzolanic effect.

[0045] The fly ash mentioned above is the fine ash collected from the flue gas after coal combustion in power plants. Fly ash is the main solid waste discharged by coal-fired power plants, has a pozzolanic effect, and is a major component in the formation of hydrated calcium silicate.

[0046] The calcium aluminate mentioned is specifically commercially available high-alumina cement, whose main chemical components are Al2O3 and CaO. It has strong activity and provides an aluminum source for cementitious materials.

[0047] The quicklime used is commercially available CaO. As an alkaline activator, the addition of quicklime adjusts the alkalinity of the anhydrous gypsum slurry and activates the slag. In the anhydrous gypsum slurry after the addition of quicklime, the lime first reacts with water, rapidly hydrating to form Ca(OH)2, creating an alkaline environment for the entire system. Only then does dihydrate gypsum form. The presence of the alkaline medium activates the potential activity of the slag, generating ettringite crystals and hydrated calcium silicate gels, which have higher strength and stability than dihydrate gypsum and lower solubility in water. These newly formed products continuously fill and encapsulate the structures of dihydrate and anhydrous gypsum, making the overall structure denser and thus improving early and later strength.

[0048] The main component of the fast-dissolving solid water glass is fast-dissolving solid sodium silicate with a modulus of 2.5 to 3.8. It acts as an early-strength alkaline activator in the system. The role of water glass is to react with anhydrous gypsum particles to precipitate Ca(OH)2, making the anhydrous gypsum slurry alkaline. This can increase the solubility of anhydrous gypsum and generate more dihydrate gypsum, which has a significant activating effect on accelerating the hydration of anhydrous gypsum.

[0049] In this embodiment of the invention, when microwave irradiation is used, the various chemical components that make up phosphogypsum have different properties, and their heating rates in the microwave field are different. This creates a significant local temperature difference between the microwave-absorbing, partially microwave-absorbing, and non-microwave-absorbing minerals. On the one hand, this generates thermal stress between the minerals, promoting crack formation at the mineral interfaces and effectively promoting the dissociation of monomers in the microwave-absorbing minerals and increasing their effective reaction area. On the other hand, the heating process causes phosphogypsum to dehydrate, undergoing crystal transformation, phase transition, or chemical reactions.

[0050] Microwave irradiation heating generates heat from within the material itself, rather than from other heating elements, resulting in rapid heating, high efficiency, energy saving, and high thermal efficiency. Using the two commonly used microwave operating frequencies of 300MHz to 945GHz in industrial microwave equipment, the relaxation time of polar molecules during polarization is approximately 10... -9 ~10 -10 The speed is orders of magnitude faster than traditional high-temperature solid-state methods. Microwave irradiation heating also has the following advantages: the heating process is simple to operate and suitable for automatic control; the microwave heating system can operate normally immediately after startup, and the heating status of the material does not change inertia. After the power is cut off, the heated material cools down rapidly, which is extremely beneficial for automatic control and continuous production; no wastewater, waste gas, or waste is generated in microwave heating and drying, and there are no radiation residues. Its microwave leakage is also much lower than the national safety standards, making it a very safe and harmless high-tech method.

[0051] Existing research indicates that a single activation method does not significantly enhance the activity of phosphogypsum and cannot overcome the transformation of phosphogypsum from an air-hardening, water-sensitive material into a hydraulic cementitious material. Microwave-activated phosphogypsum-based hydraulic materials are achieved through the activation of alkaline and salt activators under microwave support. Microwaves typically refer to high-frequency electromagnetic waves ranging from 300MHz to 945GHz. During microwave irradiation, the various minerals composing gypsum exhibit different heating rates in the microwave field due to their varying properties. Significant localized temperature differences arise between microwave-absorbing, partially microwave-absorbing, and non-microwave-absorbing minerals. This creates thermal stress between the minerals, promoting crack formation at the mineral interfaces and effectively promoting the dissociation of monomers in microwave-absorbing minerals and increasing their effective reaction area. Furthermore, the heating process causes gypsum to dehydrate and undergo crystal transformations, phase transitions, or chemical reactions. XRD analysis of the mineral composition of the composite activation system revealed that calcium silicate (mainly C3S) was generated in the system under microwave heating and composite activator conditions. This is one of the main reasons why the gypsum slurry achieved high early strength.

[0052] After mixing with activated gypsum and water, the pH value of the slurry is approximately 11, and its composite activation mechanism is relatively complex. Based on the composition of the activator, the following analysis can be made: ① Under the activation action of the salt activator, anhydrous gypsum forms a double salt, which then decomposes to obtain dihydrate gypsum crystals. ② The activator, lime, silicate cement clinker, steel slag powder, calcium aluminate, and mineral powder together constitute an alkaline activation system. Under the combined action of the composite activator, the Ca2+ dissolved in water by anhydrous gypsum... 2+ SO4 2-Ions react with active calcium, active SiO2, and active Al2O3 in lime and mineral powder to form hydration products such as hydrated calcium silicate (CSH), hydrated tetracalcium aluminoferrite (C4AF), and hydrated calcium sulfoaluminate (AFt), which are poorly soluble in water. This disrupts the concentration balance of ions in the liquid phase, thereby promoting further dissolution and hydration of anhydrous gypsum and accelerating its setting and hardening rate. While ensuring a certain early strength, the formation of hydrated calcium silicate (CSH), hydrated tetracalcium aluminoferrite (C4AF), and hydrated calcium sulfoaluminate (AFt) further enhances the development of later strength and improves water stability. This explains why the addition of microwaves with a composite activator enables gypsum slurry to achieve better setting, hardening, and physical and mechanical properties.

[0053] Its hardening mechanism is as follows:

[0054] The cementitious materials prepared using phosphogypsum, fly ash, slag, cement, and composite activator as main raw materials will undergo a hydration reaction with cement and added quicklime when exposed to water, resulting in the formation of hydrated calcium silicate (CSH), hydrated calcium aluminate (CAH), and ettringite (AFt). These reactions are shown in equations (1) to (4), respectively.

[0055] CaO + H₂O → Ca 2+ +2OH - (1)

[0056] Ca 2+ +Si(OH)4→CS-Hgel (2)

[0057] Ca 2+ +Al(OH)3→CA-Hgel (3)

[0058] Al(OH)3+Ca 2+ +CaSO4·2H2O→3CaO·Al2O3·3CaSO4·32H2O(AFt) (4)

[0059] From the above reactions, it can be seen that the obtained hydration products mainly consist of CaC-SH, CAH, AFt, and unreacted CaSO4·2H2O. Under highly alkaline conditions, the steel slag glass gradually disintegrates, and the steel slag reacts with OH... -Under the strong influence of hydration, the decomposition activation energy of the calcium-rich phase is overcome, leading to the disintegration of the glassy phase in the steel slag, which facilitates the formation of CSH and CAH. As the hydration reaction proceeds, due to the presence of phosphogypsum, AH reacts with CaSO4·2H2O to generate AFt. Simultaneously, the generated AFt can intertwine with a large amount of CSH gel and other substances to form a network structure, encapsulating and binding unreacted phosphogypsum particles. Furthermore, with the large-scale generation of hydration products, the macroscopic manifestation is that the phosphogypsum cementitious material changes from an initial flowing paste to a non-flowing hardened body with high mechanical strength.

[0060] In some embodiments, the preparation method of the microwave chemical composite phosphogypsum-based hydraulic material comprises the following raw materials in terms of mass fraction: phosphogypsum 400-800 parts, coke powder 1-5 parts, sodium sulfate 5-10 parts, calcined alunite 2-8 parts, silicate cement clinker 50-100 parts, steel slag powder 100-300 parts, mineral powder 100-300 parts, fly ash 80-160 parts, calcium aluminate 10-20 parts, quicklime 5-10 parts, and fast-dissolving solid water glass 1-5 parts.

[0061] In some embodiments, the preparation method of the microwave chemical composite phosphogypsum-based hydraulic material comprises the following raw materials in terms of mass fraction: 500-700 parts phosphogypsum, 2-4 parts coke powder, 6-8 parts sodium sulfate, 3-7 parts calcined alunite, 60-90 parts silicate cement clinker, 150-250 parts steel slag powder, 150-250 parts mineral powder, 100-150 parts fly ash, 12-15 parts calcium aluminate, 6-8 parts quicklime, and 2-4 parts fast-dissolving solid water glass.

[0062] In some embodiments, step S1, the impurity removal step, is as follows: phosphogypsum and water are fed into a flotation device in a suitable ratio, then stirred, allowed to stand, and suspended matter on the liquid surface is removed, followed by solid-liquid separation using a separation device.

[0063] In some embodiments, the microwave irradiation uses microwaves with a frequency of 300MHz to 945GHz and a microwave power of 500W to 5kW. The temperature is 140 to 150°C, and the time is 20 to 40 minutes.

[0064] In some embodiments, the coke powder is finely ground metallurgical coking coal with a carbon content >90% and a fineness of 280 mesh to 400 mesh.

[0065] In some embodiments, the steel slag powder is steel slag that has been finely ground from waste slag produced during steelmaking in steel plants after hot quenching, and has a specific surface area of ​​400–800 m². 2 / kg, with a mesh size of 380-600 mesh.

[0066] This invention also provides a phosphogypsum-based hydraulic material prepared by the above method.

[0067] This invention also provides an application of the above-mentioned phosphogypsum-based hydraulic material in the preparation of building materials.

[0068] In some embodiments, the building material is cement, concrete, or mortar. The phosphogypsum-based hydraulic material can replace all or part of cement in concrete or mortar, road water-stabilized layers, soil stabilization, slope protection, mine backfilling, unfired bricks, small components, etc.

[0069] In the preparation of mortar, road water-stabilized layer materials, and building gypsum products, the microwave-chemically activated phosphogypsum-based hydraulic material of this invention, when mixed with water, allows the gypsum to stably solidify and harden in water, activating its gelling activity and participating in the hydration reaction. In this invention, the water-cement ratio of the microwave-chemically activated phosphogypsum-based hydraulic material to water is preferably 0.19–0.35, where the "cement" (gelling material) in the water-cement ratio refers to the mass of the phosphogypsum-based hydraulic material. The microwave-chemically activated phosphogypsum-based hydraulic material of this invention exhibits high strength and good volume stability during application.

[0070] Example 1

[0071] Raw phosphogypsum and water are fed into a flotation device in a suitable ratio, then stirred, allowed to stand, and suspended matter on the surface of the liquid is removed. Solid-liquid separation is then carried out through a separation device, followed by drying.

[0072] By weight, 800 parts of phosphogypsum, 4 parts of coke powder, 10 parts of sodium sulfate, and 8 parts of calcined alum stone were thoroughly stirred at 75 rpm for 5 min. The mixture was then irradiated at 145°C for 20–40 min using a 5.0 kW, 915 MHz microwave generator. The mixture was then mixed with 50 parts of silicate silicate cement clinker, 100 parts of steel slag powder, 100 parts of mineral powder, and 150 parts of fly ash. The mixture was pulverized to 380–400 mesh and then mixed with 20 parts of calcium aluminate, 10 parts of quicklime, and 5 parts of fast-dissolving solid water glass and ground to obtain the microwave-activated phosphogypsum-based hydraulic material.

[0073] Example 2

[0074] Raw phosphogypsum and water are fed into a flotation device in a suitable ratio, then stirred, allowed to stand, and suspended matter on the surface of the liquid is removed. Solid-liquid separation is then carried out through a separation device, followed by drying.

[0075] By weight, 600 parts of phosphogypsum, 2 parts of coke powder, 8 parts of sodium sulfate, and 6 parts of calcined alum stone were thoroughly stirred at 75 rpm for 5 minutes. The mixture was then irradiated at 145°C for 20–40 minutes using a 3.0 kW, 2450 MHz microwave generator. The mixture was then mixed with 60 parts of silicate silicate cement clinker, 150 parts of steel slag powder, 150 parts of mineral powder, and 100 parts of fly ash. The mixture was pulverized to 380–400 mesh and then mixed with 16 parts of calcium aluminate, 8 parts of quicklime, and 4 parts of fast-dissolving solid water glass and ground to obtain the microwave-activated phosphogypsum-based hydraulic material.

[0076] Example 3

[0077] Raw phosphogypsum and water are fed into a flotation device in a suitable ratio, then stirred, allowed to stand, and suspended matter on the surface of the liquid is removed. Solid-liquid separation is then carried out through a separation device, followed by drying.

[0078] By weight, 500 parts of phosphogypsum, 2 parts of coke powder, 6 parts of sodium sulfate, and 4 parts of calcined alum stone were thoroughly stirred at 75 rpm for 5 minutes. The mixture was then irradiated at 145°C for 20–40 minutes using a 1.0 kW, 2450 MHz microwave generator. The mixture was then mixed with 80 parts of silicate silicate cement clinker, 200 parts of steel slag powder, 250 parts of mineral powder, and 100 parts of fly ash. The mixture was pulverized to 380–400 mesh and then mixed with 12 parts of calcium aluminate, 6 parts of quicklime, and 2 parts of fast-dissolving solid water glass and ground to obtain the microwave-activated phosphogypsum-based hydraulic material.

[0079] Application Examples 1-3

[0080] The phosphogypsum-based hydraulic materials obtained in Examples 1-3 based on microwave chemical composite activation were subjected to strength tests according to GB 17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 1.

[0081] Table 2 Strength test results for Application Examples 1-3

[0082]

[0083] As shown in Table 1, the standard mortar obtained after hardening using the microwave chemically activated phosphogypsum-based hydraulic materials described in Examples 1-3 of this invention exhibits a 3-day compressive strength of 8.2-9.8 MPa and a flexural strength of 2.2-2.4 MPa, demonstrating high early-stage strength; a 28-day compressive strength of 23.8-31.4 MPa and a flexural strength of 4.1-8.5 MPa, demonstrating high later-stage strength and excellent volume stability. Compared to the persulfurized phosphogypsum slag cement described in JCT2391-2017 "Persulfurized phosphogypsum slag cement concrete for finished products," the microwave chemically activated phosphogypsum-based hydraulic material provided by this invention shows a significant improvement in early-stage strength, indicating a substantial enhancement in phosphogypsum activity.

[0084] Application Examples 4-5

[0085] The microwave-activated phosphogypsum-based hydraulic materials obtained in Examples 2 and 3 were compared with flotation tailings at a mass ratio of 5:5 and a certain water-to-material ratio. The cubic unconfined compressive strength and softening coefficient were then tested. In Application Examples 4 and 5, the material in the water-to-material ratio refers to the total mass of the phosphogypsum-based hydraulic materials and the flotation tailings. The test results are shown in Table 2.

[0086] Table 2 Results of Unlimited Compressive Strength and Softening Coefficient in Application Examples 4-5

[0087]

[0088] As shown in Table 2, the phosphogypsum-based hydraulic material based on microwave chemical composite activation described in Examples 4-5 of this invention, after being mixed and hardened with flotation tailings slag, yields a cubic unconfined compressive strength of 1.4-2.0 MPa at 3d, indicating high early strength; a compressive strength of 30.2-40.9 MPa at 28d, indicating high later-stage strength and excellent volume stability; and a softening coefficient of 0.87-0.97 at 7d, indicating good water stability.

[0089] Application Example 6

[0090] The microwave-activated phosphogypsum-based hydraulic material obtained in Example 3 was mixed with dihydrate phosphogypsum at different mass ratios and a certain water-material ratio to form cement mortar specimens. Strength tests were conducted according to GB17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 3.

[0091] Table 3 Application Example 6 Strength Test Results

[0092]

[0093] As shown in Table 3, the microwave-activated phosphogypsum-based hydraulic material described in Example 6 of this invention, used to cure dihydrate gypsum, resulted in specimens with high early-stage strength. After curing, the specimens exhibited high early-stage strength, with a 7-day compressive strength of 6.0–10.8 MPa and a flexural strength of 2.3–3.3 MPa. At 28 days, the compressive strength was 26.6–34.9 MPa and the flexural strength was 6.0–6.1 MPa. This demonstrates high late-stage strength. The microwave-activated phosphogypsum-based hydraulic material described in Example 6 of this invention exhibits excellent curing effects on dihydrate phosphogypsum. Compared to the persulfurized phosphogypsum slag cement described in JCT2391-2017 "Persulfurized phosphogypsum slag cement concrete for finished products," the microwave-activated phosphogypsum-based hydraulic material provided by this invention shows significantly improved late-stage strength, indicating a substantial enhancement in phosphogypsum activity.

[0094] Application Example 7

[0095] The microwave-activated phosphogypsum-based hydraulic material obtained in Example 3 was mixed with crushed red sandstone soil at a mass ratio of 1:3 to form cubic specimens. The unconfined compressive strength and softening coefficient of the cubes were then tested. In Application Example 7, the material in the water-material ratio refers to the total mass of the phosphogypsum-based hydraulic material and the crushed red sandstone soil. The test results are shown in Table 4.

[0096] Table 4 Results of Unlimited Compressive Strength and Softening Coefficient in Application Example 7

[0097]

[0098] As shown in Table 4, the phosphogypsum-based hydraulic material based on microwave chemical composite activation described in Example 7 of this invention, after being mixed and hardened with broken red sandstone soil, yielded a cubic unconfined compressive strength of 0.9 MPa at 3 days, indicating high early strength; a compressive strength of 6.7 MPa at 28 days, indicating relatively high later strength and excellent volume stability; and a softening coefficient of 0.80 at 7 days, indicating good water stability.

[0099] Application Example 8

[0100] The phosphogypsum-based hydraulic material obtained in Example 3, based on microwave chemical composite activation, was mixed with coal gangue of a certain gradation to form phosphogypsum-coal gangue concrete. The material composition of the phosphogypsum-coal gangue concrete described in Example 8 of this invention is shown in Table 5.

[0101] Table 5 Application Example 8: Composition of Phospholipid Gypsum and Coal Gangue Concrete Materials

[0102]

[0103] The cubic unconfined compressive strength test was conducted on the phosphogypsum-coal gangue concrete described in Example 8 of this invention. Simultaneously, an impermeability test was performed on the phosphogypsum-coal gangue concrete according to GB / T 50082 "Standard for Test Methods of Long-Term Performance and Durability of Ordinary Concrete". The test results are shown in Table 6.

[0104] Table 6 Results of Unlimited Compressive Strength and Permeability Coefficient in Application Example 8

[0105]

[0106] As shown in Table 6, the phosphogypsum-coal gangue concrete material obtained in Example 8 of this invention has a 3-day unconfined compressive strength of 31.9 MPa, indicating high early strength; and a 28-day compressive strength of 51.0 MPa, which meets the strength design requirements of C40 concrete in GB50010-2010 "Code for Design of Concrete Structures".

[0107] Application Example 9

[0108] The microwave-activated phosphogypsum-based hydraulic material obtained in Example 3 was subjected to sulfate attack resistance tests according to GB / T749—2008 "Test Method for Sulfate Attack Resistance of Cement". 100g of composite cementitious material and 250g of medium-grade sand were weighed, and mortar specimens of 10mm×10mm×60mm were prepared with a water-cement ratio of 0.5. After curing in a standard curing chamber for 24 hours, the specimens were demolded. The demolded specimens were then placed in a 50℃ humid heat curing chamber for 7 days and then immersed in clean water and 5% Na2SO4 solution, respectively. The results of the corrosion resistance coefficient test are shown in Table 7.

[0109] Table 7 Application Example 9 Corrosion Resistance Coefficient Results

[0110]

[0111] Comparative Example 1

[0112] Microwave irradiation is not used; other operations are the same as in Example 1.

[0113] The phosphogypsum-based hydraulic materials prepared in Example 1 and Comparative Example 1 were subjected to XRD mineral composition analysis, such as... Figure 2 and Figure 3 As shown.

[0114] Depend on Figure 2 and Figure 3 It can be seen that under microwave heating and composite activator conditions, the activity of phosphogypsum is greatly improved. In particular, dihydrate gypsum dehydrates and transforms into anhydrous and hemihydrate gypsum, while tricalcium silicate is generated in the system. This is one of the main reasons why the gypsum slurry obtains high early strength.

[0115] Comparative Examples 2-3

[0116] Set the microwave power to 200W and 10kW, and perform other operations as in Example 1.

[0117] The phosphogypsum-based hydraulic materials obtained from Comparative Examples 2-3 and Example 1 based on microwave chemical composite activation were tested for strength according to GB 17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 8.

[0118] Table 8. Strength test results of Comparative Examples 2-3 and Example 1

[0119]

[0120] Table 8 shows that when the microwave power is 200W, the early and later strengths of the obtained phosphogypsum-based hydraulic material based on microwave chemical composite activation are both low, with 28-day compressive strength and flexural strength of 10.3 MPa and 2.4 MPa, respectively. This is because the degree of dehydration and reactivity of phosphogypsum are positively correlated with the microwave frequency within a certain range. When the microwave frequency is 10kW, the compressive and flexural strengths of the obtained phosphogypsum-based hydraulic material based on microwave chemical composite activation are not significantly improved compared to Application Example 1.

[0121] Comparative Example 4

[0122] The microwave irradiation temperature was set to 120℃, and other operations were the same as in Example 1. The phosphogypsum-based hydraulic materials obtained from Comparative Examples 2-3 and Example 1 based on microwave chemical composite activation were tested for strength according to GB 17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 9.

[0123] Table 9. Strength test results for Comparative Example 4 and Example 1

[0124]

[0125] As shown in Table 9, when the microwave irradiation temperature is 120℃, the 28-day compressive strength and flexural strength of the obtained phosphogypsum-based hydraulic material based on microwave chemical composite activation are 11.2 MPa and 2.6 MPa, respectively, which are much lower than those in Application Example 1. This is because when the microwave irradiation temperature is low, the dehydration degree of phosphogypsum is low, and the content of hemihydrate gypsum is low. When the temperature rises, the conversion rate of dihydrate gypsum to hemihydrate gypsum increases. When the temperature reaches 145℃, anhydrous gypsum is easily produced, resulting in tight crystal bonding and increased strength.

[0126] Comparative Example 5

[0127] The mass fraction of phosphogypsum in Example 1 was set to 1000 parts, and other operations were the same as in Example 1.

[0128] The phosphogypsum-based hydraulic materials obtained from Comparative Example 5 and Example 1 based on microwave chemical composite activation were tested for strength according to GB17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 10.

[0129] Table 10 Strength test results for Comparative Example 5 and Example 1

[0130]

[0131] As shown in Table 10, when the mass fraction of phosphogypsum is 1000 parts, the 28-day compressive strength and flexural strength of the obtained microwave chemically activated phosphogypsum-based hydraulic material are 17.6 MPa and 3.0 MPa, respectively, which is 26% lower than that of Application Example 1. This indicates that if the phosphogypsum content is too high, the relative proportion of other components will be low, making it difficult to fully utilize its gelling activity. Considering the need for high added value utilization of phosphogypsum, the mass fraction of phosphogypsum in the microwave chemically activated phosphogypsum-based hydraulic material of this invention is 400-800 parts.

[0132] Comparative Example 6

[0133] The mass fraction of silicate cement clinker in Example 2 was set to 40 parts, and other operations were the same as in Example 2.

[0134] The phosphogypsum-based hydraulic materials obtained from Comparative Example 6 and Example 2 based on microwave chemical composite activation were tested for strength according to GB17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 11.

[0135] Table 11 Strength test results for Comparative Example 6 and Example 2

[0136]

[0137] As shown in Table 11, when the cement mass fraction is 40 parts, the 28-day compressive strength and flexural strength of the obtained microwave chemically activated phosphogypsum-based hydraulic material are 22.3 MPa and 6.1 MPa, respectively, which are 19% and 28% lower than those in Application Example 2. This is because when the cement clinker content in the system is low, the provided active SiO2 and Al2O3 are limited, resulting in lower contents of hydrated calcium silicate (CSH), hydrated calcium aluminate (CAH), and ettringite (AFt), which significantly reduces both early and later strength. Therefore, in order to achieve higher mechanical properties while considering economic benefits, the mass fraction of phosphogypsum in the microwave chemically activated phosphogypsum-based hydraulic material of this invention is 50-100 parts.

[0138] Comparative Example 7

[0139] The mass fraction of calcined alum stone in Example 2 was replaced with sodium sulfate, i.e., 14 parts sodium sulfate, and the other operations were the same as in Example 2.

[0140] The phosphogypsum-based hydraulic materials obtained from Comparative Example 7 and Example 2 based on microwave chemical composite activation were tested for strength according to GB17671-1999 "Test Method for Strength of Cement Mortar". The test results are shown in Table 12.

[0141] Table 12 Strength test results for Comparative Example 7 and Example 2

[0142]

[0143] Table 12 shows that without calcined alunite, the 28-day compressive strength and flexural strength of the phosphogypsum-based hydraulic material obtained based on microwave chemical composite activation were 21.8 MPa and 5.8 MPa, respectively, lower than those in Application Example 2. This is because calcined alunite is a highly efficient activator for phosphogypsum, improving its hydration and crystallization properties. Under the activation of alkali-sulfate, it forms ettringite, causing the liquid phase SO4... 2- Increased concentration and higher supersaturation of gypsum dihydrate crystallization promote spontaneous homogeneous nucleation and crystal growth of gypsum dihydrate, resulting in good development of the strength of cement stone.

[0144] Comparative Examples 8-9

[0145] Unlike Example 2, the mass fraction of mineral powder in Comparative Example 8 was 0, and the mass fraction of steel slag powder in Comparative Example 9 was 0. All other operations were the same as in Example 2.

[0146] The phosphogypsum-based hydraulic materials obtained from Comparative Examples 8, 9 and 2 based on microwave chemical composite activation were tested for strength according to GB 17671-1999 "Test Method for Strength of Cement Mortar". The results are shown in Table 13.

[0147] Table 13 Strength test results for Comparative Example 8, Comparative Example 9 and Example 2

[0148]

[0149] Table 13 shows that, compared with Application Example 2, the 28-day compressive strength of the cement mortar specimen without mineral powder was 15.7 MPa, a decrease of 42.9% compared to Application Example 2. Without steel slag, the 28-day compressive strength of the cement mortar specimen was 11.0 MPa, a decrease of 60% compared to Application Example 2. The 28-day compressive strength and flexural strength of the materials satisfy the following: Comparative Example 8 + Comparative Example 9 < Application Example 2. This indicates that the combined addition of steel slag and mineral powder in the specified proportions significantly improves the later-stage strength of the material. Therefore, this invention uses mineral powder and steel slag powder as composite auxiliary cementitious materials to improve the mechanical properties of phosphogypsum-based hydraulic materials based on microwave chemical composite activation.

[0150] Comparative Example 10

[0151] Raw phosphogypsum and water are fed into a flotation device in a suitable ratio, then stirred, allowed to stand, and suspended matter on the surface of the liquid is removed. Solid-liquid separation is then carried out through a separation device, followed by drying.

[0152] By weight, 500 parts of phosphogypsum and 2 parts of coke powder were thoroughly stirred at 75 rpm for 5 minutes, and then irradiated at 145°C for 20–40 minutes using a 1.0 kW, 2450 MHz microwave generator. The mixture was then combined with 6 parts of sodium sulfate, 4 parts of calcined alunite, 80 parts of silicate silicate cement clinker, 200 parts of steel slag powder, 250 parts of mineral powder, and 100 parts of fly ash. The mixture was pulverized to 380–400 mesh, and then mixed with 12 parts of calcium aluminate, 6 parts of quicklime, and 2 parts of readily soluble solid water glass, and then milled to obtain the microwave-activated phosphogypsum-based hydraulic material.

[0153] The phosphogypsum-based hydraulic materials obtained in Comparative Example 10 and Example 3, based on microwave chemical composite activation, were combined with coal gangue of a certain gradation to form phosphogypsum-coal gangue concrete. The material composition of the phosphogypsum-coal gangue concrete described in Comparative Example 6 and Application Example 8 of this invention is shown in Table 5.

[0154] The phosphogypsum-coal gangue concrete described in Comparative Example 6 and Example 3 of this invention was subjected to cubic unconfined compressive strength test and impermeability test. The test results are shown in Table 11.

[0155] Table 14 Results of Unlimited Compressive Strength and Permeability Coefficient for Comparative Example 6 and Application Example 8

[0156]

[0157] As shown in Table 14, when phosphogypsum was only microwave-irradiated with coke powder, and then mixed with sodium sulfate and calcined alunite, with other operations the same as in Application Example 8, the resulting phosphogypsum-coal gangue concrete had a lower cubic compressive strength than that of Application Example 8. This is because sodium sulfate and calcined alunite salt activators can promote the generation of thermal stress in the phosphogypsum mineral components under microwave conditions. During the heating process, the phosphogypsum will undergo dehydration, crystal transformation, phase transition, or chemical reaction, resulting in higher strength.

[0158] The above description is merely a preferred embodiment of the present invention and is not intended to limit the invention. Various modifications and variations can be made to the invention by those skilled in the art. Any modifications, equivalent substitutions, or improvements made within the spirit and principles of the invention should be included within the scope of protection of the invention.

Claims

1. A method for preparing phosphogypsum-based hydraulic materials based on microwave chemical composites, characterized in that, Includes the following steps: S1, remove impurities and dry the phosphogypsum; S2, add coke powder, sodium sulfate and calcined alum stone, mix evenly, and then microwave irradiate the mixture; S4, add silicate cement clinker, steel slag powder, mineral powder and fly ash, mix evenly, and then crush to 380 mesh~480 mesh; S, add calcium aluminate, quicklime and fast-dissolving solid water glass, mix and grind to obtain the phosphogypsum-based hydraulic material; The mass fractions of each raw material are as follows: phosphogypsum 400-800 parts, coke powder 1-5 parts, sodium sulfate 5-10 parts, calcined alunite 2-8 parts, silicate cement clinker 50-100 parts, steel slag powder 100-300 parts, mineral powder 100-300 parts, fly ash 80-160 parts, calcium aluminate 10-20 parts, quicklime 5-10 parts, and fast-dissolving solid water glass 1-5 parts. The microwave irradiation uses microwaves with a frequency of 300MHz to 945GHz, a power of 500W to 5kW, a temperature of 140 to 150℃, and a duration of 20 to 40 minutes.

2. The method for preparing the microwave-chemically composited phosphogypsum-based hydraulic material according to claim 1, characterized in that, The mass fractions of each raw material are as follows: phosphogypsum 500-700 parts, coke powder 2-4 parts, sodium sulfate 6-8 parts, calcined alunite 3-7 parts, silicate cement clinker 60-90 parts, steel slag powder 150-250 parts, mineral powder 150-250 parts, fly ash 100-150 parts, calcium aluminate 12-15 parts, quicklime 6-8 parts, and fast-dissolving solid water glass 2-4 parts.

3. The method for preparing the microwave-chemically composited phosphogypsum-based hydraulic material according to claim 1, characterized in that, In step S1, the impurity removal step is as follows: phosphogypsum and water are fed into the flotation equipment in a suitable ratio, then stirred, allowed to stand, and suspended matter on the liquid surface is removed, and solid-liquid separation is performed through a separation device.

4. The method for preparing the microwave-chemically composited phosphogypsum-based hydraulic material according to claim 1, characterized in that, The coke powder is finely ground metallurgical coking coal with a carbon content >90% and a fineness of 280 mesh to 400 mesh.

5. The method for preparing the microwave-chemically composited phosphogypsum-based hydraulic material according to claim 1, characterized in that, The steel slag powder is steel slag that has been finely ground from waste slag produced during steelmaking in steel plants after hot quenching, with a specific surface area of ​​400~800 m². 2 / kg, with a mesh size of 380~600 mesh.

6. A phosphogypsum-based hydraulic material prepared by the method according to any one of claims 1 to 5.

7. The application of the phosphogypsum-based hydraulic material of claim 6 in the preparation of building materials.

8. The application according to claim 7, characterized in that, The building materials are cement, concrete, or mortar.

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