Lightweight water-resistant phosphogypsum-based cement concrete mortar and preparation process thereof

Abstract

The invention relates to the technical field of building materials, in particular to light water-resistant phosphogypsum-based cement concrete mortar and a preparation process thereof. The phosphogypsum-based cement concrete mortar comprises the following components: activated phosphogypsum, sulphoaluminate cement, magnesium phosphate cement, modified zeolite powder, lightweight aggregate, modified nano silicon dioxide, a core-shell toughening agent, a polycarboxylate superplasticizer and water, and the preparation method comprises the following steps: respectively preparing the activated phosphogypsum, the modified zeolite powder, the modified nano silicon dioxide and the core-shell toughening agent; the water resistance of the ardealite is improved through activating treatment, the interface bonding force and the structural compactness are enhanced through multi-component synergism by means of selective adsorption of the modified zeolite powder, the mechanical property of the lightweight aggregate is optimized, the toughness is improved through the core-shell toughening agent, and lightweight, high water resistance, stable performance and durability improvement of the mortar are achieved; the requirements of complex building scenes are met, and environment-friendly and practical values are achieved.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building materials, in particular to a lightweight and water-resistant phosphogypsum-based cement concrete mortar and a preparation process thereof. BACKGROUND

[0002] The phosphogypsum-based cement concrete mortar is widely used in the field of construction, and can be used for indoor and outdoor wall plastering, basement wall treatment, and thin-layer finishing engineering of various buildings, thereby providing good protection and decoration effect for building structures. Based on the above application scenarios, the phosphogypsum-based cement concrete mortar needs to have the functions of lightweight and water resistance. The prior art proposes a lightweight and water-resistant gypsum plastering mortar and a preparation method thereof (CN120247514A). The plastering mortar uses β-hemihydrate phosphogypsum as the main raw material, and forms a multi-element cementing system by combining with slag, silica fume, cement, and the like, and adds various additives such as modified vitrified microbeads, a retarder, and a water retaining agent. The plastering mortar has good strength, light weight, good water resistance, and the like, and is suitable for various environments.

[0003] However, the gypsum mortar in the prior art still has obvious deficiencies. In the treatment of phosphogypsum, the activation degree is limited, the sulfate ion dissolution rate is high, and the composition lacks a selective adsorption mechanism for sulfate ions, which easily leads to fluctuations in the performance of the mortar, and the strength retention and durability in a high-humidity environment still have room for improvement, so the mortar cannot better meet the needs of complex building scenarios. In addition, the lightweight reinforcing system is single, and only relies on modified vitrified microbeads, which is difficult to further improve the interfacial adhesion and structural compactness while achieving lightweight. SUMMARY

[0004] In view of the technical defects in the background art, the present application proposes a lightweight and water-resistant phosphogypsum-based cement concrete mortar and a preparation process thereof, which solves the above technical problems and meets the actual needs. The specific technical solution is as follows: A lightweight and water-resistant phosphogypsum-based cement concrete mortar, which is composed of the following raw materials in parts by weight: activated phosphogypsum 40-50 parts, sulphoaluminate cement 15-25 parts, magnesium phosphate cement 5-8 parts, modified zeolite powder 5-18 parts, lightweight aggregate 30-40 parts, modified nano-silicon dioxide 5-10 parts, core-shell toughening agent 3-5 parts, polycarboxylic acid water reducing agent 0.5-1 part, and water 30-40 parts.

[0005] As a further technical solution of the present application, the activated phosphogypsum is prepared by acid washing, calcining, and coating with a silicone-acrylate emulsion of phosphogypsum, and the modified zeolite powder is prepared by acid washing, calcining, and loading Mg / Fe bimetallic treatment of natural clinoptilolite.

[0006] As a further technical solution of the present invention, the modified nano-silica includes a nano-calcium carbonate core and a SiO2 layer coated on the surface of the nano-calcium carbonate core, and the lightweight aggregate is a mixture of expanded perlite and closed-cell ceramsite with a volume ratio of (2:1) - (3:1).

[0007] As a further technical solution of the present invention, the core-shell toughening agent is a composite particle of paraffin coated with recycled rubber microparticles, wherein the phase transition temperature of the paraffin is 25-30℃ and the particle size of the recycled rubber microparticles is 0.5-1mm.

[0008] A method for preparing a lightweight, water-resistant phosphogypsum-based cement concrete mortar includes the following steps: S1. Preparation of activated phosphogypsum Phosphogypsum was added to a citric acid solution and stirred at 60°C for 45 min. Then, it was filtered and washed to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum was mixed with red mud in a certain proportion, and 2 wt% metakaolin was added. Then, it was calcined at 150°C for 1.5 h to obtain a calcination intermediate. The calcination intermediate was immersed in a silicone-acrylic emulsion, stirred at 40°C for 30 min, and then dried in a fluidized bed at 120°C to obtain activated phosphogypsum. S2, Preparation of modified zeolite powder Natural clinoptilolite was heated and soaked in HCl solution for 2 hours, then removed and calcined in a muffle furnace for 3 hours to obtain pretreated zeolite. The pretreated zeolite was added to a MgCl2 / FeCl3 mixed solution, and then NaOH precipitant was added dropwise until pH=10. After aging for 4 hours, it was filtered, dried, and calcined to obtain modified zeolite powder. S3, Modified Nano-Silica Nano-CaCO3 was dispersed in sodium silicate solution, and 0.1wt% sodium polyacrylate dispersant was added. The mixture was ultrasonically treated for 30 min to obtain a CaCO3 mixed dispersion. Under 80℃ water bath conditions, 1.5mol / L H2SO4 was added dropwise to the CaCO3 mixed dispersion at a rate of 2ml / min until the pH=8.5. During the dropwise addition, the mixture was stirred at a stirring rate of 3000rpm. Then, the mixture was kept at the temperature and aged for 5 h. After filtration, the product was impregnated in silane coupling agent and then filtered to obtain modified nano-silica. S4. Preparation of core-shell toughening agent: Reclaimed rubber microspheres are vacuum treated at 120℃ for 30 min, immersed in molten paraffin and stirred for 1 h, and cooled and solidified to form paraffin-coated reclaimed rubber microspheres, thus obtaining the core-shell toughening agent; S5. Preparation of phosphogypsum-based cement concrete mortar Place 45 parts activated phosphogypsum, 20 parts sulfur-aluminum cement, 11 parts modified zeolite powder, 6.5 parts magnesium phosphate cement, 35 parts lightweight aggregate, and 7.5 parts modified nano-silica in a high-efficiency mixer and mix for 10-15 minutes until homogeneous. Then add 35 parts of water (70% of the total volume) and stir for 2-3 minutes. Next, add 4 parts of core-shell toughening agent and 0.75 parts of polycarboxylate superplasticizer and continue stirring for 1-2 minutes. Then add the remaining water to adjust the consistency and stir for 3-5 minutes until a homogeneous phosphogypsum-based cement concrete mortar with suitable fluidity is obtained.

[0009] As a further technical solution of the present invention, in step S1, the preparation method of the activated phosphogypsum is as follows: the phosphogypsum is crushed to a particle size ≤2mm, and a solution containing 1.5wt% citric acid is added at a solid-liquid ratio of 1:2. The mixture is stirred at 60℃ for 45min to dissolve soluble phosphorus and fluorine impurities. Then, it is vacuum filtered, and the filter residue is washed twice with weak acid water with pH=6 to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum is mixed with Bayer red mud at a mass ratio of 9:1, and 2wt% metakaolin is added and mixed evenly. Then, it is calcined at 150℃ for 1.5h to obtain a calcined intermediate. The calcined intermediate is immersed in 20% silicone-acrylic emulsion at a solid-liquid ratio of 1:3, stirred at 40℃ for 30min, and then dried in a fluidized bed at 120℃ to obtain activated phosphogypsum.

[0010] As a further technical solution of the present invention, in step S2, the specific preparation method of the modified zeolite powder is as follows: natural clinoptilolite is crushed to 100 mesh, then soaked in 2 mol / L HCl solution at 60°C for 2 h with a solid-liquid ratio of 1:5, then filtered, washed until neutral, and then placed in a muffle furnace at 550°C for 3 h to obtain pretreated zeolite. 1.5 mol / L MgCl2 and 0.2 mol / L FeCl3 were mixed at a molar ratio of Mg:Fe = 8:1 to obtain a MgCl2 / FeCl3 mixed solution. Pretreated zeolite was added to the MgCl2 / FeCl3 mixed solution, and then 3 mol / L NaOH precipitant was slowly added dropwise until the pH reached 10. After aging for 4 hours, the solution was filtered to obtain a composite precipitate. The composite precipitate was washed, dried, and finally calcined at 600℃ for 2 hours to obtain modified zeolite powder.

[0011] As a further technical solution of the present invention, in step S3, the specific preparation method of the modified nano silica is as follows: nano CaCO3 is dispersed in a 15% sodium silicate solution, the solid-liquid ratio of nano CaCO3 to 15% sodium silicate solution is 1:5, and then 0.1wt% sodium polyacrylate dispersant is added. The mixture is ultrasonically treated for 30 min to obtain a CaCO3 mixed dispersion. Under the condition of 80℃ water bath, 1.5mol / L H2SO4 is added dropwise to the CaCO3 mixed dispersion at a rate of 2ml / min until pH=8.5. During the dropwise addition, the mixture is stirred at a rate of 3000rpm. Then, it is kept warm and aged for 5h, filtered, and the product is placed in 3wt% γ-aminopropyltriethoxysilane and reacted at 70℃ for 1h. Then, the mixture is filtered, washed, and dried to obtain modified nano silica.

[0012] The beneficial effects of this invention are as follows: This invention effectively enhances the activation level of phosphogypsum through acid washing to remove impurities, calcination to reconstruct the mineral phases, and silicone-acrylic emulsion coating, significantly reducing sulfate dissolution rate. Simultaneously, the silicone-acrylic emulsion coating forms a hydrophobic barrier, enhancing water resistance. Modified zeolite powder, after thermal activation and pore expansion and loading with Mg / Fe bimetal, can selectively adsorb free sulfate and phosphate ions, reducing performance fluctuations. Sulfoaluminate cement, magnesium phosphate cement, and activated phosphogypsum work synergistically, combined with modified nano-silica, to effectively improve interfacial adhesion and structural density, enhancing mortar strength. The lightweight system uses a blend of expanded perlite and closed-cell ceramsite, ensuring lightweight while addressing the shortcomings of single lightweight aggregates. The core-shell toughening agent absorbs impact energy through the rubber core to improve toughness and enhances compatibility with the matrix through the paraffin shell, preventing interfacial delamination. In summary, this mortar possesses lightweight properties, high water resistance, stable mechanical properties, and good durability, while efficiently utilizing phosphogypsum, making it both environmentally friendly and practical. Detailed Implementation

[0013] The embodiments of the present invention will be described below with reference to relevant examples. The embodiments of the present invention are not limited to the following examples, and the present invention relates to relevant necessary components in this technical field, which should be regarded as well-known technology in this technical field and can be known and mastered by those skilled in this technical field.

[0014] A lightweight, water-resistant phosphogypsum-based cement concrete mortar comprises the following raw materials in parts by weight: 40-50 parts activated phosphogypsum, 15-25 parts sulfoaluminate cement, 5-8 parts magnesium phosphate cement, 5-18 parts modified zeolite powder, 30-40 parts lightweight aggregate, 5-10 parts modified nano-silica, 3-5 parts core-shell toughening agent, 0.5-1 part polycarboxylate superplasticizer, and 30-40 parts water.

[0015] This invention effectively solves the performance fluctuation problem of traditional phosphogypsum mortar by combining multi-step activation treatment of phosphogypsum with the synergistic effect of modified zeolite powder. The activated phosphogypsum is acid-washed to remove soluble phosphorus and fluorine impurities, then co-calcined with red mud and metakaolin, and finally coated with silicone-acrylic emulsion to form a hydrophobic barrier, which not only improves the activity of phosphogypsum but also significantly reduces the sulfate dissolution rate. The modified zeolite powder is acid-leached to expand pores and activated at high temperature, and then loaded with Mg / Fe bimetal, which can selectively adsorb free sulfate and phosphate ions in the system. The combination of the two reduces the performance fluctuation caused by ion dissolution from the source.

[0016] The synergistic design of the cementitious system and modified nano-silica enhances the strength and structural density of the mortar. The rapid hardening characteristics of sulfoaluminate cement, the early strength advantage of magnesium phosphate cement, and activated phosphogypsum form a multi-element cementitious network, achieving complementary strength. The modified nano-silica, with nano-calcium carbonate as the core and SiO2 as the shell, not only utilizes the nano-effect to fill microscopic pores, but its core-shell structure also enhances the interfacial adhesion with the cementitious system, promotes the uniform growth of hydration products, and further densifies the structure. This allows the mortar to maintain high compressive and flexural strength while ensuring its lightweight nature, meeting the mechanical requirements of complex scenarios.

[0017] The combination of a lightweight system and a core-shell toughening agent achieves both lightweighting and mechanical properties and toughness. The lightweight aggregate is a blend of expanded perlite and closed-cell ceramsite, which balances the strength defects of individual aggregates. The core-shell toughening agent uses recycled rubber as the core to absorb impact energy, and the paraffin shell layer enhances compatibility with the matrix and prevents interfacial delamination. This allows the mortar to possess both lightweight characteristics and good crack resistance and durability, making it suitable for complex building environments such as dampness.

[0018] As one of the preferred embodiments of the present invention, the activated phosphogypsum is obtained by acid washing, calcination and coating with silicone-acrylic emulsion of phosphogypsum, and the modified zeolite powder is prepared by acid washing, calcination and loading of Mg / Fe bimetallic treatment of natural clinoptilolite.

[0019] In the preparation of activated phosphogypsum, the phosphogypsum is first acid-washed with citric acid solution to specifically dissolve and remove soluble impurities such as phosphorus and fluorine, reducing interference with subsequent performance. Then, the acid-washed phosphogypsum is mixed with red mud in a 9:1 ratio, and 2wt% metakaolin is added. After calcination at 150℃ for 1.5h, the activity of the phosphogypsum is enhanced. Finally, the calcined product is coated with silicone-acrylic emulsion to form a dense hydrophobic film layer, which reduces the sulfate dissolution rate and enhances water resistance.

[0020] The modified zeolite powder is made by first washing natural clinoptilolite with hydrochloric acid to remove impurities and expand its pores, and then calcining and activating it at 550℃ to increase the specific surface area and adsorption sites. Subsequently, it is loaded with Mg / Fe bimetal (molar ratio 8:1). By utilizing the synergistic effect of the bimetal, it can selectively adsorb free sulfate and phosphate ions in the system, reduce the performance deviation caused by ion fluctuations, and work with activated phosphogypsum to ensure the stability of the system.

[0021] As one of the preferred embodiments of the present invention, the modified nano-silica includes a nano-calcium carbonate core and a SiO2 layer coating the surface of the nano-calcium carbonate core, and the lightweight aggregate is a mixture of expanded perlite and closed-cell ceramsite with a volume ratio of (2:1) - (3:1).

[0022] Specifically, the volume ratio of expanded perlite to closed-cell ceramic particles is preferably 2:1, the particle size of the nano-calcium carbonate core is 800nm, and the thickness of the SiO2 layer is 200nm.

[0023] The core-shell structure of modified nano-silica synergistically enhances the structural strength of concrete mortar after drying and curing. The 800nm ​​nano-calcium carbonate core provides rigid support, enhancing the strength of the particles themselves; the 200nm SiO2 layer on the surface chemically bonds with the hydration products of cement and phosphogypsum in the cementitious system, improving interfacial adhesion. Simultaneously, the nanoscale structure of the core-shell structure fills the micropores of the mortar, reducing structural defects and promoting the uniform distribution of hydration products, thereby improving overall density and mechanical properties.

[0024] The lightweight aggregate is a 2:1 volume ratio of expanded perlite and closed-cell ceramsite. The expanded perlite has a particle size of 1.0 mm, and the closed-cell ceramsite has a particle size of 0.5 mm. The expanded perlite is porous and lightweight, which can reduce the density of the mortar. The closed-cell ceramsite has high strength and low water absorption, which can make up for the insufficient strength of perlite. The two work together to ensure the lightweight characteristics of the mortar and enhance the supporting effect between the aggregates through structural complementarity, reducing the decline in mechanical properties caused by a single lightweight aggregate, and adapting to the balance between lightweight and strength required in complex scenarios.

[0025] As one of the preferred embodiments of the present invention, the core-shell toughening agent is a composite particle of paraffin coated with recycled rubber microparticles, wherein the phase transition temperature of the paraffin is 25-30℃ and the particle size of the recycled rubber microparticles is 0.5-1mm.

[0026] Specifically, the phase transition temperature of paraffin is 25℃, and the particle size of the recycled rubber microparticles is 0.1mm.

[0027] In the core-shell toughening agent, 0.1mm diameter recycled rubber microparticles serve as the core, enhancing the system's toughness and impact resistance. The use of recycled rubber also achieves waste resource utilization. The outer coating of paraffin wax with a phase change temperature of 25℃ enhances compatibility with the mortar matrix—rubber has poor affinity with inorganic cementitious materials, and paraffin wax, as an intermediate layer, reduces interfacial delamination, ensuring effective stress transfer. When the ambient temperature approaches 25℃, the phase change characteristics of paraffin wax can buffer temperature stress through minute volume changes, further reducing the risk of cracking. The two work synergistically, leveraging the core toughening role of rubber while addressing interfacial compatibility issues with paraffin wax, comprehensively improving the crack resistance and durability of the mortar.

[0028] A method for preparing a lightweight, water-resistant phosphogypsum-based cement concrete mortar includes the following steps: S1. Preparation of activated phosphogypsum Phosphogypsum was added to a citric acid solution and stirred at 60°C for 45 min. Then, it was filtered and washed to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum was mixed with red mud in a certain proportion, and 2 wt% metakaolin was added. Then, it was calcined at 150°C for 1.5 h to obtain a calcination intermediate. The calcination intermediate was immersed in a silicone-acrylic emulsion, stirred at 40°C for 30 min, and then dried in a fluidized bed at 120°C to obtain activated phosphogypsum. S2, Preparation of modified zeolite powder Natural clinoptilolite was heated and soaked in HCl solution for 2 hours, then removed and calcined in a muffle furnace for 3 hours to obtain pretreated zeolite. The pretreated zeolite was added to a MgCl2 / FeCl3 mixed solution, and then NaOH precipitant was added dropwise until pH=10. After aging for 4 hours, it was filtered, dried, and calcined to obtain modified zeolite powder. S3, Modified Nano-Silica Nano-CaCO3 was dispersed in sodium silicate solution, and 0.1wt% sodium polyacrylate dispersant was added. The mixture was ultrasonically treated for 30 min to obtain a CaCO3 mixed dispersion. Under 80℃ water bath conditions, 1.5mol / L H2SO4 was added dropwise to the CaCO3 mixed dispersion at a rate of 2ml / min until the pH=8.5. During the dropwise addition, the mixture was stirred at a stirring rate of 3000rpm. Then, the mixture was kept at the temperature and aged for 5 h. After filtration, the product was impregnated in silane coupling agent and then filtered to obtain modified nano-silica. S4. Preparation of core-shell toughening agent: Reclaimed rubber microspheres are vacuum treated at 120℃ for 30 min, immersed in molten paraffin and stirred for 1 h, and cooled and solidified to form paraffin-coated reclaimed rubber microspheres, thus obtaining the core-shell toughening agent; S5. Preparation of phosphogypsum-based cement concrete mortar Place 45 parts activated phosphogypsum, 20 parts sulfur-aluminum cement, 11 parts modified zeolite powder, 6.5 parts magnesium phosphate cement, 35 parts lightweight aggregate, and 7.5 parts modified nano-silica in a high-efficiency mixer and mix for 10-15 minutes until homogeneous. Then add 35 parts of water (70% of the total volume) and stir for 2-3 minutes. Next, add 4 parts of core-shell toughening agent and 0.75 parts of polycarboxylate superplasticizer and continue stirring for 1-2 minutes. Then add the remaining water to adjust the consistency and stir for 3-5 minutes until a homogeneous phosphogypsum-based cement concrete mortar with suitable fluidity is obtained.

[0029] The preparation method of this invention solves the problems of impurity interference, insufficient activity, and poor water resistance by three steps: acid washing to remove impurities from activated phosphogypsum, calcination to reconstruct mineral phases, and silicone-acrylic emulsion coating. The acid washing to expand pores, calcination to activate, and bimetallic loading process of modified zeolite powder gradually enhances its adsorption performance and selectivity.

[0030] The core-shell structure preparation process of modified nano-silica controls the particle size of the nano-calcium carbonate core and the thickness of the SiO2 shell, enabling it to both fill pores and enhance interfacial bonding. The paraffin coating process for the core-shell toughening agent solves the compatibility problem between recycled rubber and the inorganic matrix. Furthermore, using industrial waste such as phosphogypsum and recycled rubber as raw materials achieves resource recycling. In addition, the step-by-step feeding and gradient water addition process for mortar preparation ensures uniform material mixing, reduces performance fluctuations, and balances production efficiency with product stability.

[0031] As one of the preferred embodiments of the present invention, in step S1, the preparation method of the activated phosphogypsum is as follows: phosphogypsum is crushed to a particle size ≤2mm, and a solution containing 1.5wt% citric acid is added at a solid-liquid ratio of 1:2. The mixture is stirred at 60°C for 45 minutes to dissolve soluble phosphorus and fluorine impurities. Then, it is vacuum filtered, and the filter residue is washed twice with weak acid water at pH=6 to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum is mixed with Bayer red mud at a mass ratio of 9:1, and 2wt% metakaolin is added and mixed evenly. Then, it is calcined at 150°C for 1.5 hours to obtain a calcined intermediate. The calcined intermediate is immersed in 20% silicone-acrylic emulsion at a solid-liquid ratio of 1:3, stirred at 40°C for 30 minutes, and then dried in a fluidized bed at 120°C to obtain activated phosphogypsum.

[0032] In step S1, the phosphogypsum is first crushed to ≤2mm to increase the specific surface area for easier subsequent processing; a 1.5wt% citric acid solution is stirred at 60℃ for 45min to selectively dissolve soluble phosphorus and fluorine impurities using the weak acidity of citric acid (avoiding strong acid washing from damaging the phosphogypsum structure). A solid-liquid ratio of 1:2 ensures that the impurities are fully dissolved. Vacuum filtration and washing with pH=6 weak acid water remove residual acid and impurities to avoid secondary pollution.

[0033] After acid washing, it is mixed with Bayer red mud (containing aluminum and iron) at a ratio of 9:1, and 2wt% metakaolin (active aluminosilicate) is added. When calcined at 150℃ for 1.5h, the incorporation of red mud and metakaolin provides an aluminum source for the formation of ettringite, which enhances the reactivity of phosphogypsum. Finally, it is coated with 20% silicone-acrylic emulsion at a solid-liquid ratio of 1:3, stirred at 40℃ to ensure uniform emulsion adhesion, and dried in a fluidized bed at 120℃ to form a dense hydrophobic film layer, which not only inhibits sulfate dissolution but also enhances water resistance, thereby achieving the purification, activation and stability improvement of phosphogypsum.

[0034] As one of the preferred embodiments of the present invention, in step S2, the specific preparation method of the modified zeolite powder is as follows: natural clinoptilolite is crushed to 100 mesh, then soaked in 2 mol / L HCl solution at 60°C for 2 h with a solid-liquid ratio of 1:5, then filtered, washed until neutral, and then placed in a muffle furnace at 550°C for 3 h to obtain pretreated zeolite. 1.5 mol / L MgCl2 and 0.2 mol / L FeCl3 were mixed at a molar ratio of Mg:Fe = 8:1 to obtain a MgCl2 / FeCl3 mixed solution. Pretreated zeolite was added to the MgCl2 / FeCl3 mixed solution, and then 3 mol / L NaOH precipitant was slowly added dropwise until the pH reached 10. After aging for 4 hours, the solution was filtered to obtain a composite precipitate. The composite precipitate was washed, dried, and finally calcined at 600℃ for 2 hours to obtain modified zeolite powder.

[0035] The modified zeolite powder was prepared by enhancing its adsorption performance through gradient treatment. First, clinoptilolite was pulverized to 100 mesh to increase the specific surface area and improve the reaction contact efficiency. Then, it was soaked in 2 mol / L HCl at 60℃ for 2 h (solid-liquid ratio 1:5) to dissolve soluble impurities (such as calcium and magnesium salts) in the zeolite using the acidity of hydrochloric acid. At the same time, the surface was etched to form micropores, achieving pore expansion and preliminary purification. The powder was then washed until neutral to avoid residual acid interfering with subsequent reactions.

[0036] Calcination at 550℃ for 3 hours removes moisture and organic matter from the zeolite pores, further expanding the pore size and enhancing surface activity, forming pretreated zeolite. Subsequently, a Mg / Fe bimetallic solution is introduced: a MgCl2 / FeCl3 solution mixed in an 8:1 molar ratio. When the pH is adjusted to 10 with NaOH, Mg(OH)2 and Fe(OH)3 co-precipitate, uniformly loading onto the zeolite pores and surface. Aging for 4 hours promotes stable adhesion of the precipitate, and calcination at 600℃ for 2 hours converts hydroxides into oxides, enhancing the binding force between the bimetallic solution and the zeolite, forming stable adsorption sites that can selectively capture free sulfate and phosphate ions in the system, thus improving system stability.

[0037] As one of the preferred embodiments of the present invention, in step S3, the specific preparation method of the modified nano-silica is as follows: nano-CaCO3 is dispersed in a 15% sodium silicate solution, the solid-liquid ratio of nano-CaCO3 to 15% sodium silicate solution is 1:5, and then 0.1wt% sodium polyacrylate dispersant is added. The mixture is ultrasonically treated for 30 min to obtain a CaCO3 mixed dispersion. Under the condition of 80℃ water bath, 1.5mol / L H2SO4 is added dropwise to the CaCO3 mixed dispersion at a rate of 2ml / min until pH=8.5. During the dropwise addition, the mixture is stirred at a rate of 3000rpm. Then, it is kept warm and aged for 5h, filtered, and the product is placed in 3wt% γ-aminopropyltriethoxysilane and reacted at 70℃ for 1h. Then, the mixture is filtered, washed, and dried to obtain modified nano-silica.

[0038] In step S3, nano-CaCO3 is first dispersed in a 15% sodium silicate solution (solid-liquid ratio 1:5), and 0.1wt% sodium polyacrylate is added to prevent agglomeration through electrostatic repulsion. The mixture is then sonicated for 30 minutes to further ensure uniform dispersion and provide a stable substrate for subsequent coating.

[0039] Under an 80℃ water bath, 1.5mol / L H2SO4 was added dropwise at a rate of 2ml / min until the pH reached 8.5. The mixture was stirred at 3000rpm to gradually hydrolyze sodium silicate to generate SiO2, which uniformly coated the surface of nano-CaCO3 (forming a core-shell structure with an 800nm ​​core and a 200nm shell), avoiding uneven coating due to local over-reaction. The mixture was then aged for 5 hours to promote the densification and solidification of the SiO2 layer and enhance structural stability.

[0040] Finally, the silane coupling agent was treated with 3wt% γ-aminopropyltriethoxysilane at 70℃ for 1 hour. The organic groups of the silane coupling agent reacted with the hydroxyl groups on the surface of SiO2, introducing active groups and improving its interfacial compatibility with the cementitious materials in the mortar. This allowed the core-shell particles to fill the pores and strengthen the interfacial bonding, thus synergistically enhancing the compactness and mechanical properties of the mortar.

[0041] Example 1: Preparation of a lightweight, water-resistant phosphogypsum-based cement concrete mortar S1. Preparation of activated phosphogypsum Phosphogypsum was crushed to a particle size ≤2mm and added to an aqueous solution containing 1.5wt% citric acid at a solid-liquid ratio of 1:2 (mass ratio). The mixture was stirred at 60℃ for 45min to dissolve and remove soluble phosphorus and fluorine impurities. The mixture was then vacuum filtered, and the filter residue was washed twice with weak acid water at pH=6 to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum was mixed with Bayer red mud at a ratio of 9:1 (mass ratio), and 2wt% (relative to the total mass of acid-washed phosphogypsum and red mud) metakaolin was added. After mixing evenly, the mixture was placed in a calcining furnace and calcined at 150℃ for 1.5h to obtain a calcined intermediate. The calcined intermediate was immersed in a 20% silicone-acrylic emulsion at a solid-liquid ratio of 1:3 (mass ratio) and stirred at 40℃ for 30min to ensure that the emulsion evenly coats the surface of the intermediate. The intermediate was then dried in a fluidized bed at 120℃ to obtain activated phosphogypsum.

[0042] S2, Preparation of modified zeolite powder Natural clinoptilolite was pulverized to 100 mesh and added to a 2 mol / L HCl solution at a solid-liquid ratio of 1:5 (mass ratio). The solution was soaked at 60°C for 2 hours, filtered, and washed with deionized water until neutral to obtain pore-expanded and impurity-removed zeolite. The zeolite was then placed in a muffle furnace and calcined at 550°C for 3 hours to remove moisture and organic matter from the pores, yielding pretreated zeolite. A mixed solution was prepared by mixing 1.5 mol / L MgCl2 solution and 0.2 mol / L FeCl3 solution at a Mg:Fe ratio of 8:1 to obtain a MgCl2 / FeCl3 mixture. The pretreated zeolite was added to this mixture, and 3 mol / L NaOH solution was slowly added dropwise until the system pH reached 10. After aging for 4 hours, the mixture was filtered to obtain a composite precipitate. The precipitate was washed with deionized water until neutral, dried, and then calcined in a muffle furnace at 600°C for 2 hours to obtain modified zeolite powder (supported with Mg / Fe bimetallic oxide).

[0043] S3, Preparation of modified nano-silica Nano-CaCO3 with a particle size of 800 nm was dispersed in a 15% sodium silicate solution at a solid-liquid ratio of 1:5 (mass ratio). 0.1 wt% (relative to the mass of nano-CaCO3) of sodium polyacrylate dispersant was added, and the mixture was ultrasonically treated for 30 min to obtain a uniform CaCO3 mixed dispersion. The dispersion was placed in an 80℃ water bath, and 1.5 mol / L H2SO4 solution was added dropwise at a rate of 2 ml / min while stirring at 3000 rpm until the pH of the system reached 8.5. After stopping the addition, the mixture was kept at this temperature for 5 h to allow the sodium silicate to hydrolyze and generate SiO2, which then uniformly coated the surface of the nano-CaCO3 (forming a core-shell structure of an 800 nm core and a 200 nm SiO2 shell). The core-shell particles were obtained by filtration and placed in a 3 wt% γ-aminopropyltriethoxysilane solution. The mixture was reacted at 70℃ for 1 h, filtered, washed, and dried to obtain modified nano-silica.

[0044] S4. Preparation of core-shell toughening agent Take 0.1 mm diameter recycled rubber microparticles and place them in a vacuum drying oven at 120℃ for 30 min to remove surface impurities and moisture. Add molten paraffin (phase change temperature 25℃) to the rubber microparticles and stir for 1 h to uniformly coat the rubber surface with paraffin. After cooling and solidification, a core-shell toughening agent of paraffin-coated recycled rubber microparticles is obtained.

[0045] S5. Preparation of phosphogypsum-based cement concrete mortar Weigh the raw materials according to the following proportions by weight: 45 parts activated phosphogypsum, 20 parts sulfoaluminate cement, 11 parts modified zeolite powder, 6.5 parts magnesium phosphate cement, 35 parts lightweight aggregate (expanded perlite and closed-cell ceramsite mixed at a volume ratio of 2:1), and 7.5 parts modified nano-silica. Add the above raw materials to a high-efficiency mixer and mix for 12 minutes until uniform. Add 70% (24.5 parts) of the total water (35 parts) and stir for 2.5 minutes. Add 4 parts of core-shell toughening agent and 0.75 parts of polycarboxylate superplasticizer and continue stirring for 1.5 minutes. Add the remaining 30% of water (10.5 parts), adjust the consistency, and stir for 4 minutes to obtain a uniform, water-resistant, lightweight, water-resistant phosphogypsum-based cement concrete mortar with suitable fluidity.

[0046] Comparative Example 1 The difference between Comparative Example 1 and Example 1 is that untreated phosphogypsum (crushed to ≤2mm) was used directly to replace activated phosphogypsum, while the remaining components and processes were the same as in Example 1.

[0047] Comparative Example 2 The difference between Comparative Example 2 and Example 1 is that ordinary zeolite powder (only activated by calcination at 550°C) is used instead of modified zeolite powder, while the other components and processes are the same as in Example 1.

[0048] Comparative Example 3 The difference between Comparative Example 3 and Example 1 is that ordinary nano-SiO2 (without CaCO3 core coating) was used instead of modified nano-silica, and the amount was kept at 7.5 parts. The rest was the same as in Example 1.

[0049] Comparative Example 4 The difference between Comparative Example 4 and Example 1 is that the lightweight aggregate was replaced with single expanded perlite (35 parts, with the same volume as the original compound aggregate), and closed-cell ceramsite was removed. The rest is the same as in Example 1.

[0050] Comparative Example 5 The difference between Comparative Example 5 and Example 1 is that uncoated recycled rubber particles (0.1 mm) were used instead of the core-shell toughening agent, and the amount was kept at 4 parts, while the rest was the same as in Example 1.

[0051] Performance testing The test blocks were cured in accordance with the requirements of GB / T 17669.3-1999 "Determination of Mechanical Properties of Building Gypsum", and the compressive strength of the test blocks was tested at 7d and 28d. The 7-day tensile bond strength was tested in accordance with the specifications of JGJ / T 220-2010 "Technical Specification for Plastering Mortar"; Refer to GB / T 20473-2006 "Building Thermal Insulation Mortar" for testing softening coefficient and volume water absorption rate; Performance tests were conducted on Example 1 and Comparative Examples 1-5 according to the above standards. The test results are shown in the table below:

[0052] The comparison between Example 1 and Comparative Examples 1-5 shows that the activated phosphogypsum treatment process (acid washing, calcination, and silicone-acrylic emulsion coating) is an effective method to improve mortar performance. Comparative Example 1, due to the use of untreated phosphogypsum, has a high sulfate dissolution rate and significant impurity interference. Its 7-day and 28-day compressive strengths are only 58% and 64% of those of Example 1, respectively. Its volume water absorption rate increases to 14.2%, and its softening coefficient decreases to 0.61. This indicates that the activation treatment can effectively improve the activity of phosphogypsum, reduce ion dissolution, and enhance water resistance and strength stability.

[0053] In Comparative Example 2, after replacing ordinary zeolite powder, the 28-day compressive strength decreased by 21%, and the softening coefficient decreased to 0.76. This indicates that it reduces the adverse effects of ion erosion on strength and durability by selectively adsorbing free sulfate and phosphate ions. In Comparative Example 3, after using ordinary nano-SiO2, the 7-day tensile bond strength decreased by 27%, and the 7-day compressive strength decreased by 17%. This indicates that its core-shell structure can enhance the interfacial adhesion with the cementing system, while filling the pores and improving the overall mechanical properties.

[0054] Comparative Example 4, using a single expanded perlite, showed a 30% decrease in compressive strength at 28 days and an increase in volumetric water absorption to 9.8%, demonstrating that compounding can balance lightweighting and structural support, avoiding the problems of insufficient strength and excessive water absorption of single aggregates. Comparative Example 5, using uncoated rubber particles, showed a sharp 33% decrease in tensile bond strength at 7 days, indicating that the paraffin shell can improve the compatibility between rubber and the matrix, reduce interfacial delamination, and ensure a synergistic improvement in toughness and adhesion.

[0055] In summary, this invention, through the synergistic effects of activated phosphogypsum treatment, modified zeolite powder adsorption regulation, core-shell structured nanomaterial reinforcement, lightweight aggregate compounding, and core-shell toughening agent interface optimization, enables the mortar to simultaneously possess lightweight, high water resistance, stable mechanical properties, and good durability, meeting the needs of complex construction scenarios.

[0056] The above description is only a preferred embodiment of the present invention. It should be noted that those skilled in the art can make several improvements and modifications without departing from the principle of the present invention, and these improvements and modifications should also be considered within the scope of protection of the present invention.

Claims

1. A lightweight, water-resistant phosphogypsum-based cement concrete mortar, characterized in that, The raw materials consist of the following components by weight: 40-50 parts activated phosphogypsum, 15-25 parts sulfoaluminate cement, 5-8 parts magnesium phosphate cement, 5-18 parts modified zeolite powder, 30-40 parts lightweight aggregate, 5-10 parts modified nano silica, 3-5 parts core-shell toughening agent, 0.5-1 part polycarboxylate superplasticizer, and 30-40 parts water.

2. The lightweight, water-resistant phosphogypsum-based cement concrete mortar according to claim 1, characterized in that, The activated phosphogypsum is obtained by acid washing, calcination, and coating with silicone-acrylic emulsion. The modified zeolite powder is prepared by acid washing, calcination, and loading of Mg / Fe bimetallic treatment onto natural clinoptilolite.

3. The lightweight, water-resistant phosphogypsum-based cement concrete mortar according to claim 1, characterized in that, The modified nano-silica includes a nano-calcium carbonate core and a SiO2 layer coating the surface of the nano-calcium carbonate core. The lightweight aggregate is a mixture of expanded perlite and closed-cell ceramsite with a volume ratio of (2:1) - (3:1).

4. The lightweight, water-resistant phosphogypsum-based cement concrete mortar according to claim 1, characterized in that, The core-shell toughening agent is a composite particle of paraffin wax coated with reclaimed rubber microparticles, wherein the phase transition temperature of the paraffin wax is 25-30℃ and the particle size of the reclaimed rubber microparticles is 0.5-1mm.

5. A method for preparing lightweight, water-resistant phosphogypsum-based cement concrete mortar as described in any one of claims 1-4, characterized in that, Includes the following steps: S1. Preparation of activated phosphogypsum Phosphogypsum was added to a citric acid solution and stirred at 60°C for 45 min. Then, it was filtered and washed to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum was mixed with red mud in a certain proportion, and 2 wt% metakaolin was added. Then, it was calcined at 150°C for 1.5 h to obtain a calcination intermediate. The calcination intermediate was immersed in a silicone-acrylic emulsion, stirred at 40°C for 30 min, and then dried in a fluidized bed at 120°C to obtain activated phosphogypsum. S2, Preparation of modified zeolite powder Natural clinoptilolite was heated and soaked in HCl solution for 2 hours, then removed and calcined in a muffle furnace for 3 hours to obtain pretreated zeolite. The pretreated zeolite was added to a MgCl2 / FeCl3 mixed solution, and then NaOH precipitant was added dropwise until pH=10. After aging for 4 hours, it was filtered, dried, and calcined to obtain modified zeolite powder. S3, Modified Nano-Silica Nano-CaCO3 was dispersed in a sodium silicate solution, and 0.1 wt% sodium polyacrylate dispersant was added. The mixture was ultrasonically treated for 30 min to obtain a CaCO3 mixed dispersion. Under the condition of 80℃ water bath, 1.5 mol / L H2SO4 was added dropwise to the CaCO3 mixed dispersion at a rate of 2 ml / min until the pH=8.

5. During the dropwise addition, the mixture was stirred at a stirring rate of 3000 rpm. Then, it was kept warm and aged for 5 h, filtered, and the product was impregnated in a silane coupling agent. After filtration, the product was separated by filtration to obtain modified nano-silica. S4. Preparation of core-shell toughening agent: Reclaimed rubber microparticles are vacuum treated at 120℃ for 30 min, immersed in molten paraffin and stirred for 1 h, and cooled and solidified to form paraffin-coated reclaimed rubber microparticles, thus obtaining core-shell toughening agent. S5. Preparation of phosphogypsum-based cement concrete mortar Place 45 parts activated phosphogypsum, 20 parts sulfur-aluminum cement, 11 parts modified zeolite powder, 6.5 parts magnesium phosphate cement, 35 parts lightweight aggregate, and 7.5 parts modified nano-silica in a high-efficiency mixer and mix for 10-15 minutes until homogeneous. Then add 35 parts of water (70% of the total volume) and stir for 2-3 minutes. Next, add 4 parts of core-shell toughening agent and 0.75 parts of polycarboxylate superplasticizer and continue stirring for 1-2 minutes. Then add the remaining water to adjust the consistency and stir for 3-5 minutes until a homogeneous phosphogypsum-based cement concrete mortar with suitable fluidity is obtained.

6. The method for preparing lightweight, water-resistant phosphogypsum-based cement concrete mortar according to claim 1, characterized in that, In step S1, the preparation method of the activated phosphogypsum is as follows: phosphogypsum is crushed to a particle size ≤2mm, and a solution containing 1.5wt% citric acid is added at a solid-liquid ratio of 1:

2. The mixture is stirred at 60℃ for 45min to dissolve soluble phosphorus and fluorine impurities. Then, it is vacuum filtered, and the filter residue is washed twice with weak acid water at pH=6 to obtain acid-washed phosphogypsum. The acid-washed phosphogypsum is mixed with Bayer red mud at a mass ratio of 9:1, and 2wt% metakaolin is added and mixed evenly. Then, it is calcined at 150℃ for 1.5h to obtain a calcined intermediate. The calcined intermediate is immersed in 20% silicone-acrylic emulsion at a solid-liquid ratio of 1:3, stirred at 40℃ for 30min, and then dried in a fluidized bed at 120℃ to obtain activated phosphogypsum.

7. The method for preparing lightweight, water-resistant phosphogypsum-based cement concrete mortar according to claim 1, characterized in that, In step S2, the specific preparation method of the modified zeolite powder is as follows: natural clinoptilolite is crushed to 100 mesh, then soaked in 2 mol / L HCl solution at 60°C for 2 hours with a solid-liquid ratio of 1:5, then filtered, washed until neutral, and then placed in a muffle furnace at 550°C for 3 hours to obtain pretreated zeolite. 1.5 mol / L MgCl2 and 0.2 mol / L FeCl3 were mixed at a molar ratio of Mg:Fe = 8:1 to obtain a MgCl2 / FeCl3 mixed solution. Pretreated zeolite was added to the MgCl2 / FeCl3 mixed solution, and then 3 mol / L NaOH precipitant was slowly added dropwise until the pH reached 10. After aging for 4 hours, the solution was filtered to obtain a composite precipitate. The composite precipitate was washed, dried, and finally calcined at 600℃ for 2 hours to obtain modified zeolite powder.

8. The method for preparing lightweight, water-resistant phosphogypsum-based cement concrete mortar according to claim 1, characterized in that, In step S3, the specific preparation method of the modified nano-silica is as follows: nano-CaCO3 is dispersed in a 15% sodium silicate solution, the solid-liquid ratio of nano-CaCO3 to 15% sodium silicate solution is 1:5, and then 0.1wt% sodium polyacrylate dispersant is added. The mixture is ultrasonically treated for 30 min to obtain a CaCO3 mixed dispersion. Under the condition of 80℃ water bath, 1.5mol / L H2SO4 is added dropwise to the CaCO3 mixed dispersion at a rate of 2ml / min until pH=8.

5. During the dropwise addition, the mixture is stirred at a rate of 3000rpm. Then, it is kept warm and aged for 5h, filtered, and the product is placed in 3wt% γ-aminopropyltriethoxysilane and reacted at 70℃ for 1h. Then, the mixture is filtered, washed, and dried to obtain modified nano-silica.

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