Phosphogypsum-based concrete and preparation method thereof

Abstract

The invention discloses ardealite-based concrete and a preparation method thereof, and belongs to the technical field of building materials. The ardealite-based concrete is prepared from the following raw materials in parts by weight: 40 to 60 parts of ardealite powder, 10 to 15 parts of cement, 8 to 10.5 parts of slag powder, 5 to 7 parts of fly ash, 4 to 6 parts of desulfurized fly ash, 5 to 10 parts of natural sand, 10 to 15 parts of ardealite-based artificial aggregate, 3 to 5 parts of eggshell powder, 1.5 to 3 parts of nano silicon dioxide, 0.5 to 1.5 parts of a water reducing agent, 0.05 to 0.2 part of a retarder, 0.1 to 0.5 part of an exciting agent and 0.3 to 1 part of an antifreeze agent. According to the invention, through process innovation of multi-component collaborative design and gradient aggregate doping, the technical bottlenecks of low strength, poor durability, high environmental protection risk and the like of traditional ardealite concrete are overcome while large-scale resource utilization of ardealite is realized, and a new thought is provided for high-valued utilization of industrial solid wastes.

Description

Technical Field

[0001] The present invention relates to the technical field of building materials, and particularly to a phosphogypsum-based concrete and a preparation method thereof. Background Art

[0002] As an acidic solid waste generated during the production of wet-process phosphoric acid, the environmental problems and resource utilization dilemmas of phosphogypsum have become a global challenge. For every 1 ton of phosphoric acid produced, approximately 4.5 - 5 tons of phosphogypsum are by-produced. The accumulation of such a large amount of industrial by-products has brought serious ecological pressure. According to the statistical data of the International Fertilizer Industry Association (IFA) in 2022, the global annual emissions of phosphogypsum have exceeded 280 million tons, and the comprehensive utilization rate has long hovered below 15%. As the world's largest phosphate fertilizer producer, China has a phosphogypsum stockpile of over 800 million tons, with an annual new increment of 75 million tons, forming a giant slag yard covering an area of over 20,000 hectares. The open-piled phosphogypsum not only occupies a large amount of land resources, but the soluble phosphorus (0.5% - 2.5%), fluorides (0.3% - 1.8%) and trace heavy metals (such as As, Cd, Pb, etc.) contained in it have caused the pH value of the surrounding soil to drop to 3.5 - 4.5 and the fluoride ion concentration in the water body to exceed the standard by 10 - 50 times under the action of rain leaching, posing a serious threat to the safety of the ecosystem. In traditional disposal methods, the agricultural application of phosphogypsum (such as soil conditioner) is limited by its acidic characteristics and heavy metal migration risks. The (EU) 2019 / 1009 regulation promulgated by the European Union in 2019 has strictly restricted its agricultural use ratio; landfill disposal is facing increasingly strict environmental protection standards. The new regulation of the US EPA in 2021 requires that the anti-seepage coefficient of phosphogypsum landfills should reach 1×10 -12 cm / s, significantly increasing the disposal cost. In this context, the utilization of phosphogypsum in building materials has become the most promising technical path due to its potential for large-scale consumption, but there are still key bottlenecks in the existing technical system that need to be broken through.

[0003] In the development process of the technology for the building material utilization of phosphogypsum, early research mainly focused on the application in basic building materials. For example, simple calcination dehydration process was used to prepare plaster for rendering or gypsum board. However, such products had significant defects: in terms of mechanical properties, the 28-day compressive strength of pure phosphogypsum building materials was generally lower than 10 MPa, and there was obvious volume deformation (drying shrinkage rate > 0.15%); in terms of water resistance, the softening coefficient was usually < 0.4, and the strength loss rate exceeded 60% after 24 hours of water immersion; in terms of construction performance, the initial setting time fluctuated between 8 and 25 minutes, unable to meet the requirements of conventional construction. To improve the performance, researchers began to explore the technological innovation of composite cementitious systems. Among them, the silicate cement composite route coated phosphogypsum crystals with the C-S-H gel formed by the hydration products of cement. Although the strength could be improved when the cement addition amount was > 30%, it led to an increase in material cost by more than 40%, and the composite system was prone to sulfoaluminate erosion reaction, resulting in a reverse shrinkage of the later strength (the 90-day strength attenuation was 15% - 20%). Another technical route was the alkali-activated cementitious system, which used NaOH-sodium silicate to activate the activity of slag. Although the early strength could be increased to > 20 MPa at 3 days, the strong alkaline environment aggravated the leaching risk of soluble phosphorus and fluorine, and the heavy metal leaching concentration of the product exceeded the standard by 2 - 3 times. The organic-inorganic composite modification technology enhanced the toughness by introducing polyvinyl alcohol (PVA), polypropylene fiber, etc., but the effect could only be shown when 5% - 8% of organic components were added. This not only significantly increased the cost but also led to a decline in the fire resistance of the material. Although some progress has been made in these technical routes, none of them have fundamentally solved the performance defects of phosphogypsum building materials.

[0004] The limitations of pretreatment technologies further restricted the efficient utilization of phosphogypsum. Existing pretreatment methods mainly focused on physical modification and chemical purification. For example, the water washing method needed to consume 3 - 5 m 3 / t of clean water and produced high-concentration acidic wastewater (pH = 1.5 - 2.5), and the treatment cost was as high as 80 - 120 yuan / ton; in the dynamic calcination process, due to the poor temperature control accuracy of the traditional rotary kiln (±25 °C), the conversion rate of hemihydrate gypsum was less than 70%, and overburning and dead burning were likely to occur; the α-hemihydrate gypsum prepared by the crystal form control technology required high-pressure steam conditions (0.3 MPa, 150 °C), and the equipment investment intensity exceeded 5000 yuan per ton of production capacity. These technical bottlenecks made the pretreatment cost of phosphogypsum remain high, seriously weakening its market competitiveness as a building material.

[0005] The industrial application of phosphogypsum-based concrete still faces three core challenges. First is the stability defect of the cementitious system: soluble phosphorus (CaHPO4·2H2O) and fluorine impurities (CaF2) in phosphogypsum form an insoluble calcium phosphate salt coating layer during the hydration process. Experiments show that when the soluble phosphorus content > 0.8%, the 90-day hydration degree of the cementitious system drops to below 45%, seriously hindering the later hydration reaction process. Second is the contradiction between mechanical properties and durability: although increasing the dosage of portland cement can improve the early strength, the hydration product Ca(OH)2 of cement reacts with SO4 in phosphogypsum 2- to generate expansive ettringite (AFt), resulting in a volume expansion of > 0.15% in the concrete in a humid environment, accelerating the structural cracking. Finally is the environmental safety risk: heavy metals in phosphogypsum may be reactivated during the long-term hydration process due to the decrease in the system pH value (from 11.5 to 8.2). The research by Tsinghua University in 2021 shows that under the condition of simulated acid rain (pH = 4.3), the leaching concentration of Cd in traditional phosphogypsum concrete can reach 0.15 mg / L, exceeding the standard limit of GB 5085.3-2007 "Identification Standard for Hazardous Wastes - Identification for Leaching Toxicity" by 3 times.

[0006] In recent years, related patented technologies have tried to break through these limitations, but all have obvious defects. The invention patent application with the application number CN202111226043.8 discloses a concrete product containing phosphogypsum fly ash cement and its preparation method, adopting a ternary system of phosphogypsum - fly ash - cement. Although the strength is improved, the problem of soluble phosphorus inhibiting hydration is not solved; the invention patent application with the application number CN202310604442.6 discloses a high-amount phosphogypsum cement clinker and its preparation method. By introducing steel slag fine powder to improve the activity of phosphogypsum, the volume stability is unqualified due to the f-CaO content > 5% in the steel slag; the invention patent with the application number CN201910787918.8 discloses a preparation method of an organic coating material for phosphogypsum crystals. Through physical agitation coating and spray granulation for surface coating modification, although it is beneficial to the reuse of phosphogypsum, the cost increases by 35%. None of these technical routes can achieve the coordinated improvement of strength, durability, and environmental safety under the condition of high phosphogypsum content (> 40%). The deficiencies of the existing technical system highlight the urgency of innovation in this field: there is an urgent need to develop a new type of concrete material that can not only consume phosphogypsum on a large scale but also meet the engineering performance requirements and be environmentally friendly. Summary of the Invention

[0007] The main purpose of the present invention is to provide a phosphogypsum-based concrete and its preparation method, to solve the problems of low strength and poor durability in the existing technology, and to realize the efficient resource utilization of phosphogypsum.

[0008] To solve the above technical problems, the technical solution adopted by the present invention is as follows: A phosphogypsum-based concrete is composed of the following raw materials in parts by weight: 40-60 parts of phosphogypsum powder, 10-15 parts of cement, 8-10.5 parts of slag powder, 5-7 parts of fly ash, 4-6 parts of desulfurized ash, 5-10 parts of natural sand, 10-15 parts of phosphogypsum-based artificial aggregate, 3-5 parts of eggshell powder, 1.5-3 parts of nano-silica, 0.5-1.5 parts of water reducing agent, 0.05-0.2 parts of retarder, 0.1-0.5 parts of activator, and 0.3-1 part of antifreeze agent.

[0009] In the preferred solution, the phosphogypsum is dihydrate phosphogypsum, and the content of dihydrate phosphogypsum is not less than 90%.

[0010] In the preferred solution, the particle size of the natural sand is 2-5 mm.

[0011] In the preferred solution, the phosphogypsum-based artificial aggregate is prepared by the following steps: S1. Crush the phosphogypsum to a particle size less than 5 mm, grind the slag to a specific surface area of 400-500 m 2 / kg, and prepare a citric acid solution with a mass fraction of 5%-10%; S2. Mix the phosphogypsum in step S1 and the slag in step S1 according to a mass ratio of 2.5-3.5:1, add the citric acid solution in step S1, and mix evenly to obtain a mixture; wherein, the addition amount of the citric acid solution is 0.5%-1.5% of the total mass of the phosphogypsum and the slag; S3. Place the mixture in step S2 into a disk granulator for granulation. When granulating, evenly spray a polyvinyl alcohol solution with a mass fraction of 3%-5% into the granulator at a spraying speed of 5-8 L / min, and make the mixture roll in the disk to obtain particles with a particle size of 2-10 mm; S4. Place the particles with a particle size of 2-10 mm prepared in step S3 in a standard curing room at a temperature of 20±2 °C and a relative humidity of ≥90% for curing. For 1-3 days, use film covering for moisture preservation to avoid excessive evaporation of water. After 3 days, carry out sprinkler curing, sprinkle water 3-4 times a day, and continuously cure for 28 days to obtain the phosphogypsum-based artificial aggregate.

[0012] In a further preferred solution, the phosphogypsum-based artificial aggregate is a graded phosphogypsum-based artificial aggregate, and the mass ratio of the phosphogypsum-based artificial aggregate with a particle size of 2-5 mm to the phosphogypsum-based artificial aggregate with a particle size of 5-10 mm is 3-4:6-7.

[0013] In the preferred solution, the eggshell powder is obtained by removing the eggshell inner membrane from the eggshell, calcining it at 600-800 °C for 1-2 h, and then ball milling it to D 50It is obtained by = 5 μm. The eggshell powder has a smooth surface and is weakly alkaline, which can reduce the friction between the particles of the gelling material and improve the fluidity of the paste. At the same time, after grinding, the eggshell powder can effectively fill the pores between the cement particles and phosphogypsum in the concrete, reduce the porosity and improve the density. In addition, the main component of the eggshell powder is calcium carbonate, which is weakly alkaline and helps to solidify the phosphate ions and fluoride ions in the phosphogypsum to form insoluble salts, which is more conducive to environmental protection.

[0014] In a preferred embodiment, the water reducing agent is a polycarboxylate water reducing agent.

[0015] In a preferred embodiment, the setting retarder is composed of sodium gluconate and trimethyl phosphate in a mass ratio of 1:0.2 - 0.5. Sodium gluconate mainly delays the early crystallization of gypsum, while trimethyl phosphate effectively inhibits the formation of ettringite in the middle and late stages. The synergistic effect of the two covers the whole hydration process. This combination ratio can not only ensure an initial setting time of 120 - 150 min to meet the construction requirements, but also maintain a 28-day strength retention rate of the concrete exceeding 95%.

[0016] In a preferred embodiment, the activator is composed of sodium sulfate and lime mixed in a mass ratio of 1:1.1 - 1.5. Sodium sulfate releases SO4 2- , reacts with Al2O3 in the slag to form ettringite (AFt), improving the early strength; lime increases the pH value of the system, accelerates the dissociation of the slag vitreous body, and at the same time provides Ca 2+ .

[0017] In a preferred embodiment, the antifreeze agent is composed of calcium nitrate and sodium dodecyl sulfate in a mass ratio of 1:0.5 - 0.8. It significantly improves the frost resistance of the concrete through a dual action mechanism. Calcium nitrate, as an inorganic salt antifreeze agent, can lower the liquid phase freezing point below -15 °C and promote the early formation of ettringite, enhancing the density of the concrete; sodium dodecyl sulfate, as an air-entraining agent, can introduce uniform closed microbubbles with a diameter of 20 - 50 μm in the paste, effectively buffering the frost heaving stress. The synergistic effect of the two makes the mass loss rate of the concrete less than 2% after 150 freeze-thaw cycles in a -20 °C environment, and the dynamic modulus retention rate exceeds 90%. This ratio takes into account both the frost resistance performance and the strength development. The 28-day compressive strength is increased by more than 15% compared with the single air-entraining agent system, and the problem of efflorescence caused by excessive calcium nitrate is avoided.

[0018] The preparation method of the phosphogypsum-based concrete includes the following steps: P1. Place the phosphogypsum powder, cement, slag powder, fly ash and desulfurized ash in a forced mixer for dry mixing until the uniformity > 98%, add nano-silica and continue mixing with the dry mixture to make it uniformly dispersed in the gelling phase; P2. Dissolve the retarder, water reducer and activator in water to obtain a mixed solution. Inject the mixed solution into a forced mixer and increase the rotation speed, and stir until the fluidity of the slurry reaches 160 - 200 mm; P3. Gradient aggregate incorporation. In the first stage, add 30% of natural sand and 20% of phosphogypsum-based artificial aggregate into the forced mixer described in step P2, and stir for 90 - 150 s; In the second stage, add the remaining natural sand, 50% of phosphogypsum-based artificial aggregate and the aqueous solution of antifreeze agent, and stir for 90 - 150 s; In the third stage, add the remaining phosphogypsum-based artificial aggregate and eggshell powder, and stir for 150 - 210 s; P4. Vibration molding and curing to obtain the phosphogypsum-based concrete.

[0019] In the preferred embodiment, the rotation speed of the stirring in step P1 is 40 - 50 rpm.

[0020] In the preferred embodiment, the rotation speed of the stirring in step P2 is 55 - 65 rpm.

[0021] In the preferred embodiment, the rotation speed of the stirring in the first and second stages of step P3 is 55 - 65 rpm, and the rotation speed in the third stage is 30 - 40 rpm.

[0022] In the preferred embodiment, the frequency of the vibration molding in step P4 is 30 - 40 Hz, the amplitude is 0.5 mm, and the duration is 25 - 35 s / m 3 。

[0023] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. Through the synergistic filling effect of the gradation of phosphogypsum-based artificial aggregate, nano-silica and eggshell powder, the 28-day compressive strength of the present invention reaches 53.5 - 58.8 MPa, which is more than 40% higher than that of traditional phosphogypsum concrete (usually <40 MPa).

[0024] 2. In the present invention, the micro-filling effect of nano-silica and eggshell powder reduces the porosity to 8.3% - 9.7%, which is about 50% less than that of traditional concrete (15% - 20%), significantly improving the density and impermeability.

[0025] 3. The synergistic curing effect of eggshell powder and activator on the phosphate radical, fluoride ion and heavy metal in phosphogypsum is good, reducing environmental pollution.

[0026] 4. Through the process innovation of multi-component collaborative design and gradient aggregate incorporation, the present invention realizes the large-scale resource utilization of phosphogypsum while overcoming the technical bottlenecks of traditional phosphogypsum concrete such as low strength, poor durability and high environmental protection risks, providing a new idea for the high-value utilization of industrial solid waste. Specific Embodiments

[0027] The technical solutions of the present invention will be further described and illustrated below through embodiments. The raw materials used in the embodiments can all be purchased commercially or prepared by conventional methods.

[0028] In the embodiments of the present invention, the phosphogypsum used is dihydrate phosphogypsum, and the content of dihydrate phosphogypsum is not less than 90%. The phosphogypsum-based artificial aggregate used is prepared by the following steps: S1. Crush the phosphogypsum to a particle size of 3 mm, grind the slag to a specific surface area of 450 m 2 / kg, and prepare a citric acid solution with a mass fraction of 8%; S2. Mix the phosphogypsum in step S1 and the slag in step S1 according to a mass ratio of 3:1, add the citric acid solution in step S1, and mix evenly to obtain a mixture; wherein, the addition amount of the citric acid solution is 1% of the total mass of the phosphogypsum and the slag; S3. Place the mixture in step S2 in a disk granulator for granulation. When granulating, evenly spray a polyvinyl alcohol solution with a mass fraction of 4% into the granulator at a spraying speed of 6 L / min, and make the mixture roll in the disk to obtain particles with a particle size of 2 - 10 mm; S4. Place the particles with a particle size of 2 - 10 mm prepared in step S3 in a standard curing room at a temperature of 20 ± 2 °C and a relative humidity of ≥90% for curing. For 1 - 3 days, use film covering for moisture preservation to avoid excessive evaporation of water. After 3 days, carry out watering curing, watering 3 - 4 times a day, and continuously cure for 28 days to obtain the phosphogypsum-based artificial aggregate. The eggshell powder used is obtained by removing the eggshell inner membrane from the eggshell, calcining it at 700 °C for 1.5 h, and then ball milling it to D 50 = 5 μm.

[0029] Example 1 A phosphogypsum-based concrete is composed of the following raw materials in parts by weight: 55 parts of phosphogypsum powder, 12 parts of P·O 42.5 ordinary Portland cement, 9 parts of S95-grade slag powder, 6 parts of class II fly ash, 5 parts of desulfurized ash, 8 parts of natural sand with a particle size of 2 - 5 mm, 13 parts of phosphogypsum-based artificial aggregate (where the mass ratio of the phosphogypsum-based artificial aggregate with a particle size of 2 - 5 mm to the phosphogypsum-based artificial aggregate with a particle size of 5 - 10 mm is 3.5:6.5), 4 parts of eggshell powder, 2.5 parts of nano-silica, 1.2 parts of polycarboxylate water reducer, 0.18 parts of retarder (the mass ratio of sodium gluconate to trimethyl phosphate is 1:0.3), 0.35 parts of activator (the mass ratio of sodium sulfate to lime is 1:1.2), and 0.8 parts of antifreeze (the mass ratio of calcium nitrate to sodium dodecyl sulfate is 1:0.6).

[0030] The preparation method of the phosphogypsum-based concrete includes the following steps: P1. Place phosphogypsum powder, cement, slag powder, fly ash and desulfurized ash in a forced mixer for dry mixing. Stir at a speed of 45 rpm until the uniformity is >98%. Add nano-silica and continue to stir the dry mixture to make it evenly dispersed in the gelling phase; P2. Dissolve the retarder, water reducer and activator in water (water-binder ratio is 0.32) to prepare a mixed solution. Inject the mixed solution into the forced mixer and increase the speed to 60 rpm. Stir until the fluidity of the slurry reaches 180 mm; P3. Gradually incorporate the aggregate. In the first stage, add 30% of natural sand and 20% of phosphogypsum-based artificial aggregate into the forced mixer described in step P2, and stir at a speed of 60 rpm for 120 s. In the second stage, add the remaining natural sand, 50% of phosphogypsum-based artificial aggregate and the aqueous solution of antifreeze agent, and stir at a speed of 60 rpm for 120 s. In the third stage, add the remaining phosphogypsum-based artificial aggregate and eggshell powder, and stir at a speed of 30 rpm for 180 s; P4. Vibration molding, the molding frequency is 35 Hz, the amplitude is 0.5 mm, and the duration is 30 s / m 3 , cure for 28 days to obtain the phosphogypsum-based concrete.

[0031] The performance test results are shown in Table 1.

[0032] Table 1:

[0033] Example 2 A phosphogypsum-based concrete is composed of the following raw materials in parts by weight: 50 parts of phosphogypsum powder, 14 parts of P·O 42.5 ordinary Portland cement, 9 parts of S95 grade slag powder, 6 parts of grade II fly ash, 5 parts of desulfurized ash, 8 parts of natural sand with a particle size of 2 - 5 mm, 13 parts of phosphogypsum-based artificial aggregate (where the mass ratio of phosphogypsum-based artificial aggregate with a particle size of 2 - 5 mm to phosphogypsum-based artificial aggregate with a particle size of 5 - 10 mm is 4:6), 4 parts of eggshell powder, 2.5 parts of nano-silica, 1.2 parts of polycarboxylate water reducer, 0.18 parts of retarder (the mass ratio of sodium gluconate to trimethyl phosphate is 1:0.3), 0.4 parts of activator (the mass ratio of sodium sulfate to lime is 1:1.4), 0.8 parts of antifreeze agent (the mass ratio of calcium nitrate to sodium dodecyl sulfate is 1:0.4).

[0034] The preparation method of the phosphogypsum-based concrete includes the following steps: P1. Place phosphogypsum powder, cement, slag powder, fly ash and desulfurized ash in a forced mixer for dry mixing. Stir at a speed of 44 rpm until the uniformity is >98%. Add nano-silica and continue to stir the dry mixture to make it evenly dispersed in the gelling phase; P2. Dissolve the retarder, water reducer and activator in water (water-binder ratio is 0.32) to prepare a mixed solution. Inject the mixed solution into a forced mixer and increase the rotation speed to 60 rpm, and stir until the fluidity of the slurry reaches 180 mm; P3. Gradually incorporate the aggregates. In the first stage, add 30% of natural sand and 20% of phosphogypsum-based artificial aggregates into the forced mixer described in step P2, and stir at a rotation speed of 60 rpm for 150 s; in the second stage, add the remaining natural sand, 50% of phosphogypsum-based artificial aggregates and the aqueous solution of antifreeze agent, and stir at a rotation speed of 60 rpm for 150 s; in the third stage, add the remaining phosphogypsum-based artificial aggregates and eggshell powder, and stir at a rotation speed of 25 - 35 rpm for 210 s; P4. Vibration molding, the molding frequency is 35 Hz, the amplitude is 0.5 mm, and the duration is 30 s / m 3 , cure for 28 days to obtain the phosphogypsum-based concrete described.

[0035] The performance test results are shown in Table 2.

[0036] Table 2:

[0037] Example 3 A phosphogypsum-based concrete is composed of the following raw materials in parts by weight: 60 parts of phosphogypsum powder, 12 parts of P·O 42.5 ordinary Portland cement, 9 parts of S95-grade slag powder, 6 parts of Class II fly ash, 5 parts of desulfurized ash, 5 parts of natural sand with a particle size of 2 - 5 mm, 13 parts of phosphogypsum-based artificial aggregates (the mass ratio of phosphogypsum-based artificial aggregates with a particle size of 2 - 5 mm to phosphogypsum-based artificial aggregates with a particle size of 5 - 10 mm is 3.5:6.5), 4 parts of eggshell powder, 2.5 parts of nano-silica, 1.2 parts of polycarboxylate water reducer, 0.18 parts of retarder (the mass ratio of sodium gluconate to trimethyl phosphate is 1:0.3), 0.2 parts of activator (the mass ratio of sodium sulfate to lime is 1:1.1), 0.8 parts of antifreeze agent (the mass ratio of calcium nitrate to sodium dodecyl sulfate is 1:0.8).

[0038] The preparation method of the phosphogypsum-based concrete described includes the following steps: P1. Place the phosphogypsum powder, cement, slag powder, fly ash and desulfurized ash in a forced mixer for dry mixing, stir at a rotation speed of 45 rpm until the uniformity > 98%, add nano-silica and continue to stir the dry mixture to make it uniformly disperse in the gelling phase; P2. Dissolve the retarder, water reducer and activator in water (water-binder ratio is 0.32) to obtain a mixed solution. Inject the mixed solution into a forced mixer and increase the rotation speed to 60 rpm. Stir until the fluidity of the paste reaches 160 - 200 mm; P3. Gradually incorporate the aggregates. In the first stage, add 30% of natural sand and 20% of phosphogypsum-based artificial aggregates into the forced mixer described in step P2, and stir at a rotation speed of 60 rpm for 120 s. In the second stage, add the remaining natural sand, 50% of phosphogypsum-based artificial aggregates and the aqueous solution of antifreeze, and stir at a rotation speed of 60 rpm for 120 s. In the third stage, add the remaining phosphogypsum-based artificial aggregates and eggshell powder, and stir at a rotation speed of 30 rpm for 180 s; P4. Vibration molding, the molding frequency is 35 Hz, the amplitude is 0.5 mm, and the duration is 30 s / m 3 , cure for 28 days to obtain the phosphogypsum-based concrete described.

[0039] The performance test results are shown in Table 3.

[0040] Table 3:

[0041] It can be seen from Tables 1 - 3 that the initial setting times of all examples meet the construction requirements of 120 - 150 min. Compared with Example 1, in Example 2, due to the adjustment of the retarder ratio (the mass ratio of sodium gluconate to trimethyl phosphate is adjusted from 1:0.3 to 1:0.4), the initial setting time is the longest, reaching 142 min. In Example 3, due to the increase in the phosphogypsum content (from 55 parts to 60 parts), the initial setting time is shortened. The strength of Example 1 increases the fastest (the 3-day strength reaches 51.6% of the 28-day strength), reflecting the early synergistic effect of nano-silica and the activator. While in Example 3, due to the high phosphogypsum content, the early strength is lower, but the 28-day strength still exceeds 53 MPa. The chloride ion diffusion coefficient of Example 1 is the lowest, which is due to the pore filling effect of eggshell powder and nano-silica. From the leaching experiment data, it can be seen that the leaching concentrations of phosphate ions, fluoride ions and heavy metals are all lower than the national standard limits, meeting the environmental protection requirements. This is because the weakly alkaline calcium carbonate in eggshell powder effectively solidifies PO4³ - and F - , forming Ca3(PO4)2 and CaF2 precipitates, and at the same time synergistically with the activator to solidify the heavy metals to form insoluble salts or hydroxides.

[0042] It should be understood that the above examples are only used to illustrate the content of the present invention and not to limit the protection scope of the present invention. In addition, it should be understood that after reading the content taught by the present invention, those skilled in the art can make various changes or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims of this application.

Claims

1. A phosphogypsum-based concrete, characterized in that: The invention is composed of the following raw materials in parts by weight: 40-60 parts of phosphogypsum powder, 10-15 parts of cement, 8-10.5 parts of slag powder, 5-7 parts of fly ash, 4-6 parts of desulfurized ash, 5-10 parts of natural sand, 10-15 parts of phosphogypsum-based artificial aggregate, 3-5 parts of eggshell powder, 1.5-3 parts of nano silicon dioxide, 0.5-1.5 parts of water reducing agent, 0.05-0.2 parts of retarder, 0.1-0.5 parts of activator and 0.3-1 parts of antifreeze agent.

2. The phosphogypsum-based concrete according to claim 1, characterized in that: The phosphogypsum is dihydrate phosphogypsum, and the content of dihydrate phosphogypsum is not less than 90%.

3. The phosphogypsum-based concrete according to claim 1, characterized in that: The retarder is composed of sodium gluconate and trimethyl phosphate in a mass ratio of 1:0.2-0.5; the activator is composed of sodium sulfate and lime in a mass ratio of 1:1.1-1.5; the antifreeze agent is composed of calcium nitrate and sodium dodecyl sulfate in a mass ratio of 1:0.5-0.

8.

4. The phosphogypsum-based concrete according to claim 1, characterized in that: The phosphogypsum-based artificial aggregate is prepared by the following steps: S1. Crushing phosphogypsum to a particle size of less than 5 mm and grinding slag to a specific surface area of ​​400-500 m 2 / kg, prepare citric acid into a citric acid solution with a mass fraction of 5% to 10%; S2, mixing the phosphogypsum in step S1 and the slag in step S1 in a mass ratio of 2.5-3.5:1, adding the citric acid solution in step S1, mixing evenly, and preparing a mixture; wherein the amount of the citric acid solution added is 0.5%-1.5% of the total mass of the phosphogypsum and the slag; S3, placing the mixture in step S2 in a disc granulator for granulation, spraying a polyvinyl alcohol solution with a mass fraction of 3% to 5% evenly into the granulator at a spraying speed of 5 to 8 L / min, so that the mixture rolls in the disc to obtain particles with a particle size of 2 to 10 mm; S4. Place the particles with a particle size of 2-10 mm obtained in step S3 in a standard curing room at a temperature of 20±2°C and a relative humidity of ≥90% for curing. Use film covering for moisturizing curing for 1-3 days to avoid excessive evaporation of water. After 3 days, sprinkle water for curing 3-4 times a day for 28 days to obtain the phosphogypsum-based artificial aggregate.

5. The phosphogypsum-based concrete according to claim 4, characterized in that: The phosphogypsum-based artificial aggregate is a graded phosphogypsum-based artificial aggregate, wherein the mass ratio of the phosphogypsum-based artificial aggregate with a particle size of 2-5 mm to the phosphogypsum-based artificial aggregate with a particle size of 5-10 mm is 3-4:6-7.

6. The phosphogypsum-based concrete according to claim 1, characterized in that: The eggshell powder is obtained by removing the inner membrane of the eggshell from the eggshell, calcining it at 600-800°C for 1-2 hours, and then ball-milling it to D50=5 μm.

7. The phosphogypsum-based concrete according to claim 1, characterized in that: The particle size of the natural sand is 2-5 mm.

8. A method for preparing the phosphogypsum-based concrete according to any one of claims 1 to 7, characterized in that: The following steps are involved: P1. Phosphogypsum powder, cement, slag powder, fly ash and desulfurized ash are placed in a forced mixer for dry mixing, and stirred until the uniformity is > 98%, and nano-silicon dioxide is added to the dry mixture and continued to be stirred to make it uniformly dispersed in the gel phase; P2. Dissolve the retarder, water reducer and activator in water to prepare a mixed solution, inject the mixed solution into a forced mixer, increase the speed and stir until the slurry fluidity reaches 160~200 mm; P3, gradient aggregate incorporation, in the first stage, 30% of natural sand and 20% of phosphogypsum-based artificial aggregate are added to the forced mixer described in step P2, and stirred for 90-150 s; in the second stage, the remaining natural sand and 50% of phosphogypsum-based artificial aggregate and antifreeze aqueous solution are added, and stirred for 90-150 s; in the third stage, the remaining phosphogypsum-based artificial aggregate and eggshell powder are added, and stirred for 150-210 s; P4, vibration molding and curing to obtain the phosphogypsum-based concrete.

9. The method for preparing phosphogypsum-based concrete according to claim 8, characterized in that: The stirring speed in step P1 is 40-50 rpm; the stirring speed in step P2 is 55-65 rpm; the stirring speed in the first and second stages of step P3 is 55-65 rpm, and the stirring speed in the third stage is 30-40 rpm.

10. The method for preparing phosphogypsum-based concrete according to claim 8, characterized in that: The vibration molding described in step P4 has a frequency of 30-40 Hz, an amplitude of 0.5 mm, and a duration of 25-35 s / m 3 .

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