A phosphogypsum-based composite cementitious material and its preparation method

Abstract

This invention relates to the field of cementitious materials technology, and more particularly to a phosphogypsum-based composite cementitious material and its preparation method. The material is prepared using this method. The preparation method includes: mixing a required amount of dihydrate phosphogypsum, an alkaline activator, and water, followed by aging treatment to obtain an aged material; dry mixing the aged material, a required amount of hemihydrate phosphogypsum, and a required amount of slag powder to obtain a mixture; adding a required amount of crystal form modifier, a water-reducing agent, and water to the mixture, and stirring until homogeneous to obtain the phosphogypsum-based composite cementitious material. This invention enables large-scale resource utilization of phosphogypsum, improves early strength, significantly improves the crystal shape of hemihydrate phosphogypsum, increases strength, and solves cracking problems.

Description

Technical Field

[0001] This invention relates to the field of cementitious materials technology, and in particular to a phosphogypsum-based composite cementitious material and its preparation method. Background Technology

[0002] Phosphogypsum is an industrial byproduct of wet-process phosphoric acid production, primarily composed of dihydrate phosphogypsum (molecular formula CaSO4·2H2O). Its annual emissions are enormous, but its comprehensive utilization rate is low. Currently, the main technical routes for the resource utilization of phosphogypsum are as follows: 1) Used as a cement retarder or added in small amounts to cement; specifically, phosphogypsum dihydrate is used directly as a cement retarder or added in small amounts as a blending material to cement. This method of use involves extremely low dosages (typically <5%), making it unsuitable for large-scale disposal of phosphogypsum solid waste. 2) Production of all-solid-waste supersulfate cement; specifically, phosphogypsum dihydrate is used to activate slag powder, which reacts with the active aluminum phase in the slag powder to form ettringite, thereby generating strength. Although the later strength of this cement meets the usage requirements, the acidic impurities in phosphogypsum dihydrate inhibit the hydration reaction of the slag, resulting in low early strength (within 7 days) of the cement, which does not meet the usage requirements, and makes demolding difficult and the construction cycle long.

[0003] 3) High-temperature calcination method: Specifically, dihydrate phosphogypsum is calcined and dehydrated at 160~180℃ to produce β-hemihydrate gypsum, i.e., hemihydrate phosphogypsum, which is used as building plaster. Although the hemihydrate phosphogypsum obtained by this method has high early strength, it has poor water resistance (softening coefficient is usually <0.6). When reacting with water, the hydration rate is too fast, the heat release is concentrated, and a large amount of hemihydrate gypsum is converted into dihydrate phosphogypsum in a short time. Furthermore, the concentrated heat of hydration and high crystallization stress of hemihydrate phosphogypsum lead to volume shrinkage and cracking problems in the later stages. In addition, this method also has the problem of high energy consumption.

[0004] Therefore, it is necessary to provide a phosphogypsum-based composite cementitious material and its preparation method to solve the problems of existing phosphogypsum resource utilization: 1) low dosage, which cannot be used to dispose of phosphogypsum solid waste on a large scale; 2) the low early strength (within 7 days) of the all-solid waste supersulfate cement produced due to the presence of acidic impurities in dihydrate phosphogypsum, which does not meet the requirements for use; 3) poor water resistance of hemihydrate phosphogypsum, excessively fast hydration rate, concentrated hydration heat and high crystallization stress, which easily leads to the problem of volume shrinkage and cracking in the later stage. Summary of the Invention

[0005] The purpose of this invention is to provide a phosphogypsum-based composite cementitious material and its preparation method, the specific technical solution of which is as follows: In a first aspect, the present invention provides a method for preparing a phosphogypsum-based composite cementitious material, comprising: Step S1: Aging treatment; Mix 5-50 parts by weight of phosphogypsum dihydrate, 5-10 parts by weight of alkaline activator, and 5-10 parts by weight of water, and then age the mixture to obtain aged material. Step S2: Dry mixing; The aged material, 5-50 parts by weight of hemihydrate phosphogypsum, and 15-45 parts by weight of slag powder are dry-mixed to obtain a mixture. Step S3: Pulping; Mix 0.5 to 2 parts by weight of crystal modifier, 0.5 to 2 parts by weight of water-reducing agent, and 20 to 40 parts by weight of water, and add them to the mixture. Stir evenly to obtain phosphogypsum-based composite cementitious material. The crystal form modifier includes sodium citrate or potassium aluminum sulfate.

[0006] Optionally, the aging process involves an aging time of 24-72 hours and an aging temperature of 15-25°C.

[0007] Optionally, the alkaline activator includes steel slag powder, carbide slag, or hydrated lime.

[0008] Optionally, the slag powder is granulated blast furnace slag powder of grade S95 or higher.

[0009] Optionally, the water-reducing agent includes a polycarboxylate-based high-performance water-reducing agent.

[0010] Optionally, the mass fraction of the dihydrate phosphogypsum is 9-36 parts; the mass fraction of the hemihydrate phosphogypsum is 9-36 parts.

[0011] In a second aspect, the present invention provides a phosphogypsum-based composite cementitious material, which is prepared by the aforementioned method for preparing phosphogypsum-based composite cementitious materials.

[0012] Optionally, the 3-day flexural strength of the phosphogypsum-based composite cementitious material is not less than 1.89 MPa; the 3-day compressive strength of the phosphogypsum-based composite cementitious material is not less than 3.2 MPa.

[0013] Optionally, the 7-day flexural strength of the phosphogypsum-based composite cementitious material is not less than 4.7 MPa; the 7-day compressive strength of the phosphogypsum-based composite cementitious material is not less than 10.8 MPa.

[0014] Optionally, the 28-day flexural strength of the phosphogypsum-based composite cementitious material is not less than 8.1 MPa; the 28-day compressive strength of the phosphogypsum-based composite cementitious material is not less than 31.8 MPa.

[0015] The application of the technical solution of the present invention has at least the following beneficial effects: (1) The preparation method of the phosphogypsum-based composite cementitious material provided by the present invention has the following effects: On the one hand, it is possible to use phosphogypsum dihydrate to prepare phosphogypsum-based composite cementitious materials with high dosage (greater than or equal to 50%), thereby realizing the large-scale resource utilization of phosphogypsum.

[0016] On the other hand, it can improve the early strength of phosphogypsum-based composite cementitious materials. Specifically, the present invention first uses step S1 to age the required amount of dihydrate phosphogypsum, alkaline activator and water. This aging treatment allows the alkaline activator to consume the acidic impurities in the dihydrate phosphogypsum, thereby relieving the inhibition on the slag hydration reaction and improving the early strength of the phosphogypsum-based composite cementitious materials.

[0017] On the other hand, it can greatly improve the crystal shape of hemihydrate phosphogypsum, increase its strength, and solve the cracking problem. Specifically, the aging treatment used in this invention can also allow the calcium silicate gel (i.e., CSH gel) network generated by the hydration of the alkaline activator to encapsulate the dihydrate phosphogypsum, preventing the dihydrate phosphogypsum from inhibiting the hydration reaction of the hemihydrate phosphogypsum. The hydration reaction of the hemihydrate phosphogypsum helps to improve the early strength of phosphogypsum-based composite cementitious materials. However, considering that hemihydrate phosphogypsum has poor water resistance, a fast hydration rate, concentrated hydration heat, and high crystallization stress, it is prone to later volume shrinkage cracking. This invention uses the acid radical ions in the crystal shape modifier to selectively adsorb on specific crystal faces where the hemihydrate phosphogypsum crystals are actively growing, inhibiting their growth rate, thereby restoring the growth rate of each crystal face to a new equilibrium. Ultimately, this results in the crystal morphology changing from slender needle-like shapes to thicker, shorter, and more uniform short columnar or plate-like shapes. The short columnar or plate-like gypsum crystals have higher packing density and lower porosity, which can significantly improve flexural strength and compressive strength, and avoid later volume shrinkage cracking.

[0018] Furthermore, by encapsulating dihydrate phosphogypsum, it can continuously and slowly react with water and the active aluminum phase in slag materials when exposed to water in the later stages, generating ettringite, which continuously improves the later strength of cementitious materials, enhances their compatibility with the water environment, and increases the softening coefficient from 0.5~0.6 of ordinary gypsum materials to over 0.85, without affecting its strength performance in wet environments.

[0019] (2) The present invention combines dihydrate phosphogypsum and hemihydrate phosphogypsum in the required mass proportions because dihydrate phosphogypsum, as a "phase change heat storage material" and "flexible micro-aggregate", can alleviate the thermal shock and volume shrinkage during the hydration of hemihydrate phosphogypsum, thus helping to solve the cracking problem. In addition, the encapsulation effect of the calcium silicate gel (i.e., CSH gel) network generated by the hydration of the alkaline activator can further improve the strength of the phosphogypsum-based composite cementitious material.

[0020] (3) The present invention can use phosphogypsum dihydrate to prepare phosphogypsum-based composite cementitious materials with high dosage, which not only greatly reduces the preparation cost, but also responds to the economic development needs of environmental protection and energy conservation.

[0021] In addition to the objectives, features, and advantages described above, the present invention has other objectives, features, and advantages. The invention will now be described in further detail with reference to the figures. Attached Figure Description

[0022] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on the structures shown in these drawings without creative effort.

[0023] Figure 1 The image shows a scanning electron microscope image of the phosphogypsum-based composite cementitious material prepared in Example 1 after 28 days of curing.

[0024] Figure 2 Scanning electron microscope image of the phosphogypsum-based composite cementitious material prepared for Comparative Example 2 after 28 days of curing.

[0025] Figure 3 Images of the stone-like material prepared for Comparative Example 1 after 28 days of curing (cracks prevented scanning).

[0026] Figure 4 Images of the stone formed after 28 days of curing of the phosphogypsum-based composite cementitious material prepared in Example 1.

[0027] in, Figures 1-4 The curing conditions are a temperature of 20±2℃, a relative humidity of over 90%, and a curing period of 28 days. Detailed Implementation

[0028] 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, and not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1: A method for preparing a phosphogypsum-based composite cementitious material includes: Step S1: Aging treatment; The mixture of 5-50 parts by weight (specifically 36 parts) of phosphogypsum dihydrate, 5-10 parts by weight (specifically 10 parts) of alkaline activator, and 5-10 parts by weight (specifically 5 parts) of water is subjected to aging treatment to obtain aged material. Step S2: Dry mixing; The aged material, 5-50 parts by weight (specifically 9 parts) of hemihydrate phosphogypsum, and 15-45 parts by weight (specifically 41 parts) of slag powder are dry-mixed to obtain a mixture. Step S3: Pulping; Mix 0.5 to 2 parts by weight (specifically 2 parts) of crystal form modifier, 0.5 to 2 parts by weight (specifically 2 parts) of water reducer, and 20 to 40 parts by weight (specifically 35 parts) of water, and then add them to the mixture. Stir evenly to obtain phosphogypsum-based composite cementitious material. The crystal form modifier includes sodium citrate or potassium aluminum sulfate, specifically potassium aluminum sulfate.

[0030] The aging process involves an aging time of 24-72 hours (specifically 24 hours) and an aging temperature of 15-25°C (specifically 20°C).

[0031] The alkaline activator is carbide slag.

[0032] The slag powder is granulated blast furnace slag powder of grade S95 or above.

[0033] The water-reducing agent is a polycarboxylate-based high-performance water-reducing agent.

[0034] Example 2: Unlike Example 1, the mass fraction of the dihydrate phosphogypsum was reduced to 27 parts, and the mass fraction of the hemihydrate phosphogypsum was increased to 18 parts.

[0035] Example 3: Unlike Example 1, the mass fraction of the dihydrate phosphogypsum was reduced to 18 parts, and the mass fraction of the hemihydrate phosphogypsum was increased to 27 parts.

[0036] Example 4: Unlike Example 1, the mass fraction of the dihydrate phosphogypsum was reduced to 9 parts, and the mass fraction of the hemihydrate phosphogypsum was increased to 36 parts.

[0037] Comparative Example 1: Unlike Example 1, the mass fraction of the hemihydrate phosphogypsum was increased to 45 parts, and the mass fraction of the dihydrate phosphogypsum was zero parts.

[0038] Comparative Example 2: Unlike Example 1, the mass fraction of the dihydrate phosphogypsum was increased to 45 parts, and the mass fraction of the hemihydrate phosphogypsum was zero parts.

[0039] Comparative Example 3: Unlike Example 1, the use of the crystal form modifier was omitted.

[0040] Comparative Example 4: Unlike Example 1, the use of an alkaline activator was omitted.

[0041] The phosphogypsum-based composite cementitious materials prepared in Examples 1-4 and Comparative Examples 1-4 were sampled and tested for flexural strength and compressive strength. The test results are shown in Table 1. Both flexural strength and compressive strength tests were conducted according to the test procedures in Section 10 of GB / T 17671—2021 Cement Mortar Strength Test Method (ISO Method). The curing conditions for 3d, 7d, and 28d curing periods were a curing temperature of 20±2℃ and a relative humidity of over 90%.

[0042] Table 1 Test Results From the data in Table 1, we know that: Compared to Comparative Examples 1-4, this invention uses a combination of dihydrate phosphogypsum, hemihydrate phosphogypsum, crystal form modifier, and alkaline activator in Examples 1-4. This not only ensures that the prepared phosphogypsum-based composite cementitious material has suitable early and late flexural and compressive strengths, but also solves the cracking problem. The reasons are as follows: On the one hand, the present invention first uses step S1 to age the required amount of dihydrate phosphogypsum, alkaline activator and water. This aging process enables the alkaline activator to consume the acidic impurities in the dihydrate phosphogypsum, thereby relieving the inhibition on the slag hydration reaction and improving the early strength of the phosphogypsum-based composite cementitious material.

[0043] On the other hand, the aging treatment used in this invention also allows the calcium silicate gel (i.e., CSH gel) network generated by the hydration of the alkaline activator to encapsulate the dihydrate phosphogypsum, preventing the dihydrate phosphogypsum from inhibiting the hydration reaction of the hemihydrate phosphogypsum. The hydration reaction of the hemihydrate phosphogypsum helps to improve the early strength of the phosphogypsum-based composite cementitious material. However, considering that the hemihydrate phosphogypsum has poor water resistance, a fast hydration rate, concentrated hydration heat, and high crystallization stress, which easily leads to the problem of volume shrinkage and cracking in the later stage, this invention uses the acid radical ions in the crystal form modifier to selectively adsorb on the specific crystal faces where the hemihydrate phosphogypsum crystals are actively growing, thereby inhibiting its growth rate, so that the growth rate of each crystal face can be rebalanced, ultimately causing the crystal morphology to change from slender needle-like to thicker, shorter, and more uniform short columnar or plate-like. The short columnar or plate-like gypsum crystals have higher packing density and lower porosity, which can significantly improve flexural strength and compressive strength, and avoid the problem of volume shrinkage and cracking in the later stage. Furthermore, by encapsulating dihydrate phosphogypsum, it can continuously and slowly react with water and the active aluminum phase in the slag material when exposed to water in the later stages, generating ettringite, which continuously improves the later strength of the cementitious material.

[0044] Finally, this invention combines dihydrate phosphogypsum and hemihydrate phosphogypsum in the required mass proportions because dihydrate phosphogypsum, as a "phase change thermal storage material" and "flexible micro-aggregate," alleviates the thermal shock and volume shrinkage during the hydration of hemihydrate phosphogypsum, thus helping to solve the cracking problem. Furthermore, the encapsulation effect of the calcium silicate gel (CSH gel) network generated by the hydration of the alkaline activator can further improve the strength of the phosphogypsum-based composite cementitious material.

[0045] Combination Figures 1-4 It is known that after 28 days of standard curing, the sample of Example 1 remained intact without cracking, while the sample of Comparative Example 1 was severely cracked, so strength and SEM tests could not be carried out. In addition, the amount of ettringite and calcium silicate gel (i.e., CSH gel) produced in Example 1 was significantly greater than that in Comparative Example 2. The phosphogypsum (i.e., dihydrate phosphogypsum) was largely encapsulated by calcium silicate gel, with fewer pores and a denser structure. This microscopically reflects that the strength of Example 1 is higher than that of Comparative Example 2, which is consistent with the data in Table 1.

[0046] The above description is only a preferred embodiment of the present invention and does not limit the scope of the present invention. All equivalent structural transformations made under the inventive concept of the present invention using the contents of the present invention specification and drawings, or direct / indirect applications in other related technical fields, are included within the protection scope of the present invention.

Claims

1. A method for preparing a phosphogypsum-based composite cementitious material, characterized in that, include: Step S1: Aging treatment; Mix 5-50 parts by weight of phosphogypsum dihydrate, 5-10 parts by weight of alkaline activator, and 5-10 parts by weight of water, and then age the mixture to obtain aged material. Step S2: Dry mixing; The aged material, 5-50 parts by weight of hemihydrate phosphogypsum, and 15-45 parts by weight of slag powder are dry-mixed to obtain a mixture. Step S3: Pulping; Mix 0.5 to 2 parts by weight of crystal modifier, 0.5 to 2 parts by weight of water-reducing agent, and 20 to 40 parts by weight of water, and add them to the mixture. Stir evenly to obtain phosphogypsum-based composite cementitious material. The crystal form modifier includes sodium citrate or potassium aluminum sulfate.

2. The method for preparing a phosphogypsum-based composite cementitious material according to claim 1, characterized in that, The aging process involves an aging time of 24-72 hours and an aging temperature of 15-25°C.

3. The method of claim 1, wherein the phosphogypsum-based composite cement is prepared by mixing the phosphogypsum, the calcium sulfate hemihydrate, the calcium sulfate dihydrate, and the calcium sulfate trihydrate at a weight ratio of 1 : 1 : 1 :

1. The alkaline activator includes steel slag powder, carbide slag, or hydrated lime.

4. The method for preparing a phosphogypsum-based composite cementitious material according to claim 1, characterized in that, The slag powder is granulated blast furnace slag powder of grade S95 or above.

5. The method of claim 1, wherein the phosphogypsum-based composite cement is prepared by mixing phosphogypsum, calcium sulfate dihydrate, and a binder. The water-reducing agent includes a polycarboxylate-based high-performance water-reducing agent.

6. The method of claim 1, wherein the phosphogypsum-based composite cement is prepared by mixing phosphogypsum, calcium sulfate dihydrate, and a binder. The mass fraction of the dihydrate phosphogypsum is 9-36 parts; the mass fraction of the hemihydrate phosphogypsum is 9-36 parts.

7. A phosphogypsum-based composite cementitious material, characterized in that, The phosphogypsum-based composite cementitious material was prepared using the preparation method described in any one of claims 1 to 6.

8. The phosphogypsum-based composite cementitious material of claim 7, wherein, The 3-day flexural strength of the phosphogypsum-based composite cementitious material is not less than 1.89 MPa; the 3-day compressive strength of the phosphogypsum-based composite cementitious material is not less than 3.2 MPa.

9. The phosphogypsum-based composite cementitious material of claim 7, wherein, The 7-day flexural strength of the phosphogypsum-based composite cementitious material is not less than 4.7 MPa; the 7-day compressive strength of the phosphogypsum-based composite cementitious material is not less than 10.8 MPa.

10. The phosphogypsum-based composite cementitious material of claim 7, wherein, The 28-day flexural strength of the phosphogypsum-based composite cementitious material is not less than 8.1 MPa; the 28-day compressive strength of the phosphogypsum-based composite cementitious material is not less than 31.8 MPa.

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