A process for the conversion of gypsum dihydrate to form α-high-strength gypsum

Abstract

The present invention provides a process for the conversion of gypsum dihydrate to form α-high-strength gypsum, which relates to the field of building materials and comprises the following steps: 1) adding gypsum dihydrate into an acidic aqueous solution and stirring evenly to obtain a first mixed solution; 2) adding a composite conversion agent, calcium nitrate, potassium chloride and deionized water into the first mixed solution, stirring evenly and fully reacting to obtain a second mixed solution; 3) performing solid-liquid separation on the second mixed solution, collecting the solid phase, washing and drying to obtain α-high-strength gypsum. By crosslinking and modifying hyaluronic acid, the present invention enables the modified transparent phytic acid to have a larger molecular weight and spatial structure, and can significantly improve the conversion effect of gypsum dihydrate and increase the yield of α-high-strength gypsum when synergized with glutaric acid.

Description

Technical Field

[0001] The present invention relates to the technical field of building materials, and particularly relates to a process for converting dihydrate gypsum into α-high-strength gypsum by crystal transformation. Background Art

[0002] Gypsum, as an ancient building material, has extremely broad applications in various fields. One of them is building gypsum (i.e., β-hemihydrate gypsum, or ordinary gypsum powder), which has low physical strength, limited uses and consumption, and is mainly used for gypsum blocks, gypsum plasterboards and some decorative materials. Another one is high-strength gypsum (i.e., α-hemihydrate gypsum). Generally, it has high strength, more than 3 times that of ordinary gypsum powder, and has good mechanical properties, workability, environmental protection properties and biological properties. α-High-strength gypsum can not only replace traditional building gypsum to enhance product quality, but also has a wider range of applications, such as precision casting, high-end building materials, arts and crafts, medical treatment, aviation, shipbuilding and other fields. With the rapid development, the demand for α-type high-strength gypsum is increasing continuously, and the performance requirements for it are also getting higher and higher. At present, in China, high-strength gypsum is classified according to the 2H flexural strength and dry compressive strength, including α25, α30, α40 and α50; among them, the α25 grade. At present, some building materials products in China have a large application amount of α-type high-strength gypsum, including GRG decorative materials, gypsum-based self-leveling mortar, and raw materials for ceramic molds, etc.

[0003] The prior art has proposed several effective schemes for preparing high-strength gypsum. For example, the method for preparing α-hemihydrate gypsum from dihydrate gypsum disclosed in Patent CN105174760A is to add dihydrate gypsum and a crystal transformation agent into an aqueous solution of a mixed acid containing sulfuric acid and phosphoric acid for crystal transformation reaction, followed by solid-liquid separation. A part of the liquid phase is returned to the crystal transformation tank for continuous crystal transformation reaction, and the other part is transported to the acidolysis tank for preparing phosphoric acid by the process of sulfuric acid decomposing phosphate rock for utilization; the solid phase is washed with washing water, and the washing water is introduced into the obtained liquid phase for utilization. The solid phase obtained after washing can either be dried into α-hemihydrate gypsum powder or directly made into gypsum products such as gypsum plasterboards, gypsum blocks and gypsum components without going through the drying step. Another example is the method for preparing α-type high-strength hemihydrate gypsum disclosed in Patent CN111533475A, which proposes a scheme for preparing α-type high-strength hemihydrate gypsum by using the desulfurization by-product desulfurization gypsum of a coal-fired power plant and adopting a new type of crystallization agent; the scheme of using factory by-products as reaction raw materials can reduce the exploitation of natural gypsum mines, thus saving resources.

[0004] Due to different raw materials, crystal transformation agents, reaction conditions, etc. adopted in the existing preparation schemes, the performance of the product high-strength gypsum is uneven, that is, high-strength gypsum with uniform performance cannot be obtained; a preparation method that can control the prepared high-strength gypsum to always meet the target performance requirements is of great significance. Summary of the Invention

[0005] The object of the present invention is to provide a process for the conversion of gypsum dihydrate into α-high-strength gypsum. By using a composite crystal conversion agent prepared by the compounding of crosslinked modified hyaluronic acid, polycarboxylate water reducer, nano-calcium sulfate and glutaric acid, in combination with an acidic reaction environment, the crystal conversion effect of gypsum dihydrate is synergistically enhanced to achieve the preparation of α-high-strength gypsum that always meets the target performance requirements.

[0006] To achieve the above object, the present invention proposes the following technical solution: A process for the conversion of gypsum dihydrate into α-high-strength gypsum, comprising the following steps:

[0007] 1) Add the pretreated gypsum dihydrate into an acidic aqueous solution and stir evenly to obtain a first mixed solution;

[0008] 2) Add a composite crystal conversion agent, calcium nitrate, potassium chloride and deionized water to the first mixed solution, stir evenly and fully react to obtain a second mixed solution; wherein, the composite crystal conversion agent is a compound of modified hyaluronic acid, nano-calcium sulfate, polycarboxylate water reducer, glutaric acid and deionized water;

[0009] 3) Perform solid-liquid separation on the second mixed solution, collect the solid phase, wash and dry it to obtain α-high-strength gypsum.

[0010] Further, the modified hyaluronic acid is a multi-glycol epoxy derivative crosslinked hyaluronic acid.

[0011] Further, by mass percentage, the composite crystal conversion agent in step 2) consists of 14.5 - 20 wt.% modified hyaluronic acid, 0.2 - 1.0 wt.% nano-calcium sulfate, 2 - 5.5 wt.% polycarboxylate water reducer, 4 - 8 wt.% glutaric acid, and the balance is deionized water.

[0012] Further, the preparation process of the composite crystal conversion agent is as follows:

[0013] Dissolve the crosslinking agent tetra-armed dodecaglycol tetraepoxide in a first solvent, add hyaluronic acid, and stir and react at 40 - 55 °C in a water bath for 2 - 6 h to form a multi-glycol epoxy derivative crosslinked hyaluronic acid, and the molar ratio of the tetra-armed dodecaglycol tetraepoxide to hyaluronic acid is 1:2 - 8; wherein, the first solvent is an ethanol solution with the pH of the solution adjusted to 5.5 - 6 by hydrochloric acid, the volume fraction of ethanol in the ethanol solution is 75%, and the dosage ratio of the tetra-armed dodecaglycol tetraepoxide to the first solvent is 1 g:1.5 - 10 mL;

[0014] Modify nano-calcium sulfate with hydrogen peroxide to obtain hydroxyl-activated nano-calcium sulfate;

[0015] Using deionized water as a solvent, hydroxy-activated nano-calcium sulfate, polycarboxylate water reducer, glutaric acid, and cross-linked hyaluronic acid of polyglycol epoxy derivative are sequentially added into deionized water, and ultrasonic treatment is carried out at 40 - 60 kHz for 0.5 - 3 h to obtain a composite crystal conversion agent.

[0016] Further, the second mixed solution in step 2) includes, by weight: 100 - 120 parts of gypsum dihydrate, 0.8 - 1.8 parts of the composite crystal conversion agent, 1 - 1.5 parts of calcium nitrate, 3 - 6.5 parts of potassium chloride, and 250 - 300 parts of deionized water.

[0017] Further, the process of stirring evenly in step 2) is as follows:

[0018] According to the mass of gypsum dihydrate in the first mixed solution, the composite crystal conversion agent, calcium nitrate, potassium chloride, and deionized water are added into the first mixed solution, and stirred at a temperature of 75 - 90 °C and a stirring speed of 100 - 250 r / min for 0.5 - 2 h.

[0019] Further, the pre-treated gypsum dihydrate in step 1) is gypsum dihydrate powder, and the average particle size of the gypsum dihydrate powder does not exceed 15 μm.

[0020] Further, the process of reacting to obtain the second mixed solution in step 2) is a constant-temperature hydrothermal reaction at a temperature of 120 - 140 °C and a pressure of 0.2 - 1.0 MPa for 1.2 - 6.5 h.

[0021] Further, the acidic aqueous solution is a hydrochloric acid solution, and the pH value of the hydrochloric acid solution is 5 - 6.5.

[0022] Further, the method of solid-liquid separation of the second mixed solution in step 3) is centrifugal separation by spin-drying.

[0023] As can be seen from the above technical solutions, the technical solutions of the present invention have obtained the following beneficial effects:

[0024] The process for the conversion of gypsum dihydrate to α-high-strength gypsum disclosed in the present invention includes the following steps: 1) adding pretreated gypsum dihydrate into an acidic aqueous solution, and stirring evenly to obtain a first mixed solution; 2) adding a composite crystal conversion agent, calcium nitrate, potassium chloride and deionized water into the first mixed solution, stirring evenly and fully reacting to obtain a second mixed solution; wherein, the composite crystal conversion agent is a compound prepared by mixing modified hyaluronic acid, nano calcium sulfate, polycarboxylate water reducer, glutaric acid and deionized water; 3) performing solid-liquid separation on the second mixed solution, collecting the solid phase, washing and drying to obtain α-high-strength gypsum; wherein, the composite crystal conversion agent is composed of 14.5-20 wt.% modified hyaluronic acid, 0.2-1.0 wt.% nano calcium sulfate, 2-5.5 wt.% polycarboxylate water reducer, 4-8 wt.% glutaric acid, and the balance being deionized water, and the modified hyaluronic acid is a polyglycol epoxy derivative cross-linked hyaluronic acid. The present invention prepares α-high-strength gypsum with controllable target properties through the cooperation of a composite crystal conversion agent and an acidic reaction environment. The specific advantages are as follows:

[0025] 1. The process of converting gypsum dihydrate to α-high-strength gypsum in the present invention is carried out in an acidic environment as a whole. Acid radical ions are easily adsorbed on the surface of the generated α-high-strength gypsum crystals, resulting in a dilution effect on the crystal surface, so as to increase the mass transfer resistance of Ca 2+ and SO4 2- moving to the crystal surface, achieving the reduction of the crystal surface growth rate and guiding the α-high-strength gypsum crystals to form into thick and short columnar bodies.

[0026] 2. A composite crystal conversion agent is prepared by mixing polyglycol epoxy derivative cross-linked hyaluronic acid, nano calcium sulfate, polycarboxylate water reducer and glutaric acid. On the one hand, the polyglycol epoxy derivative cross-linked hyaluronic acid has multiple ether bonds and many hydrogen bonds and a special spatial structure. When the ionized anions thereof cooperate with the carboxylate ions ionized by the polycarboxylate water reducer and the anions ionized by glutaric acid to act on the surface of the α-high-strength gypsum crystals, they are selectively adsorbed on the (111) crystal plane to form a network-like thin film adsorption layer, significantly reducing the growth rate of the α-high-strength gypsum crystals along the C-axis direction, and then guiding the α-high-strength gypsum crystals to form into thick and short columnar bodies.

[0027] 3. As a surfactant, the polycarboxylate water reducer can, on the one hand, reduce the nucleation rate of hemihydrate gypsum and increase the growth rate of hemihydrate gypsum crystals. On the other hand, its grafting and cooperation with nano calcium sulfate activated by hydroxyl groups enable the components in the composite crystal conversion agent to be fully dispersed during the reaction process, and then develop in the direction of thickening and irregular agglomeration during the growth of hemihydrate gypsum crystals.

[0028] 4. The process for the conversion of gypsum dihydrate to α-high-strength gypsum disclosed in the present invention can accurately produce α-high-strength gypsum meeting the target performance requirements by controlling the compounding ratio of the composite conversion agent and the reaction time. Through the design of the composite conversion agent and reaction conditions, the 2H flexural strength, 2H compressive strength, and dried compressive strength of the α-high-strength gypsum prepared by the present invention can reach 7.5 MPa, 27.3 MPa, and 64.1 MPa respectively, far exceeding the performance of the current α50-grade high-strength gypsum.

[0029] It should be understood that all combinations of the foregoing concepts and additional concepts described in greater detail below can be regarded as part of the inventive subject matter of the present disclosure as long as such concepts do not conflict with each other.

[0030] The foregoing and other aspects, embodiments, and features of the teachings of the present invention can be more fully understood from the following description in conjunction with the embodiments. Other additional aspects of the present invention, such as the features and / or beneficial effects of the exemplary embodiments, will be apparent from the following description or will be learned through the practice of the specific embodiments according to the teachings of the present invention. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] The drawings are not drawn to scale in accordance with real reference objects. In the drawings, each identical or approximately identical component shown in each figure may be denoted by the same reference numeral. For the sake of clarity, not every component is labeled in each figure. Now, embodiments of various aspects of the present invention will be described by way of example and with reference to the drawings, wherein:

[0032] Figure 1 is the XRD pattern of the α-high-strength gypsum prepared in Example 1 of the present invention;

[0033] Figure 2 is the microscopic morphology diagram of the α-high-strength gypsum prepared in Example 1 of the present invention;

[0034] Figure 3 is the microscopic morphology diagram of the α-high-strength gypsum prepared in Example 8 of the present invention;

[0035] Figure 4 is the microscopic morphology diagram of the α-high-strength gypsum prepared in Comparative Example 2 of the present invention;

[0036] Figure 5 is the microscopic morphology diagram of the α-high-strength gypsum prepared in Comparative Example 2 of the present invention. DETAILED DESCRIPTION OF THE EMBODIMENTS

[0037] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are some, but not all, of the embodiments of the present invention. All other embodiments obtained by those of ordinary skill in the art based on the described embodiments of the present invention without creative efforts fall within the scope of protection of the present invention. Unless otherwise defined, the technical terms or scientific terms used herein shall have the ordinary meaning as understood by those of ordinary skill in the art to which the present invention pertains.

[0038] The terms "first", "second", and similar terms used in the description and claims of this patent application for the invention do not denote any order, quantity, or importance, but are merely used to distinguish different components. Similarly, unless the context clearly indicates otherwise, singular forms such as "a", "an", or "the" do not denote a limitation on quantity, but rather indicate the presence of at least one. The terms "comprising" or "including" and similar terms mean that the elements or items appearing before "comprising" or "including" cover the features, wholes, steps, operations, elements, and / or components listed after "comprising" or "including", and do not exclude the existence or addition of one or more other features, wholes, steps, operations, elements, components, and / or their combinations.

[0039] Based on the existing art-disclosed solutions for preparing α-high-strength gypsum, due to differences in raw materials, crystal conversion agents, reaction conditions, etc., it is impossible to obtain high-strength gypsum with uniform properties. The present invention aims to, through the selection of a composite crystal conversion agent, utilize the synergistic effect of the composite crystal conversion agent to controllably form α-high-strength gypsum into short columnar shapes, and thereby accurately prepare α-high-strength gypsum that meets the target performance requirements.

[0040] Specifically, the process for the crystal conversion of dihydrate gypsum into α-high-strength gypsum disclosed in the present invention includes the following steps:

[0041] 1) Add the pretreated dihydrate gypsum to an acidic aqueous solution and stir evenly to obtain a first mixed solution; wherein, the pretreated dihydrate gypsum is dihydrate gypsum powder, and the average particle size of the dihydrate gypsum powder does not exceed 15 μm.

[0042] 2) Add a composite crystal-changing agent, calcium nitrate, potassium chloride and deionized water to the first mixed solution, stir evenly and perform a constant temperature hydrothermal reaction at a temperature of 120-140°C and a pressure of 0.2-1.0 MPa for 1.2-6.5 hours to obtain a second mixed solution; wherein the composite crystal-changing agent is a compound of modified hyaluronic acid, nano-calcium sulfate, polycarboxylate water reducer, glutaric acid and deionized water, and the modified hyaluronic acid is a cross-linked hyaluronic acid of a polyglycol epoxy derivative; the second mixed solution comprises, by weight: 100-120 parts of dihydrate gypsum, 0.8-1.8 parts of a composite crystal-changing agent, 1-1.5 parts of calcium nitrate, 3-6.5 parts of potassium chloride and 250-300 parts of deionized water.

[0043] 3) The second mixed liquid is subjected to solid-liquid separation, and the solid phase is collected, washed, and dried to obtain α high-strength gypsum.

[0044] In the above scheme, the preparation process of the composite crystal-changing agent is as follows: dissolving the cross-linking agent four-arm dodecaglycol tetraglycidyl ether in the first solvent, adding hyaluronic acid, stirring and reacting for 2-6 hours in a water bath at 40-55°C to form a polyol epoxy derivative cross-linked hyaluronic acid, wherein the molar ratio of the four-arm dodecaglycol tetraglycidyl ether to the hyaluronic acid is 1:2-8; wherein the first solvent is an ethanol solution in which the pH of the solution is adjusted to 5.5-6 using hydrochloric acid, the volume fraction of ethanol in the ethanol solution is 75%, and the amount ratio of the four-arm dodecaglycol tetraglycidyl ether to the first solvent is 1g: 1.5-10mL; modifying nano calcium sulfate with hydrogen peroxide to obtain hydroxyl-activated nano calcium sulfate; using deionized water as solvent, sequentially adding hydroxyl-activated nano calcium sulfate, polycarboxylate water reducer, glutaric acid and polyglycol epoxy derivative cross-linked hyaluronic acid into deionized water, and treating with 40-60kHz ultrasonic wave for 0.5-3h to obtain a composite crystal-transforming agent; wherein the polycarboxylate water reducer is a liquid polycarboxylate water reducer with a solid content of 20-25% and a water reduction rate of more than 30%, which is commercially available; in terms of mass percentage, the composite crystal-transforming agent is composed of 14.5-20 wt.% modified hyaluronic acid, 0.2-1.0 wt.% nano calcium sulfate, 2-5.5 wt.% polycarboxylate water reducer, 4-8 wt.% glutaric acid, and the remainder is deionized water.

[0045] The process of crystallizing dihydrate gypsum to form α high-strength gypsum disclosed in the present invention will be further described in detail below in conjunction with specific embodiments.

[0046] Example 1

[0047] 1) Preparation of composite crystal-changing agent A

[0048] 1 mol of a cross-linking agent, tetra-dodecyl glycol tetraglycidyl ether, was dissolved in a 75% ethanol solution by volume, and 2 mol of hyaluronic acid with a molecular weight of 3.0 MDa was added, and the mixture was stirred in a water bath at 50°C for 3.5 hours to form a cross-linked hyaluronic acid with a polyol epoxy derivative; wherein the pH of the ethanol solution was 5.5, and the amount ratio of tetra-dodecyl glycol tetraglycidyl ether to the ethanol solution was 1 g:8 mL; nano-calcium sulfate was added into a 30% hydrogen peroxide solution by volume, and the mixture was heated to 65°C and stirred for 30 minutes. After the reaction was completed, the mixture was filtered and washed and dried to obtain hydroxyl-activated nano-calcium sulfate; deionized water was used as a solvent, and hydroxyl-activated nano-calcium sulfate, polycarboxylate water reducer, glutaric acid and cross-linked hyaluronic acid with a polyol epoxy derivative were added to deionized water in sequence, and ultrasonic treatment was performed at 60 kHz for 2 hours to obtain a composite crystal-transforming agent A; in terms of mass percentage, the composite crystal-transforming agent A consisted of 17 wt.% modified hyaluronic acid, 0.3 wt.% nano calcium sulfate, 3wt.% polycarboxylate water reducer, 5wt.% glutaric acid, and the balance is deionized water. In addition, four-arm dodecaglycol tetraglycidyl ether is prepared by the method disclosed in patent CN109096483B for preparing branched polyglycol epoxy derivative cross-linked sodium hyaluronate gel.

[0049] 2) Preparation of α high-strength gypsum

[0050] Add dihydrate gypsum powder with an average particle size of 15 μm to a hydrochloric acid solution with a pH value of 6, and stir at a stirring speed of 180 r / min for 1 hour to obtain a first mixed solution; add a composite crystal-changing agent, calcium nitrate, potassium chloride and deionized water to the first mixed solution, stir at a temperature of 85°C and a stirring speed of 200 r / min for 1.2 hours to mix evenly, and then carry out a constant temperature hydrothermal reaction at a temperature of 120°C and a pressure of 0.3 MPa for 3.5 hours to obtain a second mixed solution, which includes, by weight: 105 parts of dihydrate gypsum, 1.5 parts of composite crystal-changing agent A, 1.2 parts of calcium nitrate, 3.5 parts of potassium chloride, and 280 parts of deionized water; spin dry the second mixed solution and centrifuge it, collect the solid phase, wash it, and dry it to obtain α high-strength gypsum A. X-ray powder diffraction analysis and SEM micromorphology analysis were performed on the α high-strength gypsum B obtained in Example 1, and the results are as follows: Figure 1 and Figure 2 As shown, the cross-section of the α-high-strength gypsum A crystal is approximately a short columnar crystal with a regular hexagon, which is consistent with the theoretical morphology of α-high-strength gypsum obtained by molecular dynamics simulation.

[0051] Example 2

[0052] 1) Preparation of composite crystal-changing agent B

[0053] 1 mol of a cross-linking agent, tetra-dodecyl glycol tetraglycidyl ether, was dissolved in a 75% ethanol solution by volume, and 2 mol of hyaluronic acid with a molecular weight of 3.0 MDa was added, and the mixture was stirred in a water bath at 50°C for 3.5 hours to form a cross-linked hyaluronic acid with a polyol epoxy derivative; wherein the pH of the ethanol solution was 5.5, and the amount ratio of tetra-dodecyl glycol tetraglycidyl ether to the ethanol solution was 1 g:8 mL; nano-calcium sulfate was added into a 30% hydrogen peroxide solution by volume, and the mixture was heated to 65°C and stirred for 30 minutes. After the reaction was completed, the mixture was filtered and washed and dried to obtain hydroxyl-activated nano-calcium sulfate; deionized water was used as a solvent, and hydroxyl-activated nano-calcium sulfate, polycarboxylate water reducer, glutaric acid and cross-linked hyaluronic acid with a polyol epoxy derivative were added to deionized water in sequence, and ultrasonic treatment was performed at 60 kHz for 2 hours to obtain a composite crystal-transforming agent B; in terms of mass percentage, the composite crystal-transforming agent A consisted of 19 wt.% modified hyaluronic acid, 0.7 wt.% nano calcium sulfate, 5wt.% polycarboxylate water reducer, 5wt.% glutaric acid, and the balance is deionized water.

[0054] 2) Preparation of α high-strength gypsum

[0055] Add dihydrate gypsum powder with an average particle size of 10 μm to a hydrochloric acid solution with a pH value of 6, and stir at a stirring speed of 180 r / min for 1 hour to obtain a first mixed solution; add a composite crystal-changing agent, calcium nitrate, potassium chloride and deionized water to the first mixed solution, stir at a temperature of 85°C and a stirring speed of 200 r / min for 1.2 hours to uniformly mix, and then carry out a constant temperature hydrothermal reaction at a temperature of 120°C and a pressure of 0.3 MPa for 5 hours to obtain a second mixed solution, which includes, by weight, 120 parts of dihydrate gypsum, 1.2 parts of composite crystal-changing agent B, 1.2 parts of calcium nitrate, 5 parts of potassium chloride, and 300 parts of deionized water; spin-dry the second mixed solution and centrifuge it, collect the solid phase, wash it, and dry it to obtain α high-strength gypsum B.

[0056] The difference between Examples 3-9 and Comparative Examples 1-3 and Example 1 is that the composition ratios of the prepared composite crystal-transforming agents are different, as shown in Table 1 below.

[0057] Table 1 shows the composition of the composite crystal-transforming agent obtained in Examples 3-9

[0058]

[0059] The bulk density, standard consistency, initial setting time, final setting time, 2H flexural strength, 2H compressive strength and dry compressive strength of the α high-strength gypsum AI and α high-strength gypsum ac prepared in Examples 1-9 and Comparative Examples 1-4 were measured, and the results are shown in Table 2 below.

[0060] Table 2 is the performance data of α high-strength gypsum obtained in Examples 1-9

[0061]

[0062] The differences between Examples 10 - 16 and Example 1 lie in the weight parts of each component added in the second mixed solution when preparing α-high-strength gypsum. The difference between Comparative Example 5 and Example 1 lies in the constant-temperature hydrothermal reaction time for preparing α-high-strength gypsum, as shown in Table 3 below.

[0063] Table 3 shows the composition of α-high-strength gypsum prepared in Example 1, Examples 10 - 16, and Comparative Example 4.

[0064]

[0065] The bulk density, whiteness, standard consistency, initial setting time, final setting time, 2H flexural strength, 2H compressive strength, and dry compressive strength of the α-high-strength gypsum J-P and e prepared in Examples 10 - 16 and Comparative Example 5 were measured respectively, and the results are shown in Table 4 below.

[0066] Table 4 shows the performance data of α-high-strength gypsum prepared in Examples 10 - 16 and Comparative Example 4.

[0067]

[0068] Based on Tables 1 - 4, the following conclusions can be drawn:

[0069] (1) In Examples 1 - 16, the 2H flexural strength of the α-high-strength gypsum obtained by the conversion of dihydrate gypsum can reach above 2.8 MPa, the 2H compressive strength can reach above 13.3 MPa, and the dry compressive strength can reach above 32.7 MPa; under relatively optimal conditions, the 2H flexural strength can reach 7.5 MPa, the 2H compressive strength can reach 27.3 MPa, and the dry compressive strength can reach 64.1 MPa.

[0070] (2) By comprehensively comparing the data of Example 1, Examples 3-4 and Comparative Example 2, it can be seen that the addition of polycarboxylate superplasticizer can effectively improve the compressive and flexural strengths of α-high-strength gypsum. Compared with Comparative Example 2, the performance of α-high-strength gypsum added with polycarboxylate superplasticizer can reach at least α40 grade. An appropriate amount of polycarboxylate superplasticizer can significantly enhance the strength of α-high-strength gypsum. By comprehensively comparing Example 1, Examples 5-7 and Comparative Example 4, it can be seen that the addition amount of modified hyaluronic acid significantly affects the compressive and flexural strengths of α-high-strength gypsum, and increasing the addition amount of modified hyaluronic acid can improve the compressive and flexural strengths of α-high-strength gypsum. By comprehensively comparing Example 1, Example 8 and Comparative Example 1, it can be seen that the presence of a small amount of nano-calcium sulfate in the solution helps to improve the structural strength of α-high-strength gypsum. When its addition amount is too much, it will lead to a decrease in the dry compressive strength of α-high-strength gypsum. The reason is that an excessive amount of nano-calcium sulfate appears as a large number of crystal nuclei in the reaction, the nucleation rate increases, and it inhibits the forming trend of α-high-strength gypsum crystals into thick and short columnar bodies, as shown in Figure 3 ; By comprehensively comparing Example 1, Example 9 and Comparative Examples 2-4, it can be seen that glutaric acid, as an organic acid, can significantly promote the forming of α-high-strength gypsum crystals into thick and short columnar bodies when it is compounded and coordinated with modified hyaluronic acid and polycarboxylate superplasticizer, thereby improving the compressive and flexural strengths of the product. As shown in the product morphology diagrams of Comparative Examples 2 and 4 in Figure 4 and 5 , the α-high-strength gypsum crystals cannot form regular columnar bodies. Thus, it can be seen that by preferably controlling the composition and ratio of the composite crystal conversion agent within a specific range in the present invention, it can effectively act on improving the compressive and flexural properties of α-high-strength gypsum.

[0071] (3) By comprehensively comparing the data of Example 1, Examples 10-11 and Comparative Example 5, it can be seen that the composite crystal conversion agent provided by the present invention can significantly improve the performance of α-high-strength gypsum. The 2H flexural strength is increased from 2.8 MPa to 7.5 MPa, the 2H compressive strength is increased from 13.3 MPa to 26.2 MPa, and the dry compressive strength is increased from 32.7 MPa to 64.1 MPa. By comprehensively comparing Example 1, Example 12 and Example 13, calcium nitrate and potassium chloride exist in the solution as electrolytes, and the change of their contents has a slight influence on the compressive and flexural properties of α-high-strength gypsum. However, within a suitable range, the compressive and flexural properties of the prepared α-high-strength gypsum can also reach α50 grade. Thus, it can be seen that by preferably controlling the components in the second solution within a specific range in the present invention, the performance of α-high-strength gypsum can be further improved.

[0072] (4) Combining Example 1 and Examples 14 - 16, under the conditions that the composition of the composite crystal conversion agent remains unchanged and the weight parts of each component in the second solution remain unchanged, as the constant-temperature hydrothermal reaction time for preparing α-high-strength gypsum is extended, the performance of the product α-high-strength gypsum is gradually improved. α-high-strength gypsum meeting the performance requirements of the α30 level can be obtained after 1.2 h of reaction, and when the reaction time reaches 3.5 h, the performance of α-high-strength gypsum tends to be stable. Continuing to extend the reaction time, the performance improvement of α-high-strength gypsum is not significant. Thus, it can be seen that the present invention preferably controls the reaction time within a specific range, and α-high-strength gypsum meeting different target performance requirements can be prepared simultaneously.

[0073] In summary, the process for converting dihydrate gypsum into α-high-strength gypsum provided by the present invention can guide the α-high-strength gypsum crystals to form thick and short columnar bodies, and can prepare α-high-strength gypsum products with different performance grades according to the target requirements.

[0074] Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Those with ordinary knowledge in the technical field to which the present invention pertains can make various changes and modifications without departing from the spirit and scope of the present invention. Therefore, the protection scope of the present invention shall be subject to that defined by the claims.

Claims

1. A process for the conversion of gypsum dihydrate to form α-high-strength gypsum, characterized in that, The process comprises the following steps: 1) adding the pretreated dihydrate gypsum into the acidic aqueous solution and stirring evenly to obtain a first mixed solution; 2) adding a composite crystal-changing agent, calcium nitrate, potassium chloride and deionized water to the first mixed solution, stirring evenly and reacting sufficiently to obtain a second mixed solution; wherein the composite crystal-changing agent is a mixture of polyglycol epoxy derivative cross-linked hyaluronic acid, nano calcium sulfate, polycarboxylate water reducer, glutaric acid and deionized water; 3) performing solid-liquid separation on the second mixed liquid, collecting the solid phase, washing and drying it, and obtaining α high-strength gypsum; The preparation process of the composite crystal-transforming agent is as follows: dissolving a cross-linking agent, four-arm dodecaglycol tetraglycidyl ether, in a first solvent, adding hyaluronic acid, stirring and reacting for 2-6 hours in a water bath at 40-55° C. to form a polyol epoxy derivative cross-linked hyaluronic acid, wherein the molar ratio of the four-arm dodecaglycol tetraglycidyl ether to the hyaluronic acid is 1:2-8; wherein the first solvent is an ethanol solution whose pH is adjusted to 5.5-6 by hydrochloric acid, the volume fraction of ethanol in the ethanol solution is 75%, and the amount ratio of the four-arm dodecaglycol tetraglycidyl ether to the first solvent is 1g:1.5-10mL; modifying nano-calcium sulfate by hydrogen peroxide to obtain hydroxyl-activated nano-calcium sulfate; using deionized water as a solvent, sequentially adding hydroxyl-activated nano-calcium sulfate, polycarboxylate water reducer, glutaric acid and polyol epoxy derivative cross-linked hyaluronic acid into deionized water, and treating with 40-60kHz ultrasonic wave for 0.5-3h to obtain the composite crystal-transforming agent.

2. The process for converting gypsum dihydrate into α-high-strength gypsum by crystal conversion according to claim 1, characterized in that, In terms of mass percentage, the composite crystal-transforming agent in step 2) is composed of 14.5-20 wt.% of polyglycol epoxy derivative cross-linked hyaluronic acid, 0.2-1.0 wt.% of nano calcium sulfate, 2-5.5 wt.% of polycarboxylate water reducer, 4-8 wt.% of glutaric acid, and the balance is deionized water.

3. The process for converting gypsum dihydrate into α-high-strength gypsum by crystal conversion according to claim 1, characterized in that, The second mixed solution in step 2) comprises, by weight: 100-120 parts of dihydrate gypsum, 0.8-1.8 parts of a composite crystal-changing agent, 1-1.5 parts of calcium nitrate, 3-6.5 parts of potassium chloride, and 250-300 parts of deionized water.

4. The process for converting gypsum dihydrate into α-high-strength gypsum by crystal conversion according to claim 1, characterized in that The process of step 2) stirring uniformly is as follows: According to the mass of dihydrate gypsum in the first mixed solution, a composite crystal-changing agent, calcium nitrate, potassium chloride and deionized water are added to the first mixed solution, and stirred at a temperature of 75-90° C. and a stirring speed of 100-250 r / min for 0.5-2 h.

5. The process for converting gypsum dihydrate into α-high-strength gypsum by crystal transformation according to claim 1, characterized in that, The dihydrate gypsum pretreated in step 1) is dihydrate gypsum powder, and the average particle size of the dihydrate gypsum powder does not exceed 15 μm.

6. The process for converting gypsum dihydrate into α-high strength gypsum by crystal transformation according to claim 1, characterized in that, The process of obtaining the second mixed solution in step 2) is to perform a constant temperature hydrothermal reaction for 1.2 to 6.5 hours at a temperature of 120 to 140° C. and a pressure of 0.2 to 1.0 MPa.

7. The process for converting gypsum dihydrate into α-high-strength gypsum by crystal conversion according to claim 1, characterized in that, The acidic aqueous solution is a hydrochloric acid solution, and the pH value of the hydrochloric acid solution is 5-6.

5.

8. The process for converting gypsum dihydrate into α-high-strength gypsum by crystal transformation according to claim 1, characterized in that, In the step 3), the method of performing solid-liquid separation on the second mixed liquid is drying and centrifugation.

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