Magnesium oxide-based low temperature expanding agent, its preparation method and application

Abstract

The application relates to the technical field of cement-based material admixtures, in particular to a magnesium oxide-based low-temperature expansion agent and a preparation method and application thereof. The magnesium oxide-based expansion agent prepared by using active magnesium oxide as a main component, using a small amount of calcium oxide, adding modified starch, aluminum sulfate and sodium carbonate as promoting components can be used for low-temperature construction of cement-based materials.

Description

Technical Field

[0001] This invention relates to the field of cement-based material admixtures, and in particular to a magnesium oxide-based low-temperature expansion agent, its preparation method, and its application. Background Technology

[0002] Cement-based materials are commonly used bulk industrial raw materials in construction projects, and infrastructure construction cannot be separated from the use of cement concrete. During the hardening process, cement-based materials undergo volume shrinkage, such as drying shrinkage, autogenous shrinkage, temperature shrinkage, and carbonation shrinkage. When this shrinkage deformation is constrained internally or externally, tensile stress is generated within the material. When the tensile stress exceeds the tensile strength of the product, cracks will occur. It can be said that volume shrinkage is one of the important reasons for the cracking of cement-based materials, accelerated deterioration, and impact on structural durability.

[0003] To compensate for the shrinkage of cement-based materials, various types of expansive agents are often used in engineering. The crystalline expansion produced by these agents can compensate for shrinkage and prevent cracking. Magnesium oxide expansive agent is a commonly used one, which is produced by calcining and grinding magnesium-containing minerals. However, the expansive effect of magnesium oxide is greatly affected by temperature. Increased temperature enhances the hydration expansion effect of magnesium oxide, while decreased temperature reduces its expansive effect. Therefore, magnesium oxide expansive agent is often used in the production of large-volume concrete.

[0004] In recent years, with the construction of high-altitude areas and offshore wind power projects, low-temperature construction has brought more technical challenges to concrete. Studies have shown that the effect of magnesium oxide expansive agents will be greatly reduced below 20°C. There are still many technical gaps in how to improve the low-temperature performance of magnesium oxide expansive agents. Summary of the Invention

[0005] To address the problems existing in the prior art, the present invention provides a magnesium oxide-based low-temperature expansion agent. Specifically, the present invention uses active magnesium oxide as the main component, combined with a small amount of calcium oxide, and adds modified starch, aluminum sulfate and sodium carbonate as promoting components to prepare a magnesium oxide-based expansion agent that can be used in low-temperature construction of cement-based materials.

[0006] Specifically, the preparation method of the magnesium oxide-based low-temperature expanding agent of the present invention includes the following steps:

[0007] 1) Magnesite is ground into powder, calcined at 800-900℃, and then ground again to obtain active magnesium oxide.

[0008] 2) Mix starch and water evenly, adjust the pH to acidic, heat to carry out acid-catalyzed hydrolysis, neutralize, centrifuge, wash, and dry to obtain low molecular weight hydrolyzed starch.

[0009] 3) mixing low molecular weight hydrolyzed starch with water, adding epichlorohydrin, sodium hydroxide for cross-linking hydrolysis, neutralization, washing, drying, obtaining cross-linked hydrolyzed starch,

[0010] 4) dispersing cross-linked hydrolyzed starch in alkali solution, adding tetrabutylammonium bromide, propylene oxide for etherification reaction, adding anhydrous ethanol after reaction, precipitating, suction filtering, washing, drying, obtaining modified starch,

[0011] 5) mixing 80-90 parts of active magnesium oxide, 3-5 parts of calcium oxide, 5-8 parts of modified starch, 3-6 parts of aluminum sulfate, 2-4 parts of sodium carbonate by weight, obtaining the product.

[0012] The application selects light-burned magnesium oxide as the main expansion source, and active magnesium oxide is prepared by calcining magnesite and grinding. The activity of the active magnesium oxide is low at low temperature. To solve this problem, the application adds modified starch. The starch is prepared by acid-catalyzed glycosidic bond cleavage reaction, nucleophilic substitution cross-linking reaction and alkali-catalyzed ring-opening etherification reaction to obtain hydroxypropyl starch. The application finds that the modified starch significantly improves the hydrophilicity and water retention capacity of the starch. The modified starch can improve the pore solution properties in the micropores of the cement-based material, avoid the self-shrinkage caused by the rapid evaporation of water during the hardening process of the cement-based material, and has higher stability in the cement-based material added with the magnesium oxide-based low-temperature expansion agent. The modified starch can act on the surface of the active magnesium oxide, promote the low-temperature hydration of the magnesium oxide, generate magnesium hydroxide for expansion. The application adds a small amount of calcium oxide. The hydration and expansion of the calcium oxide are less sensitive to temperature. The calcium hydroxide generated by the hydration can also be slightly expanded. Most importantly, under the action of the modified starch, the calcium hydroxide generated by the calcium oxide will not be rapidly consumed by the active silicon and aluminum raw materials in the cement-based material in the early hydration stage. The active magnesium oxide and the calcium oxide constitute the expansion source of the early hydration of the cement-based material, and the active magnesium oxide plays a dominant role. In the later hydration process, the modified starch has a certain internal curing effect. The application adds aluminum sulfate and sodium carbonate. The aluminum sulfate and the sodium carbonate not only can promote the hydration of the cement-based material, improve the compactness of the hardened body, promote the growth of the mechanical properties, and further generate part of hydrotalcite from the magnesium hydroxide, but also can promote the later expansion effect.

[0013] Preferably, in step 1), the magnesite is ground to a particle size of ≤0.1 mm, and the calcination time is 1-2 h. The particle size of the active magnesium oxide is ≤20 μm.

[0014] Preferably, in step 2), the starch is at least one of corn starch, cassava starch and potato starch. The starch is derived from natural starch, and the raw material is widely available and low in price. It is green and renewable.

[0015] Preferably, in step 2), the pH value is adjusted to 3-4, heated to 45-55℃, and the reaction time is 0.5-1 h. Preferably, dilute hydrochloric acid is used for pH value adjustment, and sodium hydroxide solution is used for neutralization.

[0016] Preferably, the mass ratio of low molecular weight hydrolyzed starch, water, epichlorohydrin, sodium hydroxide in step 3) is 10: (80-90): (0.5-1): (0.2-0.5). More preferably, the mass ratio of low molecular weight hydrolyzed starch, water, epichlorohydrin, sodium hydroxide in step 3) is 10:90:1:0.3.

[0017] Preferably, the mass ratio of cross-linked hydrolyzed starch, tetrabutylammonium bromide, propylene oxide in step 4) is 10: (0.1-0.2): (0.8-1.2). Preferably, the mass ratio of cross-linked hydrolyzed starch, tetrabutylammonium bromide, propylene oxide in step 4) is 10:0.1:1.1. Preferably, the alkali solution is a sodium hydroxide solution, and the amount of alkali solution added is appropriate, which only provides an alkaline environment, which can be 6-10 times the mass of cross-linked hydrolyzed starch, or other suitable amount, and the amount of anhydrous ethanol used is appropriate, which can be 2-4 times the overall volume of the reactants.

[0018] Preferably, the etherification reaction temperature in step 4) is 45-60℃, and the reaction time is 4-8h.

[0019] In step 2), the starch is in an acidic environment, and selective hydrolysis of its α-1,4 and α-1,6 glycosidic bonds occurs, generating hydrolyzed starch with lower molecular weight and narrower polydispersity. In step 3), the hydroxyl group of the ring-opened ether bond intermediate reacts with the starch chain, forming a stable ether bond cross-linking network between the starch molecular chains, significantly enhancing the particle strength of the starch and its stability in aqueous phase. In step 4), the hydroxyl group of the cross-linked hydrolyzed starch reacts with propylene oxide in the presence of a phase transfer catalyst. The modified starch prepared by the present application can promote the expansion effect of magnesium oxide-based low-temperature expansion agent in a low-temperature environment.

[0020] The present application also relates to a magnesium oxide-based low-temperature expansion agent, specifically prepared by the above preparation method.

[0021] The present application also relates to the use of the above-mentioned magnesium oxide-based low-temperature expansion agent in the preparation of cement-based materials in a low-temperature environment.

[0022] Preferably, the amount of the magnesium oxide-based low-temperature expansion agent added is 3-8% of the mass of the cementitious material.

[0023] Preferably, the low-temperature environment is 2-10℃. More preferably, the low-temperature environment is 5-8℃.

[0024] The present application has the following technical advantages:

[0025] 1. The present application uses active magnesium oxide as the main expansion source, with stable and sustained expansion effect, and simple preparation process,

[0026] 2. The invention adds calcium oxide, modified starch, aluminum sulfate, sodium carbonate, and active magnesium oxide to form a synergistic expansion, and makes the cement-based material have good mechanical properties,

[0027] 3. The invention provides a magnesium oxide-based low-temperature expansion agent for low-temperature construction of cement-based materials, and the preparation method is simple. DETAILED DESCRIPTION

[0028] To characterize the technical effect of the invention, the expansion agent is prepared and used in the preparation of cement-based material slurry at a proportion of 4% by weight. After the slurry is formed, the test piece is placed in an environment with a temperature of 5℃ and a humidity of 50±2% for curing. The expansion rate and compressive strength at different ages are tested. The expansion rate of "+" indicates that the test piece expands, and the expansion rate of "-" indicates that the test piece shrinks.

[0029] During the test, the cement-based slurry is selected as a wind power low-temperature sleeve grouting material, and the formula is 42.5 sulfoaluminate cement 400 parts, P·O 42.5 cement 50 parts, fly ash 50 parts, standard sand 1350 parts, polycarboxylic acid water reducer 10 parts, amorphous calcium aluminate 5 parts, expansion agent 20 parts, calcium chloride 0.5 parts, and water 160 parts. The preparation process of active magnesium oxide is as follows: magnesite is ground to a particle size of ≤0.1mm, calcined at 850℃ for 1.5h, and ground to a particle size of ≤20μm. The preparation process of modified starch is as follows: 10 parts of corn starch are mixed with 100 parts of water, the pH value is adjusted to 4, heated to 50℃ for acid hydrolysis catalytic reaction for 0.5h, neutralized, centrifuged, washed, and dried to obtain low molecular weight hydrolyzed starch. 10 parts of low molecular weight hydrolyzed starch are mixed with 90 parts of water, 1 part of epoxy chloropropane and 0.3 parts of sodium hydroxide are added for cross-linking hydrolysis, neutralized, washed, and dried to obtain cross-linked hydrolyzed starch. 10 parts of cross-linked hydrolyzed starch are dispersed in 90 parts of sodium hydroxide solution, 0.1 parts of tetrabutylammonium bromide and 1.1 parts of propylene oxide are added for etherification reaction, 3 times the volume of anhydrous ethanol is added to the reaction system after the reaction is completed, and then the mixture is precipitated, filtered, washed, and dried.

[0030] Example 1

[0031] The expansion agent is prepared by mixing 85 parts of active magnesium oxide, 5 parts of calcium oxide, 6 parts of modified starch, 6 parts of aluminum sulfate, and 3 parts of sodium carbonate.

[0032] After detection, the 3d compressive strength of the test piece is 32.6MPa, the 3d expansion rate is +0.01%, the 7d compressive strength is 47.0MPa, the 7d expansion rate is +0.04%, the 28d compressive strength is 53.5MPa, and the 28d expansion rate is +0.06%.

[0033] Example 2

[0034] Expanding agent, each raw material weight parts: 90 parts of active magnesium oxide, 3 parts of calcium oxide, 7 parts of modified starch, 5 parts of aluminum sulfate, 4 parts of sodium carbonate.

[0035] It is detected that the 3d compressive strength of the test piece is 33.4MPa, the 3d expansion rate is +0.02%, the 7d compressive strength is 49.2MPa, the 7d expansion rate is +0.05%, the 28d compressive strength is 54.4MPa, and the 28d expansion rate is +0.07%.

[0036] Comparative example 1

[0037] Expanding agent, each raw material weight parts: 93 parts of active magnesium oxide, 7 parts of modified starch, 5 parts of aluminum sulfate, 4 parts of sodium carbonate.

[0038] It is detected that the 3d compressive strength of the test piece is 31.9MPa, the 3d expansion rate is +0.00%, the 7d compressive strength is 46.4MPa, the 7d expansion rate is +0.03%, the 28d compressive strength is 50.5MPa, and the 28d expansion rate is +0.05%.

[0039] Comparative example 2

[0040] Expanding agent, each raw material weight parts: 90 parts of active magnesium oxide, 10 parts of calcium oxide, 5 parts of aluminum sulfate, 4 parts of sodium carbonate.

[0041] It is detected that the 3d compressive strength of the test piece is 30.3MPa, the 3d expansion rate is-0.04%, the 7d compressive strength is 45.1MPa, the 7d expansion rate is-0.02%, the 28d compressive strength is 49.7MPa, and the 28d expansion rate is +0.01%.

[0042] Comparative example 3

[0043] Expanding agent, each raw material weight parts: 90 parts of active magnesium oxide, 3 parts of calcium oxide, 7 parts of starch, 5 parts of aluminum sulfate, 4 parts of sodium carbonate.

[0044] It is detected that the 3d compressive strength of the test piece is 32.4MPa, the 3d expansion rate is-0.05%, the 7d compressive strength is 46.2MPa, the 7d expansion rate is-0.04%, the 28d compressive strength is 50.9MPa, and the 28d expansion rate is-0.01%.

[0045] Comparative example 4

[0046] Expanding agent, each raw material weight parts: 90 parts of active magnesium oxide, 3 parts of calcium oxide, 7 parts of modified starch, 9 parts of calcium carbonate.

[0047] The 3d compressive strength of the test piece is 31.7 MPa, the 3d expansion rate is +0.01%, the 7d compressive strength is 49.2 MPa, the 7d expansion rate is +0.03%, the 28d compressive strength is 54.4 MPa, and the 28d expansion rate is +0.04%.

[0048] Comparative Example 5

[0049] In the present comparative example, no expansion agent is added, and the other raw materials of the cement-based slurry are the same as in Example 2.

[0050] The 3d compressive strength of the test piece is 29.8 MPa, the 3d expansion rate is -0.06%, the 7d compressive strength is 45.3 MPa, the 7d expansion rate is -0.04%, the 28d compressive strength is 50.2 MPa, and the 28d expansion rate is -0.03%.

[0051] Finally, it should be noted that: the above embodiments are only used to illustrate the technical solutions of the present application, and not to limit them; although the present application has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that: it can still modify the technical solutions recorded in the foregoing embodiments, or make equivalent replacement for part or all of the technical features; and these modifications or replacements do not make the essence of the corresponding technical solutions deviate from the scope of the technical solutions of the embodiments of the present application.

Claims

1. A method for producing a magnesium oxide-based low temperature expanding agent, characterized by, It comprises the following steps: 1) grinding magnesite, calcining at 800-900℃, grinding, obtaining active magnesium oxide, 2) mixing starch and water uniformly, adjusting pH value to be acidic, heating to perform acid catalytic hydrolysis reaction, neutralizing, centrifuging, washing, drying, obtaining low molecular weight hydrolyzed starch, 3) mixing low molecular weight hydrolyzed starch and water uniformly, adding epichlorohydrin and sodium hydroxide to perform cross-linking hydrolysis, neutralizing, washing, drying, obtaining cross-linked hydrolyzed starch, the mass ratio of low molecular weight hydrolyzed starch, water, epichlorohydrin and sodium hydroxide being 10:90:1:0.3; 4) dispersing cross-linked hydrolyzed starch in alkali solution, adding tetrabutylammonium bromide and propylene oxide to perform etherification reaction, adding anhydrous ethanol after the reaction, precipitating, suction filtering, washing, drying, obtaining modified starch, the mass ratio of cross-linked hydrolyzed starch, tetrabutylammonium bromide and propylene oxide being 10: 0.1:1.1， 5) mixing 80-90 parts of active magnesium oxide, 3-5 parts of calcium oxide, 5-8 parts of modified starch, 3-6 parts of aluminum sulfate and 2-4 parts of sodium carbonate uniformly, obtaining the product.

2. The method of claim 1, wherein the magnesium oxide-based low temperature expanding agent is prepared by the steps of: In step 1), the magnesite is ground to a particle size of ≤0.1mm, and the calcination time is 1-2h, and the particle size of the active magnesium oxide is ≤20μm.

3. The preparation method of the magnesium oxide-based low-temperature expanding agent according to claim 1, characterized in that, In step 2), the starch is selected from at least one of corn starch, cassava starch and potato starch.

4. The method of claim 3, wherein the magnesium oxide-based low temperature expanding agent is prepared by the steps of: In step 2), the starch is corn starch.

5. The method for preparing the magnesium oxide-based low-temperature expanding agent according to claim 1, characterized in that, In step 2), the pH value is adjusted to be 3-4, and the heating temperature is 45-55℃, and the reaction time is 0.5-1h.

6. The method of claim 1, wherein the magnesium oxide-based low temperature expanding agent is prepared by the steps of: In step 4), the etherification reaction temperature is 45-60℃, and the reaction time is 4-8h.

7. Magnesia-based low temperature expanding agent, characterized in that, Prepared by the preparation method of any one of claims 1-6.

8. The application of the magnesium oxide-based low-temperature expanding agent of claim 7 in the preparation of cement-based materials in a low-temperature environment.

9. Use according to claim 8, characterized in that, The addition amount of the magnesium oxide-based low-temperature expanding agent is 3-8% of the mass of the cementitious material.

10. Use according to claim 9, characterized in that, The addition amount of the magnesium oxide-based low-temperature expanding agent is 4% of the mass of the cementitious material.