A flexural strength resistant cement and its preparation method

Abstract

A flexural strength-resistant cement and its preparation method belong to the technical field of cement materials. The flexural strength-resistant cement comprises the following components in parts by weight: 55-80 parts of compound cement, 20-50 parts of gypsum, 5-15 parts of calcium oxide, 50-60 parts of fly ash, and 4-8 parts of a modifier; wherein, the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide, and nano-SiO₂ with a mass ratio of 4-8:3-5:1. The flexural strength-resistant cement of the present invention has both excellent flexural strength and compressive strength.

Description

Technical Field

[0001] The present invention belongs to the technical field of cement materials, and particularly relates to a flexural strength-resistant cement and a preparation method thereof. Background Art

[0002] Cement, also known as Portland cement, red hair ash, and red hair soil, is a powdery hydraulic inorganic binder. When mixed with water, it will solidify and harden. It is usually not used alone but is used to combine with sand and gravel (aggregates) to form mortar or concrete. The main raw materials of cement are lime or calcium silicate. After hardening, it can resist the erosion of fresh water or salt water. As the main binder component of concrete and cement mortar materials widely used in the fields of construction, bridges, water conservancy, and national defense, its performance directly determines the performance of concrete and cement mortar materials. However, due to the low flexural strength of existing traditional cements, they are prone to cracking and damage under the action of tensile stress generated during the shrinkage process during use, and increasingly difficult to meet the development needs of high durability and long service life of concrete buildings.

[0003] Patent CN109678402B discloses a preparation method and formula of a high compressive strength and high flexural strength-resistant silicate cement composite material, which uses micron-sized green silicon carbide and micro steel fibers to synergistically improve the composite material composed of G-class oil well cement, ultra-fine cement, and extra-fine cement. However, the compatibility of silicon carbide, micro steel fibers, and the cement system in this invention is not good, and the long-term mechanical properties of the cement composite material still need to be further improved.

[0004] Patent CN110436801B discloses a high flexural strength-resistant composite Portland cement and a preparation method thereof. The high flexural strength-resistant composite Portland cement is made from the following raw materials in parts by weight: 35-65 parts of Portland cement clinker, 17.5-40 parts of calcined clay, 10-22.5 parts of limestone, and 1.5-5 parts of gypsum. Although the flexural strength of the cement in this invention is improved to a certain extent, its 28-day flexural strength can only reach 10.2 MPa. Summary of the Invention

[0005] Aiming at the deficiencies of the prior art, the purpose of the present invention is to provide a flexural strength-resistant cement and a preparation method thereof, and the flexural strength-resistant cement has excellent flexural strength and compressive strength.

[0006] To achieve the above purpose, according to one aspect of the present invention, a flexural strength-resistant cement is provided. By weight, it includes the following components: 55-80 parts of compound cement, 20-50 parts of gypsum, 5-15 parts of calcium oxide, 50-60 parts of fly ash, and 4-8 parts of a modifier; wherein, the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide, and nano-SiO2 with a mass ratio of 4-8:3-5:1.

[0007] To the surprise of the inventors of the present invention, in the flexural strength resistant cement system of the present invention, calcium aluminate cement, gypsum, and calcium oxide react with each other to in-situ generate ettringite whiskers. These whiskers are uniformly dispersed in the Portland cement matrix and tightly combined with the matrix, promoting the development of its strength, thereby improving the flexural strength of the material. In addition, the inventors also found that in the cement system of the present invention, the content of the in-situ generated ettringite whiskers needs to be strictly controlled. If the whiskers are generated in excess, the flexural strength of the cement will instead decrease. This is because the generation of excessive whiskers will cause the matrix to crack and microcracks to form in the matrix, reducing the flexural strength of the material.

[0008] In the present invention, in the flexural strength resistant cement, the weight portion of the compound cement is 55 - 80 parts. It can be understood that its weight portion can be any specific value among 55 parts, 60 parts, 65 parts, 70 parts, 75 parts, 80 parts or any value within the range of 55 - 80 parts. In the present invention, the compound cement is Portland cement and calcium aluminate cement with a weight ratio of 13 - 25:1. It can be understood that its weight ratio can be any specific value among 13:1, 18:1, 23:1, 25:1 or any value within the range of 13 - 25:1. Preferably, the compound cement is Portland cement and calcium aluminate cement with a weight ratio of 15 - 25:1. In the present invention, the Portland cement is 52.5 grade ordinary Portland cement; the calcium aluminate cement is 42.5R grade rapid hardening calcium aluminate cement.

[0009] In the present invention, in the flexural strength resistant cement, the weight portion of the gypsum is 20 - 50 parts. It can be understood that its weight portion can be any specific value among 20 parts, 25 parts, 30 parts, 35 parts, 40 parts, 45 parts, 50 parts or any value within the range of 20 - 50 parts. In the present invention, the gypsum is anhydrite.

[0010] In the present invention, in the flexural strength resistant cement, the weight portion of the calcium oxide is 5 - 15 parts. It can be understood that its weight portion can be any specific value among 5 parts, 6 parts, 7 parts, 8 parts, 9 parts, 10 parts, 11 parts, 12 parts, 13 parts, 14 parts, 15 parts or any value within the range of 5 - 15 parts.

[0011] In the present invention, in the flexural strength resistant cement, the weight portion of the fly ash is 50 - 60 parts. It can be understood that its weight portion can be any specific value among 50 parts, 51 parts, 52 parts, 53 parts, 54 parts, 55 parts, 56 parts, 57 parts, 58 parts, 59 parts, 60 parts or any value within the range of 50 - 60 parts. In the present invention, the average particle size of the fly ash is 10 - 100 μm.

[0012] In the present invention, in the flexural strength cement, the weight portion of the modifier is 4 - 8 parts. It can be understood that the weight portion can be any specific value among 4 parts, 5 parts, 6 parts, 7 parts, 8 parts or any value within the range of 4 - 8 parts. In the present invention, the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano - SiO₂ with a mass ratio of 4 - 8:3 - 5:1. Preferably, the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano - SiO₂ with a mass ratio of 6 - 8:4 - 5:1. In the present invention, in the polycarboxylic acid grafted graphene oxide, the mass percentage of polycarboxylic acid is 30 - 35%. In the present invention, the polycarboxylic acid grafted graphene oxide is obtained by the reaction of isopentenyl polyoxyethylene ether, acrylic acid and graphene oxide. In the present invention, the sheet diameter of the graphene oxide is 0.5 - 5 μm, and the thickness is 0.8 - 1.2 nm. Preferably, the average particle size of the nano - SiO₂ is 20 - 80 nm. Preferably, the number - average molecular weight of the isopentenyl polyoxyethylene ether is 2400 g / mol.

[0013] In the present invention, the modifier prepared from anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano - SiO₂ with a specific mass ratio can significantly improve the flexural strength performance of the cement while taking into account good compressive strength performance. The reason is that the polycarboxylic acid molecular chains in the polycarboxylic acid grafted graphene oxide are adsorbed and grafted on the surface of the graphene oxide, having a steric hindrance and electrostatic repulsion effect, which improves the dispersibility of graphene oxide, in - situ generated ettringite whiskers and cement particles. At the same time, the addition of a specific amount of anhydrous sodium sulfate and nano - SiO₂ fills some pores and synergistically improves the denseness of the structure, thereby improving the flexural strength performance and compressive strength performance of the cement. The inventor further found that the mass percentage of polycarboxylic acid in the polycarboxylic acid grafted graphene oxide is crucial for the flexural strength performance. If the content is too high, the flexural strength performance will instead decrease. This is because an excessive amount of polycarboxylic acid is grafted on the surface of the graphene oxide, resulting in a decrease in the denseness of the system, thus causing the flexural strength performance to decrease.

[0014] According to another aspect of the present invention, there is also provided a preparation method of the above - mentioned flexural strength cement, including the following steps:

[0015] (1) Mix the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano - SiO₂ in proportion to obtain a modifier;

[0016] (2) Mix the sulphoaluminate cement, gypsum and calcium oxide in proportion to obtain a first mixture;

[0017] (3) Mix the first mixture, portland cement, fly ash and the modifier in proportion to obtain the flexural strength cement.

[0018] In the present invention, in the step (1), the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano-SiO₂ are uniformly mixed in a proportion to obtain a modifier. Preferably, the preparation method of the polycarboxylic acid grafted graphene oxide comprises the following steps: adding graphene oxide to the first portion of deionized water, fully stirring, adding acrylic acid, isopentenyl alcohol polyoxyethylene ether, a catalyst and the second portion of deionized water, and reacting at 45-55 °C for 6-9 h to obtain the polycarboxylic acid grafted graphene oxide. Preferably, the catalyst is ammonium cerium nitrate. More preferably, the molar ratio of the acrylic acid to the isopentenyl alcohol polyoxyethylene ether is 6-8:1, the mass ratio of the first portion of deionized water to the second portion of deionized water is 1:1-1.5, and the molar amount of the catalyst is 0.5-2% of the total molar amount of the acrylic acid and the isopentenyl alcohol polyoxyethylene ether.

[0019] In the present invention, in the step (2), the sulfoaluminate cement, gypsum and calcium oxide are uniformly mixed in a proportion to obtain a first mixture.

[0020] In the present invention, in the step (3), the first mixture, portland cement, fly ash and the modifier are uniformly mixed in a proportion to obtain the flexural strength cement.

[0021] When the flexural strength cement of the present invention is used, a certain amount of water is added for preparation, and it can be stirred evenly.

[0022] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0023] (1) The present invention uses anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano-SiO₂ with a specific mass ratio to prepare a modifier. The polycarboxylic acid molecular chains in the polycarboxylic acid grafted graphene oxide are adsorbed and grafted on the surface of the graphene oxide, having appropriate steric hindrance and electrostatic repulsion effects, improving the dispersibility of the graphene oxide, the in-situ generated ettringite whiskers and the cement particles. At the same time, the addition of a specific amount of anhydrous sodium sulfate and nano-SiO₂ fills some pores, synergistically improving the compactness of the structure, realizing good structure regulation, and thus improving the flexural strength performance of the cement. In addition, the addition of the modifier better regulates the hydration rate, thereby improving the early strength of the cement-based material.

[0024] (2) The present invention adds a certain amount of sulfoaluminate cement, gypsum and calcium oxide, and the three react with each other to in-situ generate ettringite whiskers. These whiskers are uniformly dispersed in the portland cement matrix and are tightly combined with the matrix, promoting the development of its strength, and thus improving the flexural strength performance of the material.

[0025] (3) The preparation method of the present invention is simple and convenient, the product performance is stable, and it is suitable for large-scale production. Detailed implementation manners

[0026] To enable those skilled in the art to understand the features and effects of the present invention, the following provides a general description and definition of the terms and expressions mentioned in the specification and claims. Unless otherwise specified, all technical and scientific terms used herein shall have the ordinary meanings understood by those skilled in the art with respect to the present invention. In case of conflicts, the definitions in this specification shall prevail.

[0027] The theories or mechanisms described and disclosed herein, whether correct or incorrect, shall not in any way limit the scope of the present invention, that is, the content of the present invention can be implemented without being limited by any specific theory or mechanism.

[0028] In this document, when describing embodiments or examples, it should be understood that it is not used to limit the present invention to these embodiments or examples. On the contrary, all alternatives, improvements, and equivalents of the methods and materials described in the present invention can be covered within the scope defined by the claims.

[0029] In this document, for the sake of brevity, all possible combinations of all technical features in each embodiment or example are not described. Therefore, as long as there is no contradiction in the combination of these technical features, the technical features in each embodiment or example can be combined arbitrarily, and all possible combinations should be considered as the scope described in this specification.

[0030] The present invention provides a flexural-resistant cement, which comprises the following components by weight parts: 55 - 80 parts of compound cement, 20 - 50 parts of gypsum, 5 - 15 parts of calcium oxide, 50 - 60 parts of fly ash, and 4 - 8 parts of modifier; wherein, the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide, and nano-SiO₂ with a mass ratio of 4 - 8:3 - 5:1.

[0031] In the present invention, the compound cement is a silicate cement and a sulphoaluminate cement with a weight ratio of 13 - 25:1.

[0032] In the present invention, in the polycarboxylic acid grafted graphene oxide, the mass percentage of polycarboxylic acid is 30 - 35%.

[0033] In the present invention, the silicate cement is 52.5 grade ordinary silicate cement; the sulphoaluminate cement is 42.5R grade rapid hard sulphoaluminate cement.

[0034] In the present invention, the gypsum is anhydrite.

[0035] In the present invention, the polycarboxylic acid grafted graphene oxide is obtained by reacting isopentenyl alcohol polyoxyethylene ether, acrylic acid, and graphene oxide.

[0036] In the present invention, the average particle size of the fly ash is 10 - 100 μm.

[0037] In the present invention, the average particle size of the nano-SiO₂ is 20 - 80 nm.

[0038] According to another aspect of the present invention, there is also provided a method for preparing the above-mentioned flexural strength cement, comprising the following steps:

[0039] (1) Mix the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide, and nano-SiO₂ in proportion to obtain a modifier;

[0040] (2) Mix the sulphoaluminate cement, gypsum, and calcium oxide in proportion to obtain a first mixture;

[0041] (3) Mix the first mixture, Portland cement, fly ash, and modifier in proportion to obtain the flexural strength cement.

[0042] In the present invention, the method for preparing the polycarboxylic acid grafted graphene oxide comprises the following steps: Add graphene oxide to the first portion of deionized water and stir thoroughly, then add acrylic acid, isopentenyl alcohol polyoxyethylene ether, a catalyst, and the second portion of deionized water, and react at 45 - 55 °C for 6 - 9 h to obtain the polycarboxylic acid grafted graphene oxide.

[0043] The present invention will be described in detail below through examples. It should be understood that the following examples are only used to further explain and illustrate the content of the present invention by way of example, and are not used to limit the present invention. Among them, Example 1 is the best example of the present invention.

[0044] The chemical auxiliaries used in the examples and comparative examples of the present invention are all commercially available, and the specific information is as follows:

[0045] Sulphoaluminate cement: 42.5R grade rapid hardening sulphoaluminate cement, purchased from Jianghuai Building Materials Co., Ltd., Wuxi City; Portland cement: 52.5 grade ordinary Portland cement, purchased from Jianghuai Building Materials Co., Ltd., Wuxi City; Gypsum: anhydrite, purchased from Hubei Longyuan Gypsum Co., Ltd.; Calcium oxide: purchased from Zibo Wanpu New Materials Co., Ltd.; Graphene oxide: with a sheet diameter of 0.5 - 5 μm and a thickness of 0.8 - 1.2 nm, purchased from Aladdin Reagent Co., Ltd.; Nano-SiO₂: with an average particle size of 20 - 80 nm, purchased from Aladdin Reagent Co., Ltd.; Anhydrous sodium sulfate, acrylic acid, isopentenyl alcohol polyoxyethylene ether (number average molecular weight 2400 g / mol), ammonium cerium nitrate: all purchased from Aladdin Reagent Co., Ltd.; Fly ash: with an average particle size of 10 - 100 μm, purchased from Guangdong Xinze Building Materials Co., Ltd.; Water: ordinary tap water.

[0046] Polycarboxylic acid grafted graphene oxide A: The preparation method of the polycarboxylic acid grafted graphene oxide includes the following steps: Add 48.9 g of graphene oxide to 600 mL of the first portion of deionized water and stir well. Then add 4 g of acrylic acid, 19 g of isopentenyl alcohol polyoxyethylene ether, 0.35 g of catalyst ammonium cerium nitrate, and 900 mL of the second portion of deionized water. Pass nitrogen gas and react at 55 °C for 6 h to obtain the polycarboxylic acid grafted graphene oxide A.

[0047] Polycarboxylic acid grafted graphene oxide B: The preparation method of the polycarboxylic acid grafted graphene oxide includes the following steps: Add 48.3 g of graphene oxide to 600 mL of the first portion of deionized water and stir well. Then add 4 g of acrylic acid, 16.7 g of isopentenyl alcohol polyoxyethylene ether, 0.69 g of catalyst ammonium cerium nitrate, and 600 mL of the second portion of deionized water. Pass nitrogen gas and react at 50 °C for 9 h to obtain the polycarboxylic acid grafted graphene oxide B.

[0048] Polycarboxylic acid grafted graphene oxide C: The preparation method of the polycarboxylic acid grafted graphene oxide includes the following steps: Add 42.7 g of graphene oxide to 600 mL of the first portion of deionized water and stir well. Then add 4 g of acrylic acid, 19 g of isopentenyl alcohol polyoxyethylene ether, 0.35 g of catalyst ammonium cerium nitrate, and 700 mL of the second portion of deionized water. Pass nitrogen gas and react at 55 °C for 6 h to obtain the polycarboxylic acid grafted graphene oxide C.

[0049] Polycarboxylic acid grafted graphene oxide D: The preparation method of the polycarboxylic acid grafted graphene oxide includes the following steps: Add 34.5 g of graphene oxide to 600 mL of the first portion of deionized water and stir well. Then add 4 g of acrylic acid, 19 g of isopentenyl alcohol polyoxyethylene ether, 0.35 g of catalyst ammonium cerium nitrate, and 700 mL of the second portion of deionized water. Pass nitrogen gas and react at 55 °C for 6 h to obtain the polycarboxylic acid grafted graphene oxide D.

[0050] Perform thermogravimetric analysis on the polycarboxylic acid grafted graphene oxide prepared above. The specific method is as follows: Weigh 20 mg of the sample into a crucible, place the crucible on the sample stage of the thermal analyzer, and under a nitrogen atmosphere, heat the sample from room temperature (25 °C) to 1000 °C at a heating rate of 10 °C / min. Calculate the content of the polycarboxylic acid according to the thermogravimetric analysis curve. The specific results are shown in Table 1.

[0051] Table 1 Performance data of polycarboxylic acid grafted graphene oxide .

[0052] Example 1

[0053] The flexural cement described in this embodiment includes the following components in parts by weight: 60 parts of compound cement, 25 parts of gypsum, 15 parts of calcium oxide, 55 parts of fly ash and 6 parts of modifier; wherein the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide A and nano-SiO2 in a mass ratio of 6:4:1; the compound cement is silicate cement and sulphoaluminate cement in a weight ratio of 19:1.

[0054] The preparation method of the anti-flexural cement comprises the following steps:

[0055] (1) mixing the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide A and nano-SiO2 in proportion to obtain a modifier;

[0056] (2) mixing the sulphoaluminate cement, gypsum and calcium oxide in proportion to obtain a first mixture;

[0057] (3) The first mixture, silicate cement, fly ash and modifier are mixed in proportion to obtain the flexural resistant cement.

[0058] Example 2

[0059] The flexural cement described in this embodiment includes the following components, measured in parts by weight: 55 parts of compound cement, 32 parts of gypsum, 15 parts of calcium oxide, 60 parts of fly ash and 8 parts of modifier; wherein the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide B and nano-SiO2 in a mass ratio of 4:3:1; the compound cement is silicate cement and sulphoaluminate cement in a weight ratio of 13:1.

[0060] The preparation method of the anti-flexural cement comprises the following steps:

[0061] (1) mixing the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide B and nano-SiO2 in proportion to obtain a modifier;

[0062] (2) mixing the sulphoaluminate cement, gypsum and calcium oxide in proportion to obtain a first mixture;

[0063] (3) The first mixture, silicate cement, fly ash and modifier are mixed in proportion to obtain the flexural resistant cement.

[0064] Example 3

[0065] The flexural cement described in this embodiment includes the following components in parts by weight: 80 parts of compound cement, 20 parts of gypsum, 12 parts of calcium oxide, 50 parts of fly ash and 4 parts of modifier; wherein the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide C and nano-SiO2 in a mass ratio of 8:5:1; the compound cement is silicate cement and sulphoaluminate cement in a weight ratio of 25:1.

[0066] The preparation method of the anti-flexural cement comprises the following steps:

[0067] (1) mixing the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide C and nano-SiO2 in proportion to obtain a modifier;

[0068] (2) mixing the sulphoaluminate cement, gypsum and calcium oxide in proportion to obtain a first mixture;

[0069] (3) The first mixture, silicate cement, fly ash and modifier are mixed in proportion to obtain the flexural resistant cement.

[0070] Example 4

[0071] The flexural cement described in this embodiment includes the following components in parts by weight: 66 parts of compound cement, 20 parts of gypsum, 15 parts of calcium oxide, 50 parts of fly ash and 7 parts of modifier; wherein the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide A and nano-SiO2 in a mass ratio of 4:3:1; the compound cement is silicate cement and sulphoaluminate cement in a weight ratio of 15:1.

[0072] The preparation method of the anti-flexural cement comprises the following steps:

[0073] (1) mixing the anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide A and nano-SiO2 in proportion to obtain a modifier;

[0074] (2) mixing the sulphoaluminate cement, gypsum and calcium oxide in proportion to obtain a first mixture;

[0075] (3) The first mixture, silicate cement, fly ash and modifier are mixed in proportion to obtain the flexural resistant cement.

[0076] Comparative Example 1

[0077] The preparation method of the anti-bending cement described in this comparative example is the same as that of Example 1, except that the polycarboxylic acid grafted graphene oxide A is replaced by an equal amount of polycarboxylic acid grafted graphene oxide D.

[0078] Comparative Example 2

[0079] The preparation method of the flexural-resistant cement described in this comparative example is the same as that of Example 1, except that the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide A and nano-SiO2 in a mass ratio of 5:8:1.

[0080] Comparative Example 3

[0081] The preparation method of the high-performance cement-based grouting material described in this comparative example is the same as that of Example 1, except that the modifier is polycarboxylic acid grafted graphene oxide A and nano-SiO₂ with a mass ratio of 4:1.

[0082] Comparative Example 4

[0083] The preparation method of the flexural strength cement described in this comparative example is the same as that of Example 1, except that the compound cement is portland cement and sulfoaluminate cement with a weight ratio of 35:1.

[0084] Performance test

[0085] The cement, sand and water described in Examples 1-4 and Comparative Examples 1-4 were configured with a water-cement ratio of 0.4 and a mass ratio of cement to sand of 1:3. After stirring evenly, the paste was taken out and placed in a mold (40 mm × 40 mm × 160 mm), and then cured in a thermostatic and humid box at a temperature of 25 °C and a humidity of 98% for 28 days. Performance tests were carried out in accordance with GB / T 17671-2021, and the specific results are shown in Table 2.

[0086] Table 2 Cement performance data in Examples 1-4 and Comparative Examples 1-4 。

[0087] As can be seen from Table 1, the flexural strength and compressive strength of the cement described in Examples 1-4 of the present invention are relatively high at different days. Among them, the 3-day flexural strength of the cement in Example 1 exceeds 20 MPa, the 7-day flexural strength exceeds 33 MPa, and the 28-day flexural strength exceeds 39 MPa, indicating that its early and long-term flexural strengths are relatively high. By comparing Example 1 with Comparative Example 1, it can be seen that the content of polycarboxylic acid in the polycarboxylic acid grafted graphene oxide in Comparative Example 1 is relatively high, and its flexural strength and compressive strength are significantly reduced, indicating that the content of polycarboxylic acid in the polycarboxylic acid grafted graphene oxide has a great influence on the mechanical properties of the cement system, and the more is not necessarily better. By comparing Example 1 with Comparative Example 2, it can be seen that the proportion of polycarboxylic acid grafted graphene oxide in the modifier in Comparative Example 2 is relatively high, and its flexural strength decreases significantly, indicating that the proportional relationship of the three components in the modifier is crucial for the flexural performance, and the proportional change will reduce the synergistic effect of the three, thus affecting the performance. By comparing Example 1 with Comparative Example 3, it can be seen that the modifier in Comparative Example 3 does not contain anhydrous sodium sulfate, and its flexural strength is significantly reduced, indicating that there is a synergistic effect between the three components in the modifier, which jointly improves the flexural strength of the system. By comparing Example 1 with Comparative Example 4, it can be seen that the compounding ratio of portland cement and sulfoaluminate cement in Comparative Example 4 is different, and the compressive strength and flexural strength of its system are both affected.

[0088] It can be seen that the modifier of the present invention is prepared by using anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano-SiO₂ with a specific mass ratio. The polycarboxylic acid molecular chains in the polycarboxylic acid grafted graphene oxide are adsorbed and grafted on the surface of the graphene oxide, having appropriate steric hindrance and electrostatic repulsion effects, which can improve the dispersibility of graphene oxide, the ettringite whiskers in-situ generated in the system and cement particles. At the same time, the addition of a specific amount of anhydrous sodium sulfate and nano-SiO₂ fills some pores and synergistically improves the compactness of the structure, realizing good structure regulation, thereby improving the flexural strength of the cement. The addition of the modifier better regulates the hydration rate. The hydration rate and pore structure of the cement of the present invention avoid the cracking damage caused by the freezing of water in the pore structure at low temperatures, thereby improving the early strength and low-temperature resistance of the cement-based material. In addition, a certain amount of sulfoaluminate cement, gypsum and calcium oxide are added in the present invention, and the three react with each other to in-situ generate ettringite whiskers. These whiskers are uniformly dispersed in the Portland cement matrix and combined tightly with the matrix, playing a bridging role and promoting the development of its strength, thereby improving the flexural strength of the material.

[0089] Obviously, the above embodiments of the present invention are merely examples for clearly illustrating the present invention, rather than limitations on the implementation modes of the present invention. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is impossible to list all the implementation modes here. Any obvious changes or modifications derived from the technical solutions of the present invention still fall within the protection scope of the present invention.

Claims

1. A flexural cement, characterized in that: The invention comprises the following components by weight: 55-80 parts of compound cement, 20-50 parts of gypsum, 5-15 parts of calcium oxide, 50-60 parts of fly ash and 4-8 parts of modifier; wherein the modifier is anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano-SiO2 in a mass ratio of 4-8:3-5:1; the compound cement is silicate cement and sulphoaluminate cement in a weight ratio of 13-25:1; in the polycarboxylic acid grafted graphene oxide, the mass percentage of polycarboxylic acid is 30-35%.

2. The flexural cement according to claim 1, characterized in that: The silicate cement is 52.5 grade ordinary silicate cement; the sulphoaluminate cement is 42.5R grade rapid hardening sulphoaluminate cement.

3. The flexural cement according to claim 1, characterized in that: The gypsum is anhydrite.

4. The flexural-resistant cement according to claim 1, characterized in that: The polycarboxylic acid grafted graphene oxide is obtained by reacting isopentanol polyoxyethylene ether, acrylic acid and graphene oxide.

5. The flexural-resistant cement according to claim 1, characterized in that: The average particle size of the fly ash is 10-100 μm.

6. The flexural-resistant cement according to claim 1, characterized in that: The average particle size of the nano-SiO2 is 20-80nm.

7. A method for preparing the flexural-resistant cement according to any one of claims 1 to 6, characterized in that: The steps include: (1) mixing anhydrous sodium sulfate, polycarboxylic acid grafted graphene oxide and nano-SiO2 in proportion to obtain a modifier; (2) mixing sulphoaluminate cement, gypsum and calcium oxide in proportion to obtain a first mixture; (3) The first mixture, silicate cement, fly ash and modifier are mixed in proportion to obtain flexural cement.

8. The method for preparing flexural resistant cement according to claim 7, characterized in that: The preparation method of polycarboxylic acid grafted graphene oxide comprises the following steps: adding graphene oxide to a first portion of deionized water and stirring the mixture thoroughly, adding acrylic acid, isopentanol polyoxyethylene ether, a catalyst and a second portion of deionized water, and reacting the mixture at 45-55° C. for 6-9 hours to obtain the polycarboxylic acid grafted graphene oxide.