Micro-expansive cement and preparation method thereof

Abstract

The invention relates to micro-expansive cement and a preparation method thereof, and belongs to the technical field of cement material preparation. The micro-expansive cement disclosed by the invention is prepared from the following raw materials: cement clinker, low-calcium fly ash, an expanding agent, modified carbon fibers, polyether ester elastic fibers and gypsum. The silicon dioxide is uniformly wrapped on the surface of the carbon fiber through a gas phase precipitation method to form a stable core-shell structure which can be uniformly and stably dispersed in cement slurry, the modified carbon fiber is added to limit the expansion rate of mortar, reduce the porosity and improve the compactness and strength of the cement board, and the modified carbon fiber is matched with the polyether ester elastic fiber with higher elongation to improve the strength of the cement board. The micro-expansive cement with strong crack resistance is obtained.

Description

Technical Field

[0001] The present invention belongs to the technical field of cement materials and relates to a micro-expansive cement and a preparation method thereof. Background Art

[0002] Cement is widely used in road construction due to its high strength, good water stability, good frost resistance and other advantages. When ordinary cement is made into concrete, cracks often occur in the concrete because the internal shrinkage stress exceeds the tensile strength of the concrete, which affects the structural stability and durability of the concrete. The micro-expansive cement effectively improves the problem of cracks caused by volume shrinkage of the concrete through its compensation shrinkage mechanism.

[0003] Many literatures have studied the anti-cracking performance of cement. Chinese Patent CN113024141A provides a modified carbon fiber and its preparation method and a modified carbon fiber-reinforced cement-based material. This patent incorporates carbon fiber, and through oxidizing the surface of the carbon fiber, nano-silica and carbon nanotubes are simultaneously in-situ grown on the surface of the carbon fiber and incorporated into the cement-based material, effectively improving the early shrinkage performance of the cement-based material. However, the steps for preparing the modified carbon fiber are relatively cumbersome, and the growth of silica is not easy to control; Chinese Patent CN116789424Ag discloses a preparation method of a carbon fiber cement composite material. This invention uses cement, carbon fiber, silica fume, and a dispersant as raw materials, and the dispersant is polyvinylpyrrolidone, hydroxyethyl cellulose, sodium hexametaphosphate or methyl cellulose. The addition of the dispersant solves the problem of agglomeration of carbon fibers in cement, but the dispersant also has a thickening effect, which has different degrees of influence on the expansion rate and strength of the cement mortar. There are also studies on adding polypropylene fibers, which belong to high-ductility fibers. After adding polypropylene fibers, the compressive strength has little effect in the early stage but will increase in the long term, and the flexural strength is significantly improved. However, with the increase of the fiber content, the flexural and compressive strengths show a trend of first increasing and then decreasing, and the strength of some specimens is even lower than that of the specimens without fibers. This is mainly because when the content is large, the dispersion degree of the fibers in the matrix becomes poor, and the entangled fibers are the weak points of damage, resulting in a decrease in strength. Summary of the Invention

[0004] The present invention relates to a micro-expansive cement and a preparation method thereof, belonging to the technical field of cement material preparation. The micro-expansive cement disclosed by the present invention comprises the following raw materials: cement clinker, low-calcium fly ash, an expansive agent, modified carbon fiber, polyether ester elastic fiber, and gypsum. By means of the gas-phase deposition method, silica is uniformly coated on the surface of the carbon fiber to form a stable core-shell structure, which can be uniformly and stably dispersed in the cement slurry. The addition of the modified carbon fiber restricts the expansion rate of the mortar, reduces the porosity, and improves the compactness and strength of the cement board. When combined with the polyether ester elastic fiber with a relatively large elongation rate, a micro-expansive cement with strong anti-cracking performance is obtained.

[0005] The object of the present invention can be achieved by the following technical solutions: A micro-expansive cement, the micro-expansive cement comprising the following raw materials in parts by weight: 50-80 parts of cement clinker, 15-20 parts of low-calcium fly ash, 8-12 parts of an expansive agent, 10-12 parts of modified carbon fiber, 5-7 parts of polyether ester elastic fiber, and 8-10 parts of gypsum.

[0006] Further, the expansive agent is a calcium sulfoaluminate expansive agent, and the calcium oxide content in the low-calcium fly ash is 2.5-3.5%.

[0007] Further, the preparation method of the modified carbon fiber comprises the following steps: (1) Pour concentrated nitric acid into carbon fiber so that the carbon fiber is completely immersed, then heat and stir, and then filter. The solid is washed with absolute ethanol and dried to form pretreated carbon fiber for standby; (2) Place the pretreated carbon fiber in a reaction chamber, heat it after purging oxygen by introducing argon, then mix silane with argon as the carrier gas, close the silane gas channel, then introduce oxygen, and then cool down to room temperature to obtain modified carbon fiber.

[0008] Further, in the step (1), the mass ratio of concentrated nitric acid to carbon fiber is 5-8:3, wherein the mass fraction of concentrated nitric acid is 65-68%, the heating temperature is 100-120 °C, the stirring time is 2-3 h, and the drying temperature and time are 60-80 °C and 2-3 h respectively.

[0009] Further, in the step (2), the heating temperature is 600-700 °C, the gas flow rate and time for mixing silane are 10-30 sccm and 20-30 min respectively, wherein silane is composed of equi-volume silane and disilane, and the gas flow rate and time for introducing oxygen are 8-10 sccm and 40-60 min respectively.

[0010] Further, the preparation method of the polyether ester elastic fiber is: Put terephthalate, 1,4-butanediol, a catalyst and an antioxidant into an esterification kettle, heat and stir, carry out condensation reflux, and the material obtained at the bottom of the kettle is extruded by an extruder, wound, and pulverized to obtain polyether ester elastic fiber.

[0011] Further, the mass ratio of the terephthalate, 1,4-butanediol, catalyst, and antioxidant is 3-5:1-2:0.05:0.01-0.02, wherein the catalyst is composed of cobalt acetate and tetrabutyl titanate with a mass ratio of 1:1. The temperature and time for heating and stirring are 210-220°C and 1-2 h respectively. The temperature and pressure of the extruder are 210-230°C and 7-10 MPa respectively. The winding speed is 200-400 m / min. The fineness and length of the polyether ester elastic fiber are 1-1.5 D and 1-3 mm respectively.

[0012] Further, the preparation method of the slightly expanding cement is as follows: all raw materials are mixed according to weight parts. Among them, the clinker cement and low-calcium fly ash are dried at 110-120°C for 2 h, and the gypsum is ground into powder with a mesh number of 200-300.

[0013] Advantages of the present invention: 1. In the present invention, silica is uniformly coated on the surface of carbon fiber by the gas-phase precipitation method, and the growth of silica is controlled by the oxygen gas flow rate to form a stable and tight core-shell structure, which can be uniformly and stably dispersed in the cement slurry. At the same time, the addition of modified carbon fiber improves the fluidity of the mortar, limits the expansion rate of the mortar, reduces the porosity, and improves the compactness and strength of the cement board, thereby effectively improving the crack resistance of the cement. 2. The hard segment polyester and soft segment polyether in the polyether ester elastic fiber endow the material with excellent tensile strength at both high and low temperatures. And under alkaline conditions, the negative charge on the fiber surface increases, and the repulsive interaction enhances the dispersibility of the fiber in water. Compared with polypropylene fiber, it has more excellent mechanical properties and dispersibility. Specific embodiments

[0014] To further illustrate the technical means and effects adopted by the present invention to achieve the predetermined invention purpose, the following examples are used to describe in detail the specific embodiments, structures, features and their effects of the present invention.

[0015] The cement clinker involved in the present invention is purchased from Guangzhou Qiangsheng Cement Grinding Co., Ltd.; the low-calcium fly ash is purchased from Hebei Houkang Mineral Products Co., Ltd.; the carbon fiber is purchased from Jiangxi Shuobang New Materials Technology Co., Ltd.; the gypsum is purchased from Hebei Huibaijia Building Materials Co., Ltd., with the model p130 / p160, and the antioxidant is purchased from Shenzhen Longdi Chemical Co., Ltd., with the model Irganox1076.

[0016] Example 1

[0017] A slightly expanding cement, which comprises the following raw materials in parts by weight: 50 parts of cement clinker, 15 parts of low-calcium fly ash, 8 parts of expanding agent, 10 parts of modified carbon fiber, 5 parts of polyether ester elastic fiber, and 8 parts of gypsum.

[0018] The mesh number of the cement clinker is 180 mesh, the mesh number of the low-calcium fly ash is 300 mesh, the mesh number of the expansive agent is 500 mesh, the length of the carbon fiber is 3 mm, and the aspect ratio is 1:3. The specific component analysis of the cement clinker is as follows:

[0019] The expansive agent is calcium sulfoaluminate expansive agent, and the calcium oxide content in the low-calcium fly ash is 2.5%.

[0020] The preparation method of the modified carbon fiber includes the following steps: (1) Pour concentrated nitric acid into the carbon fiber to completely immerse the carbon fiber, then heat and stir, and then filter. The solid is washed with anhydrous ethanol and dried to form pretreated carbon fiber for standby; (2) Place the pretreated carbon fiber in a reaction chamber, heat it after purging oxygen by introducing argon, then mix silane with argon as the carrier gas, close the silane gas channel, then introduce oxygen, and then cool down to room temperature to obtain modified carbon fiber.

[0021] In the step (1), the mass ratio of concentrated nitric acid to carbon fiber is 5:3, the mass fraction of concentrated nitric acid is 65%, the heating temperature is 100 °C, the stirring time is 2 h, and the drying temperature and time are 60 °C and 2 h respectively.

[0022] In the step (2), the heating temperature is 600 °C, the gas flow rate and time for mixing silane are 10 sccm and 20 min respectively. The silane is composed of equi-volume silane and disilane. The gas flow rate and time for introducing oxygen are 8 sccm and 40 min respectively.

[0023] The preparation method of the polyether ester elastic fiber is as follows: Put terephthalate, 1,4-butanediol, catalyst and antioxidant into an esterification kettle, heat and stir, and carry out condensation reflux. The material obtained at the bottom of the kettle is extruded by an extruder, wound, and pulverized to obtain polyether ester elastic fiber.

[0024] The mass ratio of terephthalate, 1,4-butanediol, catalyst and antioxidant is 3:1:0.05:0.01. The catalyst is composed of cobalt acetate and tetrabutyl titanate with a mass ratio of 1:1. The heating and stirring temperature and time are 210 °C and 1 h respectively. The temperature and pressure of the extruder are 210 °C and 7 MPa respectively. The winding speed is 200 m / min. The fineness and length of the polyether ester elastic fiber are 1 D and 1 mm respectively.

[0025] The preparation method of the slightly expansive cement is as follows: all raw materials are mixed according to parts by weight. Among them, the clinker cement and low-calcium fly ash are dried at 110°C for 2 hours, and the gypsum is ground into a powder with a mesh number of 200 meshes.

[0026] Example 2

[0027] A slightly expansive cement, which comprises the following raw materials in parts by weight: 65 parts of cement clinker, 17 parts of low-calcium fly ash, 10 parts of expansive agent, 11 parts of modified carbon fiber, 6 parts of polyether ester elastic fiber, and 9 parts of gypsum.

[0028] The mesh number of the cement clinker is 180 meshes, the mesh number of the low-calcium fly ash is 300 meshes, the mesh number of the expansive agent is 500 meshes, the length of the carbon fiber is 3 mm, and the aspect ratio is 1:3. The specific component analysis of the cement clinker is as follows:

[0029] The expansive agent is a calcium sulfoaluminate expansive agent, and the calcium oxide content in the low-calcium fly ash is 2.5%.

[0030] The expansive agent is a calcium sulfoaluminate expansive agent, and the calcium oxide content in the low-calcium fly ash is 3%.

[0031] The preparation method of the modified carbon fiber comprises the following steps: (1) Pour concentrated nitric acid into the carbon fiber so that the carbon fiber is completely immersed, then heat and stir, and then filter. The solid is washed with absolute ethanol and dried to form pretreated carbon fiber for standby; (2) Place the pretreated carbon fiber in a reaction chamber, heat it after purging oxygen with argon, then mix silane with argon as the carrier gas, close the silane gas channel, then introduce oxygen, and then cool down to room temperature to obtain modified carbon fiber.

[0032] In the step (1), the mass ratio of concentrated nitric acid to carbon fiber is 6.5:3, the mass fraction of concentrated nitric acid is 67%, the heating temperature is 110°C, the stirring time is 2.5 h, and the drying temperature and time are 70°C and 2.5 h respectively.

[0033] In the step (2), the heating temperature is 650°C, the gas flow rate and time for mixing silane are 20 sccm and 25 min respectively. Among them, silane is composed of equi-volume silane and disilane, and the gas flow rate and time for introducing oxygen are 9 sccm and 50 min respectively.

[0034] The preparation method of the polyether ester elastic fiber is as follows: Put terephthalate, 1,4-butanediol, catalyst and antioxidant into the esterification kettle, heat up and stir, condense and reflux. The material obtained at the bottom of the kettle is extruded by an extruder, wound, and pulverized to obtain polyether ester elastic fiber.

[0035] The mass ratio of the terephthalate, 1,4-butanediol, catalyst and antioxidant is 4:1.5:0.05:0.015. The catalyst consists of cobalt acetate and tetrabutyl titanate with a mass ratio of 1:1. The temperature and time of the heating and stirring are 215°C and 1.5 h respectively. The temperature and pressure of the extruder are 220°C and 8 MPa respectively. The winding speed is 300 m / min. The fineness and length of the polyether ester elastic fiber are 1.2 D and 2 mm respectively.

[0036] The preparation method of the slightly expansive cement is as follows: Mix all raw materials according to weight. The clinker cement and low-calcium fly ash are dried at 115°C for 2 h, and the gypsum is ground into powder with a mesh number of 250.

[0037] Example 3

[0038] A slightly expansive cement, which includes the following raw materials in parts by weight: 80 parts of cement clinker, 20 parts of low-calcium fly ash, 12 parts of expansive agent, 12 parts of modified carbon fiber, 7 parts of polyether ester elastic fiber, and 10 parts of gypsum.

[0039] The mesh number of the cement clinker is 180, the mesh number of the low-calcium fly ash is 300, the mesh number of the expansive agent is 500. The length of the carbon fiber is 3 mm, and the aspect ratio is 1:3. The component analysis of the cement clinker is specifically as follows:

[0040] The expansive agent is calcium sulfoaluminate expansive agent, and the calcium oxide content in the low-calcium fly ash is 3.5%.

[0041] The preparation method of the modified carbon fiber includes the following steps: (1) Pour concentrated nitric acid into the carbon fiber to completely immerse the carbon fiber, then heat and stir, and then filter. The solid is washed with absolute ethanol and dried to form pretreated carbon fiber for standby. (2) Place the pretreated carbon fiber in the reaction chamber, heat it after purging oxygen by introducing argon, then mix silane with argon as the carrier gas, close the silane gas channel, then introduce oxygen, and then cool down to room temperature to obtain modified carbon fiber.

[0042] In the step (1), the mass ratio of the concentrated nitric acid to the carbon fiber is 8:3, the mass fraction of the concentrated nitric acid is 68%, the heating temperature is 120°C, the stirring time is 3 h, and the drying temperature and time are 80°C and 3 h respectively.

[0043] In step (2), the heating temperature is 700 °C, the gas flow rate and time for mixing silane are 30 sccm and 30 min respectively, where the silane is composed of equi - volume silane and disilane, and the gas flow rate and time for introducing oxygen are 10 sccm and 60 min respectively.

[0044] The preparation method of the polyether - ester elastic fiber is as follows: Put terephthalate, 1,4 - butanediol, a catalyst and an antioxidant into an esterification kettle, heat it up and stir, with condensation and reflux. The material obtained at the bottom of the kettle is extruded through an extruder, wound, and pulverized to obtain the polyether - ester elastic fiber.

[0045] The mass ratio of the terephthalate, 1,4 - butanediol, the catalyst and the antioxidant is 5:2:0.05:0.02. The catalyst is composed of cobalt acetate and tetrabutyl titanate with a mass ratio of 1:1. The temperature and time for heating and stirring are 220 °C and 2 h respectively. The temperature and pressure of the extruder are 230 °C and 10 MPa respectively. The winding speed is 400 m / min. The fineness and length of the polyether - ester elastic fiber are 1.5 D and 3 mm respectively.

[0046] The preparation method of the slightly - expanded cement is: mix all raw materials according to weight parts. Among them, the clinker cement and low - calcium fly ash are dried at 120 °C for 2 h, and the gypsum is ground into powder with a mesh number of 300.

[0047] Comparative Example 1 The difference between Comparative Example 1 and Example 2 is only that the modified carbon fiber in Comparative Example 1 is obtained by using the preparation method in Example 1 of Patent CN113024141A. The specific steps are as follows: (1) Removal of the epoxy coating on the carbon fiber surface: Take 1 g of carbon fiber and put it into a Soxhlet extractor filled with acetone. Under the condition of a temperature of 75 °C, use acetone to wash and remove the impurities on the carbon fiber surface. The washing time is 24 h. After the reaction, cool it to room temperature, take out the carbon fiber, and then dry it at 70 °C for 2 h to obtain the carbon fiber with the surface epoxy coating removed. (2) Oxidation step of the carbon fiber: Immerse the carbon fiber obtained in step (1) into concentrated nitric acid. The mass - to - volume ratio of the carbon fiber to the concentrated nitric acid is 1 g:100 ml. Heat it to 80 °C and react for 3 h. Then take out the carbon fiber in the concentrated nitric acid, soak the obtained carbon fiber in distilled water for 5 min, take it out and then rinse it with ethanol 3 times, and dry it at 70 °C for 3 h to obtain the dried oxidized carbon fiber. (3) In-situ growth of nano-silica on the surface of carbon fiber. The oxidized carbon fiber is immersed in a KH550 solution (mass fraction is 3%, the mass ratio of carbon fiber to the volume of KH550 solution is 1g:200ml), and soaked at a constant temperature of 70°C for 3h; then the carbon fiber is taken out, washed 3 times with ethanol, dried for 2h at 70°C, and then put into an analytical pure ethanol solvent (the mass ratio of carbon fiber to the volume of ethanol is 1g:500ml). Subsequently, an ionic surfactant CTAB with a mass fraction of 1% (mass fraction of CTAB in the ethanol solution) is added to the mixture and stirred evenly, and then ammonia water (volume ratio of ammonia water to ethanol is 1:25) is added as a catalyst to provide an alkaline reaction environment and ultrasonic treatment is carried out for 30min; (4) Then, tetraethyl orthosilicate is dropped (volume ratio of tetraethyl orthosilicate to ethanol is 1:12.5), and slowly stirred at a constant temperature of 45°C for 10h, filtered, washed 3 times with methanol, and dried for 3h at 70°C to obtain the dried carbon fiber with in-situ growth of nano-silica on the surface, which is the modified carbon fiber.

[0048] Comparative Example 2 Based on Example 2, a slightly expanding cement, the slightly expanding cement comprises the following raw materials in parts by weight: 65 parts of cement clinker, 17 parts of low-calcium fly ash, 10 parts of expanding agent, 11 parts of carbon fiber, 6 parts of polyether ester elastic fiber, 9 parts of gypsum, and 0.5 part of methyl cellulose dispersant.

[0049] The mesh number of the cement clinker is 180 meshes, the mesh number of the low-calcium fly ash is 300 meshes, the mesh number of the expanding agent is 500 meshes, the length of the carbon fiber is 3mm, and the aspect ratio is 1:3. The specific component analysis of the cement clinker is as follows:

[0050] The expanding agent is a calcium sulfoaluminate expanding agent, and the calcium oxide content in the low-calcium fly ash is 2.5%.

[0051] The expanding agent is a calcium sulfoaluminate expanding agent, and the calcium oxide content in the low-calcium fly ash is 3%.

[0052] The preparation method of the polyether ester elastic fiber is as follows: Terephthalate, 1,4-butanediol, a catalyst and an antioxidant are put into an esterification kettle, heated and stirred, with condensation reflux. The material obtained at the bottom of the kettle is extruded through an extruder, wound, and pulverized to obtain the polyether ester elastic fiber.

[0053] The mass ratio between the terephthalate, 1,4-butanediol, catalyst and antioxidant is 4:1.5:0.05:0.015. The catalyst consists of cobalt acetate and tetrabutyl titanate with a mass ratio of 1:1. The temperature and time for heating and stirring are 215°C and 1.5 h respectively. The temperature and pressure of the extruder are 220°C and 8 MPa respectively. The winding speed is 300 m / min. The fineness and length of the polyether ester elastic fiber are 1.2 D and 2 mm respectively.

[0054] The preparation method of the slightly expansive cement is as follows: all raw materials are mixed according to weight parts. The clinker cement and low-calcium fly ash are dried at 115°C for 2 h, and the gypsum is ground into powder with a mesh number of 250.

[0055] Comparative Example 3 Based on Example 2, the gas flow rate of oxygen in the preparation of the modified carbon fiber is 12 sccm, and other conditions are the same as those in Example 2.

[0056] Comparative Example 4 Based on Example 2, the gas flow rate of oxygen in the preparation of the modified carbon fiber is 6 sccm, and other conditions are the same as those in Example 2.

[0057] Comparative Example 5 Based on Example 2, the polyether ester elastic fiber in the raw materials is removed and replaced with an equal weight part of polypropylene fiber, and other conditions are the same as those in Example 2 of the present invention.

[0058] Performance Test Take the modified carbon fibers prepared in Examples 1-3 and Comparative Examples 1, 3-4 as samples, and use an e-sports scanner to observe whether the fiber surface is flat and continuous; in addition, take the slightly expansive cements obtained in Examples 1-3 and Comparative Examples 1-5 and test the expansion rate, compressive strength and flexural strength of the cement according to the method of JC / T313-2009. The test results are shown in Table 1.

[0059] Table 1 Test Results

[0060] It can be concluded from the analysis in Table 1 that through electron microscopy scanning, it is found that the silica wrapped on the surface of the modified fibers in Examples 1-3 is flat and continuous. However, in Comparative Example 1, the growth process and rate of nano-silica growing on the carbon fiber surface cannot be controlled, and the nano-silica on the carbon fiber surface appears stacked. In Comparative Example 3, the gas flow rate of oxygen is increased, resulting in too fast an attachment speed of silica, and cracks are likely to appear due to vacancies on the carbon fiber surface. In Comparative Example 4, the gas flow rate of oxygen is reduced, and the attachment of silica on the carbon fiber surface decreases, forming a discontinuous structure. The linear expansion rate of the d-line in Examples 1-3 is ≥0.1%, and the 28-day linear expansion rate is ≤0.8%. Combining with the flexural strength and compressive strength, it can be judged that the addition of the modified carbon fibers in Examples 1-3 restricts the expansion rate of the mortar, reduces the porosity, improves the compactness and strength of the cement board, and effectively compensates for the cracking problem caused by the shrinkage of the base layer. The expansion rates of Comparative Examples 1-5 are greater than those of Examples 1-3. Judging from the fact that the compressive strength and flexural strength of Comparative Examples 1-5 are less than those of the examples, the expansion of Comparative Examples 1-5 will increase the porosity of the system, make the structure loose, and reduce the strength. Due to the defects in the surface structure of the modified carbon fiber and the hydrophobicity of the carbon fiber itself, it is easy to agglomerate in water, and the polypropylene fiber used in Comparative Example 5 is also prone to agglomeration in water.

[0061] The above is only a preferred embodiment of the present invention, and it does not impose any form of limitation on the present invention. Although the present invention has been disclosed above with preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or decorations equivalent to changes within the scope of the technical solution of the present invention without departing from the technical solution of the present invention. However, any brief modification, equivalent change and decoration made to the above embodiments based on the technical essence of the present invention still fall within the scope of the technical solution of the present invention.

Claims

1. A micro-expansive cement, characterized in that: The micro-expansive cement comprises the following raw materials in parts by weight: 50-80 parts of cement clinker, 15-20 parts of low-calcium fly ash, 8-12 parts of an expansive agent, 10-12 parts of modified carbon fiber, 5-7 parts of polyether ester elastic fiber, and 8-10 parts of gypsum.

2. The micro-expansive cement according to claim 1, characterized in that, The expansive agent is a calcium sulfoaluminate expansive agent, and the calcium oxide content in the low-calcium fly ash is 2.5-3.5%.

3. A micro-expansive cement according to claim 1, characterized in that, The preparation method of the modified carbon fiber comprises the following steps: (1) Pour concentrated nitric acid into carbon fiber so that the carbon fiber is completely immersed, then heat and stir, and then filter. The solid is washed with absolute ethanol and dried to form pretreated carbon fiber for standby; (2) Place the pretreated carbon fiber in a reaction chamber, heat it after purging oxygen by introducing argon, then mix silane with argon as the carrier gas, close the silane gas channel, then introduce oxygen, and then cool down to room temperature to obtain the modified carbon fiber.

4. The micro-expansive cement according to claim 3, characterized in that: In the step (1), the mass ratio of concentrated nitric acid to carbon fiber is 5-8:3, wherein the mass fraction of concentrated nitric acid is 65-68%, the heating temperature is 100-120 °C, the stirring time is 2-3 h, and the drying temperature and time are 60-80 °C and 2-3 h respectively.

5. A micro-expansive cement according to claim 3, characterized in that, In the step (2), the heating temperature is 600-700 °C, the gas flow rate and time for mixing silane are 10-30 sccm and 20-30 min respectively, wherein silane is composed of equi-volume silane and disilane, and the gas flow rate and time for introducing oxygen are 8-10 sccm and 40-60 min respectively.

6. The micro-expansive cement according to claim 1, characterized in that: The preparation method of the polyether ester elastic fiber is as follows: Put terephthalate, 1,4-butanediol, a catalyst and an antioxidant into an esterification kettle, heat up and stir, carry out condensation reflux. The material obtained at the bottom of the kettle is extruded by an extruder, wound, and pulverized to obtain the polyether ester elastic fiber.

7. The micro-expansive cement according to claim 6, characterized in that, The mass ratio among terephthalate, 1,4-butanediol, the catalyst and the antioxidant is 3-5:1-2:0.05:0.01-0.02, wherein the catalyst is composed of cobalt acetate and tetrabutyl titanate with a mass ratio of 1:

1. The heating and stirring temperature and time are 210-220 °C and 1-2 h respectively. The temperature and pressure of the extruder are 210-230 °C and 7-10 MPa respectively. The winding speed is 200-400 m / min. The fineness and length of the polyether ester elastic fiber are 1-1.5 D and 1-3 mm respectively.

8. A preparation method of the slightly expanding cement as described in claim 1, characterized in that, The preparation method of the micro-expansive cement is: mix all the raw materials according to parts by weight, wherein the clinker cement and the low-calcium fly ash are dried at 110-120 °C for 2 h, and the gypsum is ground into powder with a mesh number of 200-300.

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