Thermoshrunk fiber reinforced cementitious material, method of making and use thereof

Abstract

This invention discloses a heat-shrinkable fiber-reinforced cementitious material, its preparation method, and its application. The preparation method of the heat-shrinkable fiber-reinforced cementitious material includes: mixing standard sand, cement, calcined coal gangue powder, fly ash, and coal-fired furnace slag powder to obtain a first system; mixing first water, nano-calcium carbonate dispersion, and the first system to obtain a second system; mixing heat-shrinkable fibers with the second system to obtain mortar; casting to obtain a heat-shrinkable fiber-reinforced cementitious material precursor; and curing in a gradient water bath to obtain the heat-shrinkable fiber-reinforced cementitious material. The gradient water bath curing includes: first curing in a first curing chamber at room temperature, then curing in a second curing chamber at 85-95°C, and finally curing in a third curing chamber at room temperature. Each of the first, second, and third curing chambers contains a saturated calcium hydroxide aqueous solution at the corresponding temperature. The heat-shrinkable fiber-reinforced cementitious material prepared by this invention exhibits excellent mechanical properties.

Description

Technical Field

[0001] This invention belongs to the field of cement-based composite materials technology, specifically relating to a heat-shrinkable fiber-reinforced cement-based material, its preparation method, and its application. Background Technology

[0002] Cement, as the most widely used building cementitious material today, has powerfully promoted the development of infrastructure construction. However, the cement industry is a high-energy-consuming and high-emission industry, placing a significant burden on the environment. Statistics show that producing one ton of cement emits approximately 800-900 kilograms of carbon dioxide, accounting for about 7% of global anthropogenic carbon emissions. Under the national "dual-carbon" strategy, reducing cement usage and developing green and low-carbon cement-based materials has become an urgent need in the civil engineering field. Utilizing industrial solid waste to replace part of the cement to prepare coal-based solid waste cement-based materials can not only reduce cement usage and achieve carbon reduction, but also simultaneously solve the problem of solid waste disposal, making it one of the optimal paths to balance environmental protection and resource utilization.

[0003] However, the mechanical properties of existing coal-based solid waste cementitious materials are insufficient to meet application requirements. To address this issue, current technologies often improve the performance of these materials by adding steel fibers or polyvinyl alcohol (PVA) fibers. However, several problems remain: the high cost of PVA fibers severely restricts their application in large-scale projects, and their strong hydrophilicity reduces slurry fluidity, deteriorating workability and ease of application. Steel fibers, on the other hand, face the risk of corrosion and expansion in the complex chemical environment of solid waste systems, easily leading to matrix cracking. Furthermore, their high density can cause slurry segregation and sedimentation, resulting in uneven fiber dispersion and a relatively limited improvement in mechanical properties. Therefore, how to construct a high-content coal-based solid waste system that significantly improves the mechanical properties of coal-based solid waste cementitious materials is a pressing technical challenge in this field. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a method for preparing heat-shrinkable fiber-reinforced cementitious materials.

[0005] Another object of the present invention is to provide a heat-shrinkable fiber-reinforced cementitious material obtained by the above preparation method.

[0006] Another object of the present invention is to provide the application of the above-mentioned heat-shrinkable fiber-reinforced cementitious material as a precast concrete component.

[0007] The objective of this invention is achieved through the following technical solution.

[0008] A method for preparing a heat-shrinkable fiber-reinforced cementitious material includes the following steps:

[0009] Step 1: Mix standard sand, cement, calcined coal gangue powder, fly ash and coal-fired furnace slag powder until homogeneous to obtain the first system;

[0010] In step 1, the method for obtaining calcined coal gangue powder includes: drying coal gangue (blocks) at 80~100℃ to constant weight, then calcining it at 700~800℃ for 1~2 hours, grinding it into powder, and obtaining calcined coal gangue powder, wherein the calcined coal gangue powder includes metakaolinite phase.

[0011] Step 2: Mix the first water, the nano-calcium carbonate dispersion, and the first system until homogeneous to obtain the second system;

[0012] In step 2, the first water and the nano-calcium carbonate dispersion are mixed until homogeneous to obtain the first mixture. Under stirring conditions, the first mixture and the first system are mixed and stirred until homogeneous to obtain the second system.

[0013] In step 2, the nano-calcium carbonate dispersion includes: nano-calcium carbonate and second water, and the content of nano-calcium carbonate in the nano-calcium carbonate dispersion is 20wt%.

[0014] Step 3: Mix the heat-shrinkable fiber and the second system until homogeneous to obtain mortar;

[0015] In step 3, the heat-shrinkable fiber is a heat-shrinkable polypropylene fiber.

[0016] Step 4: Pour the mortar into the mold, cover it with a film and let it stand at room temperature, then demold to obtain the heat-shrinkable fiber-reinforced cementitious material precursor. Cure the heat-shrinkable fiber-reinforced cementitious material precursor in a gradient water bath to obtain the heat-shrinkable fiber-reinforced cementitious material. The gradient water bath curing includes: first curing in a first curing chamber at room temperature, second curing in a second curing chamber at 85~95℃, and third curing in a third curing chamber at room temperature. Each of the first, second, and third curing chambers contains a saturated calcium hydroxide aqueous solution at the corresponding temperature. During the gradient water bath curing, the heat-shrinkable fiber-reinforced cementitious material precursor is immersed in the saturated calcium hydroxide aqueous solution.

[0017] By mass fraction, the ratio of nano-calcium carbonate, first water and heat-shrinkable fiber in cement, calcined coal gangue powder, fly ash, coal slag powder, standard sand, and nano-calcium carbonate dispersion is (360~405): (10~40): (20~25): (7~23): (1300~1400): (4~7): (195~210): (2~6).

[0018] In step 4, the settling time is 22-26 hours.

[0019] In the above technical solution, the preferred ratio of nano-calcium carbonate, first water and heat-shrinkable fiber in cement, calcined coal gangue powder, fly ash, coal slag powder, standard sand, nano-calcium carbonate dispersion is (398.25~400.5):13.5:22.5:9:1350:(4.5~6.75):(198~207):(2.8~5.6).

[0020] In the above technical solution, the preferred ratio of cement, calcined coal gangue powder, fly ash, coal slag powder, standard sand, nano calcium carbonate dispersion, first water and heat shrinkable fiber is (400~400.5):13.5:22.5:9:1350:(4.5~5):(205~207):(5~5.6) by mass.

[0021] In step 4, the heat-shrinkable fiber-reinforced cementitious material precursor is cured in a gradient water bath for T days to obtain the heat-shrinkable fiber-reinforced cementitious material. The gradient water bath curing includes: first curing for 3 days in a first curing chamber at room temperature, second curing for 3 days in a second curing chamber at 85~95℃, and third curing for T-6 days in a third curing chamber at room temperature.

[0022] The heat-shrinkable fiber-reinforced cementitious material obtained by the above preparation method.

[0023] The above-mentioned heat-shrinkable fiber-reinforced cementitious materials are used as precast concrete components.

[0024] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0025] This invention involves gradient water bath curing of a heat-shrinkable fiber-reinforced cementitious material precursor obtained from mortar to produce a heat-shrinkable fiber-reinforced cementitious material. The nano-calcium carbonate dispersion optimizes micropores and strengthens the interfacial bond strength between the heat-shrinkable fibers and the matrix. The heat-shrinkable fibers provide active constraint on the matrix. Metakaolin in calcined coal gangue powder reacts with calcium hydroxide produced during cement hydration to form secondary calcium silicate gel, making the matrix denser and improving the performance of the heat-shrinkable fiber-reinforced cementitious material. The heat-shrinkable fiber-reinforced cementitious material prepared by this invention exhibits excellent flexural strength, compressive strength, and tensile strength. It significantly reduces industrial solid waste and carbon emissions while also offering excellent economic benefits and process compatibility, providing a new pathway for the preparation of green, high-performance building materials. Attached Figure Description

[0026] Figure 1 SEM images of the cross-section of the heat-shrinkable fiber-reinforced cementitious material prepared in Example 1 after 28 days of gradient water bath curing and flexural and compressive strength testing;

[0027] Figure 2 XRD pattern of the heat-shrinkable fiber-reinforced cementitious material prepared in Example 2 for 28 days of gradient water bath curing;

[0028] Figure 3 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Example 2 after flexural strength and compressive strength tests, for example 2 which was cured in a gradient water bath for 28 days.

[0029] Figure 4 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared for Comparative Example 1 after flexural strength and compressive strength tests, after gradient water bath curing for 28 days.

[0030] Figure 5 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared for Comparative Example 2 after flexural strength and compressive strength tests, after gradient water bath curing for 28 days.

[0031] Figure 6 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared for Comparative Example 3 after flexural strength and compressive strength tests, after gradient water bath curing for 28 days.

[0032] Figure 7 Macroscopic morphology of the fiber-reinforced cementitious material prepared for Comparative Example 4 after flexural strength and compressive strength tests, after gradient water bath curing for 28 days.

[0033] Figure 8 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared for Comparative Example 5 after flexural strength and compressive strength tests, after gradient water bath curing for 28 days.

[0034] Figure 9 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared by Comparative Example 6 after flexural strength and compressive strength tests, after curing in a room temperature water bath for 28 days.

[0035] Figure 10 Macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 7 after flexural strength testing, after 28 days of curing.

[0036] Figure 11 This is a photograph of the heat-shrinkable fiber after it has been scored, taken under a confocal microscope. Detailed Implementation

[0037] The technical solution of the present invention will be further described below with reference to specific embodiments.

[0038] The cement is PO 42.5 grade ordinary Portland cement, purchased from Jidong Cement Co., Ltd. The chemical composition of the cement is as follows:

[0039]

[0040] The physical properties of cement are as follows:

[0041]

[0042] The heat-shrinkable polypropylene fiber was purchased from Ningbo Shike New Material Technology Co., Ltd., and its performance specifications are as follows:

[0043]

[0044] The purchased coal gangue (in block form) was first dried at 100℃ for 24 hours to constant weight, then calcined at 750℃ for 1 hour, and ground into powder (325 mesh) to obtain calcined coal gangue powder. The physicochemical properties of the calcined coal gangue powder were tested, and the results are as follows:

[0045]

[0046] The fly ash (Grade 1 fly ash) is produced by Gongyi Longze Water Purification Materials Co., Ltd., and its chemical composition and physical properties are as follows:

[0047]

[0048] The purchased coal-fired furnace slag was first dried at 100℃ for 24 hours, then ground into powder. The physicochemical properties of the powder were tested, and the results are as follows:

[0049]

[0050] S95 grade mineral powder was purchased from Zhengzhou Jinshijia New Material Co., Ltd. (activity index: 97):

[0051]

[0052] The ordinary polypropylene fiber was purchased from Taian Tongban Fiber Co., Ltd., and its performance specifications are as follows:

[0053]

[0054] Calcium carbonate whiskers (short whiskers, 400 mesh) were purchased from Lingshou County Hongyao Mineral Products Processing Plant. Their performance specifications are as follows:

[0055]

[0056] The standard sand (ISO standard sand) was purchased from Xiamen Aisiou Standard Sand Co., Ltd., and its performance specifications are as follows:

[0057]

[0058] In the embodiments of the present invention, both ordinary polypropylene fiber and heat-shrinkable polypropylene fiber are fibers that have undergone scoring treatment. The ordinary polypropylene fiber was purchased from Taian Tongban Fiber Co., Ltd., and the heat-shrinkable polypropylene fiber was purchased from Ningbo Shike New Material Technology Co., Ltd. The scoring on the surface of the heat-shrinkable polypropylene fiber is as follows: Figure 11 As shown, the scoring process increases the anchoring force and pull-out resistance of the heat-shrinkable polypropylene fibers, enabling stress to be effectively transferred between the heat-shrinkable fiber-reinforced cementitious materials, thus significantly improving the mechanical properties of the heat-shrinkable fiber-reinforced cementitious materials.

[0059] To clarify the thermal shrinkage properties of the aforementioned ordinary polypropylene fiber and heat-shrinkable polypropylene fiber, a 40cm long ordinary polypropylene fiber (the ordinary polypropylene fiber used in the preparation of fiber-reinforced cementitious materials is 12mm long; the 40cm length was used here only for precise measurement of length variation) was immersed in water at 90℃ for 24 hours. After 24 hours of immersion, its length was measured to be 39.9cm. Similarly, a 40cm long heat-shrinkable polypropylene fiber (the heat-shrinkable polypropylene fiber used in the preparation of heat-shrinkable fiber-reinforced cementitious materials is 12.95mm long) was immersed in water at 90℃ for 24 hours. After 24 hours of immersion, its length was measured to be 39cm. Therefore, it is evident that heat-shrinkable polypropylene fiber exhibits significantly greater thermal shrinkage compared to ordinary polypropylene fiber.

[0060] In the following examples and comparative examples, the ambient temperature is 19~21℃.

[0061] In the following examples and comparative examples, both the first water and the second water are deionized water, and the only difference between the first water and the second water is the amount used.

[0062] Examples 1-3

[0063] A method for preparing a heat-shrinkable fiber-reinforced cementitious material includes the following steps:

[0064] Step 1: At room temperature, in a planetary mixer, mix standard sand, cement, calcined coal gangue powder, fly ash, and coal slag powder until homogeneous to obtain the first system. The standard sand is added in two batches (to ensure uniform mixing of materials), with the same amount of standard sand added in each batch. Specifically, the first batch of standard sand, cement, calcined coal gangue powder, fly ash, and coal slag powder is added to the planetary mixer, followed by the second batch of standard sand. The mixture is stirred at a rotation speed of 140 r / min and a revolution speed of 62 r / min for 60 s until homogeneous to obtain the first system.

[0065] Step 2: At room temperature, mix the first water, the nano-calcium carbonate dispersion, and the first system until homogeneous to obtain the second system. Specifically, mix the first water and the nano-calcium carbonate dispersion, stirring at a rotation speed of 140 r / min and a revolution speed of 62 r / min for 55 s until homogeneous to obtain the first mixture. Under stirring conditions (rotation speed of 140 r / min and revolution speed of 62 r / min), mix the first mixture and the first system, first stirring at a rotation speed of 140 r / min and a revolution speed of 62 r / min for 55 s, then stirring at a rotation speed of 285 r / min and a revolution speed of 125 r / min for 25 s until homogeneous to obtain the second system. The nano-calcium carbonate dispersion comprises: nano-calcium carbonate and second water. The content of nano-calcium carbonate in the nano-calcium carbonate dispersion is 20 wt%, and the particle size of the nano-calcium carbonate in the nano-calcium carbonate dispersion is 50 nm.

[0066] Step 3: At room temperature, mix the heat-shrinkable fiber (heat-shrinkable polypropylene fiber, 12.95 mm in length) and the second system until homogeneous to obtain mortar. The heat-shrinkable fiber is added in two stages (to reduce agglomeration), with the same amount added in both stages. Specifically, the first part of the heat-shrinkable fiber is mixed with the second system and stirred at a rotation speed of 140 r / min and a revolution speed of 62 r / min for 3 seconds until homogeneous. The second part of the heat-shrinkable fiber is then added and stirred at a rotation speed of 140 r / min and a revolution speed of 62 r / min for 2 seconds, then at a rotation speed of 285 r / min and a revolution speed of 125 r / min for 5 seconds until homogeneous. The mixture is allowed to stand for 90 seconds (during which time the walls are scraped and the mixture is turned over). Finally, the mixture is stirred at a rotation speed of 285 r / min and a revolution speed of 125 r / min for 60 seconds to obtain the mortar.

[0067] Step 4: At room temperature, pour the mortar into the mold (during the pouring process, turn on the vibrating table, pour the mortar into the mold in two batches, with the same amount poured in each batch; after each pour, insert a tool into the mortar multiple times to remove air bubbles). Use a screed to scrape off the mortar that is above the mold and smooth it out. Cover with a film and let it stand at room temperature for 24 hours. Demold to obtain the heat-shrinkable fiber-reinforced cementitious material precursor. Cure the heat-shrinkable fiber-reinforced cementitious material precursor in a gradient water bath for T days to obtain the heat-shrinkable fiber-reinforced cementitious material, where T = 7 or 28, T = T1 + T2 + T3, gradient water bath curing, includes: first curing in a first curing chamber at room temperature for T1 days, then transferring to a second curing chamber at 90℃ for a second curing period of T2 days, then transferring to a third curing chamber at room temperature for a third curing period of T3 days. When T=7, T1=3, T2=3, T3=1; when T=28, T1=3, T2=3, T3=22.

[0068] The first, second, and third curing chambers each contain a saturated calcium hydroxide aqueous solution at the corresponding temperature (the first and third curing chambers each contain a saturated calcium hydroxide aqueous solution at room temperature, while the second curing chamber contains a saturated calcium hydroxide aqueous solution at 90°C). During gradient water bath curing, the heat-shrinkable fiber-reinforced cementitious material precursor is immersed in the saturated calcium hydroxide aqueous solution.

[0069] The ratio of nano-calcium carbonate, first water, and heat-shrinkable fiber in cement, calcined coal gangue powder, fly ash, coal-fired slag powder, standard sand, and nano-calcium carbonate dispersion, by mass fraction, is A. The value of A is shown in Table 1.

[0070] Table 1

[0071]

[0072] Comparative Example 1

[0073] A method for preparing a heat-shrinkable fiber-reinforced cementitious material is basically the same as that in Example 2, except that "calcined coal gangue powder" is replaced with "cement".

[0074] Comparative Example 2

[0075] A method for preparing a heat-shrinkable fiber-reinforced cementitious material is basically the same as that in Example 2, except that: no nano-calcium carbonate dispersion is added, and the ratio of cement, calcined coal gangue powder, fly ash, coal slag powder, standard sand, first water and heat-shrinkable fiber by mass is 400.5:13.5:22.5:9:1350:4.5:225:5.6.

[0076] Comparative Example 3

[0077] A method for preparing a heat-shrinkable fiber-reinforced cementitious material is basically the same as in Example 2, except that "calcined coal gangue powder" is replaced with "slag". The slag is S95 grade mineral powder.

[0078] Comparative Example 4

[0079] A method for preparing a fiber-reinforced cementitious material is basically the same as in Example 2, except that "heat-shrinkable polypropylene fiber" is replaced with "ordinary polypropylene fiber (non-heat-shrinkable fiber)". The ordinary polypropylene fiber is 12 mm in length.

[0080] Comparative Example 5

[0081] A method for preparing a heat-shrinkable fiber-reinforced cementitious material is basically the same as that in Example 2, except that "nano-calcium carbonate in nano-calcium carbonate dispersion" is replaced with "calcium carbonate whiskers" (equal mass replacement).

[0082] Comparative Example 6

[0083] A method for preparing a heat-shrinkable fiber-reinforced cementitious material is basically the same as that in Example 2, except that "gradient water bath curing" is replaced with "room temperature water bath curing". The room temperature water bath curing in Comparative Example 6 includes: curing the heat-shrinkable fiber-reinforced cementitious material precursor obtained after demolding in a room temperature curing chamber for T days, where T=7 or 28 (room temperature water bath curing does not involve T1, T2, and T3); the curing chamber contains a saturated calcium hydroxide aqueous solution at room temperature, and the heat-shrinkable fiber-reinforced cementitious material precursor is immersed in the saturated calcium hydroxide aqueous solution at room temperature.

[0084] Comparative Example 7

[0085] A method for preparing a heat-shrinkable fiber-reinforced cementitious material is basically the same as that in Example 2, except that: during the second curing in the second curing chamber at 90°C, the heat-shrinkable fiber-reinforced cementitious material precursor is suspended in the air (the second curing chamber is filled with a steam environment formed by a saturated calcium hydroxide aqueous solution at 90°C, and the heat-shrinkable fiber-reinforced cementitious material precursor is not immersed in the saturated calcium hydroxide aqueous solution).

[0086] The mortars prepared in Examples 1-3 and Comparative Examples 1-5 were subjected to fluidity tests, and the results are shown in Table 2. (The fluidity test was conducted in accordance with GB / T2419-2005 "Method for Determination of Flowability of Cement Mortar").

[0087] Table 2

[0088]

[0089] As shown in Table 2, the mortars prepared in Examples 1-3 and Comparative Examples 1-5 have basically the same fluidity.

[0090] In the preparation method of the heat-shrinkable fiber-reinforced cementitious material, the mold is a triple mold (40mm×40mm×160mm). Each of the heat-shrinkable fiber-reinforced cementitious materials prepared in Examples 1-3, Comparative Examples 1-3, Comparative Examples 5-7, and the fiber-reinforced cementitious material prepared in Comparative Example 4 is used as a cuboid specimen. Each cuboid specimen is subjected to flexural strength and compressive strength tests. The same cuboid specimen is first tested for flexural strength, which breaks the specimen into two halves, resulting in two half-specimens. The compressive strength is then tested on the half-specimens. The flexural strength is determined according to GB / T17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)", and the compressive strength is determined according to GB / T17671-2021 "Test Method for Strength of Cement Mortar (ISO Method)".

[0091] In the preparation method of heat-shrinkable fiber-reinforced cementitious materials, the mold is dumbbell-shaped. Each of the heat-shrinkable fiber-reinforced cementitious materials prepared in Examples 1-3, Comparative Examples 1-3, Comparative Examples 5-7, and the fiber-reinforced cementitious materials prepared in Comparative Example 4 is used as a specimen and tested according to JC / T2461-2018 "Test Method for Mechanical Properties of High-Ductility Fiber-Reinforced Cementitious Composite Materials" to obtain the tensile strength.

[0092] When T=7, the flexural strength, compressive strength, and tensile strength are shown in Table 3. When T=28, the flexural strength, compressive strength, and tensile strength are shown in Table 4.

[0093] Table 3

[0094]

[0095] Table 4

[0096]

[0097] As shown in Tables 3 and 4, regardless of whether T=7 or T=28, the comprehensive performance of the heat-shrinkable fiber-reinforced cementitious materials prepared in Comparative Examples 1-3 and Comparative Example 5, as well as the fiber-reinforced cementitious materials prepared in Comparative Example 4, is not as good as that of Example 2. It can be seen that the raw materials used to prepare the mortar have a great influence on the material performance. Calcined coal gangue powder, nano-calcium carbonate, and heat-shrinkable fibers play important roles in the heat-shrinkable fiber-reinforced cementitious materials of the present invention.

[0098] As shown in Tables 3 and 4, regardless of whether T=7 or T=28, the flexural strength, compressive strength, and tensile strength of the heat-shrinkable fiber-reinforced cementitious material prepared in Example 2 are all higher than those in Comparative Examples 6 and 7. Under the same mortar conditions, the curing method also significantly affects the material properties. Compared to room temperature water bath curing, the second curing in gradient water bath curing has a dual stimulating effect: the high-temperature environment causes shrinkage stress in the heat-shrinkable fibers, thereby establishing an active constraint mechanism, which cannot be achieved by room temperature water bath curing. Compared to steam curing, water bath curing has significant advantages in heat and mass transfer. Steam, as a gaseous heat source, has lower heat transfer efficiency than liquid-phase water baths, resulting in incomplete shrinkage of the heat-shrinkable fibers, thus failing to generate sufficient pressure. Furthermore, the hydration process requires a large amount of water; insufficient water supply in a steam environment easily leads to a decrease in the relative humidity inside the matrix, while water bath curing provides sufficient water and calcium hydroxide, ensuring the hydration reaction proceeds fully. In summary, this invention proposes an optimal method for preparing heat-shrinkable fiber-reinforced cementitious materials.

[0099] The heat-shrinkable fiber-reinforced cementitious material prepared in Example 1, after being cured in a gradient water bath for 28 days (T=28) and subjected to flexural and compressive strength tests, was crushed. Small fragments corresponding to the core area of ​​the heat-shrinkable fiber-reinforced cementitious material were selected for SEM testing. Figure 1 As shown, the heat-shrinkable fiber is pulled out of the matrix, and its surface is covered with a large amount of material (such as calcium silicate gel), which indicates that there is an effective interfacial bonding force between the heat-shrinkable fiber and the matrix, and the bonding performance is good.

[0100] Figure 2 The XRD pattern of the heat-shrinkable fiber-reinforced cementitious material prepared in Example 2 after 28 days of gradient water bath curing is shown below. Figure 2 As shown, the presence of CH (calcium hydroxide) at 18.1° and 34.1° indicates that the saturated aqueous solution of calcium hydroxide effectively inhibits the dissolution and loss of CH from the matrix.

[0101] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Example 2 after 28 days of gradient water bath curing is shown in the figure after flexural strength and compressive strength tests. Figure 3 As shown, the heat-shrinkable fiber-reinforced cementitious material prepared in Example 2 maintained excellent overall integrity, with only a few very fine vertical cracks visible on the surface, and no signs of cracking, collapse or peeling.

[0102] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 1 after flexural strength and compressive strength tests is shown in the figure after 28 days of gradient water bath curing. Figure 4 As shown, the surface of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 1 exhibits multiple through-cracks and localized spalling. Because Comparative Example 1 replaced the calcined coal gangue powder with cement, the amount of secondary hydration products generated was insufficient, leading to a decrease in the bond strength between the heat-shrinkable fibers and the matrix. Therefore, under ultimate load conditions, it could not effectively suppress the rapid propagation and penetration of cracks, and its crack resistance and post-failure integrity were inferior to those of Example 2.

[0103] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 2 after flexural strength and compressive strength tests is shown in the figure below, after 28 days of gradient water bath curing. Figure 5 As shown, the surface of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 2 exhibits obvious crack propagation, and the cracks are relatively wide. This is because Comparative Example 2 did not include nano-calcium carbonate dispersion, resulting in insufficient interfacial bonding strength between the heat-shrinkable fibers and cement.

[0104] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 3 after flexural strength and compressive strength tests is shown in Figure 3, which was cured in a gradient water bath for 28 days. Figure 6As shown, long, penetrating cracks are visible on the surface of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 3. Although S95 grade mineral powder has high potential activity, the gel structure formed by the hydration products of S95 grade mineral powder is too dense and has a high elastic modulus. The introduction of S95 grade mineral powder increases the brittleness of the matrix, thus causing the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 3 to exhibit brittle characteristics. This indicates that the metakaolinite phase formed after calcination of coal gangue has better chemical compatibility with the matrix.

[0105] The macroscopic morphology of the fiber-reinforced cementitious material prepared in Comparative Example 4 after flexural strength and compressive strength tests is shown in Figure 4, which was cured in a gradient water bath for 28 days. Figure 7 As shown, the fiber-reinforced cementitious material prepared in Comparative Example 4 has numerous wide cracks on its surface, accompanied by obvious spalling and collapse, resulting in poor integrity. This is because ordinary polypropylene fibers can only provide passive bonding and cannot actively shrink and exert pressure on the matrix during the gradient water bath curing stage like heat-shrinkable polypropylene fibers. Lacking active constraint, the fiber-reinforced cementitious material cracks in the early stages of loading, and ordinary polypropylene fibers cannot effectively limit the deterioration of the cracks.

[0106] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 5 after flexural strength and compressive strength tests is shown in Figure 5, which was cured in a gradient water bath for 28 days. Figure 8 As shown, although the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 5 maintained its basic geometry, clear crack distribution and surface spalling were still visible. While calcium carbonate whiskers can provide micron-scale fibers for toughening, they lack the high specific surface area of ​​nano-calcium carbonate. Calcium carbonate whiskers are difficult to fill the tiny pores of CSH gel like nano-calcium carbonate, and easily aggregate to form internal defects. Therefore, the bonding strength of its micro-interface is weaker than that of Example 2, leading to a decrease in macroscopic crack resistance.

[0107] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 6 after flexural strength and compressive strength tests is shown in Figure 6, which was cured in a room temperature water bath for 28 days. Figure 9 As shown, the surface of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 6 has multiple wide cracks. Comparative Example 6 only used room temperature water bath curing. At room temperature, the pozzolanic reaction is slow, and without the ability to induce a deep pozzolanic reaction, the interfacial adhesion is weak. At the same time, the heat-shrinkable fibers failed to shrink, lacking the constraint mechanism provided by the active shrinkage of the heat-shrinkable fibers, resulting in its inability to effectively limit the propagation and penetration of cracks under stress.

[0108] The macroscopic morphology of the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 7 after flexural strength testing (focusing on its failure behavior and interfacial bonding state during the flexural stage) after 28 days of curing. Figure 10As shown, after flexural testing, the heat-shrinkable fiber-reinforced cementitious material prepared in Comparative Example 7 developed a wide, penetrating main crack in the middle, and the heat-shrinkable fibers that had been directly pulled out were clearly visible at the crack cross-section. Due to the defects of insufficient heat transfer and water supply in the steam curing method used in the second curing of Comparative Example 7, the reaction was incomplete, resulting in a lack of secondary hydration products at the bond between the heat-shrinkable fibers and the matrix, leading to insufficient interfacial bond strength. Simultaneously, the heat-shrinkable fibers shrank, leaving pores inside the matrix. Under flexural load, the heat-shrinkable fibers were pulled out, resulting in the crack.

[0109] In the preparation method of the heat-shrinkable fiber-reinforced cementitious material of the present invention, when the water in the cement and the second system undergoes a hydration reaction, primary hydration products (primary calcium silicate gel-CSH gel) and calcium hydroxide are generated; when the precursor of the heat-shrinkable fiber-reinforced cementitious material is formed, the coal-based solid waste material in the precursor can undergo a secondary hydration reaction with the calcium hydroxide generated (during the hydration reaction in the cement and the second system), which is a pozzolanic reaction (the coal-based solid waste material is calcined coal gangue powder, coal-fired slag powder, and fly ash, wherein the main component undergoing the secondary hydration reaction is the active component of the calcined coal gangue powder). (i.e., metakaolin), generating secondary hydration products (secondary calcium silicate gel). Both primary and secondary hydration products can improve the density and mechanical strength of the matrix (the primary and secondary hydration products are densely filled at the heat-shrinkable fibers, making the heat-shrinkable fibers and the matrix tightly bonded), ensuring the long-term steady development of the material's performance. When using saturated calcium hydroxide aqueous solution for curing, the external saturated calcium hydroxide aqueous solution can continuously diffuse into the interior of the heat-shrinkable fiber-reinforced cementitious material through the pores, and the calcium hydroxide concentration inside and outside the heat-shrinkable fiber-reinforced cementitious material tends to be consistent, ensuring the continuous occurrence of pozzolanic reaction and ensuring the continuous generation of secondary hydration products.

[0110] The superior performance of the heat-shrinkable fiber-reinforced cementitious material of this invention is attributed to the synergistic effect of nano-calcium carbonate, calcined coal gangue powder, and heat-shrinkable fibers under heat curing. Nano-calcium carbonate fills the pores in the matrix (matrix refers to the material in the heat-shrinkable fiber-reinforced cementitious material excluding the heat-shrinkable fibers), and the nano-calcium carbonate can exert a nucleation effect, strengthening the interfacial bond strength between the heat-shrinkable fibers and the matrix.

[0111] During the first curing, the grooves on the surface of the heat-shrinkable fiber form a strong physical interlocking structure with the matrix, anchoring the heat-shrinkable fiber in the matrix. During the second curing, not only is the pozzolanic reaction deeply activated, but the heat-shrinkable fiber also shrinks. Its shrinkage force is converted into pressure and applied to the matrix, constructing a "heat-shrinkable fiber active constraint" mechanism, which effectively inhibits cracking of heat-shrinkable fiber reinforced cementitious materials and significantly improves the mechanical properties of heat-shrinkable fiber reinforced cementitious materials.

[0112] The present invention has been described above by way of example. It should be noted that any simple modifications, alterations or other equivalent substitutions that can be made by those skilled in the art without creative effort without departing from the core of the present invention fall within the protection scope of the present invention.

Claims

1. A method for preparing a heat-shrinkable fiber-reinforced cementitious material, characterized in that, Includes the following steps: Step 1: Mix standard sand, cement, calcined coal gangue powder, fly ash and coal-fired furnace slag powder until homogeneous to obtain the first system; Step 2: Mix the first water, the nano-calcium carbonate dispersion, and the first system until homogeneous to obtain the second system; Step 3: Mix the heat-shrinkable fiber and the second system until homogeneous to obtain mortar; Step 4: Pour the mortar into the mold, cover it with a film and let it stand at room temperature, then demold to obtain the heat-shrinkable fiber-reinforced cementitious material precursor. Cure the heat-shrinkable fiber-reinforced cementitious material precursor in a gradient water bath to obtain the heat-shrinkable fiber-reinforced cementitious material. The gradient water bath curing includes: first curing in a first curing chamber at room temperature, second curing in a second curing chamber at 85~95℃, and third curing in a third curing chamber at room temperature. Each of the first, second, and third curing chambers contains a saturated calcium hydroxide aqueous solution at the corresponding temperature. During the gradient water bath curing, the heat-shrinkable fiber-reinforced cementitious material precursor is immersed in the saturated calcium hydroxide aqueous solution. By mass fraction, the ratio of nano-calcium carbonate, first water and heat-shrinkable fiber in cement, calcined coal gangue powder, fly ash, coal slag powder, standard sand, and nano-calcium carbonate dispersion is (360~405): (10~40): (20~25): (7~23): (1300~1400): (4~7): (195~210): (2~6).

2. The preparation method according to claim 1, characterized in that, In step 1, the method for obtaining calcined coal gangue powder includes: drying the coal gangue at 80~100℃ to constant weight, then calcining it at 700~800℃ for 1~2 hours, and grinding it into powder to obtain calcined coal gangue powder, wherein the calcined coal gangue powder includes metakaolinite phase.

3. The preparation method according to claim 1, characterized in that, In step 2, the nano-calcium carbonate dispersion includes: nano-calcium carbonate and second water, and the content of nano-calcium carbonate in the nano-calcium carbonate dispersion is 20wt%.

4. The preparation method according to claim 1, characterized in that, In step 3, the heat-shrinkable fiber is a heat-shrinkable polypropylene fiber.

5. The preparation method according to claim 1, characterized in that, In step 4, the settling time is 22-26 hours.

6. The preparation method according to claim 1, characterized in that, In step 4, the heat-shrinkable fiber-reinforced cementitious material precursor is cured in a gradient water bath for T days to obtain the heat-shrinkable fiber-reinforced cementitious material. The gradient water bath curing includes: first curing for 3 days in a first curing chamber at room temperature, second curing for 3 days in a second curing chamber at 85~95℃, and third curing for T-6 days in a third curing chamber at room temperature.

7. The preparation method according to claim 1, characterized in that, In step 2, the first water and the nano-calcium carbonate dispersion are mixed until homogeneous to obtain the first mixture. Under stirring conditions, the first mixture and the first system are mixed and stirred until homogeneous to obtain the second system.

8. The heat-shrinkable fiber-reinforced cementitious material obtained by the preparation method according to any one of claims 1 to 7.

9. The application of the heat-shrinkable fiber-reinforced cementitious material as described in claim 8 as a precast concrete component.

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