Anti-crack expansive concrete and preparation method thereof

Abstract

The invention relates to anti-crack expansive concrete and a preparation method thereof, and relates to the technical field of building materials. The concrete is prepared from the following components in parts by mass: Portland cement, fly ash, slag powder, silica fume, an expansion composition, nano calcium carbonate, sand, gravel, water, a hydration temperature rise inhibitor, a polycarboxylic acid water reducer and carboxyl modified polypropylene fibers. The preparation method comprises the steps of primary mixing and final mixing. According to the anti-crack expansive concrete disclosed by the invention, the hydration temperature rise inhibitor is added to reduce hydration heat, meanwhile, the early-stage and short-term strength is improved, and the early-stage and short-term strength loss caused by temperature cracks generated by reduction of the hydration heat is avoided.

Description

Technical Field

[0001] This invention relates to the field of building materials technology, and in particular to a crack-resistant expansion concrete and its preparation method. Background Technology

[0002] Cracking caused by the hydration reaction of concrete has long plagued the engineering community in the construction of large-volume concrete projects such as bridge piers, high-rise building foundations, and water conservancy dams. After concrete is poured, the cement hydration process releases a large amount of heat, causing a sharp rise in internal temperature and creating a significant temperature difference with the surface. This generates thermal stress, and when the stress exceeds the tensile strength of the concrete, thermal cracks will appear. At the same time, the evaporation of moisture during the concrete hardening stage causes drying shrinkage deformation, further aggravating the formation and propagation of cracks. These cracks not only reduce the load-bearing capacity of the concrete structure but also provide channels for rainwater and corrosive media to seep in, accelerating the deterioration process such as steel corrosion and concrete carbonation, severely shortening the service life of the project and increasing the cost of later maintenance.

[0003] To address the hydration cracking problem in large-volume concrete, the industry has developed a solution for temperature cracks caused by hydration heat. This involves incorporating hydration temperature inhibitors to suppress the hydration rate of tricalcium silicate and tricalcium aluminate in the cement, thereby slowing down the heat release rate, reducing the maximum temperature rise inside the concrete, and controlling the risk of temperature cracks.

[0004] Chinese invention patent application CN117945721A, published on April 30, 2024, discloses a crack-resistant concrete and its construction method. The concrete components include: 280-296 parts cement, 75-77 parts fly ash, 22-26 parts slag powder, 738-758 parts sand, 1090-1130 parts crushed stone, 160-164 parts water, 0.2-0.4 parts hydration temperature rise inhibitor, and 3.8-4.0 parts polycarboxylate superplasticizer. The hydration temperature rise inhibitor effectively limits the heat generated during the hardening process of the concrete mixture, reduces the hydration rate, and effectively prevents thermal stress cracks caused by excessive internal and external temperature differences.

[0005] In response to the above-mentioned technical solutions, the inventors discovered that while the hydration temperature rise inhibitor reduces the heat of hydration, it also reduces the degree of hydration. As a result, fewer dense structures and skeleton products are obtained, which can easily lead to a decrease in the early and short-term strength of concrete. Summary of the Invention

[0006] To address the problem that adding a hydration temperature rise inhibitor to concrete reduces the degree of hydration and decreases the early and short-term strength of the concrete, this invention provides a crack-resistant expansion concrete and its preparation method.

[0007] In a first aspect, the present invention provides a crack-resistant expansion concrete, which adopts the following technical solution: A crack-resistant expansive concrete comprises, by weight, the following components: 270-280 parts silicate cement, 70-80 parts fly ash, 50-60 parts slag powder, 8-15 parts silica fume, 5-10 parts expansive composition, 3-5 parts nano-calcium carbonate, 745-755 parts sand, 1110-1125 parts crushed stone, 155-165 parts water, 0.4-0.6 parts hydration temperature rise inhibitor, 4.0-4.5 parts polycarboxylate superplasticizer, and 0.50-0.55 parts carboxyl-modified polypropylene fiber; the expansive composition includes ettringite precursor composite powder, which comprises tricalcium aluminate and gypsum dihydrate in a mass ratio of 1:3; the nano-calcium carbonate has a particle size of 75nm ± 35nm.

[0008] By adopting the above technical solution, in the plastic stage of the initial hydration reaction 0-24 hours after concrete mixing, nano-calcium carbonate is used. On the one hand, it accelerates cement hydration through the nanocrystal nucleation effect, promotes the formation of hydrated calcium silicate gel, and forms early and short-term strength. On the other hand, the nano-calcium carbonate particles are uniformly dispersed in the interface transition zone between cement and aggregate, fill the interface micro-cracks, and form chemical bonds with cement hydration products, thereby improving the interface bonding strength and further enhancing the early and short-term strength.

[0009] The hydration of ettringite shell precursors generates ettringite, which grows in a needle-like, interwoven pattern, filling pores and reinforcing the cement skeleton, thus improving early and short-term strength. In addition, the volume expansion of the generated ettringite compensates for the early plastic shrinkage of concrete, preventing cracking. The addition of 60% ettringite precursors not only ensures the shrinkage effect but also effectively reduces the loose skeleton caused by the accumulation of ettringite crystals, ensuring the development of early and short-term strength. Silica fume reacts rapidly with Ca(OH)2 generated from cement hydration to generate high-density calcium silicate gel, which fills the interfacial gaps between cement and aggregate, improving the strength of the interfacial transition zone and enhancing impermeability.

[0010] One to seven days after concrete mixing, the peak of cement hydration heat release is reached. The temperature rise inhibitor slows down the hydration rate by forming a thin protective film on the surface of cement particles, thereby reducing the maximum temperature rise during hydration and controlling it within 50°C to avoid temperature stress cracks caused by excessive internal and external temperature differences. Nano-calcium carbonate, silica fume, and ettringite shell precursors continue to play a role in early reinforcement and pre-expansion.

[0011] Between 7 and 28 days after concrete mixing, the secondary hydration of fly ash and slag powder reaches its peak. During this process, both fly ash and slag powder continuously react with Ca(OH)2 generated from cement hydration to produce cementitious substances. These cementitious substances intertwine closely with the initial hydration products of cement, forming a denser and more stable three-dimensional network structure, which enhances the later mechanical strength of concrete. At this stage, the 40% magnesium oxide core layer gradually reacts, providing the main compensation for drying shrinkage. Nano-calcium carbonate, while improving structural density, further reduces hardening shrinkage caused by humidity changes, thus reducing cracks.

[0012] Throughout the process, polycarboxylate superplasticizers adsorb onto the particle surfaces of cementitious materials such as silicate cement, fly ash, and slag powder, allowing the cementitious materials to be uniformly dispersed in water. The strong chemical stability of carboxyl-modified polypropylene fibers bridges microcracks and works synergistically with the expansion composition and nano-calcium carbonate to control crack risk and extend the service life of concrete structures.

[0013] Optionally, the expanded composition includes a core-shell structure comprising a shell layer and a core layer, wherein the mass ratio of the shell layer to the core layer is 3:2, the core layer is magnesium oxide, and the ettringite precursor composite powder is the shell layer.

[0014] By adopting the above technical solution and using a gradient release core-shell structure with a 3:2 ratio, ettringite expands early and magnesium oxide expands late, which precisely solves the problem of early and late shrinkage cracking in concrete and also assists in strength development. At the same time, it works synergistically with nano-calcium carbonate, fiber, inhibitor, and polycarboxylate superplasticizer in the formula to achieve a balance of crack resistance, early strength and durability, which is especially suitable for large-volume construction scenarios.

[0015] Optionally, the expanding composition further includes: anhydrous aluminum sulfate, hydroxysilane-modified polypropylene fiber, and nano-silica sol, wherein the mass ratio of core-shell structure: anhydrous aluminum sulfate: hydroxysilane-modified polypropylene fiber: nano-silica sol is 100:1-2:20-30:12-18.

[0016] By adopting the above technical solution, cement hydration releases Ca(OH)2 within 0–24 hours, and anhydrous aluminum sulfate releases Al. 3+ The tricalcium aluminate and gypsum dihydrate in the gradient-release composite expansion agent synergistically generate fine ettringite crystals with a particle size ≤5μm, filling the pores of cement paste. Simultaneously, nano-silica sol reacts with Ca(OH)2 generated during cement hydration to form dense CSH gel. These two components construct an ettringite framework + CSH gel filling structure, enhancing compressive strength within 3 days. Furthermore, the reaction between anhydrous aluminum sulfate and the gradient-release composite expansion agent does not depend on highly active Ca... 2+ It maintains early strength efficiency within the temperature control range of the hydration temperature rise inhibitor, achieving temperature control without prolonged strength.

[0017] Nano-silica sol adsorbs part of the heat of hydration through its high specific surface area, and works synergistically with hydration temperature rise inhibitors to control the maximum temperature rise inside concrete below 45℃, reducing the risk of temperature cracks. The hydroxyl and silicon-oxygen bonds formed on the surface of hydroxyl silane-modified polypropylene fibers can be more firmly anchored in the concrete matrix, improving interfacial bond strength. It is compatible with carboxyl-modified polypropylene fibers added to the concrete base components. Carboxyl-modified fibers focus on improving later crack resistance, while hydroxyl silane-modified fibers improve early strength, playing a complementary role between early strength and crack resistance.

[0018] Optionally, the hydration temperature rise inhibitor is a composite hydration temperature rise inhibitor, and the preparation method of the composite hydration temperature rise inhibitor is as follows: Mix 25-35 parts of nano-silica and 10-20 parts of zinc borate evenly in a dry powder state; add 55-65 parts of 30-40 wt% hydrolyzed corn starch solution to the mixed dry powder, stir evenly and then dry; crush the dried block and sieve to obtain the final product.

[0019] By adopting the above technical solution, the hydroxyl groups on the molecular chain of starch hydrolysis products can react with Ca in cement hydration products. 2+ The formation of complexes slows down the hydration rate, thereby reducing the peak heat of hydration. Nano-silica fills the pores between cement and mineral admixtures, such as slag powder and silica fume, optimizing the density of the concrete microstructure. At the same time, nano-silica undergoes secondary hydration with Ca(OH)2 generated during cement hydration, generating more CSH gel to compensate for the weakening of strength caused by starch hydrolysis products. Zinc ions in zinc borate inhibit the metabolic activities of microorganisms, preventing starch hydrolysis products from being biodegraded in humid environments. On the other hand, zinc borate can release water of crystallization at high temperatures, diluting flammable gases and forming a glassy flame-retardant layer, meeting the flame-retardant requirements of underground engineering and tunnels.

[0020] Starch hydrolysis products delay early cement hydration, nano-silica improves density and early and short-term strength, and zinc borate improves long-term durability. The three work together to control the maximum internal temperature rise of concrete below 50°C, avoiding temperature cracks caused by thermal stress.

[0021] Pre-mixing dry powder reduces the agglomeration of individual solid powders. The 30-40 wt% hydrolyzed corn starch solution has a moderate viscosity, which can uniformly coat the surface of the pre-mixed solid particles. The starch molecular chains are adsorbed on the surface of nano-silica, forming steric hindrance and inhibiting secondary agglomeration during the drying process. At the same time, the micron-sized zinc borate particles are bridged with nano-silica, reducing the separation of the two solids in the final powder. In addition, the sealed packaging of the powder can avoid problems such as microbial deterioration and moisture evaporation separation in the slurry state, extend the shelf life, and better meet the convenience requirements of the engineering site.

[0022] Optionally, the carboxyl-modified polypropylene fiber undergoes surface roughening or indentation treatment, and the carboxyl-modified polypropylene fiber is made by mixing 3-6 mm and 6-12 mm carboxyl-modified polypropylene fibers at a mass ratio of 7:3.

[0023] By adopting the above technical solutions, carboxyl-modified polypropylene fibers with surface roughening or indentation treatment are used to improve the mechanical interlocking force with concrete. The surface roughening structure effectively transfers stress and improves crack bridging strength. In addition, the carboxyl-modified polypropylene fibers with surface roughening or indentation treatment have strong dispersibility, reducing fiber agglomeration. Their three-dimensional network structure can disperse expansion stress, allowing expansion energy to be evenly distributed in the concrete. In conjunction with hydration temperature rise inhibitors, the cement hydration rate is reduced, reducing temperature stress. The modified roughened fibers directly resist the tensile stress generated by temperature shrinkage through physical crack resistance. The combination of these two measures improves the crack resistance index of concrete in the 1-7 day range, making it particularly suitable for large-volume concrete projects.

[0024] The 3-6 mm carboxyl-modified polypropylene fibers, comprising 70% of the composition, can be uniformly dispersed in the slurry during the plastic stage of concrete. Through chemical bonding between the fibers and the carboxyl groups of the cement matrix, they effectively inhibit plastic shrinkage cracks caused by aggregate settlement. The 6-12 mm carboxyl-modified polypropylene fibers, comprising 30% of the composition, can cross micro-cracks inside the cement due to their longer fiber length. Through the bridging effect, they prevent crack propagation, and are particularly effective in inhibiting macro-cracks caused by temperature shrinkage and drying shrinkage, thus significantly improving the integrity of the concrete structure.

[0025] Optionally, the nano-calcium carbonate is prepared by compounding nano-calcium carbonate with a particle size of 50nm±10nm and nano-calcium carbonate with a particle size of 100nm±10nm in a mass ratio of 1:1.

[0026] By adopting the above technical solution, 50nm±10nm and 100nm±10nm nano-calcium carbonate are compounded in a 1:1 mass ratio to efficiently fill voids of different sizes inside concrete. Among them, 100nm±10nm nano-calcium carbonate preferentially fills the larger voids between cement particles in cement paste, while 50nm±10nm nano-calcium carbonate fills the micropores, improving density and enhancing the strength and durability of concrete.

[0027] Secondly, the present invention provides a method for preparing crack-resistant expansion concrete, which adopts the following technical solution: A method for preparing crack-resistant expansion concrete includes the following steps: Initial mixing: Add sand and gravel, dry mix for 1-2 minutes; add cement, fly ash, and slag powder, mix for 2-3 minutes; add silica fume and nano-calcium carbonate, mix for 2-3 minutes; first add the expansion composition, mix for 1-2 minutes, then add the hydration temperature rise inhibitor, mix for 1-2 minutes; add 1 / 3 of the water, mix for 2-3 minutes; the mixing speed is 120 r / min. Final Mix I: Add 1 / 3 water and polycarboxylate superplasticizer, and stir for 2-3 minutes; Final Mixing II: Add carboxyl-modified polypropylene fiber, add the remaining 1 / 3 of water, and stir for 3-4 minutes; the stirring speed is 180 r / min.

[0028] By adopting the above technical solution, the expansion agent and hydration temperature rise inhibitor are added in stages during the initial mixing stage. The expansion agent contacts the dispersed cementitious materials first to complete the initial expansion. The uniform distribution of the hydration temperature rise inhibitor precisely controls the peak value of concrete hydration heat, complementing the expansion effect of the expansion agent in time and avoiding temperature shrinkage cracks. In the final mixing stage, carboxyl-modified polypropylene fibers are added to further inhibit drying shrinkage cracks. Low-speed initial mixing reduces the agglomeration of ultrafine materials such as nano-calcium carbonate and silica fume, ensuring effective contact between the inhibitor and cement particles. Combined with the filling effect of silica fume and nano-calcium carbonate, the flexural and compressive strength of concrete is improved. At the same time, the high density structure of concrete enhances its impermeability and frost resistance.

[0029] Optionally, the polycarboxylate superplasticizer added in the final mixing step I is 50 wt% of the total polycarboxylate superplasticizer; the carboxyl-modified polypropylene fiber in the final mixing step II is pretreated in advance, and the pretreatment method is to first mix the carboxyl-modified polypropylene fiber with the 50 wt% polycarboxylate superplasticizer solution until uniform.

[0030] By adopting the above technical solution, carboxyl-modified polypropylene fibers are premixed with 50wt% polycarboxylate superplasticizer, so that the fiber surface is uniformly coated with superplasticizer. The dispersing effect of superplasticizer can destroy the electrostatic adsorption between fibers, reduce the probability of fiber clumps forming due to direct addition of carboxyl-modified polypropylene fibers to wet materials, and improve crack resistance.

[0031] In summary, the present invention has at least one of the following beneficial technical effects: 1. An expansive agent is used to compensate for the later drying shrinkage and autogenous shrinkage that cannot be solved by hydration temperature rise inhibitors, providing dual crack resistance from two dimensions: reducing temperature rise and compensating for shrinkage. Nano-calcium carbonate particles fill the interfacial micro-cracks and form chemical bonds with cement hydration products, thereby improving interfacial bond strength. In short, the components work synergistically to reduce the heat of hydration while improving early and short-term strength, avoiding strength loss caused by temperature cracks due to reduced heat of hydration and other shrinkage cracks. It is especially suitable for large-volume crack-resistant expansive concrete.

[0032] 2. By employing an expansive composition, rapid hydration is achieved, reducing microcracks caused by shrinkage stress concentration during the early strength period, protecting structural integrity, and enhancing early and short-term strength. The gradient-release core-shell structure allows ettringite to expand early and magnesium oxide to expand late, precisely addressing the problem of early and late shrinkage cracking in concrete, while also aiding in strength development. Hydroxylsilane-modified polypropylene fibers can be more firmly anchored in the concrete matrix, improving interfacial bond strength and enhancing later-stage crack resistance, thus playing a complementary role between early strength and crack resistance.

[0033] 3. By using starch hydrolysis products to delay the early hydration of cement, nano-silica to improve density and strength, and zinc borate to improve long-term durability, the three work together to control the maximum internal temperature rise of concrete below 45℃, thus avoiding temperature cracks caused by thermal stress.

[0034] 4. By using carboxyl-modified polypropylene fibers with surface roughening or indentation treatment, the crack bridging strength can be improved. In addition, the synergistic hydration temperature rise inhibitor reduces the cement hydration rate and reduces temperature stress. The modified roughened fibers directly resist the tensile stress generated by temperature shrinkage through physical crack resistance. The combination of the two improves the crack resistance index of concrete in 1 to 7 days. Detailed Implementation

[0035] The present invention will be further described in detail below with reference to the embodiments.

[0036] Unless otherwise specified, the experimental methods used in the embodiments of this application are conventional methods, and the materials used are commercially available unless otherwise specified.

[0037] Example 1: This example discloses a crack-resistant expansive concrete and its preparation method.

[0038] A crack-resistant expansive concrete comprises, by weight, 270 parts silicate cement, 70 parts fly ash, 50 parts slag powder, 8 parts silica fume, 5 parts expansive composition, 3 parts nano-calcium carbonate, 745 parts sand, 1110 parts crushed stone, 155 parts water, 0.4 parts hydration temperature rise inhibitor, 4.0 parts polycarboxylate superplasticizer, and 0.50 parts carboxyl-modified polypropylene fiber; the expansive composition includes ettringite precursor composite powder, which comprises tricalcium aluminate and gypsum dihydrate in a mass ratio of 1:3; the nano-calcium carbonate has a particle size of 75nm ± 35nm.

[0039] The fly ash can be one or a combination of two of Grade I fly ash and Grade II fly ash; in this embodiment, Grade II fly ash is selected. The slag powder can be one or a combination of two of Grade S95 and Grade S105; in this embodiment, Grade S95 is selected. The crushed stone can be one or a combination of several of granite crushed stone, basalt crushed stone, quartzite crushed stone, and limestone crushed stone; in this embodiment, basalt crushed stone is selected. The hydration temperature rise inhibitor can be one or a combination of two of inorganic salt inhibitors and starch hydrolysis (powder) inhibitors; in this embodiment, starch hydrolysis inhibitor is selected.

[0040] In this invention, the polycarboxylate superplasticizer is selected as an early-strength type polycarboxylate superplasticizer.

[0041] Its preparation method is as follows: Initial mixing: Add sand and gravel, dry mix for 1.5 min; add cement, fly ash, and slag powder, mix for 2.5 min; add silica fume and nano-calcium carbonate, mix for 2.5 min; first add the expansion composition, mix for 1.5 min, then add the hydration temperature rise inhibitor, mix for 1.5 min; add 1 / 3 of the water, mix for 2.5 min; the mixing speed is 120 r / min. Final Mix I: Add 1 / 3 water and polycarboxylate superplasticizer, and stir for 2-3 minutes; Final Mixing II: Add carboxyl-modified polypropylene fiber, add the remaining 1 / 3 of water, and stir for 3-4 minutes; the stirring speed is 180 r / min.

[0042] The 1d / 3d compressive strength was tested in accordance with GB / T50081-2019 "Standard for Test Methods of Physical and Mechanical Properties of Concrete" to evaluate the early and short-term strength development of concrete.

[0043] The peak hydration heat temperature is tested according to GB / T50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (Adiabatic Temperature Rise Method) to assess the severity of the concrete hydration reaction and the probability of temperature cracks caused by temperature rise. The higher the peak hydration heat temperature, the higher the risk of temperature cracks.

[0044] The timing of the hydration heat peak is also tested according to GB / T50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (Adiabatic Temperature Rise Method) to assess the speed of the hydration process. The later the hydration heat peak occurs, the longer the hydration heat can be released slowly, thus reducing the hydration heat peak.

[0045] The initial slump is tested in accordance with GB / T50080-2016 "Standard for Test Methods of Performance of Ordinary Concrete Mixtures" (Initial Slump Cylinder Method) to evaluate the workability of the concrete mixture and reflect its construction performance.

[0046] Example 2: This example discloses a crack-resistant expansion concrete and its preparation method.

[0047] A crack-resistant expansive concrete comprises, by weight, 280 parts silicate cement, 80 parts fly ash, 60 parts slag powder, 15 parts silica fume, 10 parts expansive composition, 5 parts nano-calcium carbonate, 755 parts sand, 1125 parts crushed stone, 165 parts water, 0.6 parts hydration temperature rise inhibitor, 4.5 parts polycarboxylate superplasticizer, and 0.55 parts carboxyl-modified polypropylene fiber; wherein the expansive composition includes ettringite precursor composite powder, and the ettringite precursor composite powder comprises tricalcium aluminate and gypsum dihydrate in a mass ratio of 1:3.

[0048] Everything else is exactly the same as in Example 1.

[0049] Example 3: This example discloses a crack-resistant expansion concrete and its preparation method.

[0050] A crack-resistant expansive concrete comprises, by weight, 275 parts silicate cement, 75 parts fly ash, 55 parts slag powder, 11.5 parts silica fume, 7.5 parts expansive composition, 4 parts nano-calcium carbonate, 750 parts sand, 1117.5 parts crushed stone, 160 parts water, 0.5 parts hydration temperature rise inhibitor, 4.2 parts polycarboxylate superplasticizer, and 0.53 parts carboxyl-modified polypropylene fiber; wherein the expansive composition includes ettringite precursor composite powder, and the ettringite precursor composite powder comprises tricalcium aluminate and gypsum dihydrate in a mass ratio of 1:3.

[0051] Everything else is exactly the same as in Example 1.

[0052] The crack-resistant expansive concretes prepared in Examples 1, 2, and 3 were tested, and the test results are shown in Table 1: Table 1: Indicator Name Example 1 Example 2 Example 3 1-day compressive strength (MPa) 15.4 15.9 15.7 3D compressive strength (MPa) 24.1 24.8 24.5 Peak temperature of hydration heat (°C) 48.9 49.3 48.7 Time of peak hydration heat (h) 42.2 39.2 41.7 Initial slump (mm) 205 211 208 Data from Examples 1, 2, and 3 show that Example 2 exhibits the best strength and initial slump among the three examples. It utilizes high-quality silica fume and nano-calcium carbonate, which rapidly react with the Ca(OH)2 generated during cement hydration within 1-3 days, producing a large amount of dense CSH gel that fills the pores inside the cement stone, thus enhancing early compressive strength. Correspondingly, due to a more complete hydration reaction, Example 2 has the highest hydration heat peak and the shortest time to reach it. The increased water usage in Example 2, combined with the water-reducing agent, improves fluidity. Example 3 represents a balanced state of all components. Overall, all three examples can meet the basic construction performance requirements of crack-resistant expansive concrete.

[0053] Example 4: This example discloses a crack-resistant expansion concrete and its preparation method.

[0054] In this embodiment, the expanded composition includes a core-shell structure, with the core layer being magnesium oxide and the shell layer being ettringite precursor composite powder.

[0055] The preparation method of the core-shell structure is as follows: S1: Mix the ettringite precursor composite powder with deionized water at a mass ratio of 1:1.5 to form a slurry; S2: Add deionized water to a reactor equipped with a stirrer and start stirring at 300 r / min. Add the ettringite precursor composite powder at a uniform rate over 60 minutes to avoid agglomeration. Continue stirring for 30 minutes, and then use ultrasonic dispersion treatment with an ultrasonic power of 300W and a frequency of 20kHz for 15 minutes to ensure that the slurry is uniformly dispersed and there are no visible agglomerations. The viscosity of the slurry is controlled at 650 mPa·s. S3: Add 7.5±2.5μm magnesium oxide particles to another stirred container, add deionized water with a solid-liquid ratio of 1:1, stir at 200r / min to form a magnesium oxide particle suspension to prevent initial agglomeration; S4: Pump the magnesium oxide particle suspension into the ettringite precursor slurry at a uniform rate over 60 minutes, and adjust the stirring speed of the reactor to 500 r / min to ensure that the slurry fully contacts the surface of the magnesium oxide particles; when the coating thickness reaches 2±1 μm, stop stirring to complete the coating. S5: Vacuum filter the coated mixed slurry, collect the filter cake, and dry it at 50°C in a vacuum drying oven to constant weight (the difference between two weighings is ≤0.1%) to obtain core-shell structured particles with magnesium oxide core layer and ettringite precursor shell layer.

[0056] Everything else is exactly the same as in Example 3.

[0057] Example 5: This example discloses a crack-resistant expansion concrete and its preparation method.

[0058] In this embodiment, the expanding composition further includes anhydrous aluminum sulfate, hydroxysilane-modified polypropylene fiber, and nano-silica sol, wherein the mass ratio of core-shell structure: anhydrous aluminum sulfate: hydroxysilane-modified polypropylene fiber: nano-silica sol is 100:1.5:25:15.

[0059] The preparation method of the hydroxysilane modified polypropylene fiber is as follows: the polypropylene fiber is immersed in an ethanol solution with a silane coupling agent concentration of 2%, treated at 50°C for 40 minutes, and then dried to obtain the fiber.

[0060] Everything else is exactly the same as in Example 4.

[0061] Example 6: This example discloses a crack-resistant expansion concrete and its preparation method.

[0062] In this embodiment, the hydration temperature rise inhibitor is a composite hydration temperature rise inhibitor, which is prepared from the following raw materials in parts by weight: 60 parts of 35wt% hydrolyzed corn starch solution, 30 parts of nano silica, and 15 parts of zinc borate.

[0063] The preparation method is as follows: 30 parts of nano-silica and 15 parts of zinc borate were mixed evenly in dry powder form; 60 parts of 35wt% hydrolyzed corn starch solution were added to the mixed dry powder, stirred evenly and then dried; the dried block was crushed and sieved to obtain the final product.

[0064] Everything else is exactly the same as in Example 5.

[0065] The crack-resistant expansive concrete prepared in Examples 4, 5, and 6 was tested, and the test results are shown in Table 2. Table 2: Indicator Name Example 4 Example 5 Example 6 1-day compressive strength (MPa) 16.3 18.6 17.9 3D compressive strength (MPa) 25.8 29.3 28.5 Peak temperature of hydration heat (°C) 46.4 44.1 40.7 Time of peak hydration heat (h) 48.2 52.2 59.2 Initial slump (mm) 211 214 224 Comparing Example 4 with Example 3, the ordinary ettringite precursor composite powder in Example 3 expanded rapidly within 1-3 days. In the core-shell structure of Example 4, the ettringite shell layer expanded slowly first, and the magnesium oxide core layer continued to expand after 7 days, compensating for drying shrinkage and making the cement stone structure denser, avoiding early cracks. Therefore, the strength was slightly increased compared to Example 3. The magnesium oxide core layer has a low thermal conductivity, which can block the heat generated by cement hydration from being transferred to the outside and slow down the heat accumulation. At the same time, the slow reaction of the ettringite shell layer can reduce the heat release rate, thereby reducing the peak heat of hydration. No significant change was observed in the initial slump, indicating that the fluidity meets the construction requirements.

[0066] Comparing Example 5 with Example 4, it can be seen that the addition of anhydrous aluminum sulfate to the expanding composition in Example 5 promotes the cement hydration reaction. It can react rapidly with the cement hydration product Ca(OH)2 to generate calcium sulfate, increasing the amount of gel formation and further improving early and short-term strength. In Example 5, the nano-silica sol fills the voids at the interface between the core-shell particles and the cement paste, improving the overall density of the cement and synergistically supporting the increase in strength. In Example 5, the hydroxysilane-modified fiber can adsorb the heat of hydration, reducing the peak value of the heat of hydration. The dense adsorption layer of the nano-silica sol hinders the diffusion of hydration ions, thus prolonging the time for the hydration peak to appear. Because the modified fiber in Example 5 has a slight water adsorption effect, the initial slump is slightly increased.

[0067] By comparing Example 6 with Example 5, it can be seen that Example 6 incorporates a composite temperature rise inhibitor, nano-silica adsorbs hydrated ions, and zinc borate and Ca... 2+The formation of stable compounds, the three synergistically reduce the degree of cement hydration, so the strength is reduced, but the peak temperature of hydration heat is significantly reduced and the time is greatly extended, meeting the requirements of ultra-high temperature control; In Example 6, in addition to the excellent water retention of corn starch, which reduces the amount of water evaporation, nano silica can improve the fluidity of the slurry, reduce the viscosity of the slurry, improve the initial slump, and further optimize the workability.

[0068] Example 7: This example discloses a crack-resistant expansion concrete and its preparation method.

[0069] In this embodiment, the carboxyl-modified polypropylene fiber is a surface-roughened modified polypropylene fiber, which is made by mixing 4.5±1.5mm and 9±3mm carboxyl-modified polypropylene fibers in a mass ratio of 7:3.

[0070] Everything else is exactly the same as in Example 6.

[0071] Example 8: This example discloses a crack-resistant expansion concrete and its preparation method.

[0072] In this embodiment, the nano-calcium carbonate is prepared by compounding nano-calcium carbonate with a particle size of 50nm±10nm and nano-calcium carbonate with a particle size of 100nm±10nm in a mass ratio of 1:1.

[0073] Everything else is exactly the same as in Example 7.

[0074] Example 9: This example discloses a crack-resistant expansion concrete and its preparation method.

[0075] In the final mixing step of the preparation method in this embodiment, the polypropylene fiber with carboxyl modified group is pretreated in advance; the pretreatment method is to mix the modified polypropylene fiber with 50wt% polycarboxylate superplasticizer until uniform.

[0076] Its preparation method is as follows: Initial mixing: Add sand and gravel, dry mix for 1.5 min; add cement, fly ash, and slag powder, mix for 2.5 min; add silica fume and nano-calcium carbonate, mix for 2.5 min; first add the expansion composition, mix for 1.5 min, then add the hydration temperature rise inhibitor, mix for 1.5 min; add 1 / 3 of the water, mix for 2.5 min; the mixing speed is 120 r / min; Pretreatment: The modified polypropylene fiber is mixed with 50 wt% polycarboxylate superplasticizer until homogeneous; Final Mixing I: Add 1 / 3 water and 50wt% polycarboxylate superplasticizer, and stir for 2.5 minutes; Final Mixing II: Add 50wt% of polycarboxylate superplasticizer pretreated polypropylene fiber, add the remaining 1 / 3 of water, and stir for 3.5 min; the stirring speed is 180 r / min.

[0077] Everything else is exactly the same as in Example 8.

[0078] The crack-resistant expansion concretes prepared in Examples 7-9 were tested, and the test results are shown in Table 3: Table 3: Indicator Name Example 7 Example 8 Example 9 1-day compressive strength (MPa) 18.3 18.9 19.7 3D compressive strength (MPa) 29.6 31.8 34.1 Peak temperature of hydration heat (°C) 42.3 42.9 41.9 Time of peak hydration heat (h) 55.2 54.9 59.4 Initial slump (mm) 219 227 233 Comparing Example 7 with Example 6, the surface-roughened modified fiber increases the interfacial bonding strength and reduces the crack incidence, thus slightly improving the strength. The contact area between the surface-roughened fiber and the cement paste is increased compared to ordinary modified fiber, resulting in increased frictional resistance during paste flow and a slight decrease in initial slump. In Example 7, the peak temperature of hydration heat is increased, which may be due to reduced fluidity and more intense hydration reaction in some areas.

[0079] Comparing Example 8 with Example 7, the compounded nano-calcium carbonate in Example 8 can fill the interfacial pores between the surface roughened fibers and the cement paste, as well as the capillary pores of the cement hydration products themselves, reducing interfacial porosity and improving the overall density of the cement; at the same time, the nanoparticles act as crystal nuclei to promote CSH gel growth; in addition, the Ca released by the nano-calcium carbonate 2+ It can activate active SiO2 in cement, accelerate the hydration process of tricalcium silicate and aluminum sulfur trioxide, and further achieve synchronous optimization of early and short-term strength and later strength.

[0080] Nano-calcium carbonate, acting as a nucleus for the hydration reaction, shortens the cement hydration cycle, accelerates the heat release rate, and increases the peak temperature of hydration heat, but maintains a low level, demonstrating a balanced effect of promoting strength without significant heat increase. The uniform dispersion of nanoparticles in the slurry can reduce the friction between cement particles and fibers. At the same time, the nanoparticles reduce the adsorption of free water by fibers, effectively improving the fluidity of the slurry and causing the initial slump to recover.

[0081] By comparing Example 9 with Example 8, the modified fiber pretreatment avoids the direct adsorption of water by polypropylene fibers, resulting in more complete cement hydration and improved compressive strength. Through pre-coating and a double coating system with composite hydration temperature rise inhibitor, the diffusion of hydration ions is hindered, the heat release rate is reduced, and the peak reaction time of cement hydration is extended, providing sufficient time for heat dissipation and significantly reducing the risk of cracking in large-volume concrete.

[0082] Comparative Example 1: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0083] This comparative example discloses a type of concrete and its preparation method. This comparative example does not contain an expansive composition, but is otherwise identical to Example 3.

[0084] Comparative Example 2: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0085] This comparative example discloses a type of concrete and its preparation method. This comparative example does not contain hydration temperature rise inhibitors, but is otherwise completely the same as Example 3.

[0086] Comparative Example 3: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0087] This comparative example discloses a type of concrete and its preparation method. The expansion composition in this comparative example is tricalcium aluminate alone, and everything else is exactly the same as in Example 3.

[0088] The crack-resistant expansion concrete prepared in Comparative Examples 1 to 3 was tested, and the test results are shown in Table 4: Table 4: Indicator Name Comparative Example 1 Comparative Example 2 Comparative Example 3 1-day compressive strength (MPa) 10.3 15.1 12.6 3D compressive strength (MPa) 17.1 23.2 20.6 Peak temperature of hydration heat (°C) 55.4 67.9 51.6 Time of peak hydration heat (h) 30.2 23.2 39.2 Initial slump (mm) 183 193 201 By comparing Comparative Example 1 and Example 3, the ettringite precursor composite powder of Example 3 slowly generates ettringite within 1 to 3 days, causing expansion, which can accurately compensate for the early plastic shrinkage and drying shrinkage of concrete and avoid the generation of internal microcracks; while the composition of Comparative Example 1 without expansion has an increased possibility of the number of internal microcracks, which greatly weakens the structural bearing capacity and causes a decrease in early and short-term strength; at the same time, because ettringite can fill the existing interface voids, if it is not present, there will be a loose structure, which will further aggravate the strength loss.

[0089] In Example 3, the formation of ettringite requires the consumption of some Ca(OH)2 and water, which can slow down the cement hydration rate. In Comparative Example 1, there is no such buffer, the cement hydration is intense, the heat is released in a concentrated manner, resulting in a higher peak temperature.

[0090] By comparing Comparative Example 2 and Example 3, it was found that Comparative Example 2 had no hydration temperature rise inhibitor and a higher hydration peak temperature. The high temperature caused the microstructure of the cement hydration reaction to deteriorate, and the strength at 1 day and 3 days was slightly lower than that of Comparative Example 3. The higher hydration heat peak temperature of Comparative Example 2 accelerated the water evaporation rate, which in turn led to a decrease in the initial slump and weakened workability.

[0091] By comparing Comparative Example 3 with Example 3, the ettringite precursor composite powder in Example 3 achieved slow and continuous expansion, which matched the shrinkage process of concrete. Comparative Example 3 used a single tricalcium aluminate, which had a faster expansion rate, resulting in internal expansion stress and the formation of microcracks. This led to a significant decrease in 1d / 3d compressive strength, and other indicators also showed a slight decrease, which may be due to the weakening effect caused by the faster expansion rate.

[0092] Comparative Example 4: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0093] This comparative example discloses a type of concrete and its preparation method. In this comparative example, the amount of silica fume added is halved, that is, reduced from 8-15 parts to 4-7.5 parts. In this example, 5.75 parts of silica fume are added, and the rest is exactly the same as in Example 3.

[0094] Comparative Example 5: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0095] This comparative example discloses a type of concrete and its preparation method. In this comparative example, the amount of nano-calcium carbonate added is doubled, that is, increased from 3-5 parts to 6-10 parts. In this example, 8 parts of nano-calcium carbonate are added, and the rest is exactly the same as in Example 3.

[0096] Comparative Example 6: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0097] This comparative example discloses a type of concrete and its preparation method. In this comparative example, micron-sized calcium carbonate is used instead of nano-sized calcium carbonate. A crack-resistant expansive concrete is also disclosed, whose composition by mass is the same as that in Example 3, except that the 75nm±35nm nano-sized calcium carbonate is replaced with calcium carbonate with a particle size of 3μm±2μm, while the proportion remains unchanged.

[0098] Everything else is exactly the same as in Example 3.

[0099] The crack-resistant expansion concrete prepared in comparative examples 4 to 6 was tested, and the test results are shown in Table 5: Table 5: Indicator Name Comparative Example 4 Comparative Example 5 Comparative Example 6 1-day compressive strength (MPa) 13.9 12.9 11.4 3D compressive strength (MPa) 24.1 19.4 28.1 Peak temperature of hydration heat (°C) 48.3 49.6 51.2 Time of peak hydration heat (h) 42.2 36.2 34.2 Initial slump (mm) 199 175 188 By comparing Comparative Example 4 and Example 3, in the components of this invention, silica fume mainly fills the fine voids of cement hydration products. In Comparative Example 4, the silica fume was halved, resulting in insufficient filling effect, which led to more internal voids in the concrete and a corresponding decrease in early compressive strength. Silica fume can also react with Ca(OH)2 generated by cement hydration to form additional CSH gel, enhancing the adhesion of the interfacial transition zone. In Comparative Example 4, the silica fume was insufficient, and the reaction had not fully compensated for the pore defects by 3 days, which together led to a decrease in early and short-term strength. At the same time, the peak temperature of hydration heat was slightly reduced, and the time to peak appearance was slightly prolonged.

[0100] By comparing Comparative Example 5 with Example 3, although nano-calcium carbonate can fill micropores, when the amount added in Comparative Example 5 is doubled, the van der Waals forces between particles are enhanced, making it easy to form a large number of agglomerates, which cannot enter the micropores. At the same time, excessive nano-calcium carbonate adsorbs a large amount of free water, resulting in insufficient moisture in the early stage of cement hydration reaction, further aggravating the strength loss. Meanwhile, the initial slump is severely reduced, and the workability is severely reduced.

[0101] Although an appropriate amount of nano-calcium carbonate can act as a crystal nucleus to promote hydration, excessive agglomerates cause the crystal nuclei to be concentrated and distributed, accelerating the local hydration reaction and increasing the overall peak temperature of hydration heat.

[0102] By comparing Comparative Example 6 with Example 3, the micron-sized calcium carbonate particles in Comparative Example 6 are much larger than nano-sized particles, and therefore cannot fill the small areas of cement hydration products. Instead, they form gaps at the interface, reducing early and short-term strength. The peak hydration heat temperature increases, which is presumably related to the adsorption effect of nanoparticles, i.e., it can reduce the calcium content. 2+ With SiO4 4- The nanoparticles slowed down the hydration reaction, while there were no nanoparticles in Comparative Example 6. Micron particles have poorer dispersibility than nanoparticles and have greater frictional resistance with cement particles, resulting in a decrease in initial slump.

[0103] The above are all preferred embodiments of the present invention and are not intended to limit the scope of protection of the present invention. Therefore, all equivalent changes made in accordance with the structure, shape and principle of the present invention should be covered within the scope of protection of the present invention.

Claims

1. A crack-resistant expansion concrete, characterized in that, The composition comprises, by weight, 270-280 parts silicate cement, 70-80 parts fly ash, 50-60 parts slag powder, 8-15 parts silica fume, 5-10 parts expansion composition, 3-5 parts nano-calcium carbonate, 745-755 parts sand, 1110-1125 parts crushed stone, 155-165 parts water, 0.4-0.6 parts hydration temperature rise inhibitor, 4.0-4.5 parts polycarboxylate superplasticizer, and 0.50-0.55 parts carboxyl-modified polypropylene fiber; the expansion composition includes ettringite precursor composite powder, which comprises tricalcium aluminate and gypsum dihydrate in a mass ratio of 1:3; the nano-calcium carbonate has a particle size of 75nm ± 35nm.

2. The crack-resistant expansion concrete according to claim 1, characterized in that, The expanded composition includes a core-shell structure, which comprises a shell layer and a core layer, wherein the mass ratio of the shell layer to the core layer is 3:2, the core layer is magnesium oxide, and the shell layer is ettringite precursor composite powder.

3. The crack-resistant expansion concrete according to claim 2, characterized in that, The expanded composition further includes anhydrous aluminum sulfate, hydroxysilane-modified polypropylene fiber, and nano-silica sol, wherein the mass ratio of core-shell structure: anhydrous aluminum sulfate: hydroxysilane-modified polypropylene fiber: nano-silica sol is 100:1~2:20~30:12~18.

4. A crack-resistant expansion concrete according to any one of claims 1-3, characterized in that, The hydration temperature rise inhibitor is a composite hydration temperature rise inhibitor, and the preparation method of the composite hydration temperature rise inhibitor is as follows: Mix 25-35 parts of nano-silica and 10-20 parts of zinc borate evenly in a dry powder state; add 55-65 parts of 30-40 wt% hydrolyzed corn starch solution to the mixed dry powder, stir evenly and then dry; crush the dried block and sieve to obtain the final product.

5. The crack-resistant expansion concrete according to claim 4, characterized in that, The carboxyl-modified polypropylene fiber undergoes surface roughening or indentation treatment. The carboxyl-modified polypropylene fiber is made by mixing 3-6 mm and 6-12 mm carboxyl-modified polypropylene fibers at a mass ratio of 7:

3.

6. The crack-resistant expansion concrete according to claim 5, characterized in that, The nano-calcium carbonate is composed of nano-calcium carbonate with a particle size of 50nm±10nm and nano-calcium carbonate with a particle size of 100nm±10nm in a mass ratio of 1:

1.

7. A method for preparing crack-resistant expansive concrete according to any one of claims 1-6, characterized in that, Includes the following steps: Initial mixing: Add sand and gravel, dry mix for 1-2 minutes; add cement, fly ash, and slag powder, mix for 2-3 minutes; add silica fume and nano-calcium carbonate, mix for 2-3 minutes; first add the expansion composition, mix for 1-2 minutes, then add the hydration temperature rise inhibitor, mix for 1-2 minutes; add 1 / 3 of the water, mix for 2-3 minutes; the mixing speed is 120 r / min. Final Mix I: Add 1 / 3 water and polycarboxylate superplasticizer, and stir for 2-3 minutes; Final Mixing II: Add carboxyl-modified polypropylene fiber, add the remaining 1 / 3 of water, and stir for 3-4 minutes; the stirring speed is 180 r / min.

8. The method for preparing crack-resistant expansive concrete according to claim 7, characterized in that, The polycarboxylate superplasticizer added in the final mixing step I is 50 wt% of the total polycarboxylate superplasticizer; the carboxyl-modified polypropylene fiber in the final mixing step II is pretreated in advance, and the pretreatment method is to first mix the carboxyl-modified polypropylene fiber with the 50 wt% polycarboxylate superplasticizer solution until uniform.