A crack-resistant expansive concrete and a method for preparing the same

Abstract

The application relates to an anti-cracking expansive concrete and a preparation method thereof, and relates to the technical field of building materials.The anti-cracking expansive concrete is composed of the following components in mass fractions: Portland cement, fly ash, slag powder, silica fume, an expansive composition, nano calcium carbonate, sand, gravel, water, a hydration temperature rising inhibitor, a polycarboxylic acid water reducing agent and carboxyl-modified polypropylene fibers; the preparation method comprises initial mixing and final mixing steps; the anti-cracking expansive concrete protected by the application can reduce the hydration heat while adding the hydration temperature rising inhibitor, can improve the early and short-term strength, and can avoid the early and short-term strength loss caused by the temperature cracks generated due to the reduction of the hydration heat.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of building materials, in particular to an anti-cracking expansive concrete and a preparation method thereof. BACKGROUND

[0002] In the construction of mass concrete projects such as bridge piers, high-rise building foundations, and water conservancy dams, the problem of cracking caused by the hydration reaction of concrete has long plagued the engineering community. After the concrete is poured, the hydration process of cement releases a large amount of heat, causing a sharp rise in internal temperature and a significant temperature difference with the surface, which in turn generates thermal stress. When the stress exceeds the tensile strength of the concrete, temperature cracks will occur. At the same time, water evaporation during the hardening stage of the concrete will cause drying shrinkage deformation, further exacerbating the generation and propagation of cracks. These cracks not only reduce the load-bearing capacity of the concrete structure, but also provide a penetration channel for rainwater, corrosive media, and other substances, accelerating the deterioration process of steel corrosion and concrete carbonization, and severely shortening the service life of the project and increasing the cost of later maintenance and repair.

[0003] To solve the problem of hydration cracking of mass concrete, the industry has developed a hydration temperature inhibitor to inhibit the hydration rate of tricalcium silicate and tricalcium aluminate in cement, slow down the heat release speed, reduce the maximum temperature rise inside the concrete, and control the risk of temperature cracking.

[0004] Chinese patent application with publication number CN117945721A and publication date of April 30, 2024 proposes an anti-cracking concrete and its construction method, wherein the concrete components include: cement 280-296 parts, fly ash 75-77 parts, slag powder 22-26 parts, sand 738-758 parts, gravel 1090-1130 parts, water 160-164 parts, hydration temperature inhibitor 0.2-0.4 parts, polycarboxylic acid water reducer 3.8-4.0 parts. The use of a hydration temperature 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 temperature difference between the inside and outside.

[0005] For the above technical solution, the inventors found that the hydration temperature inhibitor not only reduces the hydration heat but also reduces the hydration degree, so that the dense structure and skeleton product are less likely to be obtained, which can easily lead to a decrease in the early and short-term strength of the concrete. SUMMARY

[0006] To solve the problem of reduced hydration degree and decreased early and short-term strength of the concrete after adding a hydration temperature inhibitor in the concrete material, the present application provides an anti-cracking expansive concrete and a preparation method thereof.

[0007] In a first aspect, the present application provides an anti-cracking expansive concrete, which adopts the following technical solution:

[0008] An anti-cracking expansive concrete, the mass fraction of its composition includes: Portland cement 270-280 parts, fly ash 70-80 parts, slag powder 50-60 parts, silica fume 8-15 parts, expansive composition 5-10 parts, nano calcium carbonate 3-5 parts, sand 745-755 parts, crushed stone 1110-1125 parts, water 155-165 parts, hydration temperature rise inhibitor 0.4-0.6 parts, polycarboxylate superplasticizer 4.0-4.5 parts, carboxyl modified polypropylene fiber 0.50-0.55 parts;The expansive composition includes ettringite precursor composite powder, the ettringite precursor composite powder includes tri calcium aluminate and dihydrate gypsum with a mass ratio of 1:3;The particle size of the nano calcium carbonate is 75nm±35nm.

[0009] By adopting the above technical scheme, within 0-24 hours after the concrete is mixed, in the plastic stage of the initial hydration reaction, nano calcium carbonate is used, on the one hand, the nano crystal nucleus effect is used to accelerate the cement hydration and promote the generation of hydrated calcium silicate gel to form early and short-term strength, on the other hand, the nano calcium carbonate particles are uniformly dispersed in the interface transition zone between the cement and the aggregate, fill the interface microcracks, and form chemical bonding with the cement hydration products, so that the interface bonding strength is improved, and the early and short-term strength is further improved.

[0010] The ettringite shell precursor is hydrated to generate ettringite, the ettringite grows in the form of needle rods, fills the pores and enhances the cement skeleton, improves the early and short-term strength, in addition, the ettringite generated by the reaction expands in volume, compensates the early plastic shrinkage of the concrete, avoids cracking, and the addition amount of 60% ettringite precursor not only ensures the shrinkage effect, but also effectively reduces the loose skeleton caused by the accumulation of ettringite crystals, and guarantees the development of early and short-term strength;The silica fume quickly reacts with Ca(OH)2 generated by the cement hydration to generate high-density calcium silicate gel, fill the interface gap between the cement and the aggregate, improve the strength of the interface transition zone, and at the same time, enhance the impermeability.

[0011] Within 1-7 days after the concrete is mixed, the cement hydration reaches the heat release peak, the hydration temperature rise inhibitor forms a thin protective film on the surface of the cement particles to slow down the hydration rate, thereby reducing the maximum hydration temperature to within 50℃, avoiding the temperature stress cracks caused by the large temperature difference between the inside and the outside; The nano calcium carbonate, the silica fume and the ettringite shell precursor continue to play the roles of pre-strengthening and pre-expanding.

[0012] The secondary hydration of fly ash and slag powder enters the peak period 7-28 days after the mixing of concrete, and both of them continue to react with Ca(OH)2 generated by cement hydration in this process to generate cementitious substances. These cementitious substances will be closely intertwined with the initial hydration products of cement to form a more compact and stable three-dimensional network structure, which improves the late mechanical strength of concrete; in this stage, 40% of the magnesium oxide core layer gradually reacts to provide the main compensation for dry shrinkage; while the nano calcium carbonate further reduces the hardening shrinkage caused by humidity changes and reduces cracks by improving the compactness of the structure.

[0013] Throughout the process, the polycarboxylate superplasticizer is adsorbed on the surface of cementitious materials such as portland cement, fly ash, and slag powder, allowing the cementitious materials to be uniformly dispersed in water; the strong chemical stability of the carboxyl-modified polypropylene fiber bridges micro-cracks and cooperates with the expansion composition and nano calcium carbonate to control the risk of cracking and prolong the service life of the concrete structure.

[0014] Optionally, the expansion composition includes a core-shell structure, 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 a calcium aluminate precursor composite powder.

[0015] By adopting the above technical solution, the 3:2 ratio of the gradient release type core-shell structure allows the early expansion of calcium aluminate and the late expansion of magnesium oxide, accurately solving the early and late shrinkage and cracking problems of concrete, assisting the development of strength, and efficiently cooperating with the nano calcium carbonate, fiber, inhibitor, and polycarboxylate superplasticizer in the formula to achieve the balance of crack resistance, early strength, and durability, especially suitable for large-volume construction scenarios.

[0016] Optionally, the expansion composition further includes anhydrous aluminum sulfate, hydroxyl silane modified polypropylene fiber, and nano silicon sol, and the mass ratio of the core-shell structure: anhydrous aluminum sulfate: hydroxyl silane modified polypropylene fiber: nano silicon sol is 100: 1-2: 20-30: 12-18.

[0017] By adopting the above technical solution, cement hydration releases Ca(OH)2 within 0-24 hours, anhydrous aluminum sulfate releases Al 3+ , and the alumina tricalcium and gypsum dihydrate in the shell layer of the gradient release type composite expansion agent cooperatively generate fine calcium aluminate crystals with a particle size of ≤5 μm to fill the pores of the cement paste; at the same time, the nano silicon sol reacts with Ca(OH)2 generated by cement hydration to generate dense C-S-H gel, and both of them construct a calcium aluminate skeleton + C-S-H gel filling structure to improve the 3d compressive strength; in addition, the reaction of anhydrous aluminum sulfate and the gradient release type composite expansion agent does not depend on high-activity Ca 2+ , and the early strength efficiency is maintained within the temperature control range of the hydration temperature inhibitor, achieving temperature control without strength delay.

[0018] The nano-silica sol controls the maximum temperature rise in the concrete below 45℃ by adsorbing part of the hydration heat through high specific surface area and cooperating with the hydration temperature rise inhibitor, and reduces the risk of temperature cracks; the hydroxyl groups on the surface of the hydroxyl silane modified polypropylene fibers can be more firmly anchored in the concrete matrix to improve the interfacial bonding strength; and the carboxyl modified polypropylene fibers added in the concrete base component are adapted to the hydroxyl silane modified polypropylene fibers, the carboxyl modified fibers focus on improving the later-stage crack resistance, and the hydroxyl silane modified fibers improve the early strength, and play a complementary role of early strength-crack resistance.

[0019] Optionally, the hydration temperature rise inhibitor is a composite hydration temperature rise inhibitor, and a preparation method of the composite hydration temperature rise inhibitor is as follows:

[0020] 25-35 parts of nano-silicon dioxide and 10-20 parts of zinc borate are uniformly mixed in a dry powder state; 55-65 parts of a 30-40 wt% hydrolyzed corn starch solution is added to the mixed dry powder, and after uniform stirring, the dry powder is baked; and the baked block is crushed and sieved to obtain the product.

[0021] By using the above technical solution, the hydroxyl groups on the molecular chain of the starch hydrolysate can form a complex with Ca 2+ in the cement hydration product, delay the hydration rate, and thus reduce the hydration heat peak value; the nano-silicon dioxide fills the pores between the cement and the mineral admixtures such as slag powder and silica fume, and optimizes the microstructure density of the concrete; at the same time, the nano-silicon dioxide and the Ca(OH)2 generated by the cement hydration undergo secondary hydration to generate more C-S-H gel, which makes up for the weakening of the strength of the starch hydrolysate; the zinc ions in the zinc borate inhibit the metabolic activity of microorganisms, avoiding the biodegradation of the starch hydrolysate in a humid environment; on the other hand, the zinc borate can release crystallization water at high temperatures, dilute combustible gases and form a glassy flame-retardant layer, meeting the demand for flame retardation in underground engineering, tunnels and the like.

[0022] The starch hydrolysate delays the early hydration of the cement, the nano-silicon dioxide improves the density, early and short-term strength, and the zinc borate improves the long-term durability, and the three are synergistic to control the maximum temperature rise in the concrete below 50℃, avoiding temperature cracks caused by temperature difference stress.

[0023] The dry powder is pre-mixed to reduce the respective agglomeration of the solid powders, the 30-40 wt% hydrolyzed corn starch solution has moderate viscosity and can be uniformly coated on the surface of the pre-mixed solid particles, the starch molecular chain is adsorbed on the surface of the nano-silicon dioxide to form steric hindrance, inhibiting secondary agglomeration in the drying process, and at the same time, the micron-sized zinc borate particles are bridged with the nano-silicon dioxide to reduce the stratification of the two kinds of solids in the final powder; in addition, the powder form with sealed packaging can avoid problems such as microbial deterioration, moisture evaporation stratification and the like in the slurry state, prolong the storage period, and better meet the convenience demand of the construction site.

[0024] Optionally, the carboxyl-modified polypropylene fibers are subjected to surface roughening or indentation treatment, and the carboxyl-modified polypropylene fibers are mixed with carboxyl-modified polypropylene fibers of 3-6 mm and 6-12 mm at a mass ratio of 7:3.

[0025] By using the above technical solution, the carboxyl-modified polypropylene fibers are subjected to surface roughening or indentation treatment, the mechanical interlocking force with the concrete is improved, the stress is effectively transmitted through the surface roughening structure, and the crack bridging strength is improved; in addition, the carboxyl-modified polypropylene fibers subjected to surface roughening or indentation treatment have strong dispersibility, reduce fiber agglomeration, and the three-dimensional network structure can disperse and expand stress, so that the expansion energy is uniformly distributed in the concrete; the hydration temperature rise inhibitor reduces the hydration rate of cement, reduces the temperature stress, the modified rough surface fibers directly resist the tensile stress caused by temperature shrinkage through physical crack resistance, and the combination of the two improves the anti-cracking index of the concrete in 1-7d, and is especially suitable for mass concrete engineering.

[0026] The 3-6 mm carboxyl-modified polypropylene fibers account for 70%, which can be uniformly dispersed in the slurry in the plastic stage of the concrete, and the carboxyl groups of the fibers and the cement matrix are chemically bonded, effectively inhibiting the plastic shrinkage cracks caused by aggregate settlement; the 6-12 mm carboxyl-modified polypropylene fibers account for 30%, and due to the longer fiber length, they can span the micro-cracks inside the cement, prevent crack propagation through the bridging effect, and have significant inhibition effect on macroscopic cracks caused by temperature shrinkage and drying shrinkage, thereby greatly improving the integrity of the concrete structure.

[0027] Optionally, the nano calcium carbonate is compounded by nano calcium carbonate with a particle size of 50nm±10nm and nano calcium carbonate with a particle size of 100nm±10nm at a mass ratio of 1:1.

[0028] By using the above technical solution, the 50nm±10nm and 100nm±10nm nano calcium carbonate compounded at a mass ratio of 1:1 can efficiently fill different size voids inside the concrete; the 100nm±10nm nano calcium carbonate preferentially fills the larger voids between cement particles in the cement slurry, and the 50nm±10nm nano calcium carbonate fills the tiny pores, thereby improving the compactness and improving the strength and durability of the concrete.

[0029] In a second aspect, the application provides a preparation method of anti-cracking and expansive concrete, which adopts the following technical solution:

[0030] The preparation method of the anti-cracking and expansive concrete comprises the following steps:

[0031] Primary mixing: put in sand, gravel, dry mixing 1-2 min; add cement, fly ash, slag powder, stirring 2-3 min; put in silica fume, nano calcium carbonate, stirring 2-3 min; first add the expansion composition, stirring 1-2 min, then add the hydration temperature inhibitor, stirring 1-2 min; add 1 / 3 of the water, stirring 2-3 min; wherein the stirring speed is 120 r / min;

[0032] Final mixing I: add 1 / 3 of the water and polycarboxylate superplasticizer, stirring 2-3 min;

[0033] Final mixing II: put in carboxyl modified polypropylene fiber, add the remaining 1 / 3 of the water, stirring 3-4 min; wherein the stirring speed is 180 r / min.

[0034] By adopting the above technical scheme, the expansion agent and the hydration temperature inhibitor are added step by step in the primary mixing stage, the expansion agent is first contacted with the dispersed cementitious material to complete the initial expansion, the uniform distribution of the hydration temperature inhibitor accurately controls the hydration heat peak value of the concrete, the expansion effect of the expansion agent and the time complementation are formed to avoid temperature shrinkage cracks; the carboxyl modified polypropylene fiber is put in the final mixing stage to further inhibit the drying shrinkage cracks; the low-speed primary mixing reduces the agglomeration of the nano calcium carbonate, silica fume and other superfine materials to ensure the effective contact of the inhibitor with the cement particles; in cooperation with the filling effect of the silica fume and the nano calcium carbonate, the flexural and compressive strengths of the concrete are improved; at the same time, the high density structure of the concrete improves the impermeability and frost resistance of the concrete.

[0035] Optionally, the polycarboxylate superplasticizer added in the final mixing I step is 50wt% of the total polycarboxylate superplasticizer; the carboxyl modified polypropylene fiber in the final mixing II step is pretreated in advance, and the pretreatment method is that the carboxyl modified polypropylene fiber is first mixed with a 50wt% polycarboxylate superplasticizer solution until uniform.

[0036] By adopting the above technical scheme, the carboxyl modified polypropylene fiber is premixed with 50wt% polycarboxylate superplasticizer, so that the fiber surface is uniformly coated with the superplasticizer; the dispersion effect of the superplasticizer can destroy the electrostatic adsorption between the fibers, reduce the probability of forming a fiber ball when the carboxyl modified polypropylene fiber is directly put into the wet material, and improve the anti-cracking effect.

[0037] In summary, the present application includes at least one of the following beneficial technical effects:

[0038] 1. Dual anti-cracking from two dimensions of temperature rise reduction and shrinkage compensation by using expansion agent to solve the late drying shrinkage and autogenous shrinkage that cannot be solved by hydration temperature rise inhibitor; nano calcium carbonate particles fill the interface microcracks and form chemical bonding with cement hydration products, so that the interface bonding strength is improved; in summary, the components cooperate synergistically to reduce hydration heat while improving early and short-term strength, avoiding strength loss due to temperature cracks and other shrinkage cracks caused by reducing hydration heat, especially suitable for mass anti-cracking expansive concrete.

[0039] 2. By using expansion composition, rapid hydration can be achieved, microcracks caused by shrinkage stress concentration during early strength period can be reduced, structural integrity can be protected, and early and short-term strength can be improved; gradient release type core-shell structure allows ettringite to expand early and magnesium oxide to expand late, accurately solving the early and late shrinkage and cracking problems of concrete, and assisting strength development; hydroxyl silane modified polypropylene fibers can be more firmly anchored in the concrete matrix, improving the interface bonding strength; improving the late anti-cracking property, playing the complementary role of early strength-anti-cracking.

[0040] 3. By using starch hydrolysate to delay early hydration of cement, nano silicon dioxide to improve density and strength, and zinc borate to improve long-term durability, the three synergistically control the internal maximum temperature rise of concrete below 45℃, avoiding temperature cracks caused by temperature difference stress.

[0041] 4. By using carboxyl modified polypropylene fibers with surface roughening or indentation treatment, the crack bridging strength is improved; in addition, the hydration temperature rise inhibitor reduces the cement hydration rate, reduces the temperature stress, and the modified rough surface fiber directly resists the tensile stress generated by temperature shrinkage through physical crack resistance, and the combination of the two improves the anti-cracking index of concrete in 1-7d. DETAILED DESCRIPTION

[0042] The application will be further described in detail below in combination with examples.

[0043] In the examples of the present application, the experimental methods used are conventional methods unless otherwise specified, and the materials used, etc., are commercially available unless otherwise specified.

[0044] Example 1: The present embodiment discloses an anti-cracking expansive concrete and a preparation method thereof.

[0045] An anti-cracking expansive concrete, a mass fraction of a composition of which comprises: Portland cement 270 parts, fly ash 70 parts, slag powder 50 parts, silica fume 8 parts, expansive composition 5 parts, nano calcium carbonate 3 parts, sand 745 parts, crushed stone 1110 parts, water 155 parts, hydration temperature rise inhibitor 0.4 parts, polycarboxylate superplasticizer 4.0 parts, carboxyl modified polypropylene fiber 0.50 parts; the expansive composition comprises ettringite precursor composite powder, the ettringite precursor composite powder comprises tri calcium aluminate and gypsum dihydrate in a mass ratio of 1:3; the particle size of the nano calcium carbonate is 75nm±35nm.

[0046] The fly ash can be selected from one or a combination of grade I fly ash and grade II fly ash, and grade II fly ash is selected in the embodiment; the slag powder can be selected from one or a combination of S95 grade and S105 grade, and S95 grade is selected in the embodiment; the crushed stone can be selected from one or a combination of granite crushed stone, basalt crushed stone, quartzite crushed stone and limestone crushed stone, and basalt crushed stone is selected in the embodiment; the hydration temperature rise inhibitor can be selected from one or a combination of inorganic salt inhibitor and starch hydrolysis (powdered) inhibitor, and starch hydrolysis inhibitor is selected in the embodiment.

[0047] The polycarboxylate superplasticizer in the application is selected from early strength type polycarboxylate superplasticizer.

[0048] The preparation method is as follows:

[0049] Primary mixing: put in sand and crushed stone, dry mix for 1.5min; add cement, fly ash and slag powder, stir for 2.5min; add silica fume and nano calcium carbonate, stir for 2.5min; first add the expansive composition, stir for 1.5min, then add the hydration temperature rise inhibitor, stir for 1.5min; add 1 / 3 of the water, stir for 2.5min; wherein the stirring speed is 120r / min;

[0050] Final mixing I: add 1 / 3 of the water and polycarboxylate superplasticizer, stir for 2-3min;

[0051] Final mixing II: add carboxyl modified polypropylene fiber, add the remaining 1 / 3 of the water, stir for 3-4min; wherein the stirring speed is 180r / min.

[0052] 1d / 3d compressive strength is detected according to GB / T50081-2019 "Standard for Testing Methods of Physical and Mechanical Properties of Concrete", to evaluate the early and short-term strength development of the concrete.

[0053] The hydration heat peak temperature is detected according to GB / T50080-2016 Standard Test Methods for Properties of Fresh Ordinary Concrete (Adiabatic Temperature Rise Method), and the hydration reaction intensity and the temperature crack probability caused by temperature rise are evaluated. The higher the hydration heat peak temperature is, the higher the risk of temperature crack is.

[0054] The hydration heat peak appearance time is also detected according to GB / T50080-2016 Standard Test Methods for Properties of Fresh Ordinary Concrete (Adiabatic Temperature Rise Method), and the speed of hydration process reaction is evaluated. The later the hydration heat peak appearance time is, the longer the hydration heat can be released slowly, and the hydration heat peak can be reduced.

[0055] The initial slump is detected according to GB / T50080-2016 Standard Test Methods for Properties of Fresh Ordinary Concrete (Initial Slump Cylinder Method), and the workability of the concrete mixture and the construction performance are evaluated.

[0056] Embodiment 2: The embodiment discloses an anti-cracking expansive concrete and a preparation method thereof.

[0057] An anti-cracking expansive concrete, which comprises the following components in parts by mass: 280 parts of Portland cement, 80 parts of fly ash, 60 parts of slag powder, 15 parts of silica fume, 10 parts of an expansive composition, 5 parts of nano calcium carbonate, 755 parts of sand, 1125 parts of crushed stone, 165 parts of water, 0.6 parts of a hydration temperature rise inhibitor, 4.5 parts of a polycarboxylate superplasticizer, and 0.55 parts of carboxyl-modified polypropylene fibers; the expansive composition comprises a ettringite precursor composite powder, and the ettringite precursor composite powder comprises 1:3 of tricalcium aluminate and dihydrate gypsum in terms of mass ratio.

[0058] The other aspects are completely the same as those of Embodiment 1.

[0059] Embodiment 3: The embodiment discloses an anti-cracking expansive concrete and a preparation method thereof.

[0060] An anti-cracking expansive concrete, which comprises the following components in parts by mass: 275 parts of Portland cement, 75 parts of fly ash, 55 parts of slag powder, 11.5 parts of silica fume, 7.5 parts of an expansive composition, 4 parts of nano calcium carbonate, 750 parts of sand, 1117.5 parts of crushed stone, 160 parts of water, 0.5 parts of a hydration temperature rise inhibitor, 4.2 parts of a polycarboxylate superplasticizer, and 0.53 parts of carboxyl-modified polypropylene fibers; the expansive composition comprises a ettringite precursor composite powder, and the ettringite precursor composite powder comprises 1:3 of tricalcium aluminate and dihydrate gypsum in terms of mass ratio.

[0061] The other aspects are completely the same as those of Embodiment 1.

[0062] The anti-cracking expansive concrete prepared in Embodiment 1, Embodiment 2 and Embodiment 3 is detected, and the detection results are shown in Table 1:

[0063] Table 1:

[0064] Index name Example 1 Example 2 Example 3 1d compressive strength (MPa) 15.4 15.9 15.7 3d compressive strength (MPa) 24.1 24.8 24.5 Temperature of peak hydration heat (°C) 48.9 49.3 48.7 Time of peak hydration heat occurrence (h) 42.2 39.2 41.7 Initial slump (mm) 205 211 208

[0065] From the data of Example 1, Example 2 and Example 3, it can be seen that Example 2 is the best in strength and initial slump among the three examples, high silica fume and nano calcium carbonate are used to quickly react with Ca(OH)2 generated by cement hydration within 1-3d to generate a large amount of dense C-S-H gel to fill the internal pores of the cement stone and improve the early compressive strength; correspondingly, because the hydration reaction is more complete, the hydration heat peak value of Example 2 is the highest and the time of the hydration heat peak value appears is the shortest; Example 2 improves the fluidity due to the increase of water consumption and the cooperation with the water reducing agent; Example 3 belongs to the balanced state of each component; overall, the three examples can all meet the basic construction performance of the anti-cracking expansive concrete.

[0066] Example 4: The present example discloses an anti-cracking expansive concrete and a preparation method thereof.

[0067] The expansive composition in the present example includes a core-shell structure, the core layer is magnesium oxide, and the shell layer is ettringite precursor composite powder.

[0068] The preparation method of the core-shell structure is as follows:

[0069] S1: The ettringite precursor composite powder is mixed with deionized water at a mass ratio of 1:1.5 to form a slurry;

[0070] S2: The deionized water is added to a reaction kettle with a stirring device, and the stirring is started at 300r / min. The ettringite precursor composite powder is added at a uniform speed within 60 minutes to avoid agglomeration. The stirring is continued for 30 minutes, and then ultrasonic dispersion treatment is adopted, with an ultrasonic power of 300W and a frequency of 20kHz, for 15 minutes to ensure that the slurry is uniformly dispersed and there is no visible particle agglomeration, and the viscosity of the slurry is controlled at 650mPa·s;

[0071] S3: The 7.5±2.5μm magnesium oxide particles are added to another stirring container, and deionized water is added at a solid-liquid ratio of 1:1, and stirred at 200r / min to form a magnesium oxide particle suspension to prevent initial agglomeration;

[0072] S4: The magnesium oxide particle suspension is pumped into the ettringite precursor slurry at a uniform speed within 60 minutes, and the stirring speed of the reaction kettle is adjusted to 500r / min to make the slurry fully contact the surface of the magnesium oxide particles; when the thickness of the coating layer reaches 2±1μm, the stirring is stopped, and the coating is completed;

[0073] S5: The coated mixed slurry is vacuum filtered, and the filter cake is collected and dried in a vacuum drying oven at 50℃ to constant weight (the difference between the two weightings is ≤0.1%) to obtain magnesium oxide core layer-calcium aluminate precursor shell layer core-shell structure particles.

[0074] The other aspects are identical to example 3.

[0075] Example 5: This example discloses a crack-resistant expansive concrete and a preparation method thereof.

[0076] The expansive composition in this example further comprises: anhydrous aluminum sulfate, hydroxyl silane modified polypropylene fiber and nano-silica sol, wherein the mass ratio of core-shell structure: anhydrous aluminum sulfate: hydroxyl silane modified polypropylene fiber: nano-silica sol is 100: 1.5: 25: 15.

[0077] The hydroxyl silane modified polypropylene fiber is prepared by immersing polypropylene fiber in an ethanol solution with a silane coupling agent concentration of 2% at 50℃ for 40 min, and then drying.

[0078] The other aspects are identical to example 4.

[0079] Example 6: This example discloses a crack-resistant expansive concrete and a preparation method thereof.

[0080] The hydration temperature rise inhibitor in this example is a composite hydration temperature rise inhibitor, which is prepared from the following raw materials in mass fraction: 60 parts of 35wt% hydrolyzed corn starch solution, 30 parts of nano-silicon dioxide, and 15 parts of zinc borate.

[0081] The preparation method is as follows:

[0082] 30 parts of nano-silicon dioxide and 15 parts of zinc borate are mixed uniformly in a dry powder state; 60 parts of 35wt% hydrolyzed corn starch solution is added to the mixed dry powder, and after stirring uniformly, it is dried; the dried block is crushed and sieved to obtain the product.

[0083] The other aspects are identical to example 5.

[0084] The crack-resistant expansive concrete prepared in example 4, example 5 and example 6 is detected, and the detection results are shown in table 2:

[0085] Table 2:

[0086] Index name Example 4 Example 5 Example 6 1d compressive strength (MPa) 16.3 18.6 17.9 3d compressive strength (MPa) 25.8 29.3 28.5 Temperature of peak hydration heat (°C) 46.4 44.1 40.7 Time of peak hydration heat occurrence (h) 48.2 52.2 59.2 Initial slump (mm) 211 214 224

[0087] 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.

[0088] 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.

[0089] 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.

[0090] Example 7: This example discloses a crack-resistant expansion concrete and its preparation method.

[0091] 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.

[0092] Everything else is exactly the same as in Example 6.

[0093] Example 8: This example discloses a crack-resistant expansion concrete and its preparation method.

[0094] 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.

[0095] Everything else is exactly the same as in Example 7.

[0096] Example 9: This example discloses a crack-resistant expansion concrete and its preparation method.

[0097] 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.

[0098] Its preparation method is as follows:

[0099] 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;

[0100] Pretreatment: The modified polypropylene fiber is mixed with 50 wt% polycarboxylate superplasticizer until homogeneous;

[0101] Final Mixing I: Add 1 / 3 water and 50wt% polycarboxylate superplasticizer, and stir for 2.5 minutes;

[0102] 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.

[0103] Everything else is exactly the same as in Example 8.

[0104] The crack-resistant expansion concretes prepared in Examples 7-9 were tested, and the test results are shown in Table 3:

[0105] Table 3:

[0106] Index name Example 7 Example 8 Example 9 1d compressive strength (MPa) 18.3 18.9 19.7 3d compressive strength (MPa) 29.6 31.8 34.1 Temperature of peak hydration heat (°C) 42.3 42.9 41.9 Time of peak hydration heat occurrence (h) 55.2 54.9 59.4 Initial slump (mm) 219 227 233

[0107] 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.

[0108] 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.

[0109] 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.

[0110] 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.

[0111] Comparative Example 1: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0112] 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.

[0113] Comparative Example 2: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0114] 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.

[0115] Comparative Example 3: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0116] 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.

[0117] The crack-resistant expansion concrete prepared in Comparative Examples 1 to 3 was tested, and the test results are shown in Table 4:

[0118] Table 4:

[0119] Index name Comparative Example 1 Comparative Example 2 Comparative Example 3 1d compressive strength (MPa) 10.3 15.1 12.6 3d compressive strength (MPa) 17.1 23.2 20.6 Temperature of peak hydration heat (°C) 55.4 67.9 51.6 Time of peak hydration heat occurrence (h) 30.2 23.2 39.2 Initial slump (mm) 183 193 201

[0120] 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.

[0121] 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.

[0122] 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.

[0123] 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.

[0124] Comparative Example 4: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0125] 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.

[0126] Comparative Example 5: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0127] 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.

[0128] Comparative Example 6: This comparative example discloses a crack-resistant expansive concrete and its preparation method.

[0129] 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.

[0130] Everything else is exactly the same as in Example 3.

[0131] The crack-resistant expansion concrete prepared in comparative examples 4 to 6 was tested, and the test results are shown in Table 5:

[0132] Table 5:

[0133] Index name Comparative Example 4 Comparative Example 5 Comparative Example 6 1d compressive strength (MPa) 13.9 12.9 11.4 3d compressive strength (MPa) 24.1 19.4 28.1 Temperature of peak hydration heat (°C) 48.3 49.6 51.2 Time of peak hydration heat occurrence (h) 42.2 36.2 34.2 Initial slump (mm) 199 175 188

[0134] 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.

[0135] 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.

[0136] 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.

[0137] 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.

[0138] 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, expansive concrete, characterized in that, The mass parts of the components include: 270-280 parts of Portland cement, 70-80 parts of fly ash, 50-60 parts of slag powder, 8-15 parts of silica fume, 5-10 parts of expansion composition, 3-5 parts of nano calcium carbonate, 745-755 parts of sand, 1110-1125 parts of gravel, 155-165 parts of water, 0.4-0.6 parts of composite hydration temperature inhibitor, 4.0-4.5 parts of polycarboxylate superplasticizer, and 0.50-0.55 parts of carboxyl-modified polypropylene fiber; the particle size of the nano calcium carbonate is 75nm±35nm; The expansion composition comprises a core-shell structure, anhydrous aluminum sulfate, hydroxyl silane-modified polypropylene fiber and nano silicon sol, wherein the mass ratio of the core-shell structure, the anhydrous aluminum sulfate, the hydroxyl silane-modified polypropylene fiber and the nano silicon sol is 100:1-2:20-30:12-18; The core-shell structure comprises a shell layer and a core layer, and 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; The ettringite precursor composite powder comprises 1:3 of tricalcium aluminate and dihydrate gypsum; The preparation method of the composite hydration temperature inhibitor is as follows: 25-35 parts of nano silicon dioxide and 10-20 parts of zinc borate are uniformly mixed in a dry powder state; 55-65 parts of a 30-40wt% hydrolyzed corn starch solution is added to the mixed dry powder, which is uniformly stirred and then dried; the dried block is crushed and sieved to obtain the composite hydration temperature inhibitor.

2. The anti-cracking expansive concrete according to claim 1, wherein, The carboxyl-modified polypropylene fiber is subjected to surface roughening or indentation treatment, and the carboxyl-modified polypropylene fiber is mixed with 3-6mm and 6-12mm carboxyl-modified polypropylene fibers at a mass ratio of 7:

3.

3. The anti-cracking expansive concrete according to claim 2, wherein, The nano calcium carbonate is compounded by 1:1 of nano calcium carbonate with a particle size of 50nm±10nm and nano calcium carbonate with a particle size of 100nm±10nm.

4. A method of producing the anti-cracking expansive concrete according to any one of claims 1 to 3, characterized in that, The method comprises the following steps: Primary mixing: add sand and gravel, dry mix for 1-2min; add cement, fly ash and slag powder, stir for 2-3min; add silica fume and nano calcium carbonate, stir for 2-3min; first add the expansion composition, stir for 1-2min, then add the hydration temperature inhibitor, stir for 1-2min; add 1 / 3 of the water, stir for 2-3min; wherein the stirring speed is 120r / min; Final mixing I: add 1 / 3 of the water and polycarboxylate superplasticizer, stir for 2-3min; Final mixing II: add the carboxyl-modified polypropylene fiber, add the remaining 1 / 3 of the water, and stir for 3-4min; wherein the stirring speed is 180r / min.

5. A method of producing a crack-resistant expansive concrete according to claim 4, characterized in that, The polycarboxylate superplasticizer added in the final mixing I step accounts for 50wt% of the total polycarboxylate superplasticizer; the carboxyl-modified polypropylene fiber in the final mixing II step is pretreated in advance, and the pretreatment is that the carboxyl-modified polypropylene fiber is first mixed with a 50wt% polycarboxylate superplasticizer solution until uniform.