Medium-heat Portland cement and preparation method thereof

Abstract

The invention belongs to the technical field of building materials, and particularly relates to moderate heat Portland cement and a preparation method thereof, and the cement is composed of Portland cement clinker, gypsum, blast furnace slag and two novel inorganic modified compounds. According to the invention, calcium aluminum silicon nitrogen oxide nanosheets and calcium zirconate coated lithium zinc borosilicate core-shell microspheres are innovatively introduced as functional modifiers. The two-dimensional structure of the two-dimensional structure plays dual roles of nucleation and shielding in the cement hydration process, so that the early strength development can be promoted, and the heat release process can be delayed; the latter generates controllable expansion in a specific hydration stage by virtue of a core-shell structure, so that the shrinkage stress is effectively compensated, and the anti-cracking performance is remarkably improved. The preparation method comprises the working procedures of raw material crushing, segmented grinding, homogenizing stirring and the like. The obtained cement product has excellent early strength, long-term durability and volume stability while keeping the low hydration heat characteristic, and is suitable for construction of major water conservancy projects and infrastructures which have strict requirements on temperature control of mass concrete.

Description

Technical Field

[0001] This invention belongs to the field of building materials technology, specifically relating to a medium-heat silicate cement and its preparation method. Background Technology

[0002] In the construction of large-scale water conservancy projects, large-volume concrete structures, and important infrastructure projects, intermediate-heat silicate cement has become a key cementing material due to its relatively low heat of hydration. Traditional production processes primarily reduce the heat of hydration by controlling the clinker mineral composition and limiting the content of highly reactive minerals such as tricalcium silicate and tricalcium aluminate, thereby slowing down the hydration reaction and preventing cracking of the concrete structure due to temperature stress. This type of cement has been widely used in several large-scale water conservancy projects in my country, and its technical standards have clear limitations on early-stage heat of hydration. However, this technical approach, which sacrifices early strength, has significant shortcomings: the slow development of early strength directly affects construction progress and formwork turnover efficiency; the level of heat of hydration control is still insufficient to meet the temperature control requirements of ultra-large-volume concrete structures; and long-term crack resistance and durability need further improvement. To improve these properties, auxiliary cementing materials such as fly ash, slag powder, or silica fume are often added, but these materials often introduce new problems while improving certain properties. For example, while ultrafine silica fume can improve the density of the paste, it significantly increases water consumption and inhibits early strength development; calcined clay, although possessing certain activity, may exacerbate drying shrinkage and increase the risk of long-term cracking. These traditional modification methods have failed to fundamentally resolve the contradictory relationship between early strength and heat release control in low-heat cement.

[0003] To overcome the aforementioned technical challenges, researchers have developed various inorganic modifying materials to optimize cement performance. For example, using high specific surface area nanomaterials as hydration nucleating agents can promote the formation of hydration products and improve early strength, but it often simultaneously accelerates early hydration heat release, which is detrimental to temperature control. Various expanding agents are used to compensate for shrinkage, but the timing and amount of expansion are difficult to control precisely, potentially leading to later stability issues. Furthermore, existing modifying materials have limited functionality and lack the ability to synergistically regulate different hydration stages, failing to ensure strength development while reducing early heat release, and also struggling to balance long-term volume stability and erosion resistance. In particular, the lack of multifunctional composite materials capable of precisely controlling the hydration process at the molecular scale presents a bottleneck for further improvements in cement performance. Therefore, there is an urgent need in this field to develop a novel cement system capable of synergistically regulating the entire hydration process to achieve synergistic optimization of multiple performance indicators such as low heat generation, high strength, and high durability.

[0004] Based on this, the present invention proposes a technical solution that introduces two novel inorganic modifying compounds. This solution utilizes layered nanomaterials with specific surface modifications to provide effective nucleation sites in the early stages of cement hydration, promoting strength development while simultaneously delaying the exothermic hydration process through their surface properties. Furthermore, microspheres with a core-shell structure generate controllable expansion at specific hydration stages, effectively compensating for shrinkage stress and significantly improving crack resistance. The two functional materials complement each other in performance and are sequentially activated, together forming a complete hydration process regulation system. Ultimately, this achieves a breakthrough in the comprehensive performance of intermediate-heat silicate cement, providing a superior material choice for major engineering projects. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a medium-heat silicate cement and its preparation method.

[0006] In a first aspect, the present invention provides a method for preparing intermediate-heat silicate cement, comprising the steps of: S1. Crush silicate cement clinker to obtain crushed silicate cement clinker; dry gypsum at 78-82℃ to obtain dried gypsum; add crushed silicate cement clinker, dried gypsum and blast furnace slag into a ball mill and grind to obtain cement powder. S2. Add calcium aluminum silicon nitrogen oxide nanosheets and calcium zirconate-coated lithium borosilicate zinc core-shell microspheres to cement powder, grind them to obtain a mixed material; stir the mixed material in a mixer.

[0007] In this invention, the hydration process of intermediate-heat silicate cement exhibits a multi-stage, multi-dimensional synergistic regulatory mechanism due to the introduction of two functional compounds. In the early stages of hydration, calcium aluminum silicate nitrogen oxide nanosheets, with their large specific surface area and surface-modified nucleation sites, significantly promote the hydration of tricalcium silicate to form CSH gel. This promoting effect stems from their reduction of the nucleation barrier of hydration products. Simultaneously, the hydrophobic layer on the nanosheet surface temporarily shields some cement particles from contact with water, delaying the rapid hydration, particularly of tricalcium aluminate. This helps to mitigate the early hydration exothermic peak, improve the workability of the freshly mixed slurry, and reduce the risk of early temperature cracking. In the middle and later stages of hydration, calcium zirconate-coated lithium borosilicate zinc core-shell microspheres begin to play a crucial role. At this point, the cement slurry has formed preliminary structural strength, the microsphere shell is slowly eroded in an alkaline environment, and the internal active components are released in a controlled manner. The released borates and other substances participate in the reaction, generating expansive products. This expansion is strictly regulated by the core-shell structure and the dense outer shell, matching the hydration shrinkage process in time and spatially distributing at microscopic defects in the slurry, thereby effectively generating prestress, compensating for shrinkage, and refining pores. Furthermore, the nanosheets continue to exert their micro-filling effect; their two-dimensional sheet-like structure can intersect between hydration products, effectively segmenting large pores and guiding the CSH gel to grow more orderly, resulting in a denser slurry microstructure. The interfaces between the two modified compounds and cement hydration products are tightly bound through chemical bonding and physical interlocking, enhancing the mechanical properties and durability of the interfacial transition zone. Finally, through the regulation of early hydration exothermics and nucleation processes by the nanosheets, and the optimization of mid-to-late-stage volume stability by the core-shell microspheres, this invention successfully achieves precise management of the entire cement hydration process, endowing the cement stone structure with comprehensive performance improvements including low heat of hydration, high early strength, excellent crack resistance, and durability.

[0008] As a preferred embodiment of the present invention, in step S1, the grinding time is 30-40 min.

[0009] As a preferred technical solution of the present invention, in step S2, the mixed materials are stirred in the mixer for 60-80 minutes.

[0010] As a preferred embodiment of the present invention, the method for preparing the calcium aluminum silicon nitride nanosheets includes: A1. Dissolve calcium nitrate, aluminum isopropoxide, and tetraethyl orthosilicate in anhydrous ethanol; add urea and hexadecyltrimethylammonium bromide to obtain a mixed solution; transfer the mixed solution to a high-pressure reactor and react it at 175-185℃; after the reaction is complete, allow it to cool naturally to room temperature, collect the precipitate by centrifugation, wash the precipitate with anhydrous ethanol, and dry it under vacuum to obtain the precursor; A2. The precursor was heated to 795-805℃ under a nitrogen atmosphere and held at that temperature. Then, it was heat-treated at 895-905℃ under an ammonia atmosphere to obtain nitrided nanosheets. After the nitrided nanosheets were cooled to room temperature, they were dispersed in isopropanol, dodecyltriethoxysilane was added, and the mixture was refluxed at 68-72℃. The solid product was collected by centrifugation, washed with ethanol, and dried under vacuum at 78-82℃.

[0011] In this invention, the synthesis of calcium aluminum silicon nitride nanosheets is mainly based on a solvothermal reaction combined with high-temperature nitridation. The mechanism begins with the breaking and recombination of the molecular structures of aluminum isopropoxide and tetraethyl orthosilicate under heating and the influence of protons (mainly derived from the water of crystallization of calcium nitrate) within the system. Specifically, the Al-OC and Si-OC bonds (alkoxy bonds) break and combine with hydroxyl groups (from the water of crystallization) in the system, forming an amorphous aluminum-oxygen-silicon (Al-O-Si) three-dimensional network gel through a condensation reaction. Simultaneously, calcium ions dissociated from the added calcium nitrate are effectively fixed within this gel network through electrostatic interactions. The addition of urea decomposes during subsequent heat treatment, providing a nitrogen source. The ammonia gas produced by its decomposition reacts with the oxide precursor, achieving partial substitution of oxygen atoms by nitrogen atoms, thereby forming a more thermodynamically stable nitride structure. In this process, hexadecyltrimethylammonium bromide acts as a structure-directing agent. Its hydrophilic end binds to inorganic species through electrostatic interactions, and the steric hindrance effect of the hydrophobic long chain restricts the three-dimensional growth of the crystal, inducing its preferential growth along the two-dimensional direction, initially forming a nanosheet morphology. Subsequently, heat treatment is carried out under strictly controlled atmosphere and temperature. First, in a nitrogen atmosphere, the precursor undergoes further polymerization and crystallization, and the nitrogen atmosphere effectively prevents the oxidation of the active components. Then, high-temperature treatment is carried out in an ammonia atmosphere, which is a key step in deepening the nitridation reaction. The highly active nitrogen species generated by ammonia cracking penetrate deeper into the crystal lattice, promoting the transformation of the amorphous phase into well-crystallized calcium aluminum silicon nitride. Its crystal structure can be compared with the defective pyrochlore structure, which is a three-dimensional network composed of tetrahedra and octahedra connected by common vertices. The surfactant regulation and specific crystal facet energy differences in this synthesis lead to the emergence of the two-dimensional sheet-like structure. Finally, the nanosheets were surface-modified using dodecyltriethoxysilane. The alkoxy groups in dodecyltriethoxysilane condensed with the hydroxyl groups on the nanosheet surface to form strong Si-O covalent bonds, thereby grafting hydrophobic long-chain alkyl groups onto the nanosheet surface. This modification process endowed the nanosheets with better dispersibility in the organic medium of cement paste. More importantly, the hydrophobic layer can physically shield the contact between water molecules and cement particles in the early stages of hydration, providing a temporary water-blocking effect and delaying early hydration heat release.

[0012] As a preferred embodiment of the present invention, in step A1, the mass ratio of calcium nitrate, aluminum isopropoxide, tetraethyl orthosilicate, urea and hexadecyltrimethylammonium bromide is 25-30:8-10:10-12:25-30:6-8.

[0013] As a preferred embodiment of the present invention, in step A2, the temperature is raised to 795-805℃ and the holding time is 1-2 hours.

[0014] As a preferred embodiment of the present invention, the preparation method of the calcium zirconate-coated lithium borosilicate zinc core-shell microspheres includes: B1. Lithium nitrate, zinc nitrate, boric acid, and tetraethyl orthosilicate are dissolved in a mixed solvent of deionized water and ethanol. Polyvinylpyrrolidone is added, and the pH is adjusted to 9-10. Spray drying is performed to obtain precursor microspheres. B2. The precursor microspheres are heated to 495-505℃ in air, held at that temperature, and then sintered at 695-705℃ to obtain sintered core microspheres. Zirconium oxychloride and calcium nitrate are dissolved in deionized water, and polyacrylic acid is added to obtain a mixture. The sintered core microspheres are added to the mixture, stirred at room temperature, and then separated by centrifugation to obtain coated microspheres. The coated microspheres are heated to 900-1000℃ in air and held at that temperature. They are then naturally cooled to room temperature.

[0015] In this invention, the core mechanism of the formation of calcium zirconate-coated lithium zinc borosilicate core-shell microspheres lies in the stepwise construction of core-shell structures with different reactivity. The core is first prepared by spray drying to rapidly dehydrate a salt solution containing lithium, zinc, boron, and silicon, forming metastable precursor microspheres with uniform chemical composition. Polyvinylpyrrolidone (PVP) serves as a dispersant and template; its molecular chains prevent excessive particle aggregation through steric hindrance and help maintain the spherical morphology. Subsequently, segmented sintering is performed at specific temperatures. At lower temperatures, residual organic matter and water of crystallization are mainly removed; at higher temperatures, solid-state reactions occur, with boron oxide from boric acid decomposition reacting with zinc oxide, lithium oxide, and silicon dioxide to form an amorphous lithium zinc borosilicate glassy phase core. This amorphous structure possesses potential reactivity due to its high Gibbs free energy. The outer shell is constructed through sol-gel encapsulation and secondary sintering. A solution prepared from zirconium and calcium sources is mixed with the core microspheres. By precisely controlling the pH and ionic strength of the solution, calcium zirconate sol is uniformly coated onto the core surface through electrostatic adsorption and hydrogen bonding. The addition of polyacrylic acid improves the stability of the sol, preventing premature flocculation or adhesion of the core microspheres due to van der Waals forces. During the final sintering process, the coating layer undergoes dehydration, condensation, and crystallization, forming a dense calcium zirconate shell. This shell has a perovskite-type crystal structure, and its density controls the permeation rate of water and ions, thereby regulating the release kinetics of the active substances in the core. This core-shell design allows the microspheres to remain relatively inert in the early stages of cement hydration. As hydration progresses, hydroxide ions in the pore fluid gradually erode the shell, allowing the active components such as boron, silicon, and zinc in the core to be slowly released. Boron can react with calcium ions in the cement hydration products to generate expansive crystalline products; simultaneously, the released zinc and lithium ions may participate in the hydration reaction or alter the crystallization habit of the hydration products. This moderate expansion, triggered during the critical period of cement paste strength development, can effectively compensate for the chemical shrinkage and drying shrinkage of the cement matrix, thereby reducing the generation of microcracks and improving crack resistance and long-term durability.

[0016] As a preferred embodiment of the present invention, in step B1, the mass ratio of lithium nitrate, zinc nitrate, boric acid, tetraethyl orthosilicate and polyvinylpyrrolidone is 6-8:8-10:15-18:15-18:8-10.

[0017] As a preferred embodiment of the present invention, in step B2, the temperature is raised to 900-1000℃ and the holding time is 3-5 hours.

[0018] In a second aspect, the present invention provides a method for preparing medium-heat silicate cement, comprising the following raw materials in parts by weight: 80-90 parts silicate cement clinker; 3-5 parts gypsum; 5-15 parts blast furnace slag; 0.5-2 parts calcium aluminum silicon nitrogen oxide nanosheets; and 1-3 parts calcium zirconate-coated lithium borosilicate zinc core-shell microspheres.

[0019] Compared with the prior art, the present invention has the following beneficial effects: (1) This invention has achieved a breakthrough in the core aspect of hydration heat control. By introducing functionalized calcium aluminum silicon nitride nanosheets, this material plays a key role in intelligent regulation in the early stage of cement hydration. Its unique two-dimensional nanostructure provides a large number of nucleation sites for hydration products, but after surface modification, its hydrophobic properties can physically shield some cement particles from contact with water for a certain period of time, thereby effectively delaying the intense exothermic process of early hydration. At the same time, calcium zirconate-coated lithium borosilicate zinc core-shell microspheres control the slow release of internal active components through their dense outer shell, avoiding concentrated exothermic reactions caused by local rapid reactions.

[0020] (2) The technical solution of the present invention has achieved satisfactory results in terms of mechanical properties, especially in terms of early strength development. Traditional medium-heat cement often has to sacrifice early strength in order to pursue low heat characteristics, resulting in a prolonged demolding cycle and affecting the construction progress. However, the calcium aluminum silicon nitrogen oxide nanosheets in the present invention not only regulate the heat of hydration, but their highly active surface and crystal structure similar to traditional cement minerals make them a high-quality nucleation substrate, which greatly promotes the rapid formation and orderly deposition of calcium silicate, thereby effectively accelerating the formation and compaction of the slurry structure.

[0021] (3) This invention demonstrates outstanding technical effects in improving the long-term durability and volume stability of cement-based materials. The design of calcium zirconate-coated lithium borosilicate zinc core-shell microspheres ensures that their expansion function is triggered in the middle and late stages of cement hydration, that is, at the critical stage when the slurry strength develops to a certain extent and begins to show a shrinkage trend. The core material is slowly released through a pre-set channel, reacting with the alkaline components in the slurry to produce an appropriate amount of expansion products, thereby accurately and effectively compensating for the shrinkage caused by the hydration and drying process, and greatly reducing the risk of early cracking of concrete. In addition, the two modified compounds optimize the microstructure of the slurry at different scales. The nanosheets fill the nanoscale pores, while the microspheres improve the micron-scale particle size distribution, together promoting a significant reduction in the proportion of harmful pores in the cement paste, forming a more dense and less permeable matrix structure. This optimized microstructure not only endows cement stone with excellent impermeability, but also significantly enhances its ability to resist environmental degradation such as sulfate attack and chloride ion penetration, thereby ensuring the ultra-long service life and durability of concrete structures made with this cement in harsh environments. Detailed Implementation

[0022] To facilitate understanding of the present invention, the following embodiments are provided. Those skilled in the art should understand that these embodiments are merely illustrative and should not be construed as limiting the scope of the invention.

[0023] The sources of some components in the examples and comparative examples are as follows: The hexadecyltrimethylammonium bromide was purchased from Guangzhou Yuanda New Materials Co., Ltd.

[0024] The dodecyltriethoxysilane was purchased from Shandong Jieying New Materials Co., Ltd.

[0025] The polyacrylic acid was purchased from Shandong Wanhua Tianhe New Materials Co., Ltd. Example

[0026] This embodiment provides a method for preparing medium-heat silicate cement, the steps of which include: Preparation of calcium aluminum silicon nitride nanosheets: First, accurately weigh 30.00 g of calcium nitrate, 10.00 g of aluminum isopropoxide, and 12.00 g of tetraethyl orthosilicate. Dissolve all three in 200 mL of anhydrous ethanol and magnetically stir at 300 rpm for 30 min at 25 °C until a completely clear and transparent solution is formed. Then, add 30.00 g of urea and 8.00 g of hexadecyltrimethylammonium bromide to the solution sequentially, and continue stirring for 60 min to ensure thorough mixing of all components, obtaining a homogeneous mixed solution. Transfer this mixed solution to a 500 mL high-pressure reactor lined with polytetrafluoroethylene (PTFE). After sealing, place the reactor in an oven and program the temperature to 180 °C at a rate of 2 °C / min, maintaining this temperature for 20 h for a solvothermal reaction. After the reaction, allow the reactor to cool naturally to room temperature (25 °C). The precipitate was collected by centrifugation at 8000 rpm for 10 min. This precipitate was then ultrasonically washed three times with 200 mL of anhydrous ethanol, followed by centrifugation after each wash. The washed precipitate was placed in a vacuum drying oven and dried at 80 °C and -0.1 MPa for 6 h to obtain precursor powder. Next, the precursor powder was placed in a tube furnace and heated to 800 °C at a rate of 5 °C / min under a nitrogen atmosphere with a flow rate of 100 mL / min, and held at this temperature for 1.5 h. Subsequently, the atmosphere was switched to ammonia at a flow rate of 100 mL / min, and the heat treatment continued at 900 °C for 1 h to complete the nitriding process, yielding crude nitrided nanosheets. After the tube furnace cooled to 25 °C, the nitrided nanosheets were removed, dispersed in 200 mL of isopropanol, and 4.50 g of dodecyltriethoxysilane was added. Surface modification was performed by reflux in an oil bath at 70 °C for 12 h. After the reaction was complete, the solid product was collected again by centrifugation at 8000 rpm and washed three times with 200 mL of ethanol. Finally, the obtained solid was dried at 80 °C and -0.1 MPa for 6 h to obtain the final product, calcium aluminum silicon nitride nanosheets, which were then stored in a desiccator for later use.

[0027] Preparation of calcium zirconate-coated lithium borosilicate zinc core-shell microspheres: First, accurately weigh 7.00 g of lithium nitrate, 9.00 g of zinc nitrate, 17.00 g of boric acid, and 17.00 g of tetraethyl orthosilicate, and dissolve them together in a mixed solvent consisting of 200 mL of deionized water and 200 mL of ethanol. Stir at 25 °C until completely dissolved. Add 9.00 g of polyvinylpyrrolidone to this solution and continue stirring until completely dissolved. Subsequently, slowly add a 10% sodium hydroxide solution dropwise to precisely adjust the pH of the mixed solution to 9.5. The solution is then processed using a spray dryer with an inlet temperature of 180 °C, an outlet temperature of 80 °C, a feed rate of 5 mL / min, and a compressed air flow rate of 600 L / h to atomize and dry the solution, obtaining precursor microsphere powder. The powder was placed in a muffle furnace and heated to 500°C at a rate of 5°C / min under static air atmosphere, and held for 1 hour to completely remove organic matter. Subsequently, it was heated to 700°C at the same rate and sintered at this temperature for 2 hours to obtain sintered core microspheres. Next, the outer shell coating solution was prepared: 8.00 g of zirconium oxychloride and 4.00 g of calcium nitrate were accurately weighed and dissolved in 200 mL of deionized water. Then, 6.00 g of polyacrylic acid was added as a dispersant, and the mixture was stirred until a homogeneous and transparent mixture was formed. 10.00 g of the sintered core microspheres were slowly added to this mixture, and the mixture was stirred continuously at 200 rpm for 12 hours at 25°C to ensure complete wetting. Afterward, the coated wet microspheres were obtained by centrifugation at 5000 rpm. Finally, these wet microspheres were placed back into a muffle furnace and heated to 950°C at a rate of 5°C / min under static air atmosphere, and held at this temperature for 4 hours to densify the shell. After the process was completed, the muffle furnace was allowed to cool naturally to 25°C, and the microspheres were removed to obtain the final product, calcium zirconate-coated lithium borosilicate zinc core-shell microspheres, which were then placed in a desiccator for later use.

[0028] Preparation of intermediate-heat silicate cement: First, 850.0g of silicate cement clinker was crushed using a jaw crusher, controlling the output particle size to be less than 5mm, to obtain crushed silicate cement clinker. Simultaneously, 40.0g of dihydrate gypsum was placed in a forced-air drying oven and dried at 80℃ for 4 hours to reduce its moisture content to below 1%, obtaining dried gypsum. The crushed silicate cement clinker, dried gypsum, and 100.0g of granulated blast furnace slag were then placed together in a small laboratory ball mill (grinding media: steel balls, material-to-ball mass ratio: 1:2). The ball mill was run at a fixed frequency of 45Hz for 35 minutes, with pauses every 10 minutes to turn the material and ensure uniformity, ultimately yielding cement powder with a specific surface area of ​​approximately 350m² / kg. Then, accurately weigh 12.0 g of the prepared calcium aluminum silicon nitrogen oxide nanosheets and 20.0 g of the prepared calcium zirconate-coated lithium borosilicate zinc core-shell microspheres, and add them to the above cement powder. Restart the ball mill and mix and grind for 15 min under the same conditions (45 Hz, ball-to-material ratio 1:2) to obtain a uniformly mixed material. Finally, transfer this mixed material to a three-dimensional motion mixer and run it at 30 rpm for 70 min to perform thorough homogenization. After homogenization, remove the material to obtain the finished medium-heat silicate cement of this invention, and seal and store it. Example

[0029] This embodiment provides a method for preparing medium-heat silicate cement, the steps of which include: Preparation of calcium aluminum silicon nitride nanosheets: First, accurately weigh 25.00 g of calcium nitrate, 8.00 g of aluminum isopropoxide, and 10.00 g of tetraethyl orthosilicate. Dissolve all three in 150 mL of anhydrous ethanol and magnetically stir at 300 rpm for 30 min at 25 °C until a completely clear and transparent solution is formed. Then, add 25.00 g of urea and 6.00 g of hexadecyltrimethylammonium bromide to the solution sequentially, and continue stirring for 60 min to ensure thorough mixing of all components, obtaining a homogeneous mixed solution. Transfer this mixed solution to a 500 mL high-pressure reactor lined with polytetrafluoroethylene (PTFE). After sealing, place the reactor in an oven and program the temperature to 175 °C at a rate of 2 °C / min, maintaining this temperature for 24 h for a solvothermal reaction. After the reaction is complete, allow the reactor to cool naturally to room temperature (25 °C). The precipitate was collected by centrifugation at 8000 rpm for 10 min. This precipitate was then ultrasonically washed three times with 150 mL of anhydrous ethanol, followed by centrifugation after each wash. The washed precipitate was placed in a vacuum drying oven and dried at 80 °C and -0.1 MPa for 6 h to obtain precursor powder. Next, the precursor powder was placed in a tube furnace and heated to 800 °C at a rate of 5 °C / min under a nitrogen atmosphere with a flow rate of 100 mL / min, and held at this temperature for 1.5 h. Subsequently, the atmosphere was switched to ammonia at a flow rate of 100 mL / min, and the heat treatment was continued at 900 °C for 1 h to complete the nitriding process, yielding crude nitrided nanosheets. After the tube furnace cooled to 25 °C, the nitrided nanosheets were removed, dispersed in 150 mL of isopropanol, and 3.75 g of dodecyltriethoxysilane was added. Surface modification was performed by reflux in an oil bath at 70 °C for 12 h. After the reaction was complete, the solid product was collected again by centrifugation at 8000 rpm and washed three times with 150 mL of ethanol. Finally, the obtained solid was dried at 80 °C and -0.1 MPa for 6 h to obtain the final product, calcium aluminum silicon nitride nanosheets, which were then stored in a desiccator for later use.

[0030] Preparation of calcium zirconate-coated lithium borosilicate zinc core-shell microspheres: First, accurately weigh 6.00 g of lithium nitrate, 8.00 g of zinc nitrate, 15.00 g of boric acid, and 15.00 g of tetraethyl orthosilicate, and dissolve them together in a mixed solvent consisting of 150 mL of deionized water and 150 mL of ethanol. Stir at 25 °C until completely dissolved. Add 8.00 g of polyvinylpyrrolidone to this solution and continue stirring until completely dissolved. Subsequently, slowly add a 10% sodium hydroxide solution dropwise to precisely adjust the pH of the mixed solution to 9.0. The solution is then processed using a spray dryer with an inlet temperature of 180 °C, an outlet temperature of 80 °C, a feed rate of 5 mL / min, and a compressed air flow rate of 600 L / h to atomize and dry the solution, obtaining precursor microsphere powder. The powder was placed in a muffle furnace and heated to 500°C at a rate of 5°C / min under static air atmosphere, and held for 1 hour to completely remove organic matter. Subsequently, it was heated to 700°C at the same rate and sintered at this temperature for 2 hours to obtain sintered core microspheres. Next, the outer shell coating solution was prepared: 8.00 g of zirconium oxychloride and 4.00 g of calcium nitrate were accurately weighed and dissolved in 150 mL of deionized water. Then, 6.00 g of polyacrylic acid was added as a dispersant, and the mixture was stirred until a homogeneous and transparent mixture was formed. 10.00 g of the sintered core microspheres were slowly added to this mixture, and the mixture was stirred continuously at 200 rpm for 12 hours at 25°C to ensure complete wetting. Afterward, the coated wet microspheres were obtained by centrifugation at 5000 rpm. Finally, these wet microspheres were placed back into a muffle furnace and heated to 1000°C at a rate of 5°C / min under static air atmosphere, and held at this temperature for 4 hours to densify the shell. After the process was completed, the muffle furnace was allowed to cool naturally to 25°C, and the microspheres were removed to obtain the final product, calcium zirconate-coated lithium borosilicate zinc core-shell microspheres, which were then placed in a desiccator for later use.

[0031] Preparation of intermediate-heat silicate cement: First, 800.0g of silicate cement clinker was crushed using a jaw crusher, controlling the output particle size to be less than 5mm, to obtain crushed silicate cement clinker. Simultaneously, 30.0g of dihydrate gypsum was placed in a forced-air drying oven and dried at 78℃ for 4 hours to reduce its moisture content to below 1%, obtaining dried gypsum. The crushed silicate cement clinker, dried gypsum, and 50.0g of granulated blast furnace slag were placed together in a laboratory small ball mill (grinding media: steel balls, material-to-ball mass ratio: 1:2). The ball mill was run at a fixed frequency of 45Hz for 30 minutes, with pauses every 10 minutes to turn the material and ensure uniformity, ultimately obtaining cement powder. Then, 5.0g of the prepared calcium aluminum silicon nitrogen oxide nanosheets and 10.0g of the prepared calcium zirconate-coated lithium borosilicate zinc core-shell microspheres were accurately weighed and added to the cement powder. The ball mill was restarted and mixed and ground for 15 minutes under the same conditions (45Hz, ball-to-material ratio 1:2) to obtain a uniformly mixed material. Finally, this mixed material was transferred to a three-dimensional motion mixer and run at 30 rpm for 60 minutes to achieve thorough homogenization. After homogenization, the material was removed, yielding the finished medium-heat silicate cement of this invention, which was then sealed and stored. Example

[0032] This embodiment provides a method for preparing medium-heat silicate cement, the steps of which include: Preparation of calcium aluminum silicon nitride nanosheets: First, accurately weigh 28.00 g of calcium nitrate, 9.00 g of aluminum isopropoxide, and 11.00 g of tetraethyl orthosilicate. Dissolve all three in 180 mL of anhydrous ethanol and magnetically stir at 300 rpm for 30 min at 25 °C until a completely clear and transparent solution is formed. Then, add 28.00 g of urea and 7.00 g of hexadecyltrimethylammonium bromide to the solution sequentially, and continue stirring for 60 min to ensure thorough mixing of all components and obtain a homogeneous solution. Transfer this solution to a 500 mL high-pressure reactor lined with polytetrafluoroethylene (PTFE). After sealing, place the reactor in an oven and program the temperature to 185 °C at a rate of 2 °C / min, maintaining this temperature for 18 h for a solvothermal reaction. After the reaction, allow the reactor to cool naturally to room temperature (25 °C). The precipitate was collected by centrifugation at 8000 rpm for 10 min. This precipitate was then ultrasonically washed three times with 180 mL of anhydrous ethanol, followed by centrifugation after each wash. The washed precipitate was placed in a vacuum drying oven and dried at 80 °C and -0.1 MPa for 6 h to obtain precursor powder. Next, the precursor powder was placed in a tube furnace and heated to 800 °C at a rate of 5 °C / min under a nitrogen atmosphere with a flow rate of 100 mL / min, and held at this temperature for 1.5 h. Subsequently, the atmosphere was switched to ammonia at a flow rate of 100 mL / min, and the heat treatment was continued at 900 °C for 1 h to complete the nitriding process, yielding crude nitrided nanosheets. After the tube furnace cooled to 25 °C, the nitrided nanosheets were removed, dispersed in 180 mL of isopropanol, and 4.20 g of dodecyltriethoxysilane was added. Surface modification was performed by reflux in an oil bath at 70 °C for 12 h. After the reaction was complete, the solid product was collected again by centrifugation at 8000 rpm and washed three times with 180 mL of ethanol. Finally, the obtained solid was dried at 80 °C and -0.1 MPa for 6 h to obtain the final product, calcium aluminum silicon nitride nanosheets, which were then stored in a desiccator for later use.

[0033] Preparation of calcium zirconate-coated lithium borosilicate zinc core-shell microspheres: First, accurately weigh 8.00 g of lithium nitrate, 10.00 g of zinc nitrate, 18.00 g of boric acid, and 18.00 g of tetraethyl orthosilicate, and dissolve them together in a mixed solvent consisting of 180 mL of deionized water and 180 mL of ethanol. Stir at 25 °C until completely dissolved. Add 10.00 g of polyvinylpyrrolidone to this solution and continue stirring until completely dissolved. Subsequently, slowly add a 10% sodium hydroxide solution dropwise to precisely adjust the pH of the mixed solution to 10.0. The solution is then processed using a spray dryer with an inlet temperature of 180 °C, an outlet temperature of 80 °C, a feed rate of 5 mL / min, and a compressed air flow rate of 600 L / h to atomize and dry the solution, obtaining precursor microsphere powder. The powder was placed in a muffle furnace and heated to 500°C at a rate of 5°C / min under static air atmosphere, and held for 1 hour to completely remove organic matter. Subsequently, it was heated to 700°C at the same rate and sintered at this temperature for 2 hours to obtain sintered core microspheres. Next, the outer shell coating solution was prepared: 8.00 g of zirconium oxychloride and 4.00 g of calcium nitrate were accurately weighed and dissolved in 180 mL of deionized water. Then, 6.00 g of polyacrylic acid was added as a dispersant, and the mixture was stirred until a homogeneous and transparent mixture was formed. 10.00 g of the sintered core microspheres were slowly added to this mixture, and the mixture was stirred continuously at 200 rpm for 12 hours at 25°C to ensure complete wetting. Afterward, the coated wet microspheres were obtained by centrifugation at 5000 rpm. Finally, these wet microspheres were placed back into a muffle furnace and heated to 900°C at a rate of 5°C / min under static air atmosphere, and held at this temperature for 4 hours to densify the shell. After the process was completed, the muffle furnace was allowed to cool naturally to 25°C, and the microspheres were removed to obtain the final product, calcium zirconate-coated lithium borosilicate zinc core-shell microspheres, which were then placed in a desiccator for later use.

[0034] Preparation of intermediate-heat silicate cement: First, 900.0g of silicate cement clinker was crushed using a jaw crusher, controlling the output particle size to be less than 5mm, to obtain crushed silicate cement clinker. Simultaneously, 50.0g of dihydrate gypsum was placed in a forced-air drying oven and dried at 82℃ for 4 hours to reduce its moisture content to below 1%, obtaining dried gypsum. The crushed silicate cement clinker, dried gypsum, and 150.0g of granulated blast furnace slag were placed together in a laboratory small ball mill (grinding media: steel balls, material-to-ball mass ratio: 1:2). The ball mill was run at a fixed frequency of 45Hz for 40 minutes, with pauses every 10 minutes to turn the material and ensure uniformity, ultimately obtaining cement powder. Then, 20.0g of the prepared calcium aluminum silicon nitrogen oxide nanosheets and 30.0g of the prepared calcium zirconate-coated lithium borosilicate zinc core-shell microspheres were accurately weighed and added to the above cement powder. The ball mill was restarted and mixed and ground for 15 minutes under the same conditions (45Hz, ball-to-material ratio 1:2) to obtain a uniformly mixed material. Finally, this mixed material was transferred to a three-dimensional motion mixer and run at 30 rpm for 80 minutes to achieve thorough homogenization. After homogenization, the material was removed, yielding the finished medium-heat silicate cement of this invention, which was then sealed and stored.

[0035] Comparative Example 1 The difference between this comparative example and Example 1 is that this comparative example does not include the two functional compounds: calcium aluminum silicon nitride nanosheets and calcium zirconate-coated lithium borosilicate zinc core-shell microspheres.

[0036] Comparative Example 2 The difference between this comparative example and Example 1 is that this comparative example only adds calcium aluminum silicon nitride nanosheets, and does not add calcium zirconate-coated lithium borosilicate zinc core-shell microspheres.

[0037] Comparative Example 3 The difference between this comparative example and Example 1 is that this comparative example only adds calcium zirconate-coated lithium borosilicate zinc core-shell microspheres, without adding calcium aluminum silicon nitride nanosheets.

[0038] The performance of the medium-heat silicate cements obtained in Examples 1-3 and Comparative Examples 1-3 was tested in accordance with national and industry standard testing specifications.

[0039] The heat of hydration of cement was tested using the heat of solution method. A constant temperature environment heat of solution meter was used. 10.00g of the prepared cement sample was mixed with 50.00g of deionized water and placed in the inner cylinder of the calorimeter. The temperature of the outer cylinder was precisely controlled at 20±0.1℃. The temperature change curve during the dissolution process was recorded by a precision temperature sensor. The heat value released during the hydration process was calculated according to the thermodynamic formula. The cumulative heat of hydration values ​​at 3d and 7d were recorded respectively. The cement mortar strength test was conducted according to the standard mortar strength testing procedure. A standard mass ratio of 450.0g cement, 1350.0g standard sand, and 225.0mL distilled water was used. A planetary cement mortar mixer was used to mix at a low speed of 140±5r / min for 30s, followed by a 90s pause, and then at a high speed of 285±10r / min for 60s. The mixture was poured into a triplet mold measuring 40mm×40mm×160mm and tested on a standard vibrating table at a frequency of 50Hz. The specimen was vibrated for 120 seconds to form the sample. After curing in a standard curing chamber at 20±1℃ and relative humidity ≥90% for 24±2 hours, the specimen was demolded and immediately placed in a constant temperature water bath at 20±1℃ to continue curing until the specified test age. The compressive strength was tested at 3 days and 28 days. The compressive strength test was conducted using a microcomputer-controlled electro-hydraulic servo pressure testing machine. The load was uniformly applied at a loading rate of 2400±200 N / s until the specimen failed. The maximum failure load was recorded and the compressive strength value was calculated. The drying shrinkage performance test adopted the length change measurement method. After the cement mortar specimens were cured to the specified age under standard curing conditions, their reference length was measured using a precision length comparator with an accuracy of 0.001 mm. The specimens were then transferred to a constant temperature and humidity chamber at a temperature of 20±2℃ and a relative humidity of 60±5% for continuous curing. The length change of the specimens was measured periodically, and the observation was continued for 56 days, and the final drying shrinkage value was recorded. For all test items, no less than 3 valid parallel specimens were set, and the final test result was taken as the arithmetic mean of the test data of all parallel specimens as the reported value.

[0040] The performance test data above are shown in Table 1.

[0041] Table 1 Performance Test Results

[0042] The test results in Table 1 above clearly show that Examples 1-3 of the present invention effectively solve the three major technical problems existing in traditional medium-heat silicate cement by synergistically using calcium aluminum silicon nitrogen oxide nanosheets and calcium zirconate-coated lithium borosilicate zinc core-shell microspheres.

[0043] Regarding the control of heat of hydration, the heat of hydration after 3 days in Examples 1-3 was 218-232 kJ / kg, and the heat of hydration after 7 days was 248-265 kJ / kg, which was significantly lower than 258 kJ / kg and 298 kJ / kg in Comparative Example 1, demonstrating that the synergistic effect of the two modified compounds can effectively reduce the heat of hydration in the early and middle stages. In Comparative Examples 2 and 3, although the use of either compound alone could partially reduce the heat of hydration, the effect was not as good as the synergistic use of both.

[0044] Regarding early strength development, the 3-day compressive strength of Examples 1-3 reached 22.5-25.3 MPa, which was much higher than the 15.7 MPa of Comparative Example 1. This was mainly attributed to the efficient nucleation sites provided by the calcium aluminum silicon nitride nanosheets, which promoted the formation of early hydration products. In contrast, the early strength of Comparative Example 3, which only used calcium zirconate to coat lithium zinc borosilicate core-shell microspheres, was only 16.5 MPa, further confirming the key role of nanosheets in early strength development.

[0045] Regarding long-term crack resistance, the 56-day drying shrinkage value of Examples 1-3 was 270-298 × 10⁻⁶. -6 It is significantly lower than the 350×10 of Comparative Example 1. -6 This is due to the moderate expansion of calcium zirconate-coated lithium borosilicate zinc core-shell microspheres during a specific hydration stage, which effectively compensates for shrinkage; while in Comparative Example 2, which only used nanosheets, the shrinkage value still reached 340 × 10⁻⁶. -6 This indicates that the lack of expansion compensation from microspheres makes it difficult to effectively control long-term shrinkage. Of particular note is that Example 1, while achieving a compressive strength of 53.6 MPa after 28 days, still maintained low heat of hydration and shrinkage values, successfully achieving a balance between low heat, high strength, and high crack resistance—a comprehensive effect that cannot be achieved by a single modified compound.

Claims

1. A method for preparing intermediate-heat silicate cement, characterized in that the steps include... include: S1. Crush the silicate cement clinker to obtain crushed silicate cement clinker; The gypsum is dried at 78-82℃ to obtain the dried gypsum. Crushed silicate cement clinker, dried gypsum, and blast furnace slag are added to a ball mill and ground to obtain cement powder. S2. Add calcium aluminum silicon nitrogen oxide nanosheets and calcium zirconate-coated lithium borosilicate zinc core-shell microspheres to cement powder, grind them to obtain a mixed material; stir the mixed material in a mixer.

2. The method for preparing intermediate-heat silicate cement according to claim 1, characterized in that, In step S1, the grinding time is 30-40 minutes.

3. The method for preparing intermediate-heat silicate cement according to claim 1, characterized in that, In step S2, the mixed materials are stirred in the mixer for 60-80 minutes.

4. The method for preparing intermediate-heat silicate cement according to claim 1, characterized in that, The method for preparing the calcium aluminum silicon nitride nanosheets includes: A1. Dissolve calcium nitrate, aluminum isopropoxide, and tetraethyl orthosilicate in anhydrous ethanol; add urea and hexadecyltrimethylammonium bromide to obtain a mixed solution; transfer the mixed solution to a high-pressure reactor and react it at 175-185℃; after the reaction is complete, allow it to cool naturally to room temperature, collect the precipitate by centrifugation, wash the precipitate with anhydrous ethanol, and dry it under vacuum to obtain the precursor; A2. The precursor was heated to 795-805℃ under a nitrogen atmosphere and held at that temperature. Then, it was heat-treated at 895-905℃ under an ammonia atmosphere to obtain nitrided nanosheets. After the nitrided nanosheets were cooled to room temperature, they were dispersed in isopropanol, dodecyltriethoxysilane was added, and the mixture was refluxed at 68-72℃. The solid product was collected by centrifugation, washed with ethanol, and dried under vacuum at 78-82℃.

5. The method for preparing intermediate-heat silicate cement according to claim 4, characterized in that, In step A1, the mass ratio of calcium nitrate, aluminum isopropoxide, tetraethyl orthosilicate, urea, and hexadecyltrimethylammonium bromide is 25-30:8-10:10-12:25-30:6-8.

6. The method for preparing intermediate-heat silicate cement according to claim 4, characterized in that, In step A2, the temperature is raised to 795-805℃ and held for 1-2 hours.

7. The method for preparing intermediate-heat silicate cement according to claim 1, characterized in that, The preparation method of the calcium zirconate-coated lithium borosilicate zinc core-shell microspheres includes: B1. Lithium nitrate, zinc nitrate, boric acid, and tetraethyl orthosilicate are dissolved in a mixed solvent of deionized water and ethanol. Polyvinylpyrrolidone is added, and the pH is adjusted to 9-10. Spray drying is performed to obtain precursor microspheres. B2. The precursor microspheres are heated to 495-505℃ in air, held at that temperature, and then sintered at 695-705℃ to obtain sintered core microspheres. Zirconium oxychloride and calcium nitrate are dissolved in deionized water, and polyacrylic acid is added to obtain a mixture. The sintered core microspheres are added to the mixture, stirred at room temperature, and then separated by centrifugation to obtain coated microspheres. The coated microspheres are heated to 900-1000℃ in air and held at that temperature. They are then naturally cooled to room temperature.

8. The method for preparing intermediate-heat silicate cement according to claim 7, characterized in that, In step B1, the mass ratio of lithium nitrate, zinc nitrate, boric acid, tetraethyl orthosilicate, and polyvinylpyrrolidone is 6-8:8-10:15-18:15-18:8-10.

9. The method for preparing intermediate-heat silicate cement according to claim 7, characterized in that, In step B2, the temperature is raised to 900-1000℃ and held for 3-5 hours.

10. A medium-heat silicate cement prepared by the method for preparing medium-heat silicate cement according to any one of claims 1-9, characterized in that, The raw materials include the following parts by weight: 80-90 parts silicate cement clinker; 3-5 parts gypsum; 5-15 parts blast furnace slag; 0.5-2 parts calcium aluminum silicon nitrogen oxide nanosheets; and 1-3 parts calcium zirconate-coated lithium borosilicate zinc core-shell microspheres.

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