Anti-freezing nano SiO2-BF mortar material and preparation method thereof

Abstract

The invention belongs to the technical field of mortar materials, and particularly discloses an anti-freezing nano SiO2-BF mortar material and a preparation method thereof.The anti-freezing nano SiO2-BF mortar material comprises cement, nano SiO2, basalt fiber, river sand, water and a water reducing agent, the mass ratio of the cement to the river sand to the water is (1.94-2): 5.4: 1, the mixing amount of the nano SiO2 is 0%-3% of the mass of the cement, the mixing amount of the basalt fiber is 0%-3% of the mass of the water reducing agent, and the mixing amount of the basalt fiber is 0%-3% of the mass of the water reducing agent. The volume mixing amount of the basalt fiber is 0-0.4% of the total volume of the mortar, and the addition amount of the water reducing agent is 0.25% of the total amount of the cementing material. According to the mix proportion scheme, the synergistic enhancement effect of basalt fibers and nano SiO2 can be fully exerted, the compressive strength and breaking strength of the mortar are remarkably improved, the requirement of a building structure for high-strength mortar is met, and the high-strength mortar is especially suitable for cold regions. Meanwhile, the anti-freezing performance is excellent, the water absorption rate can be effectively reduced, the damage of freezing and thawing cycles to mortar is reduced, the service life of a building structure is prolonged, and the maintenance cost is reduced.

Description

Technical Field

[0001] This invention relates to the field of mortar materials technology, specifically to an antifreeze nano-SiO2-BF mortar material and its preparation method. Background Technology

[0002] Cement-based mortars are widely used in the global construction industry due to their significant advantages, such as simple manufacturing, low energy consumption, and low production costs. They are applied to various construction activities, including masonry, structural reinforcement, plastering, and repair work. However, when buildings are exposed to harsh environments, especially in northern my country where there are significant temperature differences between day and night and frequent temperature fluctuations, cement-based materials are highly susceptible to freeze-thaw damage. This damage leads to the destruction of the material's internal microstructure, significantly reducing the safety and stability of the building structure, resulting in serious economic losses and even threatening personal safety. Therefore, effectively improving the freeze-thaw resistance of cement-based materials has become a critical issue that urgently needs to be addressed in the building materials field, and it has extremely important practical significance.

[0003] Basalt fiber, as a natural volcanic rock material, has gradually emerged in the field of civil engineering due to its high strength, excellent durability, and good thermal stability, and is widely used in concrete and mortar. Numerous scholars both domestically and internationally have conducted in-depth research on this material, achieving fruitful results. Studies have shown that appropriate incorporation of basalt fiber can significantly enhance the mechanical properties and durability of mortar. However, when the fiber content exceeds a certain threshold, it can negatively impact the strength of the mortar. Currently, although there are many related studies, most focus on the static mechanical properties of basalt fiber-reinforced mortar and its influence on mortar bonding performance. Research on the durability of fiber-reinforced mortar in complex environments is relatively limited, and a systematic theoretical and application framework has not yet been established.

[0004] Furthermore, with the increasing penetration of nanotechnology into the field of building materials, nanomaterials such as nano-SiO2 have attracted widespread attention from scholars both domestically and internationally due to their unique high activity and excellent filling effect, and have been widely applied in the performance improvement of cement-based materials. Numerous experimental studies have shown that nano-SiO2 can effectively fill the internal pores of mortar, significantly reduce water absorption, decrease crack formation, and generate more cementitious substances through chemical reactions with cement hydration products, thereby comprehensively improving the mechanical and durability properties of mortar. However, the application of nanomaterials also faces some challenges, such as the dosage, dispersibility, and compatibility with the matrix material. These issues, to some extent, limit the full realization of the performance of nanomaterials in mortar, requiring further in-depth research and optimization.

[0005] Patent CN120623818 A discloses a basalt fiber-modified cement-based penetrating crystalline waterproof coating and its preparation method. The waterproof coating is made of silicate cement, surface-modified basalt fiber, gradient crystallization promoter, nano-SiO2, polycarboxylate superplasticizer, quartz sand, functional additives, and water. Patent CN120647249 A discloses a nanomaterial-based anti-freeze-crack saline soil solidifying agent and its preparation method, including the following steps: mixing cement and basalt fiber evenly to obtain a premix; adding silica fume, nano-SiO2, and polyacrylamide to the premix, and continuing to mix evenly to obtain the nanomaterial-based anti-freeze-crack saline soil solidifying agent. Both patents use magnesium oxide as an expanding agent. From a molecular level perspective, the 118% volume expansion generated by MgO hydration is superimposed on the frost heave stress, and Mg... 2+ Replace CSH interlayer Ca 2+ This reduces the interlayer bonding energy by about 15%, inducing early microcracks. The present invention adopts a "zero MgO" design, which can obtain better freeze resistance and durability at both the molecular and macroscopic scales without the need to introduce this side effect component.

[0006] Meanwhile, existing technologies lack scientific and systematic mix design schemes for the admixture of basalt fiber and nano-SiO2 in mortar. Current admixture ratios are mostly based on experience or single performance indicators, failing to balance the interaction between the two and making it difficult to determine the optimal admixture combination to achieve the best balance between mortar's freeze-thaw resistance and mechanical properties. This results in mortar performance failing to meet expectations in practical applications, especially in complex environments in cold regions, making it difficult to satisfy the stringent requirements of building structures for high-performance mortars. Summary of the Invention

[0007] To address the aforementioned problems, this invention aims to establish a scientific and systematic mix design scheme for frost-resistant nano-SiO2-BF mortar. Through extensive experiments and data analysis, the optimal combination of basalt fiber and nano-SiO2 dosages is determined. The goal is to balance the interaction between the two, fully leverage their synergistic effect, and achieve the best balance between the mortar's frost resistance and mechanical properties, thus meeting the demand for high-performance mortar in complex environments of cold regions.

[0008] The first objective of this invention is to provide a frost-resistant nano-SiO2-BF mortar material, comprising cement, nano-SiO2, basalt fiber, river sand, water, and a water-reducing agent. The mass ratio of cement, river sand, and water is (1.94-2):5.4:1. The amount of nano-SiO2 is 0%-3% of the cement mass, the volume of basalt fiber is 0%-0.4% of the total mortar volume, and the amount of water-reducing agent is 0.25% of the total amount of cementitious materials.

[0009] Preferably, the mass ratio of cement, river sand, and water is 2:5.4:1.

[0010] Preferably, the cement is ordinary Portland cement of grade PO42.5 that meets national standards, and its chemical composition and physical performance indicators should meet relevant requirements to ensure that the cement quality is stable and reliable.

[0011] Preferably, the average particle size of the nano-SiO2 is 20 nm.

[0012] Preferably, the basalt fiber has a length of 12 mm, a single filament diameter of 13 μm, and a density of 2.6 g / cm³. 3 Basalt fibers should be dry, free of oil and impurities to ensure good adhesion to the cement matrix.

[0013] Preferably, the river sand is natural river sand from Harbin City, Heilongjiang Province, with a fineness modulus of 2.5 and an apparent density of 2630 kg / m³. 3 The bulk density is 1573 kg / m³. 3 River sand should be sieved to remove impurities and mud lumps to ensure good gradation.

[0014] Preferably, the water-reducing agent is a polycarboxylate high-efficiency water-reducing agent, which has the advantages of low dosage, high water reduction rate, and low shrinkage, and can improve the fluidity and early strength of mortar.

[0015] Using the above technical solution, this invention uses PO42.5 grade ordinary Portland cement as the matrix material, with clearly defined chemical composition and physical performance indicators. Nano-SiO2 with an average particle size of 20nm is selected; the basalt fibers have a length of 12mm, a single filament diameter of 13μm, and a density of 2.6g / cm³. Simultaneously, natural river sand is used as fine aggregate, and an appropriate amount of polycarboxylate superplasticizer is added to improve the fluidity of the mortar.

[0016] A second objective of this invention is to provide a method for preparing the above-mentioned antifreeze nano-SiO2-BF mortar material, comprising the following steps: S1: Mix the water-reducing agent with water and stir. S2: Add nano-SiO2 to the mixture obtained in step S1 and stir continuously to ensure uniform dispersion; S3: Add cement and continue mixing, adding river sand evenly during the mixing process; S4: Add basalt fibers in batches to the mixture obtained in step S3, and stir continuously to make the mortar uniform, so as to obtain the frost-resistant nano-SiO2-BF mortar material.

[0017] Using the above technical solution, a planetary cement mixer is used to prepare the mortar. First, the water-reducing agent is mixed with water and stirred for 30 seconds; then nano-SiO2 is added and stirred at high speed for 60 seconds to ensure uniform dispersion; next, cement is added and stirred at low speed for 30 seconds; during the stirring process (the second 30 seconds), river sand is added evenly; for mortar containing basalt fibers, the basalt fibers are added in batches to prevent agglomeration, and stirred at low speed for 30 seconds after each addition; then stirred at high speed for 30 seconds, paused for 90 seconds to clean the residue on the pot wall and fan blades, and finally stirred at high speed for 60 seconds to make the mortar uniform.

[0018] The beneficial effects of this invention are: (1) The mix proportion scheme of the present invention can give full play to the synergistic reinforcement effect of basalt fiber and nano SiO2, significantly improve the compressive strength and flexural strength of mortar, meet the requirements of building structure for high-strength mortar, and is especially suitable for cold regions.

[0019] (2) The nano-SiO2-BF mortar of the present invention has excellent antifreeze properties, which can effectively reduce water absorption, reduce the damage of freeze-thaw cycles to mortar, extend the service life of building structures, and reduce maintenance costs.

[0020] (3) The mixing ratio scheme of the present invention is reasonable, easy to operate, and suitable for mortar preparation in actual engineering. By optimizing the amount and ratio of materials, the material cost and construction difficulty can be reduced while meeting the performance requirements. Attached Figure Description

[0021] Figure 1 The water absorption rate of mortars with different amounts of nano-SiO2 and different amounts of basalt fiber according to the present invention is shown in the curves at 28 days and 90 days.

[0022] Figure 2 The BF0NS0 group mortar of this invention was scaled up 500 times after 75 freeze-thaw cycles. Figure 2 a) and magnified 1000 times ( Figure 2 b) SEM image.

[0023] Figure 3 The BF0NS2 group mortar of this invention was scaled up 500 times after 75 freeze-thaw cycles. Figure 3 a) and magnified 1000 times ( Figure 3 b) SEM image.

[0024] Figure 4 The BF0.2NS0 group mortar of the present invention was scaled up 500 times after 75 freeze-thaw cycles. Figure 4 a) and magnified 1000 times ( Figure 4 b) SEM image.

[0025] Figure 5The BF0.2NS2 group mortar of this invention was scaled up 500 times after 75 freeze-thaw cycles. Figure 5 a) and magnified 1000 times ( Figure 5 b) SEM image.

[0026] Figure 6 This is a diagram showing the fracture of basalt fibers inside the mortar after 75 freeze-thaw cycles of the BF0.2NS2 group mortar of the present invention.

[0027] Figure 7 This is a diagram showing the internal structure of the mortar under 75 freeze-thaw cycles with different mix proportions according to the present invention. Detailed Implementation

[0028] To make the technical means, creative features, objectives and effects of this invention easier to understand, the invention will be further described below in conjunction with specific embodiments.

[0029] This invention provides the following technical solution: a frost-resistant nano-SiO2-BF mortar material, comprising cement, nano-SiO2, basalt fiber, river sand, water, and a water-reducing agent. The mass ratio of cement, river sand, and water is (1.94-2):5.4:1 (non-limiting examples: 1.94:5.4:1, 1.95:5.4:1, 1.96:5.4:1, 1.97:5.4:1, 1.98:5.4:1, 1.99:5.4:1, 2:5.4:1, etc.). The amount of nano-SiO2 is 0%-3% of the cement mass (non-limiting examples: 0%, 1%, 2%, 3%, etc.). The volume content of basalt fiber is 0%-0.4% of the total mortar volume (non-limiting examples: 0%, 0.1%, 0.2%, 0.3%, 0.4%, etc.). The amount of water-reducing agent is 0.25% of the total amount of cementitious materials.

[0030] Specifically, the mass ratio of cement, river sand, and water is 2:5.4:1.

[0031] Specifically, the cement is ordinary Portland cement of grade PO42.5 that meets national standards. Its chemical composition and physical performance indicators should meet relevant requirements to ensure the stable and reliable quality of the cement.

[0032] Specifically, the average particle size of nano-SiO2 is 20 nm.

[0033] Specifically, the basalt fiber has a length of 12 mm, a single filament diameter of 13 μm, and a density of 2.6 g / cm³. 3 Basalt fibers should be dry, free of oil and impurities to ensure good adhesion to the cement matrix.

[0034] Specifically, the river sand used is natural river sand from Harbin City, Heilongjiang Province, with a fineness modulus of 2.5 and an apparent density of 2630 kg / m³.3 The bulk density is 1573 kg / m³. 3 River sand should be sieved to remove impurities and mud lumps to ensure good gradation.

[0035] Specifically, the water-reducing agent is a polycarboxylate high-efficiency water-reducing agent, which has the advantages of low dosage, high water reduction rate, and low shrinkage, and can improve the fluidity and early strength of mortar.

[0036] The preparation method of the above-mentioned antifreeze nano-SiO2-BF mortar material includes the following steps: S1: Mix the water-reducing agent with water and stir. S2: Add nano-SiO2 to the mixture obtained in step S1 and stir continuously to ensure uniform dispersion; S3: Add cement and continue mixing, adding river sand evenly during the mixing process; S4: Add basalt fibers in batches to the mixture obtained in step S3, and stir continuously to make the mortar uniform, so as to obtain the frost-resistant nano-SiO2-BF mortar material.

[0037] Specifically, a planetary cement mixer is used to prepare the mortar. First, the water-reducing agent is mixed with water and stirred for 30 seconds; then nano-SiO2 is added and stirred at high speed for 60 seconds to ensure uniform dispersion; next, cement is added and stirred at low speed for 30 seconds; during the stirring process (the second 30 seconds), river sand is added evenly; for mortar containing basalt fibers, the basalt fibers are added in batches to prevent agglomeration, and stirred at low speed for 30 seconds after each addition; then stirred at high speed for 30 seconds, paused for 90 seconds to clean the residue on the pot wall and fan blades, and finally stirred at high speed for 60 seconds to make the mortar uniform.

[0038] Example Based on experimental research, the following mix proportions (material usage per cubic meter of mortar) were determined: Cement: 500kg±10kg; Water: 250kg±5kg; Sand: 1350kg±10kg; Nano SiO2: The amount to be used is calculated based on the dosage (0%-3%), accurate to ±0.1kg; Basalt fiber: The amount to be used is calculated based on the dosage (0%-0.4%), accurate to ±0.01kg; Water-reducing agent: 1.25kg±0.02kg (BF—basalt fiber, NS—nano SiO2). For detailed mix proportions, please refer to Table 1.

[0039] Table 1 Mortar Mix Design Test case (1) Specimen molding: The well-mixed mortar is poured into a mold pre-coated with lubricating oil in two layers. Each layer is vibrated 60 times with a vibrating table. After vibration, the excess mortar is scraped off and the surface of the specimen is smoothed.

[0040] (2) Curing: After the molded specimens are cured in a standard curing room with a temperature of 20℃±2℃ and a humidity of 95% or higher for 24 hours, they are demolded and then cured in the standard curing room until the specified age (such as 7d, 28d, 90d, etc.).

[0041] (3) Performance testing Mechanical property tests were conducted on mortars with different mix proportions, including compressive strength and flexural strength tests at 7 days and 28 days; durability tests were also conducted, covering water absorption tests, freeze-thaw resistance tests, and microstructure analysis, to verify the effectiveness and superiority of the mix proportion schemes.

[0042] (1.1) Mechanical property testing (1.1.1) Compressive strength test: In accordance with JGJ / T70-2009 "Standard for Basic Performance Test Methods of Building Mortar", the compressive strength of 70.7mm×70.7mm×70.7mm cube specimens was tested using an ATM fully automatic universal testing machine. The loading speed was 0.25KN / s. The arithmetic mean of the three specimens was taken as the average value of the mortar cube compressive strength of the group of specimens. For details, please refer to Table 2.

[0043] Table 2 Compressive Strength of Mortar (1.1.2) Flexural strength test: Referring to the "Test Method for Strength of Cement Mortar (ISO Method)", the flexural strength of a 40mm×40mm×160mm prism specimen was tested by center loading method. The top loading cylinder was used to continuously apply pressure to the side of the specimen at a loading rate of 50N / s±10N / s until the specimen broke. The average value of three specimens was taken as the flexural strength value. See Table 3 for details.

[0044] Table 3 Flexural Strength of Mortar (1.2) Durability test (1.2.1) Water Absorption Test: According to JGJ / T 70-2009 standard, a water absorption test was conducted on specimens measuring 70.7mm × 70.7mm × 70.7mm. Specimens cured for 28 days were placed in an oven at (78±3)℃ and dried for (48±0.5)h. After cooling for 2h, they were weighed to obtain a constant dried weight. Then, the specimens were immersed in a water bath for (48±0.5)h, removed, and the surface moisture was wiped off before weighing. The corresponding water absorption rate was calculated according to the formula, and the average value of three specimens was taken as the final result. For details, see [link to relevant documentation]. Figure 1 .

[0045] (1.2.2) Freeze-thaw resistance test: The freeze-thaw resistance test was conducted according to JGJ / T70-2009 standard. Two sets of specimens cured for 26 days were taken out of the curing room and immersed in water at 15℃-20℃ for 48 hours. After that, the specimens were removed, dried, weighed, and numbered. The specimens were then placed in a freezer for a freeze-thaw cycle test. The freezer temperature was controlled at -15℃ to -20℃. The specimens were placed in the freezer when the temperature was below -15℃. The freezing time was not less than 4 hours. After freezing, the specimens were removed and placed in a water bath at 15℃-20℃ for thawing. The thawing time was not less than 4 hours. After thawing, one freeze-thaw cycle was completed. The compressive strength of the specimens was tested every 25 cycles, and the strength loss rate was calculated to evaluate the freeze-thaw resistance of the mortar. See Table 4 for details.

[0046] Table 4. Mortar compressive strength and loss rate under different freeze-thaw cycles (1.2.3) Microstructure Analysis: Representative samples were selected from the mortar specimens and cut into 10mm×10mm×10mm pieces. After ultrasonic cleaning to remove surface impurities, the samples were thoroughly dried and then sputter-coated with gold to enhance conductivity. The treated samples were placed in the scanning electron microscope (SEM) chamber, and the accelerating voltage was set to 10kV. The working distance and other parameters were adjusted, and an appropriate magnification (100x~1000x) was selected to observe the microstructure and record the images. Quantitative analysis of the SEM images was performed using image analysis software to study the microstructural characteristics of the mortar, such as pores and cracks, in order to reveal the influence mechanism of basalt fibers and nano-SiO2 on the frost resistance of the mortar. For details, please refer to [link to relevant documentation]. Figures 2-7 .

[0047] (4) Results Analysis (4.1) Significantly Improved Mechanical Properties: Referring to Tables 2 and 3, the compressive and flexural strengths of the mortar were significantly improved by incorporating appropriate amounts of nano-SiO2 and basalt fiber into the mortar. At 7d and 28d ages, when the nano-SiO2 content was 2% and the basalt fiber content was 0.2% (i.e., the BF0.2NS2 group), the compressive and flexural strengths of the mortar reached their maximum values, respectively, which were 33.37% (7d), 27.75% (7d) and 17.00% (28d), 16.90% (28d) higher than those of the blank control group (i.e., the BF0NS0 group), and were superior to mortars containing only one of the two materials. This indicates that the mix proportion scheme of the present invention can fully utilize the synergistic reinforcing effect of nano-SiO2 and basalt fiber, significantly improving the mechanical properties of the mortar and meeting the requirements of building structures for high-strength mortar.

[0048] (4.2) Excellent antifreeze properties: Reference Figure 1The nano-SiO2-BF mortar of this invention exhibits excellent antifreeze properties. When the nano-SiO2 content is 2% and the basalt fiber content is 0.2% (i.e., the BF0.2NS2 group), the water absorption rate of the mortar reaches the lowest values ​​of 6.73% and 6.33% at 28 days and 90 days, respectively, which are 23.6% and 15.7% lower than the blank control group (i.e., the BF0NS0 group). This effectively reduces the porosity inside the mortar and reduces the damage caused by water penetration and freeze expansion during freeze-thaw cycles.

[0049] By comparison Figure 1 a and Figure 1 c. Figure 1 b and Figure 1 The four graphs clearly show that the water absorption rate of the mortar at 90 days is significantly lower than that at 28 days. This phenomenon is likely due to fiber agglomeration and nano-SiO2 aggregation within the mortar during the initial reaction, with the CSH gel generated during the reaction encapsulating the nano-SiO2 particles. Over time, due to the pozzolanic effect and the delayed reaction with the formed calcium hydroxide crystals, more CSH gel is generated. The reaction is complete around 90 days, and the cementitious material formed by the secondary reaction fills the voids and adheres to the fiber surface, reducing porosity.

[0050] Secondly, by Figure 1 a and Figure 1 c shows that the water absorption rate of mortar generally exhibits a trend of first decreasing and then increasing with the increase of nano-SiO2 content. In the early stage, as the content increases, the aggregation of nano-SiO2 leads to an increase in the water absorption rate of the mortar. However, as the number of days increases, most of the aggregated nano-SiO2 will further react, reducing the porosity and permeability of the mortar. Figure 1 b and Figure 1As can be seen from graph d, the water absorption of mortar initially decreases and then increases with the increase of basalt fiber content. With the appropriate amount of fiber added, the more cementitious substances generated by the reaction of nano-SiO2 inside the mortar work synergistically with the fibers, adhering to the fiber surface, which can optimize the pore structure and reduce porosity, thus reducing the water absorption rate of the mortar. As the fiber content continues to increase, due to the material properties, agglomeration is more likely to occur inside the mortar, and the increased number of agglomeration sites leads to increased pore size and thus increased water absorption rate. The figure also shows that the minimum water absorption rate corresponds to 0.2% BF and 2% NS. The water absorption rates of mortar made from a mixture of 0.2% basalt fiber and 2% nano-SiO2 at 28 days and 90 days are 6.73% and 6.33%, respectively. Compared with the BF0NS0 group mortar, the water absorption rates at 28 days and 90 days are reduced by 23.6% and 15.7%, respectively. Compared with the BF0.2NS0 group mortar, the water absorption rates are reduced by 11.7% and 5.1%, respectively. Compared with the BF0NS2 group mortar, the water absorption rates are reduced by 20.2% and 13.0%, respectively. In summary, appropriate amounts of basalt fiber and nano-SiO2 can effectively improve the porosity of mortar, optimize the pore structure of mortar, and reduce mortar permeability, thereby reducing the water absorption rate of the mixed mortar. However, excessive addition of these two materials will have a negative impact on the mortar, leading to excessive pores and increased water absorption, which is not conducive to the service of the mortar in harsh environments.

[0051] At the molecular level, during the 90-day period, the secondary volcanic ash reaction of NS continues to consume Ca(OH)2, generating CASH gel to fill the original >50nm harmful pores; after the BF surface is initially coated by CSH, the porosity of the interfacial region decreases, causing the capillary water absorption coefficient k to decrease proportionally according to the Lucas-Washburn equation: k∝φ·r², where φ is the porosity and r is the average pore diameter; at 90 days, the k value decreases by about 6% compared to 28 days.

[0052] Referring to Table 4, the compressive strengths of the BF0.2NS2 group after 25, 50, and 75 freeze-thaw cycles were 44.5 MPa, 41.3 MPa, and 40.5 MPa, respectively, which were 21.25%, 31.95%, and 43.11% higher than those of the blank control group (i.e., the BF0NS0 group), and superior to mortars with either material alone. Its strength loss rates were 3.47%, 10.41%, and 12.15%, respectively, showing a lower level compared to other groups. With the addition of a certain amount of nano-SiO2, a secondary hydration reaction can occur to generate more CSH gel to fill the pores inside the mortar, improving the bond strength between aggregate and cement paste, reducing the porosity and osmotic pressure of the mortar, and decreasing the permeation path of free water. Furthermore, the addition of fibers can form a good bond between the cement matrix and act as a reinforcement within the mortar, increasing the tensile stress inside the mortar. This is beneficial for the redistribution of internal stress during freeze-thaw cycles. To a certain extent, the fiber mesh inhibits the generation and propagation of microcracks. The combined effect of nano-SiO2 and basalt fibers improves the compressive strength of the mortar after freeze-thaw cycles and reduces the strength loss rate. However, with increasing basalt fiber and nano-SiO2 content, they are not easily dispersed within the mortar, creating more porosity. Under freeze-thaw cycles, this accelerates the mortar's deterioration rate, reduces compressive strength, and increases the strength loss rate.

[0053] For example, the compressive strength of BF0NS0 group mortar is 39.4 MPa before freeze-thaw cycles. After 25, 50, and 75 freeze-thaw cycles, the compressive strength of the mortar becomes 36.7 MPa, 31.2 MPa, and 28.3 MPa, respectively, with strength loss rates of 6.85%, 20.81%, and 28.17%. This is because during freezing, water in the capillaries inside the mortar expands in volume when it freezes. This expansion stress causes micro-cracks to appear inside the mortar. When the expansion stress exceeds the tensile strength of the mortar, cracks will form. Secondly, when the temperature around the mortar decreases, the surface of the mortar... The surface temperature drops rapidly, while the internal temperature drops relatively slowly. When the ambient temperature reaches 0℃, the various stresses caused by the freezing of water in the pores on the mortar surface can lead to micro-cracks inside the mortar. With each freeze-thaw cycle, larger pores are created within the mortar. The freezing of the solution in these larger pores increases the salt concentration, creating a concentration difference with the solution in the surrounding smaller pores and generating osmotic pressure, which damages the mortar's interior. Furthermore, the freezing of some water in the mortar pores forces the solution in the unfrozen pores to migrate outwards, generating hydrostatic pressure. If this pressure exceeds the tensile strength of the mortar, it will also fail. Therefore, with each freeze-thaw cycle, the rate of mortar deterioration accelerates, leading to rapid failure under external pressure.

[0054] (4.3) Optimized microstructure: Reference Figures 2-7 Scanning electron microscopy analysis revealed that the nano-SiO2-BF mortar of this invention exhibits significantly reduced internal porosity, a marked decrease in the number of cracks, and a significantly improved microstructural density. Basalt fibers and nano-SiO2 work synergistically within the mortar; the basalt fibers act as a bridging agent, effectively inhibiting the generation and propagation of microcracks, while the nano-SiO2 fills the pores within the mortar, making the matrix more compact. This optimized microstructural characteristic effectively suppresses the development of freeze-thaw damage, significantly improving the mortar's freeze-thaw resistance, durability, and impermeability.

[0055] After undergoing 75 freeze-thaw cycles Figure 2 (i.e. blank control group) The mortar internal structure produces a large number of pores and cracks, which is caused by the frost heave that occurs inside the mortar during the freeze-thaw process. As the cracks continue to expand, the internal density of the mortar decreases. Figure 3 Adding 2% nano-SiO2 to mortar results in smaller particle size and more complete hydration within the mortar as the number of freeze-thaw cycles increases, leading to the generation of more cementitious substances and relatively smaller pores and cracks. Figure 4 Adding 0.2% basalt fiber to the mortar can clearly show the connecting effect of the fiber, further inhibiting the damage of the mortar under freeze-thaw action and playing a role in sharing external stress. Figure 5 It is a mortar mixed with 0.2% basalt fiber and 2% nano-SiO2. It can be seen that after the freeze-thaw cycle, the cracks inside the mortar are significantly reduced, and due to the addition of basalt fiber, the hydration reaction of the nearby mortar causes more hydration products to adhere to the fiber surface.

[0056] like Figure 6 As shown, the failure mode of the fiber inside the mortar is as follows. The mortar is subjected to stress after freeze-thaw cycles and fails. It can be seen that the fiber inside the mortar can bear part of the tensile stress and the fracture surface of the fiber is relatively flat, indicating that the fiber and the cement matrix are well bonded and there is no slippage phenomenon but direct fracture. With the addition of nano SiO2, the hydration reaction is promoted, which further improves the stability between the fiber and the mortar matrix, resists external destructive forces, and significantly improves the freeze-thaw resistance.

[0057] Figure 7The changes in the internal porosity of the mortar were more intuitively reflected. The area ratio of the red area shown in the figure was calculated as follows: 25.63% for the BF0NS0 group, 15.32% for the BF0NS2 group, 15.21% for the BF0.2NS0 group, and 12.03% for the BF0.2NS2 group. It can be seen that with the incorporation of nano-SiO2 and basalt fiber, compared with the blank control group (i.e., the BF0NS0 group), the internal porosity and cracks of the mortar were significantly reduced. This indicates that nano-SiO2 effectively filled the pore structure inside the mortar, continuously reacted with cement particles to generate CSH gel, promoted the bonding between the fiber and the matrix, resulting in a denser structure and reduced damage from freeze-thaw cycles. The freeze-thaw resistance of the fiber-nano blended mortar was significantly improved.

[0058] At the molecular level, the smooth fracture surface indicates that the chemical bonding strength at the fiber-matrix interface is greater than the tensile strength of the fiber itself, meaning that a large number of Si-O-Ca covalent bonds are formed at the interface. This morphology directly proves that the NS pozzolanic reaction increases the CSH content at the interface, thereby increasing the interfacial fracture energy G_IC by approximately 0.8 J / m. 2 This inhibits the propagation of freeze-thaw microcracks along weak areas of the interface.

[0059] This invention, through extensive experimental research, proposes a scientifically sound and reasonable mix proportion scheme for frost-resistant nano-SiO2-BF mortar. This scheme clarifies the optimal combination of basalt fiber and nano-SiO2 dosages, providing clear guidance for engineering practice, reducing the number of experiments required for mix proportion optimization, lowering R&D costs, and also offering a reliable solution for predicting mortar performance under complex environmental conditions, thus possessing significant practical application value.

[0060] Through extensive experimental research, the optimal combination of basalt fiber and nano-SiO2 dosages was determined. When the nano-SiO2 dosage is 2% and the basalt fiber dosage is 0.2%, the compressive strength, flexural strength, water absorption, and frost resistance of the mortar all achieve the best balance, which can give full play to the synergistic reinforcing effect of the two and achieve the optimal improvement of mortar performance.

[0061] The foregoing has shown and described the basic principles, main features, and advantages of the present invention. It will be apparent to those skilled in the art that the present invention is not limited to the details of the exemplary embodiments described above, and that the invention can be implemented in other specific forms without departing from its spirit or basic characteristics. Therefore, the embodiments should be considered exemplary and non-limiting in all respects, and the scope of the invention is defined by the appended claims rather than the foregoing description. Thus, it is intended that all variations falling within the meaning and scope of equivalents of the claims be included within the present invention.

[0062] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. A frost-resistant nano-SiO2-BF mortar material, comprising cement, river sand, water, and a water-reducing agent, wherein the mass ratio of cement, river sand, and water is (1.94-2):5.4:1, and the amount of water-reducing agent added is 0.25% of the total amount of cementitious materials, characterized in that, It also includes nano-SiO2 and basalt fiber, wherein the amount of nano-SiO2 is 0%-3% of the cement mass, and the volume of basalt fiber is 0%-0.4% of the total volume of mortar.

2. The antifreeze nano-SiO2-BF mortar material as described in claim 1, characterized in that, The mortar material includes cement, nano-SiO2, basalt fiber, river sand, water, and water-reducing agent. The mass ratio of cement, river sand, and water is 2:5.4:

1. The amount of nano-SiO2 is 0%-3% of the cement mass. The volume of basalt fiber is 0%-0.4% of the total mortar volume. The amount of water-reducing agent is 0.25% of the total amount of cementitious materials.

3. The antifreeze nano-SiO2-BF mortar material as described in claim 2, characterized in that, The cement is ordinary Portland cement of grade PO42.5 that meets national standards.

4. The antifreeze nano-SiO2-BF mortar material as described in claim 3, characterized in that, The average particle size of the nano-SiO2 is 20 nm.

5. The antifreeze nano-SiO2-BF mortar material as described in claim 4, characterized in that, The basalt fibers have a length of 12 mm, a single filament diameter of 13 μm, and a density of 2.6 g / cm³. 3 .

6. The antifreeze nano-SiO2-BF mortar material as described in claim 5, characterized in that, The river sand has a fineness modulus of 2.5 and an apparent density of 2630 kg / m³. 3 The bulk density is 1573 kg / m³. 3 .

7. The antifreeze nano-SiO2-BF mortar material as described in claim 6, characterized in that, The water-reducing agent is a polycarboxylate superplasticizer.

8. A method for preparing the antifreeze nano-SiO2-BF mortar material as described in any one of claims 1-7, characterized in that, Includes the following steps: S1: Mix the water-reducing agent with water and stir. S2: Add nano-SiO2 to the mixture obtained in step S1 and stir continuously to ensure uniform dispersion; S3: Add cement and continue mixing, adding river sand evenly during the mixing process; S4: Add basalt fibers in batches to the mixture obtained in step S3, and stir continuously to make the mortar uniform, so as to obtain the frost-resistant nano-SiO2-BF mortar material.

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