High-Fluidization, Early Strength Cement Grouting Material Enhanced by Nano-SiO2: Formula and Mechanisms

Cement grouting material is one of the most important materials in civil construction at present, for seepage prevention, rapid repair, and reinforcement. To achieve the ever-increasing functional requirements of civil infrastructures, cement grouting materials must have the specific performance of high fluidization, early strength, and low shrinkage. In recent years, nanomaterials have been widely used to improve the engineering performance of cement grouting materials. However, the mechanisms of nanomaterials in grouting materials are not clear. Hence, a high-fluidization, early strength cement grouting material, enhanced by nano-SiO2, is developed via the orthogonal experimental method in this study. The mechanisms of nano-SiO2 on the microstructure and hydration products of the HCGA, in the case of different curing ages and nano-SiO2 contents, are analyzed through scanning electron microscopy tests, X-ray diffraction tests, differential scanning calorimetry tests, and Fourier transform infrared spectroscopy tests.


Introduction
In civil engineering, grouting is one of the most efficient and common methods for seepage prevention, rapid repair, and reinforcement [1,2]. Owing to the advantage in mature technology and satisfactory cost performance, cement-based materials are widely used in grouting [3]. To achieve the ever-increasing functional requirements of civil infrastructures, cement grouting materials must have the following specific characteristics: (a) high fluidization (to ensure that the grouting materials can fill into the defects of the engineering structure easily and fully); (b) early strength (to shorten the engineering period); (c) low shrinkage (to prevent shrinkage cracks at an early age) [4]. In this case, various innovative materials have been used to attempt to prepare modified cement-based grouting materials. Liu et al. [4], Li et al. [5], Li et al. [6], Wu et al. [7], and Zhang et al. [8] adopted aluminate cement, magnesium phosphate cement, sulphoaluminate cement, potassium magnesium phosphate cement, and ultrafine sulphoaluminate cement to improve the early strength and fluidization of cement grouting materials, respectively, which could obtain a significant improvement effect. However, the source of these new types of cements is limited, which might not meet the requirement of engineering applications. Zhou et al. [9], Celik et al. [10], Zhang et al. [11], and Guo et al. [12] adopted water glass, bottom ash, microfine fly ash, and ultrafine cement to modify the fluidization of grouting materials,

Methods
In this study, the orthogonal experimental method is used to determ benchmark formulas of the cement grouting material, owing to the advan conveniently analyzing the interrelations among different test factors and scien reducing the experimental workload [32,33]. Subsequently, the effects of na content on the engineering performance of the benchmark formulas are ana determine the final high-fluidization, early strength cement grouting mater fluidity (flowing time), flexural strength (1 day, 3 days, and 7 days), compressive (1 day, 3 days, and 7 days), and dry-shrinkage rate (7 days and 28 days) are ad evaluate the engineering performance of the cement grouting materials. experiments are implemented in accordance with the Chinese specification Methods of Cement and Concrete for Highway Engineering' [34].
Moreover, the SEM (FEI Quanta 250, Anton Paar GmbH, Graz, Austria) te (AXS, Bruker Corporation, Billerica, USA) test, DSC (SDT 650, TA Instrumen Castle, USA) test, and FTIR (Nicolet 5700, Thermo Fisher Scientific -CN, Shangha test are adopted to reveal the mechanisms of the proposed high-fluidizatio strength cement grouting material via microstructure and hydration products. T test is used for the detailed analysis of the micro-morphology of the hydration The XRD test is used to investigate the types of hydration products with a scannin

Methods
In this study, the orthogonal experimental method is used to determine the benchmark formulas of the cement grouting material, owing to the advantage in conveniently analyzing the interrelations among different test factors and scientifically reducing the experimental workload [32,33]. Subsequently, the effects of nano-SiO 2 content on the engineering performance of the benchmark formulas are analyzed to determine the final high-fluidization, early strength cement grouting material. The fluidity (flowing time), flexural strength (1 day, 3 days, and 7 days), compressive strength (1 day, 3 days, and 7 days), and dry-shrinkage rate (7 days and 28 days) are adopted to evaluate the engineering performance of the cement grouting materials. All the experiments are implemented in accordance with the Chinese specification of "Test Methods of Cement and Concrete for Highway Engineering" [34].
Moreover, the SEM (FEI Quanta 250, Anton Paar GmbH, Graz, Austria) test, XRD (AXS, Bruker Corporation, Billerica, USA) test, DSC (SDT 650, TA Instruments, New Castle, USA) test, and FTIR (Nicolet 5700, Thermo Fisher Scientific -CN, Shanghai, China) test are adopted to reveal the mechanisms of the proposed high-fluidization, early strength cement grouting material via microstructure and hydration products. The SEM test is used for the detailed analysis of the micro-morphology of the hydration product. The XRD test is used to investigate the types of hydration products with a scanning speed of 10 • /min and a scanning angle of 10-65 • (angle measurement error < 0.01 • and angle repeatability < 0.0001 • ). The DSC test is used to analyze the content of hydration products via the weight change and heat change, ranging from 0 • C to 600 • C, with a heating rate of 15 • C/min (nitrogen atmosphere). The FTIR test is used to investigate functional group characteristics in a spectral range of 400-4000 cm −1 in transmission mode using the potassium bromide pressed-disk technique.
The samples used in the SEM tests, XRD tests, DSC tests, and FTIR tests are prepared as follows:

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According to the standard method [34], the beam samples with a size of 4 cm × 4 cm × 16 cm are prepared by curing the target age (1 day, 3 days, or 7 days).

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The sheet samples with a size of 2 cm × 2 cm × 1 cm are prepared by cutting the beam samples, and are put into absolute ethyl alcohol for seven days (the absolute ethyl alcohol must be replaced everyday).

•
The treated sheet samples are prepared to cubic blocks with an approximate size of 1 cm ×

Design of the Orthogonal Experiments
The orthogonal experimental factors and their levels are listed in Table 6. The experimental schemes are presented in Table 7.

Orthogonal Experiment Analysis
The results of the orthogonal experiments are listed in Table 8. According to Table 8, the ranges for each experimental factor and the corresponding average values for each experimental level are calculated to analyze the orthogonal experimental results, as presented in Table 9. The range is equal to the difference of the average values among different experimental levels for the same experimental factor, as expressed in Equation (1). The influence of the experimental factor increases as the range increases. The process of the orthogonal experimental analysis is shown in Figure 3.
where TD max and TD min are the maximum value and minimum value of the target property index in the case of different experimental levels of a certain experimental factor, respectively. According to Table 8, the ranges for each experimental factor and the corresponding average values for each experimental level are calculated to analyze the orthogonal experimental results, as presented in Table 9. The range is equal to the difference of the average values among different experimental levels for the same experimental factor, as expressed in Equation (1). The influence of the experimental factor increases as the range increases. The process of the orthogonal experimental analysis is shown in Figure 3.
where TDmax and TDmin are the maximum value and minimum value of the target property index in the case of different experimental levels of a certain experimental factor, respectively.     The ranges of different properties are shown in Figure 4. According to Figures 3 and 4, the key factor () and the secondary factor () for different properties are listed in Table 10. The ranges of different properties are shown in Figure 4. (e) (f) According to Figures 3 and 4, the key factor (✔✔) and the secondary factor (✔) for different properties are listed in Table 10.           As shown in Figure 5, the flow time decreases as the water-cement ratio increases, and the water-reducing agent and accelerating agent decrease. Moreover, when the water-cement ratio is more than 0.56 and the accelerating agent is less than 2.5%, the above trend of the flow time gradually begins to flatten. Hence, considering that the fluidity should range from 9 s to 13 s, according to the Chinese specification "Technical Specification for Road Semi-Flexible Pavement" [35], the water-cement ratio is suggested to be more than 0.56, the water-reducing agent is suggested to be less than 1.2%, and the accelerating agent is suggested to be less than 2.5%. As shown in Figure 6, the 1-day compressive strength linearly increases as the accelerating agent increases and the watercement ratio decreases. Moreover, the 1-day flexural strength increases as the accelerating agent increases; first, it gradually increases and then rapidly decreases as the watercement ratio increases. When the water-cement ratio is equal to 0.56, the 1-day flexural strength achieves the highest value. Hence, the water-cement ratio is suggested to be 0.53-0.56, and the accelerating agent should be selected as a high level. As shown in Figures 7  and 8, the 3-day and 7-day strengths decrease as the water-cement ratio increases. Considering that a higher strength is better, the water-cement ratio should be selected as a low level. As shown in Figure 9, the 7-day and 28-day dry-shrinkage rates decrease as the water-cement ratio, expansion agent, and water-reducing agent increase. However, when the water-cement ratio and expansion agent are more than 0.56 and 8%, respectively, the descending trend gradually begins to flatten. Hence, the water-cement ratio and expansion agent are suggested to be more than 0.56 and 8%, respectively. The water-reducing agent should be selected as a high level. Note that the optimal proportion of water-reducing agent for the dry-shrinkage rate is contrary to that for the fluidity. However, considering the importance of the water-reducing agent on the fluidity is more significant than the dry-shrinkage rate. The suggested content of water-reducing agent is 1.0%-1.2%. In summary, according to the above analysis of different properties, the effective composition of cement grouting material can be considered to be the following: water-cement ratio = 0.53-0.56, accelerating agent = 2.0%-2.5%, water-reducing agent = 1.0%-1.2%, and expansion agent > 8%.

The High-Fluidization, Early Strength Cement Grouting Enhanced by Nano-SiO2
According to the conclusion of Section 3.2, four benchmark formulas are proposed for further verification, as given in Table 11. The results of the engineering performance of the four formulas are presented in Table 12. The performance standard of cement grouting materials shown in Table 12 comes from the Chinese specification "Technical Specification for Road Semi-Flexible Pavement" [35]. As shown in Figure 5, the flow time decreases as the water-cement ratio increases, and the water-reducing agent and accelerating agent decrease. Moreover, when the watercement ratio is more than 0.56 and the accelerating agent is less than 2.5%, the above trend of the flow time gradually begins to flatten. Hence, considering that the fluidity should range from 9 s to 13 s, according to the Chinese specification "Technical Specification for Road Semi-Flexible Pavement" [35], the water-cement ratio is suggested to be more than 0.56, the water-reducing agent is suggested to be less than 1.2%, and the accelerating agent is suggested to be less than 2.5%. As shown in Figure 6, the 1-day compressive strength linearly increases as the accelerating agent increases and the water-cement ratio decreases. Moreover, the 1-day flexural strength increases as the accelerating agent increases; first, it gradually increases and then rapidly decreases as the water-cement ratio increases. When the water-cement ratio is equal to 0.56, the 1-day flexural strength achieves the highest value. Hence, the water-cement ratio is suggested to be 0.53-0.56, and the accelerating agent should be selected as a high level. As shown in Figures 7 and 8, the 3-day and 7-day strengths decrease as the water-cement ratio increases. Considering that a higher strength is better, the water-cement ratio should be selected as a low level. As shown in Figure 9, the 7-day and 28-day dry-shrinkage rates decrease as the water-cement ratio, expansion agent, and water-reducing agent increase. However, when the water-cement ratio and expansion agent are more than 0.56 and 8%, respectively, the descending trend gradually begins to flatten. Hence, the water-cement ratio and expansion agent are suggested to be more than 0.56 and 8%, respectively. The water-reducing agent should be selected as a high level. Note that the optimal proportion of water-reducing agent for the dryshrinkage rate is contrary to that for the fluidity. However, considering the importance of the water-reducing agent on the fluidity is more significant than the dry-shrinkage rate. The suggested content of water-reducing agent is 1.0%-1.2%. In summary, according to the above analysis of different properties, the effective composition of cement grouting material can be considered to be the following: water-cement ratio = 0.53-0.56, accelerating agent = 2.0%-2.5%, water-reducing agent = 1.0%-1.2%, and expansion agent > 8%.

The High-Fluidization, Early Strength Cement Grouting Enhanced by Nano-SiO 2
According to the conclusion of Section 3.2, four benchmark formulas are proposed for further verification, as given in Table 11. The results of the engineering performance of the four formulas are presented in Table 12. The performance standard of cement grouting materials shown in Table 12 comes from the Chinese specification "Technical Specification for Road Semi-Flexible Pavement" [35]. As shown in Table 12, the fluidity of Y-3 and Y-4 is significantly better than Y-1 and Y-2. Moreover, the flexural strength of Y-4 at an early curing age is higher than Y-3, especially for the 1-day flexural strength. Hence, Y-4 is determined to be the optimal formula.
To further improve the engineering performance, nano-SiO 2 (see Figure 9) is mixed into the proposed benchmark formula (Y-4). The engineering performance of the cement grouting materials with different contents of nano-SiO 2 is presented in Table 13. Six specimens are successfully tested for each data. The coefficients of variation (COV) are presented in Table 14. According to the Chinese test specification "Test Methods of Cement and Concrete for Highway Engineering (JTG E30-2005)" [34], the COVs of the fluidity, strength, and shrinkage rate must be less than 10%, 10%, and 15%, respectively. It can be found that the COVs all meet the requirements of the Chinese test specification, showing the availability of the test results.
where σ is the standard deviation and µ is the average value. As shown in Table 13, it can be found that the nano-SiO 2 has a significant effect on the 1-day strength, 3-day strength, and fluidity, especially for the 1-day strength. Every 1% increase in the content of nano-SiO 2 translates into, on average, a 6.21%, 10.43%, 1.99%, and 3.71% increase in the 1-day flexural strength, 1-day compressive strength, 3-day flexural strength, and 3-day compressive strength, respectively, and translates into a 7.61% fall in the fluidity. This indicates that nano-SiO 2 can significantly improve the early age strength and slightly weaken the fluidity.
In addition, it should be noted that, although the chemical nature of nano-SiO 2 is stable, a possible hazard is breathing in dust because the fine nano-SiO 2 particles are easy to float in the air. Hence, the handlers must wear masks during construction.

Hydration Mechanisms of HCGA
The effects of curing age (1-day, 3-day, and 7-day) and nano-SiO 2 content (0%, 1%, 2%, and 3%) on the microstructure and hydration products of HCGA are analyzed in this section.   As shown in Table 13, it can be found that the nano-SiO2 has a significant effect on the 1-day strength, 3-day strength, and fluidity, especially for the 1-day strength. Every 1% increase in the content of nano-SiO2 translates into, on average, a 6.21%, 10.43%, 1.99%, and 3.71% increase in the 1-day flexural strength, 1-day compressive strength, 3-day flexural strength, and 3-day compressive strength, respectively, and translates into a 7.61% fall in the fluidity. This indicates that nano-SiO2 can significantly improve the early age strength and slightly weaken the fluidity.
In addition, it should be noted that, although the chemical nature of nano-SiO2 is stable, a possible hazard is breathing in dust because the fine nano-SiO2 particles are easy to float in the air. Hence, the handlers must wear masks during construction.
As shown in Figure 10, the following observations can be made. The CSH (calcium silicate hydrate) gels and AFt crystals (ettringite) can be observed in each HCGA, whether the nano-SiO 2 is added or not. However, there are some obvious voids in the microstructure of the HCGA without nano-SiO 2 . These voids gradually decrease as the content of nano-SiO 2 increases. It can be speculated that the nano-SiO 2 is helpful in improving the hydration of the cement grouting material.
Moreover, the CH(Ca(OH) 2 ) crystals provide an effect to guarantee the stable existence of cement hydration products. The CH crystals in the HCGA without nano-SiO 2 are mainly generated as layered joints at the interface of cement stone, which cannot be wrapped by CSH gels, resulting in restriction of the strength formation. As the content of nano-SiO 2 increases, the number and size of the layered CH crystals gradually decrease, and the CSH gels accordingly increase, indicating that nano-SiO 2 is beneficial to accelerate the consumption of CH crystals and the formation of CSH gels. In addition, with the addition of nano-SiO 2 , the CSH gel and AFt crystals are gradually connected to each other, and form an interlaced skeleton structure. The phenomena also explain why the 1-day flexural and compressive strengths of the HCGA increase as the content of nano-SiO 2 increases.
Hence, it can be speculated that the mechanism of nano-SiO 2 on the early strength of the HCGA is to accelerate the generation of CH crystals, to reach saturation at a faster rate and urge the CHS gels to generate early, while the mechanism is irrelevant to the AFt crystals. In addition, owing to the accelerated reaction of CH crystals and CHS gels, caused by nano-SiO 2 , the number and size of voids can be effectively controlled.
As shown in Figure 10, the following observations can be made. The CSH (calcium silicate hydrate) gels and AFt crystals (ettringite) can be observed in each HCGA, whether the nano-SiO2 is added or not. However, there are some obvious voids in the microstructure of the HCGA without nano-SiO2. These voids gradually decrease as the content of nano-SiO2 increases. It can be speculated that the nano-SiO2 is helpful in improving the hydration of the cement grouting material.
Moreover, the CH(Ca(OH)2) crystals provide an effect to guarantee the stable existence of cement hydration products. The CH crystals in the HCGA without nano-SiO2 are mainly generated as layered joints at the interface of cement stone, which cannot be wrapped by CSH gels, resulting in restriction of the strength formation. As the content of nano-SiO2 increases, the number and size of the layered CH crystals gradually decrease, and the CSH gels accordingly increase, indicating that nano-SiO2 is beneficial to accelerate the consumption of CH crystals and the formation of CSH gels. In addition, with the addition of nano-SiO2, the CSH gel and AFt crystals are gradually connected to each other, and form an interlaced skeleton structure. The phenomena also explain why the 1-day flexural and compressive strengths of the HCGA increase as the content of nano-SiO2 increases.
Hence, it can be speculated that the mechanism of nano-SiO2 on the early strength of the HCGA is to accelerate the generation of CH crystals, to reach saturation at a faster rate and urge the CHS gels to generate early, while the mechanism is irrelevant to the AFt crystals. In addition, owing to the accelerated reaction of CH crystals and CHS gels, caused by nano-SiO2, the number and size of voids can be effectively controlled.

The Curing Age of 3-Day
The microstructures of the HCGA with different nano-SiO2 contents, at the curing age of 3 days, are shown in Figure 11. (c) (d) Figure 11. Microstructure of HCGA with different contents of nano-SiO 2 at 3 d curing age.
As shown in Figure 11, compared to the microstructure at the curing age of 1 day, the number of voids and the amount of layered CH crystals in the HCGA at the curing age of 3 days significantly decreases in the field of the microscope, and the amount of CSH gel accordingly increases. This indicates that the hydration degree of the HCGA is further strengthened. Moreover, as the content of nano-SiO 2 increases, it can also be found that the CHS gels increase and the layered CH crystals decrease, proving that the effect of nano-SiO 2 on early hydration still remains. However, the difference in the microstructures in the case of different contents of nano-SiO 2 , at the curing age of 1 day, is less than that at the curing age of 3 days, showing that the effect of nano-SiO 2 gradually grows less as the curing age increases.
As shown in Figure 11, compared to the microstructure at the curing age of 1 day, the number of voids and the amount of layered CH crystals in the HCGA at the curing age of 3 days significantly decreases in the field of the microscope, and the amount of CSH gel accordingly increases. This indicates that the hydration degree of the HCGA is further strengthened. Moreover, as the content of nano-SiO2 increases, it can also be found that the CHS gels increase and the layered CH crystals decrease, proving that the effect of nano-SiO2 on early hydration still remains. However, the difference in the microstructures in the case of different contents of nano-SiO2, at the curing age of 1 day, is less than that at the curing age of 3 days, showing that the effect of nano-SiO2 gradually grows less as the curing age increases.

The Curing Age of 7-Day
The microstructure of cement grouting materials with different nano-SiO2 contents at 7 days is shown in Figure 12. As shown in Figure 12, the hydration products are closely connected to form a relatively dense and stable microstructure. This shows that the hydration of the HCGA has tended to be completed at the curing age of 7 days. In addition, the differences in the microstructure in the case of different contents of nano-SiO2 are not significant, indicating that the nano-SiO2 has little effect on the hydration of the HCGA at the curing age of 7 days.
In previous studies [15,18], nano-SiO2 can also play a significant role in early strength at the curing age of 7 days for common cement-based materials. In contrast, the effect of nano-SiO2 weakened at the curing age of 3 days and disappeared at the curing age of 7 days for the HCGA proposed in this study. It can be speculated that the reaction period As shown in Figure 12, the hydration products are closely connected to form a relatively dense and stable microstructure. This shows that the hydration of the HCGA has tended to be completed at the curing age of 7 days. In addition, the differences in the microstructure in the case of different contents of nano-SiO 2 are not significant, indicating that the nano-SiO 2 has little effect on the hydration of the HCGA at the curing age of 7 days.
In previous studies [15,18], nano-SiO 2 can also play a significant role in early strength at the curing age of 7 days for common cement-based materials. In contrast, the effect of nano-SiO 2 weakened at the curing age of 3 days and disappeared at the curing age of 7 days for the HCGA proposed in this study. It can be speculated that the reaction period of nano-SiO 2 is not fixed, which is related to the hydration rate. The effect of nano-SiO 2 on the strength will occur ahead, as the hydration rate quickens. Figure 13 shows the XRD results of the HCGA in the case of different contents of nano-SiO 2 at the curing age of 1 day, 3 days, and 7 days. In Figure 13, C 2 S and C 3 S represent dicalcium silicate and tricalcium silicate, respectively. of nano-SiO2 is not fixed, which is related to the hydration rate. The effect of nano-SiO2 on the strength will occur ahead, as the hydration rate quickens. Figure 13 shows the XRD results of the HCGA in the case of different contents of nano-SiO2 at the curing age of 1 day, 3 days, and 7 days. In Figure 13, C2S and C3S represent dicalcium silicate and tricalcium silicate, respectively. As shown in Figure 13, the constituents of the HCGA in the case of different contents of nano-SiO2 are similar in the XRD images. At the curing age of 1 day, the intensity of the diffraction peak of C3S decreases as the content of nano-SiO2 increases, showing that nano-SiO2 accelerates the consumption of C3S to generate CH crystals and CSH gels, to realize the early strength. When the diffraction angle is 35°, the changes in the CH crystals are similar to when the diffraction angle is 28° [36,37], owing to the formation of the CSH gels, caused by the reaction of nano-SiO2 and CH crystals. This is the reason that the diffraction peak of CH crystals decreases as the content of nano-SiO2 increases. In addition, the differences in the derivative peak of C2S in the case of different contents of nano-SiO2 are limited, showing that nano-SiO2 has little effect on the long-term strength of the HCGA. The above phenomena show that nano-SiO2 mainly takes part in the hydration reaction of C3S to improve the early strength in the HCGA, while it is irrelevant to the C2S.

X-ray Diffraction Analysis
Moreover, the diffraction peaks at the curing age of 3 days and 7 days are similar to those at the curing age of 1 day, indicating that there is no new hydration reaction during the curing age of 3 days and 7 days. The intensities of the diffraction peaks of C2S and C3S decrease as the curing age increases. This implies that the hydration of the HCGA is still ongoing at the curing age of 3 days and 7 days. In addition, the difference in the diffraction peaks in the case of different contents of nano-SiO2 at the curing age of 7 days shows that nano-SiO2 has little effect on hydration at the curing age of 7 days. As shown in Figure 13, the constituents of the HCGA in the case of different contents of nano-SiO 2 are similar in the XRD images. At the curing age of 1 day, the intensity of the diffraction peak of C 3 S decreases as the content of nano-SiO 2 increases, showing that nano-SiO 2 accelerates the consumption of C 3 S to generate CH crystals and CSH gels, to realize the early strength. When the diffraction angle is 35 • , the changes in the CH crystals are similar to when the diffraction angle is 28 • [36,37], owing to the formation of the CSH gels, caused by the reaction of nano-SiO 2 and CH crystals. This is the reason that the diffraction peak of CH crystals decreases as the content of nano-SiO 2 increases. In addition, the differences in the derivative peak of C 2 S in the case of different contents of nano-SiO 2 are limited, showing that nano-SiO 2 has little effect on the long-term strength of the HCGA. The above phenomena show that nano-SiO 2 mainly takes part in the hydration reaction of C 3 S to improve the early strength in the HCGA, while it is irrelevant to the C 2 S. Moreover, the diffraction peaks at the curing age of 3 days and 7 days are similar to those at the curing age of 1 day, indicating that there is no new hydration reaction during the curing age of 3 days and 7 days. The intensities of the diffraction peaks of C 2 S and C 3 S decrease as the curing age increases. This implies that the hydration of the HCGA is still ongoing at the curing age of 3 days and 7 days. In addition, the difference in the diffraction peaks in the case of different contents of nano-SiO 2 at the curing age of 7 days shows that nano-SiO 2 has little effect on hydration at the curing age of 7 days.

Differential Scanning Calorimetry
The mass loss curve (TG curve, red) and heat flow curve (DSC curve, black) of the HCGA are shown in Figure 14. In the curves, there are two obvious segments for the weight loss and enthalpy change. The first thermal decomposition peak and the corresponding weight loss that appeared at lower than 150 • C mainly represent the evaporation of free water [38], abbreviated as I-stage. The second thermal decomposition peak and the corresponding weight loss that appeared at 350-600 • C represent the decomposition of CH crystals [39], abbreviated as II-stage. Moreover, the enthalpy change and weight loss in the DSC curves are extracted to further analyze the effects of nano-SiO 2 content and curing age, as shown in Figure 15.

Differential Scanning Calorimetry
The mass loss curve (TG curve, red) and heat flow curve (DSC curve, black) of the HCGA are shown in Figure 14. In the curves, there are two obvious segments for the weight loss and enthalpy change. The first thermal decomposition peak and the corresponding weight loss that appeared at lower than 150 °C mainly represent the evaporation of free water [38], abbreviated as I-stage. The second thermal decomposition peak and the corresponding weight loss that appeared at 350-600 °C represent the decomposition of CH crystals [39], abbreviated as II-stage. Moreover, the enthalpy change and weight loss in the DSC curves are extracted to further analyze the effects of nano-SiO2 content and curing age, as shown in Figure 15.  As shown in Figures 14 and 15, at the curing age of 1 day and 3 days, the weight l and enthalpy change in the I-stage decrease by 3.34% and 0.97%, on average, for every increase in the content of nano-SiO2, respectively, while, in the II-stage, they accordin increase by 12.04% and 0.51%. The less free water there is, the more bound water there and the more complete the hydration reaction is. This implies that nano-SiO2 promo the hydration reaction of the HCGA at an early curing age. Moreover, the increase weight loss in the II-stage indicates the accelerated generation of CH crystals. This sho that nano-SiO2 is conducive, to accelerate the generation of CH crystals to reach saturat at a faster rate, verifying the conjecture in Section 4.1.1. In addition, the change in wei loss and enthalpy change at the curing age of 3 days is, on average, 35.83% and 5.33% l than that at the curing age of 1 day, respectively, implying that the effect of nano-SiO2 the hydration reaction at the curing age of 3 day is lower than that at the curing age o day. When the curing age is 7 days, the difference in the weight loss and enthalpy chan in the case of different contents of nano-SiO2 is not significant, showing that nano-SiO2 little influence at the curing age of 7 days.
In addition, peak-splitting, for both the observed peaks, can be found in some D curves. The DSC curve obtained by the chemical reaction should be a single smooth pe under ideal test conditions. However, the peak shape may be deformed, resulting fr overlapping reactions in the process of sample preparation and testing, owing to As shown in Figures 14 and 15, at the curing age of 1 day and 3 days, the weight loss and enthalpy change in the I-stage decrease by 3.34% and 0.97%, on average, for every 1% increase in the content of nano-SiO 2 , respectively, while, in the II-stage, they accordingly increase by 12.04% and 0.51%. The less free water there is, the more bound water there is, and the more complete the hydration reaction is. This implies that nano-SiO 2 promotes the hydration reaction of the HCGA at an early curing age. Moreover, the increase in weight loss in the II-stage indicates the accelerated generation of CH crystals. This shows that nano-SiO 2 is conducive, to accelerate the generation of CH crystals to reach saturation at a faster rate, verifying the conjecture in Section 4.1.1. In addition, the change in weight loss and enthalpy change at the curing age of 3 days is, on average, 35.83% and 5.33% less than that at the curing age of 1 day, respectively, implying that the effect of nano-SiO 2 on the hydration reaction at the curing age of 3 day is lower than that at the curing age of 1 day. When the curing age is 7 days, the difference in the weight loss and enthalpy change in the case of different contents of nano-SiO 2 is not significant, showing that nano-SiO 2 has little influence at the curing age of 7 days.
In addition, peak-splitting, for both the observed peaks, can be found in some DSC curves. The DSC curve obtained by the chemical reaction should be a single smooth peak under ideal test conditions. However, the peak shape may be deformed, resulting from overlapping reactions in the process of sample preparation and testing, owing to the unevenness of raw materials, the uncertainty of cement hydration, and the thermal decomposition reaction in an inert atmosphere. Moreover, considering the aging of the apparatus used in this study, the above phenomenon is more significant.

Fourier Transform Infrared Spectroscopy
The results of the FTIR tests are shown in Figure 16. The results of the FTIR tests are shown in Figure 16. The vibration peak mainly corresponds to the water molecules and Si-O-T (T = Si a Al) in CSH gels. At 4000-400 cm −1 , the FTIR vibration bands of the HCGA with differ contents of nano-SiO2 are almost the same. The peak values of tensile vibration a flexural vibration of bound water also do not change significantly. This indicates that types of hydration products are the same in the case of different curing ages and nan SiO2 contents. The absorption peak at 3643-3645 cm −1 is caused by the -OH stretchi vibration of Ca(OH)2 [40,41]. It can be found that the wave number slightly increases the content of nano-SiO2 increases, showing that the bond energy of -OH in Ca(OH) improved; that is to say that the amount of CH crystals increases as the content of nan SiO2 increases. This is consistent with the aforementioned analysis on the hydrati process. In addition, the absorption peak at 1639-1646 cm −1 is due to the bending vibrati caused by -OH in water molecules. The absorption peak at 1480-1485 cm −1 is due to CO3 2− antisymmetric stretching vibration. This implies that the calcium hydroxide in cement grout reacts with the carbon dioxide in the air to form calcium carbonate duri the preparation of the samples. The range of 400-1400 cm −1 is generally identified a fingerprint area.

Conclusions
A high-fluidization, early strength cement grouting material, enhanced by nano-S (HCGA), is developed via the orthogonal experimental method in this study. Moreov The vibration peak mainly corresponds to the water molecules and Si-O-T (T = Si and Al) in CSH gels. At 4000-400 cm −1 , the FTIR vibration bands of the HCGA with different contents of nano-SiO 2 are almost the same. The peak values of tensile vibration and flexural vibration of bound water also do not change significantly. This indicates that the types of hydration products are the same in the case of different curing ages and nano-SiO 2 contents. The absorption peak at 3643-3645 cm −1 is caused by the -OH stretching vibration of Ca(OH) 2 [40,41]. It can be found that the wave number slightly increases as the content of nano-SiO 2 increases, showing that the bond energy of -OH in Ca(OH) 2 is improved; that is to say that the amount of CH crystals increases as the content of nano-SiO 2 increases. This is consistent with the aforementioned analysis on the hydration process. In addition, the absorption peak at 1639-1646 cm −1 is due to the bending vibration caused by -OH in water molecules. The absorption peak at 1480-1485 cm −1 is due to the CO 3 2− antisymmetric stretching vibration. This implies that the calcium hydroxide in the cement grout reacts with the carbon dioxide in the air to form calcium carbonate during the preparation of the samples. The range of 400-1400 cm −1 is generally identified as a fingerprint area.

Conclusions
A high-fluidization, early strength cement grouting material, enhanced by nano-SiO 2 (HCGA), is developed via the orthogonal experimental method in this study. Moreover, the mechanisms of nano-SiO 2 on the microstructure and hydration products, in the case of different curing ages and nano-SiO 2 contents, are analyzed through SEM tests, XRD tests, DSC tests, and FTIR tests.

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The formula of the HCGA is water-cement ratio = 0.56, water-reducing agent = 1.2%, accelerating agent = 2.5%, expansion agent = 8%, and nano-SiO 2 = 1%. The flexural and compressive strength of the HCGA at the curing age of 1 day is higher than 3.5 MPa and 12 MPa, respectively, while the fluidity and shrinkage rate is less than 11 s and 0.15%, respectively; • Nano-SiO 2 can significantly improve the flexural and compressive strength of the HCGA at an early curing age, while it will slightly weaken the fluidity. The enhancement of nano-SiO 2 on the strength becomes weak when the content of nano-SiO 2 exceeds 1%. Hence, considering economic costs, it is recommended that the recommended content of nano-SiO 2 is 2%. In addition, the effects of nano-SiO 2 decrease as the curing age increases, which has little significance at the curing age of 7 days. The mechanism of nano-SiO 2 on the early strength of the HCGA is to accelerate the generation of CH crystals, to reach saturation at a faster rate and urge the CHS gels to generate early, while it is irrelevant to the AFt crystals; • The types of hydration products of the HCGA are almost the same in the case of different curing ages and nano-SiO 2 contents. Nano-SiO 2 mainly takes part in the hydration reaction of tricalcium silicate, to improve the early strength in the HCGA, while it is irrelevant to the dicalcium silicate. The reaction period of nano-SiO 2 is not fixed, which is related to the hydration rate. Compared to common cement-based materials, the effect of nano-SiO 2 on the strength will occur ahead, as the hydration rate quickens in the HCGA (early strength materials).
In addition, thermogravimetric analysis can be used for the quantitive analysis of hydration products. Owing to the limitation of the obtained data in this study, more in-depth quantitative analysis, based on DSC tests, will be addressed in future studies.