Study on Mechanical Properties and Microstructure characteristics of Fly Ash Mixed Wet Shotcrete under Low Temperature Curing Environment

To systematically examine the mechanical properties and microstructure of fly ash-doped wet shotcrete under low-temperature curing conditions, experimental studies were conducted on the compressive strength. The investigations encompassed varying activator dosage, fly ash dosage, and curing temperature for a thorough examination of the material’s performance. The results show that the addition of activator can significantly improve the compressive strength of fly ash wet shotcrete under low temperature curing, but the optimal dosage of different activator types is not consistent. The selection of NaAlO2 as the activator has the most significant effect on the compressive strength of the samples. Under low temperature curing environment, the increase of curing age can also significantly improve the compressive strength of the sample, especially the early strength. The order of influence degree of three factors on compressive strength is that alkaline activator has the highest influence degree, curing temperature has the second, fly ash content has the least influence degree, and activator content is the significant influence factor of early compressive strength. A prediction model for the compressive strength of wet shotcrete doped with fly ash was established in the low temperature curing environment, and the effect of increasing the dosage of activator on the compressive strength of the sample increased with the decrease of curing temperature. With the increase of the content of activator and curing temperature, the size of the void structure in the sample is gradually reduced, which makes the density of the microstructure gradually increase, and thus improves the macroscopic compressive strength of the sample.


Introduction
Wet shotcrete is a slurry with certain flow performance prepared from required aggregates, cement, water and admixtures, and the concrete is sprayed to the surface of rock mass at a high speed by means of air compression through corresponding spraying equipment, and the concrete supporting structure is condensed in a short time [1][2][3].Wet shotcrete can seal the tensile cracks and joints of rock mass and improve the overall strength and stability of rock mass, and has been gradually widely used in underground mining engineering and tunnel engineering [4][5].In addition, in the process of preparing 2 wet shotcrete, adding a certain amount of mineral admixtures can not only save cement, but also further improve the overall mechanical properties of concrete [6][7].However, in the actual application process of underground mines in some high-cold areas, the mechanical properties of wet shotcrete will inevitably be affected by low temperature due to the low temperature underground temperature.Therefore, it is necessary to carry out targeted research on the activation of mineral admixtures and the mechanical properties of wet shotcrete containing mineral admixtures under low temperature environment.
At present, many domestic scholars have carried out systematic research on the mechanical properties of wet shotcrete and the influence of mineral admixtures on the mechanical properties of concrete.Zhang et al. [8] carried out a study on the multi-functional pumping wet shotcrete coagulation performance mixed with micro-encapsulated phase change materials, and pointed out that the addition of an appropriate amount of micro-encapsulated phase change materials can improve the performance of wet shotcrete.Cristobal Javier Manquehual et al. [9] carried out a study on the leaching of steel fiber shotcrete in fresh water and salt water, and pointed out that the difference of leaching water would affect the mechanical properties of the samples.Li et al. [10] carried out a study on the dynamic properties of wet shotcrete in the early curing stage and pointed out that the difference of curing time had an impact on the dynamic compressive strength of samples.Liu et al. [11] carried out a study on the optimization of the ratio parameters of wet shotcrete containing polypropylene fiber, and pointed out that adding an appropriate amount of polypropylene fiber can significantly improve the mechanical properties of concrete.Wang et al. [12] carried out a study on improving the mechanical properties of ultra-high performance concrete with steel fiber, and pointed out that steel fiber can effectively improve the compressive strength of concrete.Hang et al. [13] studied the effects of fiber and rubber materials on dynamic mechanical behavior and damage evolution of shotcrete, and pointed out that the addition of fiber and rubber particles can effectively improve the compressive strength of concrete.Fang et al. [14] carried out an experimental study on the mechanical properties of Yellow River sediment concrete in Kaifeng section, and concluded that the compressive strength of concrete test block at 28 days of age increased by 5% compared with ordinary concrete when the proportion of sand replaced by Yellow River sediment was 10%.Ge et al. [15] conducted a study on the microstructure and mechanical properties of fiber reinforced concrete after high temperature, and concluded that the tensile strength of fiber reinforced concrete specimens first increased and then decreased when the temperature changed from 20℃ to 500℃.Chen Qingqing et al. [16] pointed out that compared with ordinary recycled concrete, the static uniaxial compressive strength and splitting tensile strength of sodium silicate modified recycled concrete were improved to a certain extent.
Based on the above literature, it can be seen that domestic scholars have made many beneficial achievements in the mechanical properties of concrete and the influence of mineral admixtures on the mechanical properties of concrete, which can provide certain theoretical guidance for the research on the mechanical properties of concrete and the recycling of mineral waste.However, there are few researches on the activation of fly ash in wet shotcrete under temperature effect and the mechanical properties of fly ash concrete under different temperatures, so it is necessary to carry out further research work.Therefore, in order to systematically explore the active excitation and mechanical properties of fly ash in concrete under temperature effect, this paper carried out indoor research on the influence of activator type, dosage and temperature on the mechanical properties of fly ash concrete.Firstly, the effect of activator type and dosage on fly ash in fly ash concrete under low temperature curing was studied, and the best activator type and dosage was selected.Secondly, after determining the type and dosage of activator, orthogonal tests on mechanical properties of fly ash wet shotcrete containing activator at different temperatures were carried out to analyze the effects of activator dosage, curing temperature and fly ash content on the compressive strength of fly ash wet shotcrete.Finally, the microstructure of fly ash concrete under different curing temperatures was analyzed by SEM, and the influence mechanism of activator and curing temperature on mechanical properties of fly ash wet shotcrete was revealed.

Experimental Material
In the test, mine independent crushing waste rock is used as the coarse aggregate for preparing concrete, and tailing sand is used as the corresponding fine aggregate with waste stone particles.The corresponding particle size distribution is shown in figure 1. Fly ash comes from secondary fly ash produced by a company.The chemical composition of fly ash is shown in table 1.From the observation of table 1, it can be seen that the higher content of SiO2 and Al2O3 indicates that the quality of fly ash is higher, and a certain amount of CaO is conducive to the formation of hydraulic minerals such as tricalcium silicate and calcium sulfoaluminate.The composition of waste rock, tailings and cement is shown in table 2.

Experimental Scheme
This test is mainly divided into two parts.First, fly ash activity excitation test.Firstly, four activators are selected, namely NaAlO2, NaOH, Na2SO4 and NaCl, and the dosage is designed to be 2%, 3% and 4% of the total mass of the gelling material.Fly ash is in the form of single addition, the addition amount is 10%, and the addition method is equal quality instead of cement content.The specific scheme is shown in table 3. Second, the mechanical properties test of fly ash wet shotcrete with activator under low temperature effect, mainly explore the influence of temperature change on the excitation effect and the compression strength of fly ash wet shotcrete.In this part of the study, the curing temperature, the amount of initiator and the amount of fly ash were taken as three variables, and the orthogonal test of three factors and three levels was carried out.The specific scheme was shown in table 4. In addition, during the whole test, the cement content was 310kg/m 3 and the water-cement ratio was 0.42.4, wet shotcrete samples are prepared.The specific process is as follows: First, waste rock, tailing sand, cement and fly ash are mixed and stirred evenly in proportion, and alkaline activator is dissolved in water and added into the material.Then, the material was stirred by a hand-held mixer for 3 minutes, and then the wet shotcrete sample was prepared by a square mold with length, width and height of 70.7mm.The cast mold is placed in the laboratory for 24h and then demoulded, and then the sample is cured in the curing box to the specified age, which is designed to be 1d, 3d, 7d and 28d curing ages.In addition, it should be noted that when the first part of the test was carried out, the temperature of the curing box was designed to be 5℃, and the best type and dosage of stimulator was obtained by analyzing the test results.When the samples reached the designed curing age, WEW-600D press was used to test the compressive strength of the samples under different curing ages.The displacement control mode was adopted in the test loading, and the loading rate was 1.5mm/min.
The microstructure of the sample was studied by scanning electron microscopy (SEM), and the influence mechanism of curing temperature on mechanical properties of wet shotcrete was revealed.Equipment Model and parameters: JSM-6510A electron microscope, resolution of 6 nm, maximum acceleration voltage of 30 kV.

Rational Optimization of the Type and Dosage of Activator
By comparing the compressive strength of the sample with different activators, the best type and dosage of activator can be selected.Figure 2 shows the change characteristics of the compressive strength of the wet shotcrete samples doped with fly ash under different chemical activators with curing age.
It can be seen from figure 2 (a) that the compressive strength of the sample with NaAlO2 added is significantly higher than that of the sample without activator added at all ages, indicating that the incorporation of NaAlO2 can significantly improve the compressive strength of the sample.When the content of activator was 2%, the samples on 1d, 3d, 7d and 28d were increased by 61.9%, 89.7%, 135.7% and 54.8%, respectively, compared with the samples without activator.When the content of activator was 3%, the 1d, 3d, 7d and 28d samples increased by 71.4%, 110.2%, 189.3% and 63.7%, respectively, compared with the samples without activator.When the content of activator was 4%, the 1d, 3d, 7d and 28d samples increased by 76.2%, 128.2%, 210.7% and 79.1%, respectively, compared with the samples without activator.Therefore, when NaAlO2 is used as the activator, the optimal addition amount is 4%, and the compressive strength of the sample increases the most.
It can be seen from figure 2 (b) that the compressive strength of the sample with NaOH added is significantly higher than that of the sample without activator added at all ages, indicating that the incorporation of NaOH can also significantly improve the compressive strength of the sample.When the content of activator was 2%, the 1d, 3d, 7d and 28d samples increased by 47.6%, 61.5%, 85.7% and 41.9%, respectively, compared with the samples without activator.When the content of activator was 3%, the 1d, 3d, 7d and 28d samples increased by 70.4%, 107.7%, 123.2% and 55.6%, respectively, compared with the samples without activator.When the content of activator was 4%, the samples of 1d, 3d, 7d and 28d increased by 57.1%, 71.8%, 108.9% and 45.9%, respectively, compared with the samples without activator.Therefore, when NaOH is used as the activator, the optimal addition amount is 3%, and the compressive strength of the sample increases the most.
As can be seen from figure 2 (c), the compressive strength of the sample with Na2SO4 added at each age is also significantly higher than that of the sample without activator added, indicating that the addition of Na2SO4 can also significantly improve the compressive strength of the sample.When the content of activator was 2%, the 1d, 3d, 7d and 28d samples increased by 4.80%, 15.4%, 12.5% and 14.5%, respectively, compared with the samples without activator.When the content of activator was 3%, the 1d, 3d, 7d and 28d samples increased by 47.6%, 69.2%, 62.5% and 62.1%, respectively, compared with the samples without activator.When the content of activator was 4%, the samples of 1d, 3d, 7d and 28d increased by 42.9%, 51.3%, 30.4% and 41.3%, respectively, compared with the samples without activator.Therefore, when Na2SO4 is used as the activator, the optimal addition amount is 3%, and the compressive strength of the sample increases the most.
According to figure 2 (d), the compressive strength of samples with NaCl added is also significantly higher than that of samples without activator added at all ages, indicating that the addition of NaCl can also significantly improve the compressive strength of samples.When the content of activator was 2%, the 1d, 3d, 7d and 28d samples increased by 4.79%, 5.12%, 10.7% and 13.7%, respectively, compared with the samples without activator.When the content of activator was 3%, the 1d, 3d, 7d and 28d samples increased by 9.5%, 12.8%, 17.8% and 5.6%, respectively, compared with the samples without activator.When the content of activator was 0.4%, the samples of 1d, 3d, 7d and 28d increased by 19.1%, 38.4%, 16.1% and 3.2%, respectively, compared with the samples without activator.Therefore, when NaCl is used as the activator, the optimal addition amount is 4%, and the increase of compressive strength in the early stage of the sample is the highest, but the increase of compressive strength is small.Based on the above analysis, it can be seen that when the dosage range of activator is 0%~4%, the optimal dosage range corresponding to different activator types is different, that is, the optimal dosage of NaAlO2, NaOH, Na2SO4 and NaCl is 4%, 4%, 3% and 3%, respectively, and the strength increase of the sample added with NaAlO2 is the most obvious.Therefore, NaAlO2 can be selected as the activator to study the mechanical properties of fly ash wet shotcrete at low temperature.

Experimental Result
Table 5 shows the compressive strength of wet shotcrete samples doped with fly ash at different curing ages under low temperature curing environment.In addition, in order to investigate the degree of influence of initiator content (A), fly ash content (B) and curing temperature (C) on the compressive strength of fly ash doped wet shotcrete samples and find out the corresponding significant influencing factors, range and variance analysis were conducted for the compressive strength of each age in table 5

Growth Law of Compressive Strength of Specimens under Low Temperature Curing
Figure 3 describes the change of compressive strength of wet shotcrete samples doped with fly ash under low temperature curing with curing age.As can be seen from figure 3, with the extension of curing age, the compressive strength of the sample shows an increasing trend, indicating that even under low temperature curing environment, the increase of curing age can significantly improve the compressive strength of the sample, especially the early strength.However, from the overall change trend, when the curing age increased from 1d to 3d, the compressive strength of the sample increased significantly, and the change trend of strength was basically the same.When the curing period increased from 3d to 7d, the compressive strength of the sample further increased, and its increase rate was significantly higher than that from 1d to 3d.When the curing age increased from 7 days to 28 days, the compressive strength increased with the increase, but the increase rate decreased significantly, indicating that after the curing age exceeded 7 days, the hydration response of the cementified material under low temperature environment gradually became complete, and the increase rate of compressive strength gradually decreased, and finally became stable.

Variation Characteristics of Compressive Strength of Specimens under Low Temperature Curing With Influencing Factors
Figure 4 shows the variation characteristics of the compressive strength of the sample with the three factors at low temperature.It can be seen from figure 4 (a) that under different curing ages, the compressive strength of the sample shows an obvious increasing trend with the increase of the dosage of alkaline activator, and the slope of the compressive strength curves of 7d and 28d is significantly higher than that of the 1d and 3d compressive strength curves.It shows that the increase of alkali activator content is more conducive to improving the 7d and 28d compressive strength of fly ash wet shotcrete under low temperature environment.It can be seen from figure 4 (b) that under different curing ages, the compressive strength of the sample also shows a tendency to increase with the increase of fly ash content.However, when the curing ages are 1d and 3d, the compressive strength of the sample does not increase significantly; however, when the curing ages exceed 3d, the compressive strength shows a significant increase trend.When the curing age is 1d and 3d, the hydration reaction is in the early stage, the alkaline environment of the slurry is limited, PH value and hydroxide ion concentration are low, and the dissolution of silica-aluminum raw materials is limited, so the increase of fly ash content has no significant effect on the early strength.With the extension of curing time, hydration reaction is gradually enhanced, at this time, the PH value and hydroxide ion concentration of slurry increase, and the dissolution of silica-aluminum raw materials increases, resulting in a larger increase in compressive strength [17].It can be seen from figure 4 (c) that under different curing ages, the compressive strength of the sample also shows an increasing trend with the increase of temperature.However, when the curing ages are 1d and 3d, the increase of compressive strength is not significant; however, when the curing age exceeds 3d, the compressive strength shows an obvious increasing trend with the increase of temperature.

Sensitivity and Significance Analysis of Influencing Factors of Compressive Strength
By analyzing the range and variance of compressive strength in each age of the sample, the influence degree of the three factors on compressive strength and the corresponding significant influencing factors can be qualitatively analyzed.According to the range analysis in table 6, for the 1d compressive strength of the sample, the order of influence of the three factors on the 1d compressive strength is the content of initiator (1.82) > curing temperature (0.51) > the content of fly ash (0.41).As can be seen from the variance analysis results of 1d compressive strength in table 7, the significance level p value of initiator content is 0.042, lower than 0.05, indicating that initiator content is a significant influence factor on 1d compressive strength, while the significance levels of fly ash content and curing temperature are both higher than 0.05, indicating that these two factors are non-significant influence factors.
Within the parameters of the experimental design, the order of the influence of the three factors on the 3d and 7d compressive strength is also the content of activator > curing temperature > fly ash content.According to the variance analysis results of 3d and 7d compressive strength in table 7, the significance level p value of the initiator content is 0.040 and 0.041, respectively, both lower than 0.05, indicating that the initiator content is also a significant factor affecting the 3d and 7d compressive strength.As for the 28d compressive strength, the range analysis results show that the order of influence of the three factors is the content of activator (3.27) > curing temperature (2.18) > the content of fly ash (1.90).It can be seen from the variance analysis results of 28d compressive strength in table 7 that the p values of significance levels of activator content, curing temperature and fly ash content are 0.011, 0.013 and 0.025, respectively, which are all lower than 0.05, indicating that the three factors are significant influencing factors of 28d compressive strength.
Therefore, within the parameters of this test, the three factors have the highest degree of influence on compressive strength, followed by curing temperature and fly ash content, and the content of initiator is a significant factor affecting the early compressive strength (1d, 3d and 7d).

Regression Prediction Model of Compressive Strength of Specimens Under Low Temperature Curing
In order to quantitatively analyze the influence law of the amount of initiator, the amount of fly ash and the curing temperature on the compressive strength, the regression prediction model of compressive strength considering the interactive influence of multiple factors was independently constructed.The model expression was shown as follows: In the above formula: y is the compressive strength, MPa; x is the content of activator, %; x is the fly ash content, %; Combined with the compressive strength data measured by the test, matlab was used to carry out nonlinear regression fitting to the data, and the corresponding model coefficient was obtained, and then the prediction model of the compressive strength of wet shotcrete mixed with fly ash under low temperature curing was established, as shown in formula (2) ~ (5).
Prediction model of compressive strength of sample 1d: Prediction model of 7d compressive strength of sample: Prediction model of 28d compressive strength of sample: By constructing the prediction model of the compressive strength of the wet shotcrete doped with fly ash at different curing ages, the ratio parameters in table 5 were substituted into formulas (2) to (5) for calculation, and the measured and predicted values of the compressive strength were obtained, as shown in figure 5.It can be seen from figure 5 that the measured value of compressive strength has the same change characteristics as the predicted value, and the maximum errors of 1d, 3d, 7d and 28d compressive strength are 6.1%, 8.9%, 8.2% and 6.7% respectively.It can be seen that the maximum errors are all within 10%.It shows that the strength model can accurately predict the compressive strength of wet shotcrete doped with fly ash at low temperature, and can provide guidance for the design of field parameters.

Influence of Interaction Between Activator and Curing Temperature on Compressive Strength of Sample
Combined with the range analysis results, it can be seen that the three factors have the highest degree of influence on the compressive strength of the sample, followed by the curing temperature, and the least degree of influence on the content of fly ash.Therefore, the interaction between the activator and the curing temperature on the compressive strength of the sample is further explored.Combined with the prediction model of compressive strength, a 3D visualization model of compressive strength was established to reveal the interaction between the activator content and curing temperature on the strength.
Figure 6 shows the influence of the interaction between the content of activator and curing temperature on compressive strength when the content of fly ash is 10%.According to the analysis of figure 6 (a), when the curing temperature is 3℃, the 1d compressive strength of the sample increases by 52.1% as the activator increases from 2% to 4%, while when the curing temperature is 9℃, the 1d compressive strength of the sample increases by 24.6%.According to the analysis of figure 6 (b), when the curing temperature is 3℃, the 3d compressive strength of the sample increases by 61.7% as the activator increases from 2% to 4%, while when the curing temperature is 9℃, the 3d compressive strength of the sample increases by 35.2%.The analysis of figrue 6 (c) shows that when the curing temperature is 3℃, the 7d compressive strength of the sample increases by 55.8% as the activator increases from 2% to 4%, while when the curing temperature is 9℃, the 7d compressive strength of the sample increases by 24.6%.The analysis of figure 6 (d) shows that when the curing temperature is 3℃, the 28d compressive strength of the sample increases by 55.8% as the activator increases from 2% to 4%, while when the curing temperature is 9℃, the 28d compressive strength of the sample increases by 24.6%.
Therefore, the sensitivity of the compressive strength of the sample to the amount of activator decreases gradually with the increase of curing temperature, indicating that the lower the curing temperature, the more significant the effect of the increase of the amount of activator on the compressive strength of the sample, which reflects that in the preparation of fly ash wet shotcrete in mines in cold regions, the mechanical properties of the concrete can be enhanced by adding an appropriate amount of activator.

Microstructure Characteristics of Wet Shotcrete with Fly Ash
Under different influence factors, there are significant differences in the macroscopic mechanical properties of the samples, which is essentially the result of the differences in the internal microstructure of the samples.Therefore, taking the sample with a curing age of 28d as an example, the change characteristics of the microstructure of the sample under different curing temperatures and the dosage of activator were analyzed, and the influence mechanism of the two factors on its mechanical properties was revealed.
Figure 7 shows the microstructure change characteristics of samples with different activator content when curing temperature is 3℃ and fly ash content is 10%.As can be seen from figure 7, there are significant differences in the microstructure of samples under different dosage of activator, indicating that the change of the dosage of activator can have a significant impact on the microstructure.When the content of activator is 2%, the hydrated products such as C-S-H gel and calcium hydroxide crystal are generated in the sample, which makes the aggregate particles bond with each other to form a skeleton structure, but there is a very obvious void structure in the sample, and the microstructure is poor.When the content of activator is 3%, a large number of hydration products are produced inside the sample to adhere to the particle surface, and there is an obvious void structure inside the sample, but compared with the sample with the content of activator is 2%, the density of the microstructure is improved.When the content of activator is 4%, a large number of hydration products such as C-S-H gel and calcium hydroxide crystal are also produced in the sample, and there is no obvious void structure in the sample compared with the sample with the content of activator is 2% and 3%.It shows that the increase of the content of activator can improve the density of the microstructure of the sample, and form a more stable skeleton support system inside the sample, so as to improve the macroscopic mechanical properties of the sample.Figure 8 shows the microstructure change characteristics of samples at different curing temperatures when the content of activator is 2% and the content of fly ash is 10%.As can be seen from figure 8, there are obvious differences in the internal microstructure of the sample under different curing temperatures, indicating that the change of curing temperature can also have a significant impact on the microstructure of the sample.When the curing temperature was 3℃, hydrated products such as C-S-H gel and calcium hydroxide crystal were formed in the sample, which enabled the aggregate particles to bond with each other to form a skeleton structure, but there were very obvious void structures in the sample, and the microstructure was poor in density.When the curing temperature is 6℃, a large number of hydration products are also produced inside the sample to adhere to the particle surface, and there is an obvious void structure inside the sample, but compared with the curing temperature of 3℃, the density of the microstructure is improved.When the curing temperature is 9℃, a certain void structure is generated in the inner pages of the samples, but compared with the samples at the curing temperature of 3℃ and 6℃, the scale of the void structure is significantly lower, indicating that the increase of curing temperature can improve the density of the microstructure of the samples, so as to form a more stable skeleton support system inside the samples, and thus improve the macro-mechanical properties of the samples.

Conclusion
Based on laboratory tests, this paper carried out experimental research on the mechanical properties of alkali excited fly ash wet shotcrete under low temperature curing environment.On the basis of obtaining the best type of activator, it systematically analyzed the law of the influence of activator content, curing temperature and fly ash content on the compressive strength of fly ash wet shotcrete, and built a strength prediction model.The main conclusions are as follows: (1) The addition of NaAlO2, NaOH, Na2SO4 and NaCl can effectively improve the compressive strength of fly ash wet shotcrete, but the addition of NaAlO2 increases the strength most significantly.Therefore, NaAlO2 can be selected as an activator to improve the mechanical properties of wet shotcrete under low temperature environment.(2) Under low temperature curing environment, the increase of curing age can also significantly improve the compressive strength of the sample, especially the early strength.However, after the curing age exceeds 7 days, the increase of compressive strength gradually decreases and finally becomes stable.
(3) Combined with range and variance analysis, it can be seen that the three factors have the highest degree of influence on compressive strength, followed by curing temperature, and the least degree of influence on fly ash content.Moreover, the content of initiator is a significant factor affecting the early compressive strength (1d, 3d and 7d).
(4) A prediction model of the compressive strength of wet shotcrete doped with fly ash under low temperature curing environment is established, and the model can accurately predict the compressive strength of wet shotcrete doped with fly ash under low temperature environment.In addition, the sensitivity of the sample's compressive strength to the amount of activator decreased with the increase of curing temperature, indicating that the lower the curing temperature, the more significant the effect of increasing the amount of activator on the compressive strength of the sample.
(5) The change of initiator content and curing temperature can have a significant impact on the microstructure of the sample.Specifically, with the increase of initiator content and curing temperature, the size of the void structure inside the sample gradually decreases, which gradually increases the density of the microstructure, and thus improves the macroscopic compressive strength of the sample.

Figure 1 .
Figure 1.Characteristics curve negative cumulative distribution of aggregate particles.

Figure 2 .
Figure 2. Compressive strength of samples with curing age under different chemical activators.

Figure 3 .
Figure 3. Variation characteristics of compressive strength of samples under low temperature curing with curing age.

Figure 4 .
Figure 4. Variation characteristics of compressive strength of samples with three factors at low temperature.

3 x
is the curing temperature, ℃; 0 b is the model regression coefficient.

Figure 5 .
Figure 5. Evolution curves of measured and predicted compressive strength under different curing ages.

Figure 6 .
Figure 6.Influence of interaction between activator and curing temperature on compressive strength.

Figure 7 .
Figure 7. Microstructure characteristics of samples with different dosage of alkaline activator.

Figure 8 .
Figure 8. Microstructure characteristics of samples at different curing temperatures.

Table 3 .
Activation test scheme of fly ash under different activators.

Table 4 .
Factors and levels of orthogonal test scheme.

Table 5 .
. The results are shown in table 6 and table 7. Compressive strength of fly ash mixed wet shotcrete under low temperature curing.

Table 6 .
Range analysis of compressive strength of samples under low temperature curing.Ki is the average value of the test results for each factor at Level i, and R is the range value. Note:

Table 7 .
Variance analysis of compressive strength of samples under low temperature curing.