In order to further research the influence of the microstructure properties of the SCSGP on the correlation between each independent influence factor and the macroscopic / microstructure characteristics. In this study, based on macroscopic / microstructure analysis, the SCSGPs were divided into workability, porosity, MIP and SEM results. And establish the relationship between each macroscopic / microstructure analysis and the 28 - days compressive strength, and drew a regression curve for comparison. First, the differences mechanical parameters (i.e., workability and porosity) of SCSGPs were discussed. Then through the MIP and SEM analysis to determine the relationship between the mechanical properties and microstructure characteristics of SCSGPs.
4.1 Workability
Panizza et al. indicated that the workability and compressive strength can be reasonably described by a power function [32]. Fig. 3 compared the correlation between compressive strength and workability at curing times of 1 and 28 days. Workability were 90 – 645 min, and when the curing time of 1 day and 28 days, the compressive strength were 2.33 – 51.17 MPa and 3.52 – 66.08 MPa, respectively. The results showed that the relationship between workability and compressive strength was nonlinear. It can be seen from Fig. 3(a) that when the SCS replacement levels increased from 0% to 40%, the r2 value of the regression equation was increased from 0.14 to 0.87, and the correlation between its workability and compressive strength gradually increases. It shows that as the amount of SCS replacement levels increases, the impact of compressive strength was increases on workability. And under the same compressive strength, the setting time of the SCSGP increased with the amount of SCS replacement levels increased. In addition, when the curing times of 28 days and the SCS replacement levels of 40%, the r2 value was reduced to 0.81, as shown in Fig. 3(b). It is mainly due to the presence of too much SCS content, which reduced the solubility and hindered the geopolymerization. The unreacted particles of MK and SCS may exist in the structure of the SCSGP, and structural defect factors that affect the final mechanical properties [33]. Therefore, the correlation between the workability and the compressive strength of the SCSGP was gradually decreases. The relationship between the workability and compressive strength was conformed to the power function regression equation (2), where y was the final setting time (min), x was the compressive strength (MPa); a and b were two regression coefficients.
According to the equation (2), the power function regression analysis results between workability and compressive strength were summarized in Table 7. When the curing time of 1 day, the results showed that it can be observed the coefficient b of the SCS replacement levels of 20% was the highest, indicating the compressive strength has the greatest impact on workability. Due to the synergistic effect between MK and SCS, the coefficient b of the power function regression analysis result of the SCS replacement levels of 20% was the most influential; when the curing time was increased to 28 days, which can be observed that the coefficient b of the SCS replacement levels of 30% to 40% were the highest. Because the more SCS content will reduce the solubility, the unreacted particles of MK and SCS may exist in the structure of the SCSGP, and hindering the synergistic effect between MK and SCS [34], leading to structural defects, which in turn affects the final mechanical strength [33].
4.2 Porosity
Fig. 4 compares the correlation between the porosity and compressive strength. When the curing time of 1 day, the porosity of SCSGPs were 48.93–69.47%, and the compressive strength of SCSGPs were 2.33–51.17 MPa; when the curing time of 28 days, the porosity of SCSGPs were 45.58–67.55%, and the compressive strength of SCSGPs were 3.52–66.08 MPa. Fig. 4(a) showed that the porosity and compressive strength had a negative correlation. Keke et al. research showed that geopolymers had the lower porosity and denser structure, and the results of the pore structure was consistent with the change in the compressive strength [35]. At the same compressive strength, when the SCS replacement levels increased from 0% to 20%, the r2 value of the regression equation increases from 0.75 to 0.83, and the linear regression had a trend of high accuracy. According to Asrani et al., which indicated that the linear regression has a high correlation accuracy and a good correlation [36], so that the compressive strength has a good correlation with the porosity. When the curing time of 28 days and the SCS replacement levels of 40%, the r2 value of the regression equation increased from 0.66 to 0.73, which showed that the correlation increased with the curing time, as shown in Fig. 4(b). When the amount of SCS replacement levels exceeds 20%, the SCS will quickly form oligomers and precipitate on the surface of the raw material particles, thereby hindering the content of MK and SCS dissolved during the equilibrium period, resulting in the reducing of the geopolymerization rate after the condensation point [34]. Therefore, when the SCS replacement levels of 40%, the correlation of linear regression analysis between the porosity and the compressive strength was increased. The relationship between the porosity and compressive strength was conformed to the linear regression equation (3), where y was the porosity (%) and x was the compressive strength (MPa); a and b were the two regression coefficients.
According to the equation (3), the linear regression analysis results between porosity and compressive strength were summarized in Table 8. When the curing time of 1 day and the SCS replacement levels of 30%, which showed that the coefficient was the highest (- 0.33), indicating that the compressive strength has the greatest impact on porosity [37]. When the coefficient value was higher, which indicated that the two parameters had a great influence on each other [32; 38]. From the results of the above linear regression analysis, it can be known that the synergy effect of MK and SCS has an influence interval of 20% SCS replacement levels. When the curing time was increased to 28 days, which observed that the coefficient a of the SCS replacement levels of 30% was increased to -0.26. Due to the curing time increases to 28 days, which showed that its excessive SCS content will have a negative impact on the strength, result in the porosity increasing, and the linear regression analysis coefficient of a will increased. The research results were consistent with the Zhang et al. research [33].
4.3 MIP analysis
The pore distribution of five types SCSGPs (mixtures No. 26 – 30) were shown in Fig. 5. It can be seen from the figure, which showed that the large pores gradually move to the small pores with the curing time increased, and the pore distribution was changed by the reaction products gradually filling the large pores [39]. When the curing time of 28 days, it can be seen that the pores were mainly distributed at 10-20 nm, as shown in Fig. 5(b). Fig. 6 compares the correlation between the Si / Al ratio and the pore volume. The results showed that the Si / Al ratio and the pore volume were a positive correlation. Hu et al. research showed that the high Si / Al molar ratio was promoted the dissolution and polycondensation of aluminosilicate [34]. When the curing of 1 day and the SCS replacement levels increases from 0% to 10%, the r2 value of the regression equation increases from 0.40 to 0.70, this is due to the synergistic effect between MK and SCS, which makes the high degree of reaction of geopolymerization, forms more reaction products, and increasing the pore volume. When the curing age of 28 days and the SCS replacement levels of 40%, the regression equation of r2 = 0.53 was the lowest, but when the Si / Al molar ratio was too high ( > 2.5), which will lead to decrease the degree of reaction and reaction products. Therefore, the results of this study were consistent with those researches of Fernandez-Jimenez et al. and He et al. [40, 41]. The relationship between the porosity and Si / Al molar ratio was conformed to the linear regression equation (3), where y was the porosity (%) and x was the Si / Al molar ratio; a and b were the two regression coefficients.
According to the equation (4), the linear regression analysis results between porosity and Si / Al molar ratio were summarized in Table 9. The results showed that when the curing time of 1 day, which can be observed that the SCS replacement levels increases from 10% to 20%, the coefficient was decreases from 28.98 to 13.67, indicating that the effect of Si / Al ratio on the pore volume gradually decreases. When the SCS replacement levels of 40% and the curing time of 28 days, which observed the coefficient was decreased to 3.62. Due to the influence of excessive SCS, which caused the reaction rate of SCSGP decreased [33]. Therefore, when the Si / Al was relatively high, the coefficient of a decreased with the influence of the parameters. The results of this study were consistent with He et al. research [41].
4.4 Microstructure analysis
The SEM images of three types of SCSGPs (mixtures No. 26, 27, and 30) were shown in Fig. 7 and Fig. 8. When the curing time of 1 day and the SCS replacement levels of 0%, the figure showed that the uniform plate particles was dispersed in the microstructure, which may be unreacted metakaolin (Fig. 7 (a)). According to Kljajević et al. research, the alkaline solution was the alkali activates the surface particles of metakaolin, which caused to dissolve and release Si4+ and Al3+ ions. Subsequently, the ions were participated in the geopolymerization reaction, resulting in the formation and growth of geopolymer gels [42]. Therefore, it can be clearly seen that the appearance of geopolymer gels and some unreacted metakaolin particles exist in the structure; when the curing age of 28 days, the N – A – S – H gels were gradually filling the pores between the geopolymers, which showed that the structure of the mixture No. 26 was relatively dense, and the compressive strength tends to increase. At the early curing time (1 day) and SCS replacement levels of 10%, the microstructure of the bonding area and the interface of SCSGP were denser (Fig. 7 (b)). Sun et al. indicated that the interface strength between binders were high, and it helps to increase the strength [43]. At the later curing age (28 days), it was observed that the mixture No. 27 sample was mainly filled with amorphous gel products (such as N – A – S – H gels), resulting in a uniform and dense appearance of structure.
Fig. 7 (c) and Fig. 8 (c) were the SEM microstructure of the mixture No. 30 samples at different curing times of 1 to 28 days. When the curing age of 1 day and the SCS replacement levels of 40%, the figure showed that the microstructure of pores and uniform plate particles were increased. Although, the MK and SCS had a synergistic effect, but the considerable amount of MK and SCS were still presented in the form of uniform plate particles (Fig. 7 (c)), and as structural defects exist in the geopolymer network structure, forming more macroporous structure. According to the observation results, when the curing age of 28 days, which observed that the strength of sample was reduced. Due to excessive SCS content, which will hinder the synergistic effect between MK and SCS [34], causing the aluminosilicate to precipitate and cover the surface of MK and SCS particles, thereby reducing the dissolution reaction activity, forming a loose structure with large pores and reducing its compressive strength development.