## 3 Results and Discussion
### Microstructure  
There was a direct and intimate connection found between the macroperformance and microstructure of the AASC in this study. Figure 9 shows an SEM image of an AASC with a silicate modulus of 1.2 and Na\({}_{2}\)O content of 8% (M1.2N8) after 28 days of curing at 1000 times magnification. As can be seen, a dense structure and uniform hydration products are obvious in the AASC. No obvious interfacial transition zone was found in the AASC; this is consistent with the strong interactions between the aggregate and paste, which play such an important role in providing compressive strength in the concrete [8]. The reaction products of the AASC were symmetrical, close-grained and cemented with the surrounding binding products, and the specimen did not exhibit massive Ca(OH)\({}_{2}\) formations like ordinary Portland cement concrete. Additionally, in the AASC, crystalloids could be observed to complete and link with each other. All of these qualities make for excellent integrated and close-grained properties in AASC, improving its mechanical properties.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Properties** & **Values** & **Mean Value** & **SD** & **CV** \\ \hline \multirow{4}{*}{Compressive strength (MPa)} & 17.5 & & & \\  & 15.4 & & & \\  & 18.7 & 17.2 & 1.27 & 0.074 \\  & 16.6 & & & \\  & 17.9 & & & \\ \hline \multirow{4}{*}{Flexural strength (MPa)} & 1.3 & & & \\  & 1.2 & & & \\ \cline{1-1}  & 1.1 & 1.2 & 0.10 & 0.083 \\  & 1.2 & & & \\  & 1.1 & & & \\ \hline \multirow{4}{*}{Water absorption (\%)} & 10.43 & & & \\  & 9.38 & 9.95 & 0.53 & 0.054 \\ \cline{1-1}  & 10.04 & & & \\ \hline Moisture content (\%) & 6.24 & 6.19 & 0.05 & 0.009 \\ \hline \hline \end{tabular} Note: SD is standard deviation and CV is coefficient of variation.  
\end{table}
Table 4: Mechanical and physical properties of AASCHBs cast from Mix M1.2N8.  
The EDS results of the AASC specimens are shown in Figure 10. The reaction products in the AASC showed amounts of Ca, Si, Na, Al, and O elements, and the Ca:Si and Ca:Na ratios were in the range of 0.64-0.83 and 0.56-0.66, respectively, which is much lower than the Ca:Si ratio of C-S-H in neat Portland cement pastes (between 1.2 and 2.3) [48] and within the Ca:Si ratio of C-S-H in alkali-activated slag (between 0.5 and 1.1) [49]. The main hydration products of AASC are C-S-H (or N-A-S-H) gel, alkaline aluminosilicate and zeolite mineral, and [SiO\({}_{4}\)]\({}^{4-}\) and [AlO\({}_{4}\)]\({}^{5-}\) tetrahedrons forming polymeric aluminum-oxygen-silicon network structure products synthesized by mutual oxygen, and they contain no Ca(OH)\({}_{2}\) at room temperature [1, 49, 50, 51, 52]. Close-grained hydration products without visible transition strips make the structure of AASC more compact, giving it excellent mechanical performance.  
The observed density of the pore structure of AASC provides a great deal of its strength. Previously, the authors of Reference [12] indicated that the pore size distribution of alkali-activated slag mortars included three ranges (<50 nm, 50-10,000 nm, >10,000 nm) within which the pore structure <50 nm played a significant role in the drying shrinkage: the higher the volume of pores <50 nm, the greater was the drying shrinkage resulting from the higher contracting stress influenced by the finer capillary pores [53].  
Figure 10: Energy-dispersive spectrometry (EDS) images of AASC with a silicate modulus of 1.2 and Na\({}_{2}\)O content of 8% after 28 days: (**a**) Spectrum 1; (**b**) Spectrum 2.  
Figure 9: Scanning electron microscope (SEM) image of AASC with a silicate modulus of 1.2 and Na\({}_{2}\)O content of 8% after 28 days.