## 3 Results and Discussion
### Reaction Products
The formation of that intermediate product would be explained by the synchronised dissolution of the aluminium and silicon ions in the alkaline medium in the initial stages. As the aluminium content was fairly low, upon its depletion the silicon would continue to react, affording the gel a higher proportion of silicon and shifting the FTIR band to higher values (1014 cm\({}^{-1}\)). The carbonate band on the spectra for the activated pastes were both shifted to slightly higher or lower wave numbers (1400 to 1500 cm\({}^{-1}\)) and somewhat more intense. A shoulder appearing at 875 cm\({}^{-1}\) confirmed the formation of the carbonates detected with XRD in the alkali-activated MA157 clay pastes. More specifically, vibration bands characteristic of calcite were observed at approximately 1432, 876 and 710 cm\({}^{-1}\)[42; 43] in the 4, 6 and 8 M NaOH-activated pastes, whereas the signal at approximately 1450 cm\({}^{-1}\) might be attributed to sodium carbonate [43]. The bands associated with  
Figure 4: FTIR spectra for anhydrous MA157 clay and (**a**) two days and (**b**) 28 days pastes alkali-activated with 4, 6 and 8 M NaOH. the presence of calcite were most intense on the spectrum for the 28 days sample activated with 4 M NaOH (Figure 4b), a finding consistent with the XPRD results (Figure 3). Figure 5 depicts the two days and Figure 6 the 28 days micrographs for the 4, 6 and 8 M NaOH-activated pastes. A cementitious matrix consisting primarily in Si, Al and Ca was identified in all. The EDX analyses showed that the proportions of these elements, calcium in particular, varied depending on activator concentration and curing time. That finding was associated with the possible formation of a mix of (N,C)-A-S-H/N-(C)-A-S-H gels. The formation of that mix of gels and the high early mechanical strength at ambient temperature were primarily the outcome of the presence of calcium in these dehydroxylated clays [32; 44; 45]. Activator alkalinity also affected the type and proportion of the C-A-S-H (Al-enriched C-S-H) and/or (N,C)-A-S-H (calcium-bearing N-A-S-H) gels formed, as well as the amount and composition of the secondary reaction products (such as carbonates or AFm phases) (Figure 3). As a rule the microstructure of the 2 days, 4 M NaOH-activated paste was less uniform than in the other materials (Figure 5a). The phases in the form of Ca-high plates in the Si, Al and Ca-containing cementitious matrix suggested the formation of an Al-tobermite-like compound. In the 28 days 4 M-activated paste, the matrix was less compact and more granular (Figure 6a). That observation was associated with the significant carbonation in the 4 M NaOH-activated pastes (as revealed by the XRD and FTIR results) that apparently induced decalcification of the C-A-S-H-like cementitious gel. The EDX analyses denoted a lower calcium content in the 28 days than in the two days gels. Similar decalcification was described by [PERSON] et al. [46] in a study of C-A-S-H gel carbonation in a study of the alkaline activation of blast furnace slag. At the two higher alkalinities, the microstructure was more uniform and compact. Nonetheless, the excess sodium from the higher concentration of the activating solution (especially in the 8 M NaOH-activated samples) induced calcium and sodium carbonate salt precipitation, as confirmed by the EDX analyses (Figure 5(c2)). The resulting calcite crystals were primarily located in, and filling, the matrix pores. Figure 5: Two day SEM micrographs and EDX spectra for (**a**) 4 M, (**b**) 6 M (**b1,b2**) and (**c**) 8 M (**c1,c2**) NaOH-activated pastes. The EDX-determined variability in gel composition was graphed on CaO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) and Na\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) ternary diagrams (Figure 7) to study the effect of activator concentration on the type or types of gels formed.