## 3 Results and Discussion
### Reaction Products
The findings showed that in all the cases studied, the alkaline activation of carbonate-high clays with NaOH solutions entailed the co-precipitation of a mix of C-A-S-H- and (N,C)-A-S-H-like gels, which had a higher SiO\({}_{2}\) and a lower Al\({}_{2}\)O\({}_{3}\) content (i.e., a higher SiO\({}_{2}\)/Al\({}_{2}\)O\({}_{3}\) ratio) than other gels described in the literature [32]. Chemically speaking, these gels are more like the ones forming in blast furnace slag [47, 48] than in dehydroxylated kaolinite alkaline activation [14, 27]. Figure 6: Twenty-eight day SEM micrographs and EDX spectra for (**a**) 4 M, (**b**) 6 M (**b1**,**b2**) and (**c**) 8 M (**c1**,**c2**) NaOH-activated pastes. When the activator was 4 M NaOH and the time two days, although scattered, calcium clearly prevailed. The points with >60% CaO were associated with the Ca-high, plate-like particles visible in Figure 5a. Given their chemical composition and shape, those phases would appear to be consistent with an aluminium-enriched C-S-H such as Al-theormorite [32, 49, 50]. The clusters observed on the micrographs for the two days, 6 and 8 M NaOH-activated pastes (Figure 7a) were characterised by a non-uniform distribution of CaO, ranging from 25% to 60% and associated with the precipitation of a mix of an Al-enriched C-S-H gel (C-A-S-H) and a calcium-bearing N-A-S-H gel (N,C)-A-S-H). With time, higher activator alkalinity (especially visible for 8 M NaOH) induced (N,C)-A-S-H gel evolution toward a C-A-S-H gel. Earlier studies [32] showed that N-A-S-H gel may persist at lower pH (<12), whereas in the presence of calcium, ionic exchange prompts the formation of (N,C)-A-S-H gel, in which calcium may be taken up until it ultimately replaces all the sodium in the cementitious gel's three-dimensional structure. The 28 days, 8 M NaOH-activated MA157 paste exhibited a chemical composition primarily attributable to a C-A-S-H gel, although at 0.24 \(\pm\) 0.07 the Al\({}_{2}\)O\({}_{3}\)/SiO\({}_{2}\) molar ratio was lower than the 0.3 to 0.5 range normally observed for such gels [32]. That was due the higher SiO\({}_{2}\) than Al\({}_{2}\)O\({}_{3}\) content in the starting clay. In contrast, the matrix for that same paste exhibited a CaO/SiO\({}_{2}\) molar ratio of 0.64 \(\pm\) 0.17, consistent with the range suggested in the literature (0.6 \(<\) CaO/SiO\({}_{2}\)\(<\) 1) for C-A-S-H gels [32, 48]. The chemical composition of the 28 days, 6 M NaOH-activated pastes also moved toward higher CaO contents in the CaO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) ternary diagrams (Figure 7b) than the two days pastes (Figure 7a). In contrast, the matrices for the 28 days, 4 M NaOH-activated pastes were characterised by a lower CaO/SiO\({}_{2}\) ratio than the two days materials (Figure 7a,b) and a higher Na\({}_{2}\)O uptake in the gel as the reaction progressed (Figure 7d). Those findings may be attributed to the higher carbonation observed in the latter pastes. As the XRPD and FTIR data showed, the 28 days, 4 M NaOH-activated  
Figure 7: EDX data for the two and 28 days, 4, 6 and 8 M NaOH-activated pastes graphed on (**a**,**b**) CaO-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\); and (**c**,**d**) Na\({}_{2}\)O-Al\({}_{2}\)O\({}_{3}\)-SiO\({}_{2}\) ternary diagrams. pastes had a high calcite content, a result of the interaction between atmospheric CO\({}_{2}\) and the calcium ions in the material. Inasmuch as these pastes had no portlandite, the calcium was necessarily sourced from the C-S-H/C-A-S-H gel initially formed [46]. The mechanisms governing alkali-activated materials are presently under study by a number of authors, for many factors are involved, including type of starting materials [51], activator type [47], relative humidity [52] and CO\({}_{2}\) concentration [53]. The reasons underlying the higher carbonation in the 4 M than in the 6 M or 8 M NaOH-activated pastes are fairly complex. [PERSON] et al.