When slag pastes were activated with NaOH, very small quantities of N-A-S-H gel initially formed, as evidenced by the 27 Al NMR spectra (Figure ). Relatively quickly, after only a few hours of hydration, the heat (Figure ) produced by the sample increased as an ordered C-A-S-H gel and hydrotalcite precipitated. These events occurred simultaneously when the activator dosage was above 4.0 wt% of Na2O but when the activator dosage was reduced from 4.0 wt% of Na2O to 2.0 wt%, a division of the heat flow peak was observed. This is probably a sign of a separation of the formation of hydrotalcite and C-A-S-H into two steps. It could thus be interesting to further study these samples with NMR and XRD to verify this hypothesis. In any case, the formation of C-A-S-H was accompanied by a rapid increase of the mechanical strength and of the quantity of bound water (Figure and). These reactions also slowed down rapidly, as shown by the low increase of the total heat, of the mechanical strength, and of the quantity of bound water after two days. In the Na2Si1.7O4.4-activated pastes, a N-A-S-H amorphous gel was also the first phase to precipitate, but in larger proportions than with NaOH, as evidenced by the NMR results . This product usually forms in alkali-activated binders with low calcium contents, such as alkali-activated fly ash . The precipitation of the N-A-S-H gel, instead of a C-A-S-H, can be explained here by the very high concentration of silicon and sodium brought about by the activating solution, compared to the calcium concentration which only came from the slag dissolution. The formation of this first phase led to the first heat flow peak (Figure ) and to a quick setting (see Figure in the Supplementary Materials) but only small values of mechanical strength and bound water (Figures and).