Finally, Na2CO3-activation first led to the precipitation of a N-A-S-H gel with Si and Al in environments similar to those of Na2Si1.7O4.4-activated pastes, as shown by NMR spectra (Figure , 15, and 18). Nonetheless, the second phase that precipitated was not a C-A-S-H gel, as with the silicate activation, but calcite, as shown by the XRD results (Figure ). In fact, the CO3 2-anions brought about by the activator have a great affinity for Ca 2+ ions, as suggested by the low solubility of CaCO3 of only 14 mg/L. The calcium was thus consumed by the calcite phase and was first not available to form a C-A-S-H gel. The formation of these two phases, calcite and N-A-S-H, led to the first peak of heat flow, which appeared after a day of hydration (Figure ), and to low amounts of bound water (Figure ). However, no mechanical strength developed at that time (Figure ), which explains the variability and length of the measured setting times (see Figure in the Supplementary Materials).