AN EXPERIMENTAL REVIEW OF THE REACTION PATHS FOLLOWED BY ALKALI-ACTIVATED SLAG CEMENTS

Solid-state NMR, XRD, mechanical strength measurements, quantification of bound water and isothermal calorimetry were combined to contrast the reaction paths followed by slag under the influence of three alkaline activators: NaOH, Na2CO3, and Na2Si1.7O4.4. NaOH-activation led to the concomitant formation of a very ordered C-A-S-H gel and of hydrotalcite, giving rise to early mechanical strength. For Na2Si1.7O4.4, a N-A-S-H gel formed first due to the high quantities of silicon in solution. This led to quick setting but no mechanical strength. Later, an amorphous C-A-S-H gel provided mechanical strength, while an aluminate phase precipitated. Finally, Na2CO3-activation led also to the formation of N-A-S-H favored by the initial consumption of calcium to form calcite. This did not bring any real structuration and mechanical strength to the paste. Only after a few days of hydration did the mechanical strength improve with the precipitation of amorphous C-A-S-H, an ill-defined hydrated aluminate phase, and gaylussite.