An experimental review of the reaction paths followed by alkali-activated slag pastes

NMR, XRD, mechanical strength measurements, bound water quantification and isothermal calorimetry were combined to contrast the degree of reaction and the paths followed by slag under the influence of three activators: NaOH, Na₂CO₃, and Na₂Si_1.7O_4.4. 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 Na₂Si_1.7O_4.4, 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 also led to the formation of N-A-S-H, formation favored by the initial consumption of calcium to form calcite. This did not bring any real structuration and mechanical strength. 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 of gaylussite.