Modifications to reaction mechanisms, phase assemblages and mechanical properties of alkali-activated slags induced by gypsum addition

Alkali-activated slags (AASs) are often reported to exhibit fast settings, undermining their utilisation as sustainable binders from a technological point of view. Although gypsum was shown in previous studies to change the setting time of AASs, understanding the mechanism behind the changes in the reactivity and microstructure of the AASs in the presence of gypsum remains unclear which limits its utilisation in AASs. For the activator conditions used in this research (i.e., Na₂O/raw material = 4.5 % and SiO₂/Na₂O = 1.0), gypsum was observed to dissolve quickly in the system, promoting the precipitation of portlandite and thenardite phases which led to a reduction in the pH of the pore solution causing a slow dissolution of slags. Furthermore, gypsum addition increased the setting time and delayed the compressive strength development of the paste, accompanied by a reduction in the total measured heat. The description of the changes in phase assemblages and kinetic of the system provided here is fundamental for predicting the behaviour of gypsum in AASs and promoting the industrial wide-uptake of AAS technology.