Effect of NaOH molarity on the formation of hybrid cements from sulfate-bearing clay and Pietra Serena sludge

A sulfate-bearing kaolinitic clay and sludge deriving from Pietra Serena sandstone cultivation have been used to synthesize binary (80/20, 50/50) alkali cements. NaOH solutions 4,6 and 8 M were used as activators to determine the effect of alkalinity on the hydration products. The results show that mechanical strength increases linearly with the pH of the activator. Lower molarity favors the formation of ettringite and calcium-silicate-hydrate/calcium-aluminum–silicate-hydratecompounds; higher molarity favors monosulfate and cancrinite/sodalite zeolites. Both systems can retain sulfate. The occurrence of ettringite and monosulfate in the early stage of the reaction together with the cementing gel could be enough to enhance the strength with a milder alkaline activation.