Alkali activation of PFA:effect on strength development

The compressive strength development of mortars with 30%, 50% and 100% by mass replacement of Portland cement by pulverised fuel ash, activated by alkalis was studied. The considered activators were based on NaOH, Ca(OH) 2, and waterglass (2:1 SiO 2:Na 2O) in various concentrations. Two different temperature curing regimes, i.e. 20°C and 60°C water storage after moulding at 24 hours, were tested. 40mm cube strength was measured at 1, 3, 7, 28, and 91 days. The results show lower strength development rates than PC control and indeed literature references report paste strength development cured at temperatures typically between 65 and 85°C. The results underline the difficulty of alkali activating PFA at practical temperatures that could be used in precast concrete production.