The reaction of calcium during the formation of metakaolin phosphate geopolymer binder

This work reports the role of calcium during the acidic aluminum phosphate-activation of metakaolin (MK) and the characteristics of the resulting binder. Calcium hydroxide and calcium silicate have been used as substitutes for metakaolin at different percentages (0–9 wt%). Their dissolution behavior and the physical properties along with the chemistry of the binder were emphasized. The results show that the heat released during the dissolution of the calcium sources enhances the dissolution rate of MK. However, a replacement rate higher than 6 wt% hinders the subsequent dissolution of MK. The binder consists of a silico-aluminophosphate network. Sufficient calcium content modifies its composition to a hybrid network of calcium phosphate dispersed in aluminophosphate and silico-aluminophosphate networks. The replacement rate of up to 6 wt% by highly soluble calcium hydroxide was beneficial for improving the compressive strength at any age, while the sparsely soluble calcium silicate decreases the compressive strength whatever the replacement rate.