Role of the silica source on the geopolymerization rate: A thermal analysis study

Increasing the amount of amorphous silica in a mixture containing silica and quartz favors a polycondensation reaction (i.e., geopolymerization) and improves the mechanical properties of the synthesized materials. The study aimed to investigate the polycondensation reaction during the consolidation step of geopolymer formation and examine the various equilibriums at different temperatures. In total, eleven compositions with various amounts of amorphous silica S (high reactivity) and quartz Q (low reactivity) (from 100%Q to 100%S) were synthesized in basic media with metakaolin. The synthesized samples were characterized by thermal analyses and mercury porosimetry. Correlations between the loss of water and the molar ratio of each composition were investigated. The existence of four reactions during the consolidation process was demonstrated: (i) the reorganization of the species; (ii) the dissolution of the metakaolin; (iii) the formation of oligomers; and (iv) the reaction of polycondensation. Moreover, two types of networks were shown, a silicate solution network for quartz-rich samples and a geopolymeric network for amorphous silica-rich samples. The nature of the primary network and the reactivity of the synthesized sample depend on the reactivity of the silica source used.