Alkali activation of fly ash: Effect of the SiO2/Na2O ratio: Part I: FTIR study

In this paper the main reaction products formed in alkali activated fly ash systems have been studied by using Infrared Spectroscopy. The main variables of work were: soluble silica content in the alkaline activating solution (0% SiO2; 1.62% SiO2; 5.40% SiO2 and 9.07% SiO2) and thermal curing time (8 h, 60 and 180 days). The results obtained have shown that the main reaction product formed in all cases was an alkaline aluminosilicate-type gel regarded to be a zeolite precursor. By means of the Infrared Spectroscopy it has been shown that there is an asymmetric stretching band (T–O), associated with that aluminosilicate-type gel, which can be interpreted in terms of that gel showing two different types of Si–O bonds: bridge and terminal bonds. The presence of soluble silica modifies reaction kinetics and prompts the formation of more Si-rich gels with a higher percentage of bridge bonds. The presence of soluble silica also affects the rate of crystallization and the type of zeolites formed (Na-Chabacite, zeolite Y, zeolite P). Finally when the thermal curing time increases, the gel undergoes polymerization and its structure becomes highly ordered (with a predominance of bridge over terminal bonds)