Unravelling the stability of solvated silicate species in sodium silicate activator solution of different silica modulus by FTIR and NMR studies and their chemical reactivity on metakaolin and fly ash geopolymerisation

A hybrid solution of sodium silicate and NaOH is often employed in the production of alkali-activated material. When the silica modulus (Ms) of activator solution (AS) is lower than 1.5, tend to crystallise when stored for a longer duration, but this range is capable of producing a strong and durable geopolymers. Considering the importance of stability, a comprehensive study was conducted to evaluate the physio-chemical and structural changes when the AS (Ms 0.8,1,1.1) were stored at ambient, low temperature as well freeze–thaw for short and long durations from 1 day to 2 years. FTIR data revealed the presence of silicate species as Q0, Q1, Q2 (three-membered ring) and Q3 (3,4 membered ring) in the frequency range of 900–1100 cm−1. The NMR signal of nuclei 29Si, which is found to be in a linear and cyclic form from the chemical shift of signals corresponding to Q0, Q1 and Q2C respectively at −71, −79 and −83 ppm and Q2, Q3c, Q3, Q4 at −86, −89, −97 and 103 ppm, corroborated with FTIR results. The chemical activity was tested on fly ash (FA) and metakaolin (MK) precursor independently and the strength of the matured sample does not differ much with the AS stored under different conditions, which implied that silicate groups are present initially in the solution with good connectivity favouring the release of adequate hydroxyl ions for the dissolution of the raw material. The presence of oligomers in the silicate solution may be an important factor influencing the reactivity.