Effect of change in the silica modulus of sodium silicate solution on the microstructure of fly ash geopolymers

Abstract

This study focuses on the impact of change in the silica modulus (SiO2/Na2O) (0.6–1.6) of the sodium silicate solution (SSS) on the microstructure, mechanical properties and thermal stability of fly ash geopolymer (FAGP). The variation in the compressive strength of FAGP was studied on increasing the temperature from 200 °C to 800 °C. On increasing the silica modulus (Ms) of the SSS from 0.8 to 1.6, the compressive strength of the FAGP mortars increases, but the FAGP mortar with Ms 1.2 experienced flash setting. The compressive strength does not show any uniform trend with increasing temperature. Among fabricated geopolymers, SSS with Ms 1.4 attained a strength of 28 MPa at 800 °C. The changes in Si and Al environment were studied using 29Si, 27Al magic angle spinning nuclear magnetic resonance (MAS-NMR) and Fourier transform infrared (FT-IR) frequency band of Si–O–T (T: Si/Al). The positive shift in 29Si NMR signal with higher Ms confirmed the formation of an Al-rich binding gel and the chemical shift at 56 ppm in 27Al NMR corresponds to a AlQ4(4Si) poly (sialate-siloxo)-type network... The structural changes arising from the integration of Al was further confirmed by shifting the Si–O-T asymmetric stretching vibration to lower wave number in FT-IR compared to raw fly ash. The microstructure was studied using scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDAX), X-ray diffraction (XRD) and thermal properties using thermal gravimetric analysis/differential thermal analysis (TGA/DTA.