Microstructural Characteristics of Geopolymers Based on Alkali-Activated Fly Ash

Aluminosilicate geopolymers with a high silica molar ratio have been prepared by mixing Croatian coal-ash (ASTM Class F) and sodium silicate solution (water glass, technical grade). The samples were cured at 85 degrees C for 8 h and 24 h in different experimental conditions. Fourier transform infrared spectroscopy (FTIR), X-ray diffraction (XRD) and scanning electron microscopy (SEM/EDS) have been used to identify the new phases formed in geopolymeric binders, and to follow the progress of their formation. It was observed that the glassy component of fly ash used reacted with water glass. The main reaction product formed in the geopolymeric materials was amorphous alkali aluminosilicate gel that induces a shift in the T-O (T=Si or Al) asymmetric stretching band towards lower frequency (1074.58 cm(-1)) in relation to the ones in the original fly ash. SEM/EDS detected a highly inhomogeneous glass-like matrix, constituted mainly of Na-Si-Al phases in a bulk region, together with unreacted spheres of fly ash particles.