Synthesis of Al-substituted 11Å tobermorite using waste glass cullet: A study on the microstructure

Abstract

Calcium silicate hydrate (CSH), the key by-product of Portland cement hydration, due to the structural resemblance, is mainly represented as a defect tobermorite analog. To provide an extensive overview on the impact of Al incorporation in CSH, synthesis of Al-substituted 11 Å tobermorite using waste glass cullet (WGC) was investigated and compared with an Al-free reference. The optimal parameters of 180 °C curing temperature, 5h curing time, 0.5 M of activator concentration, and Ca/Si of 0.83 were found via XRD to synthesize pure crystalline tobermorite. SEM micrographs also showed that Al incorporation changed the tobermorite morphology from platy-like shape to acicular crystals. The WGC synthesized tobermorite exhibited similar thermal behavior to reference tobermorite as observed via TGA/DSC such as the free water release at 55 °C, interlayer water release at 131 °C, Si–O–H bond cleavage at 680 °C, and recrystallization to wollastonite at 820 °C. 29Si and 27Al NMR analysis showed that Al substitution preferably occurred at bridging (Q2) and branching (Q3) positions. The results from this study could significantly contribute to the application of Al-based wastes in concrete and mortar construction to improve the microstructure of the hydration by-products.