Recycling of waste glass in cement mortars: Mechanical properties under high temperature loading

The disposal of large amount of waste glass at landfills has increasingly posed an environmental issue in Hong Kong. This study focused on recycling of waste glass cullet and powder into cement mortar products. The physical and chemical evolutions of cement pastes prepared with glass powder and fly ash subjected to elevated temperatures and loading were evaluated. The residual strength of the cement pastes was determined after cooling and the microstructure was analysed from mineralogy and morphology views. The improvement of the residual compressive strength was noticeable after 800°C exposure thanks to the incorporation of glass power in the cement pastes. The results showed that a new crystalline phase, devitrite (Na₂Ca₃Si₆O₁₆), was formed from the softened amorphous silica of the glass powder, which might contribute to improving the residual compressive strength. This was also confirmed by the observation that pores and micro cracks of the cement pastes were filled after 800°C exposure. By means of positive effect of the glass power in improving the high temperature resistance of cement-based materials, dry-mix cement mortars were designed with the glass powder as a replacement of cement and the glass cullet as a replacement of natural aggregates for testing the compressive strength at 800°C ("hot test"). The use of the glass powder and the glass aggregates in mortars could both improve the high temperature resistance. However, excessive use of glass materials softened the mortars, which could not sustain high temperature loadings. The contents of the glass powder and the glass aggregates should be cautiously controlled to balance the two opposing effects.