Influence of Waste Glass Powder Addition on the Microstructure and Mechanical Properties of Autoclaved Building Materials

Lime quartz samples in which ground quartz sand was gradually substituted with waste glass powder (GP) were obtained under hydrothermal conditions to determine the influence of GP addition on the microstructure (observed by SEM), phase composition (analyzed by XRD), and compressive strength of autoclaved building materials. An additional series containing analytical grade NaOH and no GP was formed to evaluate the effect of sodium ions on tobermorite formation and its impact on the mechanical properties of the samples. GP addition hindered the formation of tobermorite during autoclaving. Instead, a higher amount of an amorphous and semi-crystalline C–S–H phase formed, leading to the densification of the composite matrix. Nevertheless, tobermorite-like structures were found during both XRD and SEM analyses, proving that the presence of small amounts of Al³⁺ ions allowed, to an extent, for the stabilization of the phase despite the high sodium content. The compressive strength values indicate that the presence of alkali in the system and the resulting formation of additional portions of C–S–H have a beneficial influence on the mechanical properties of autoclaved composites. However, the effect fades with increasing glass powder content which, together with a slight expansion of the samples, suggests that at high sand substitution levels, an alkali–silica reaction takes place.