Precursor-dependent reactivity of construction flat glass waste in fly ash-based geopolymers binders: Mechanical, microstructural and thermal insights

This study investigates the precursor-dependent reactivity of construction flat glass waste as a sustainable silica source in fly ash-based geopolymer binders. Waste glass powder (WGP) was introduced at 0–30 wt% by replacing two Moroccan fly ashes (FA1 and FA2) to evaluate the influence of precursor characteristics on its reactivity. The highest compressive strength was achieved at 10 wt% WGP for both systems (20.64 MPa for FA1 and 21.16 MPa for FA2). Beyond this level, FA1-based geopolymers exhibited a significant decrease in strength, whereas FA2-based systems maintained stable mechanical performance up to 30 wt%. Microstructural and thermal analyses (XRD, FTIR, SEM-EDS, TGA/DTG, and DTA) confirmed geopolymer formation in both systems and revealed distinct interaction mechanisms. The results suggest that the higher reactivity of FA2 may be associated with its finer particle size and greater contribution of the amorphous phase, which appears to support enhanced dissolution of WGP and its potential incorporation into the geopolymer network, resulting in a more homogeneous matrix and improved thermal stability. In contrast, in FA1-based systems, WGP seems to act primarily as an inert filler at higher substitution levels. These findings demonstrate that the reactivity of construction flat glass waste in geopolymer binders is strongly governed by the physicochemical characteristics of the fly ash, highlighting the critical role of precursor selection in controlling geopolymerization mechanisms and optimizing sustainable cement-free materials.