Evaluating Glass Wool Waste as a Supplementary Silica Source in Hybrid Metakaolin/Fly Ash-Based Alkali-Activated Binders: Mitigating Strength Regression

This research addresses the critical challenge of “strength regression” observed in alkali-activated binders synthesized from glass wool wastes. In our preliminary studies, while sodium-based activation provided impressive initial strength, the specimens suffered a systematic and significant decline in mechanical performance at 3, 7 and 28 days, exhibiting a 74.3% strength reduction down to 24.61 MPa. Investigative analysis revealed that this instability is closely correlated with the physical degradation and micro-cracking observed in SEM micrographs, which is consistent with the literature regarding high silica-to-alumina network imbalances. To resolve these structural flaws, the precursor blend was modified by incorporating Class F fly ash and metakaolin to rebalance the Si/Al ratio. The addition of these aluminosilicate sources facilitated the consumption of excess sodium ions through enhanced geopolymerization and provided a micro-filling effect that refined the pore structure. Our findings demonstrate that this optimization not only prevents the subsequent loss of strength but also ensures stable compressive strength development up to 28 days without subsequent regression, reaching an ultimate average strength of 110.81 MPa. This approach provides a viable pathway for transforming insulation glass wool waste into high-performance, durable construction materials.