Reactivity and activation of waste glass in cementitious systems: A critical review of mechanisms, performance and environmental implications

Recycling waste glass offers a route for resource recovery and low-carbon construction, but its use in cementitious systems is limited by variable reactivity and unsatisfactory performance. This review examines glass composition, structure–reactivity relationships, reactivity tests, and activation strategies, and uses these findings to develop a semi-quantitative reaction-control framework. The framework uses two composition-based descriptors: a calculated non-bridging-oxygen-per-tetrahedron index (NBO/T*) to describe network depolymerisation, and a gel-stabilisation index (GSI = (Al + Ca)/Si) to describe the availability of Al and Ca for stable gel formation. Three regimes are identified: Si-rich/gel-limited, stable gel-forming, and modifier-/side-phase instability. The compiled alkali-activated/geopolymer data suggest an empirical gel-formation transition at GSI ≈ 0.45–0.60, whereas the destabilisation boundary remains system-dependent. This framework explains why strength activity index, dissolution tests and alkali-activation studies can give conflicting results. Activation can increase dissolution, but performance improves only when dissolved silicate species are stabilised into load-bearing gels. The review also shows that reactivity tests, including the rapid, relevant and reliable (R³) test, should be interpreted together with particle size, glass composition, and durability risks. Overall, waste-glass valorisation should be guided by compositional balance and gel-forming capacity, rather than by activation intensity alone.