Experimental and Thermodynamic Study of Alkali-Activated Waste Glass and Calcium Sulfoaluminate Cement Blends: Shrinkage, Efflorescence Potential, and Phase Assemblages

Abstract

Urban waste glass powder (GP) has been identified as the Si-rich (>70%) but Ca- and Al-deficient precursor material for use in alkali-activated materials (AAMs). To facilitate the recycling and utilization of GP, this study explored the possibility of using calcium sulfoaluminate cement (CSA) as both reactive alumina source and shrinkage-reducing agent to improve the strength and durability properties of alkali-activated waste GP. The experimental and thermodynamic simulation showed that 0%–50% of CSA substitution could contribute to the formation of calcium (sodium) aluminosilicate hydrate [C─ (N─ )A─ S─ H] and N─ A─ S─ H gels due to the release of Ca and Al from CSA. Nonetheless, more than 50% of CSA resulted in the change of reaction paths and the precipitation of AFt and AFm as the major phases, which reduced the porosity significantly. A higher CSA replacement ratio generally increased the early strength development rate, while the delayed strength gain could be observed due to the slower but progressive dissolution of GP. More importantly, CSA effectively reduced the overall shrinkage of hardened AAM pastes, and the mixtures with up to 50% of CSA had lower shrinkage than the reference ordinary portland cement sample. However, the GP-rich mixtures showed viscoelastic/viscoplastic response to the capillary pressure–induced stress upon drying, possibly due to the structural rearrangement of C─ (N─ )A─ S─ H and N─ A─ S─ H. It was also found that the moderate amount of CSA in mixtures could considerably reduce the leaching of free alkali and therefore lower the potential environmental impacts. This study can improve the limited understanding on the hydrate assemblages and properties of AAMs based on GP and CSA, and expand the toolkit of cementitious materials based on recycled urban waste glass.