Geopolymer Concrete for Sustainable Design and Construction: Age-Dependent Behavior, Microstructure, and Predictive Models for Strength and Elastic Modulus

This study investigated the age-dependent compressive strength (fc), static modulus of elasticity (Es), and microstructure of geopolymer concrete (GPC) incorporating varying proportions of fly ash (FA) and ground granulated blast furnace slag (GGBFS). GGBFS was introduced as a partial replacement for FA at levels of 0%, 20%, 40%, 60%, and 80%. Testing was conducted at six ages: 7, 28, 60, 90, 150, and 180 days. Additionally, an ordinary portland cement concrete (OPCC) mix with the same binder content was also studied for comparison. Experimental results showed that all GPC mixes exhibited consistent gains in fc and Es with age. The inclusion of GGBFS significantly enhanced early-age and long-term fc and Es. Optimal performance was observed in mixes with 40% and 60% GGBFS, which surpassed OPCC strength after 90 days. Microstructural analysis using scanning electron microscopy and energy dispersive spectroscopy revealed that increasing GGBFS content improved the matrix compactness by enhancing the formation of calcium-alumino-silicate-hydrate (C─ A─ S─ H) gel in synergy with sodium-alumino-silicate-hydrate (N─ A─ S─ H) gel, which resulted in superior mechanical properties. Additionally, new predictive models for fc and Es were developed, incorporating the effects of age and GGBFS content, and validated against experimental data and published literature. Further, the sustainability aspect of GPC in terms of CO2 reduction revealed that all GPC mixes demonstrated significantly better sustainability performance compared to OPCC. Overall, this study provides a comprehensive understanding of the mechanical and microstructural behavior of GPC with FA and GGBFS, offering valuable insights for its application in sustainable construction and guiding the design of optimized GPC mixes.