Synergistic enhancement of mechanical and durability properties in alkali-activated low-grade calcined clay pastes by slag addition

The recycling of low-grade waste clay into alkali-activated calcined clay (AACC) pastes represents a promising strategy for sustainable waste management. The synergistic effects of ground granulated blast-furnace slag (GGBS) on the properties of AACC pastes synthesised from low-grade calcined clay were investigated in this work. The influence of slag content (0–60%) on setting time, density, ultrasonic pulse velocity and early mechanical properties was systematically evaluated to identify an optimal mix formulation. The results showed that the incorporation of GGBS significantly shortened the setting times and enhanced early-age strength. The AACC pastes with 50% GGBS demonstrated optimal performance, combining rapid setting with high compressive strength. Long-term water immersion tests revealed excellent volume and strength stability; however, a notable reduction in deformation resistance was observed. Microstructural analysis using low-field nuclear magnetic resonance showed that prolonged water exposure increased overall porosity and promoted the conversion of gel and capillary pores into larger macropores, leading to a coarsened pore structure. These findings provide valuable insights into the durability and microstructural evolution of slag-enhanced AACC pastes, supporting their potential application as sustainable construction materials.