Role of silica fume in the microstructure and mechanical properties of calcium hydroxide (CH)-activated cementless concrete

ABSTRACT Ordinary Portland cement (OPC) is a major source of embodied CO 2 in concrete, while sodium-activated cementless systems often suffer from rapid setting that limits constructability. To address these issues, this study investigates calcium hydroxide (CH)-activated cementless concrete, examining the role of silica fume and curing regime in governing microstructure and mechanical performance. OPC was fully replaced with ground granulated blast furnace slag (GGBFS), with CH fixed at 7 wt.% and silica fume incorporated at 0–30 wt.%. CH activation effectively mitigated rapid setting, with initial setting times of 16.0–26.2 h. Steam curing accelerated alkali activation and CH consumption, while silica fume enhanced microstructural refinement through combined filler and pozzolanic effects. Under steam curing, strength and energy absorption increased progressively with silica fume content. Despite a slight rise in total cost, normalized cost and material-based CO 2 emissions decreased, indicating the potential of this CH-activated system as a low-carbon alternative.