Used Coffee Grounds as Fine Aggregate in Lightweight Geopolymer Composites: Influence on Engineering Properties and Microstructures

Abstract

Developing lightweight and sustainable construction materials is important for reducing the carbon footprint of the construction industry. This study investigated the feasibility of incorporating used coffee grounds (UCG) as a partial replacement for bottom ash (BA), as fine aggregates, in fly ash–ground granulated blast-furnace slag (FA–GGBFS) geopolymer composites. Six mixtures with UCG contents from 0 to 25% by volume were prepared while maintaining constant binder and activator ratios. The effects of UCG on flowability, fresh unit weight, apparent porosity, compressive strength, thermal conductivity, and microstructure were examined. Results showed that 5% UCG improved workability and achieved the highest compressive strength (39.4 MPa, about 15% higher than the control), while maintaining similar thermal conductivity (0.694 W/m·°C). At higher UCG levels (≥15%), porosity increased, and compressive strength decreased markedly, whereas thermal insulation improved, with conductivity decreasing to 0.394 W/m·°C (estimated based on density). SEM observations indicated a compact matrix and dense interfacial transition zone at low UCG contents, while higher replacement levels produced a more porous and discontinuous microstructure. Within the investigated range, 5% UCG yielded the most balanced overall mechanical performance, whereas replacement levels up to 10% provided a practical compromise between weight reduction and thermal insulation, despite a measurable strength penalty relative to the control.