Impact of hemp shiv content and binder type on the thermal and mechanical properties of geopolymer composites

This study investigates the impact of incorporating hemp shiv (HS) fibers into geopolymer composites formulated with metakaolin (MK), coal combustion fly ash (FA1), and biomass fly ash (FA2) on their thermal and mechanical properties. HS content was varied (0, 200, 250, 300, and 350 wt% of the binder) to assess its effect on dry density, porosity, thermal conductivity, and microstructure. Results show that increasing HS content significantly increased porosity, leading to reduced density and thermal conductivity across all binder types. FA2-based composites exhibited the highest porosity (68.6%) and the lowest thermal conductivity (0.068 W.m −1 .K −1 ) at 350 wt% HS, reinforcing their suitability for thermal insulation applications. SEM analysis revealed substantial interfacial voids and cracking, particularly in FA-based composites, contributing to enhanced insulation properties but reducing compressive strength. Incomplete geopolymerization was observed in high-fiber-content composites, likely due to the absorption of activator solution by HS fibers, which affected matrix cohesion. Despite these structural changes, all composites remained within NF EN 998–1 standards for rendering and plastering mortars, with FA2-based composites demonstrating the greatest potential for thermal insulation applications, contributing to sustainability in construction. • HS increase total porosity in geopolymer composites up to 68.6 %. • FA2-based composites achieve lowest thermal conductivity at 0.068 W.m −1 .K −1 . • MK composites exhibit higher compressive strength compared to FA1 and FA2 binders.