Alkali-activated geopolymer hempcrete: thermal and mechanical performance with energy and environmental assessment

Hempcrete is widely recognised as a sustainable building material; however, its application is often constrained by low mechanical strength and limited thermal energy efficiency. This study investigates the use of alkali-activated geopolymer binders based on fly ash and metakaolin to enhance the mechanical, thermal, and environmental performance of hempcrete. Geopolymer hempcrete composites were produced using sodium hydroxide and sodium silicate activators and characterised for physical, mechanical, thermal, and microstructural properties. X-ray diffraction confirmed the formation of amorphous aluminosilicate gel phases, with metakaolin-based systems exhibiting a higher degree of geopolymerization. Compressive strength reached up to 3.8 MPa, satisfying ASTM requirements for non-load-bearing masonry units. Thermal characterisation revealed low dry-state thermal conductivity and high thermal energy storage capacity (up to 5.5 MJ/m2), with approximately 50% of stored energy accumulated within the first 6 h and only 4–16% energy loss after 24 h. Moisture was identified as the dominant factor affecting thermal transport. Energy calculations for an assumed 0.2 m thick wall suggest that geopolymer hempcrete may contribute to reduced heating demand and associated CO2 emissions under cold-climate conditions, although the analysis is based on simplified wall-scale modelling. Overall, geopolymer hempcrete demonstrates strong potential as a multifunctional, low-carbon building envelope material.