Mechanical and Thermal Performance of LDPE-modified Fly Ash–GGBS Geopolymer Mortars for Sustainable Building Applications

Introduction Environmental problems are becoming increasingly prevalent in the construction industry, mainly due to the significant carbon emissions associated with Ordinary Portland Cement (OPC) production and the growing accumulation of non-biodegradable plastic waste. In this context, the present study focuses on the use of discarded Low-Density Polyethylene (LDPE) as a partial replacement for fine aggregate in geopolymer mortars. Methods Three distinct binder systems were considered: fly ash-based Geopolymer Mortar (GPM), blended Fly ash–GGBS Geopolymer Mortar (FG), and traditional OPC mortar, with LDPE replacement levels ranging from 0% to 50%. The experimental investigation was carried out to evaluate compressive strength, thermal properties including thermal conductivity and specific heat capacity, and dynamic thermal parameters such as time lag (φ) and decrement factor (f). Results The results showed that as the LDPE content increased, the thermal performance of all mixtures consistently improved, while the compressive strength gradually decreased. The fly ash–GGBS blended geopolymer mortar with 50% LDPE showed better thermal performance among all the mixtures, with a low thermal conductivity of 0.29 W/m·K, a decrement factor of 0.029, and a time lag of approximately 4.10 hours. Discussion The inclusion of LDPE improved internal porosity, which reduced thermal conductivity and enhanced dynamic thermal performance. This demonstrates the ability of fly ash–GGBS geopolymer mortar to maintain a balance between mechanical strength and thermal insulation. Conclusion These findings suggest that LDPE-incorporated geopolymer mortars, especially fly ash–GGBS-based systems, can serve as energy-efficient and sustainable alternatives for non-structural building envelope applications.