<p>This study investigates the impact of diss fibres on the mechanical and thermal properties of geopolymer mortar (GM) compared to Portland mortar (PM), addressing environmental concerns in construction materials. Geopolymers, synthesized from aluminosilicate materials and alkaline activators, offer a sustainable alternative to Portland cement with lower CO₂ emissions and enhanced durability. The experimental results reveal that incorporating diss fibres significantly affects both mechanical strength and thermal insulation characteristics. Compressive strength significantly decreased for both GM and PM with diss fibre addition. Flexural strength followed a similar trend, although GM demonstrated superior performance, with an average strength up to seven times higher than that of PM. Notably, the inclusion of diss fibres markedly improved thermal insulation properties, achieving a 64% reduction in thermal conductivity at a 30% fibre replacement level. PM provided slightly better insulation than GM at 20% and 25% diss fibre rates, but the difference became insignificant at 30%. Both mortar types displayed similar thermal diffusivities and effusivities, indicating enhanced thermal damping ability. According to RILEM classification, GM containing up to 30% diss fibres qualifies as a structural and insulating material, while PM achieves this classification only with a maximum fibre content of 20%. These findings suggest that diss fibre-reinforced geopolymer composites offer a promising sustainable alternative to traditional building materials, combining improved thermal properties with eco-friendly characteristics. The environmental impact assessment, that calculated the total CO₂ emissions, revealed that the geopolymer composite had a 22% reduction in CO₂ emissions compared to the traditional Portland cement composite.</p>