Environmental Impacts of Kapok–Cotton Seed Fibers and Waste Tire Rubber Powder-Incorporated Hybrid Mortar System: A Cradle-to-Gate Life Cycle Assessment

Owing to their well-established formulations and high performance, traditional cement-based materials are among the most widely used construction materials worldwide. However, their production generates substantial greenhouse gas emissions and relies heavily on the unsustainable consumption of limited natural resources. Increasingly stringent environmental regulations have therefore made reducing their environmental footprint a priority for both industrial sector and academic institutions. This study investigated the effects of incorporating kapok–cotton seed fibers (KCSFs) and waste tire rubber powder (WTRP) into conventional cement mortars on their mechanical properties, thermal conductivity, and life cycle environmental impacts. For this purpose, five mortar mixtures containing different content ratios of KCSFs and WTRP were prepared. Their mechanical properties, thermal conductivity, microstructure, mineralogical and pore characteristics were comprehensively evaluated, together with a comparative life cycle assessment (LCA). The results showed that all mixtures maintained 28-day compressive strengths above 40 MPa, while flexural strength increased by up to approximately 18% and thermal conductivity decreased by up to approximately 32% compared with the reference mortar. Moreover, the HybTP-0.5 mixture demonstrated an approximately 8.5% lower global warming potential (GWP) than the reference mortar. However, environmental trade-offs were observed, with the Hyb-0.5 mixture exhibiting increases of approximately 118% in eutrophication potential (EP) and 19% in terrestrial ecotoxicity (TE), while the corresponding increases for HybTP-0.5 were approximately 115% and 8%, respectively. These findings indicate that the environmental benefits varied across impact categories and mixture compositions. Overall, the findings indicate that the combined incorporation of KCSFs and WTRP improved flexural strength and reduced thermal conductivity while maintaining compressive strengths above 40 MPa, although the environmental outcomes varied across impact categories. These results highlight the potential of incorporating bio-based and waste-derived constituents into conventional cement mortars without requiring substantial modifications to established mixture-design and production practices.