Residual mechanical properties of fiber-reinforced lightweight concrete with LECA and recycled porcelain ceramic aggregates after elevated temperature exposure

Exposure to elevated temperatures significantly deteriorates the mechanical performance of concrete, thereby limiting the application of lightweight structural systems under such conditions. This study develops a sustainable lightweight structural concrete incorporating lightweight expanded clay aggregate (LECA) as a full replacement for natural coarse aggregate, recycled porcelain ceramic waste as fine aggregate, and Portland cement or calcium aluminate cement (CAC), with and without hybrid polyolefin-glass fibers. The proposed material system was designed to reduce density and improve residual performance after thermal exposure, while evaluating whether Portland cement-based mixtures could achieve performance comparable to that of CAC-based concrete. A total of 150 specimens were prepared and exposed to temperatures of 25, 400, and 800 °C under a controlled heating regime. After cooling, compressive strength, flexural strength, water absorption, and weight loss were evaluated, while thermal conductivity was measured at ambient temperature. Microstructural evolution following heating was examined by scanning electron microscopy (SEM). At 800 °C, normal-weight concrete retained only 26% of its initial compressive strength, whereas the lightweight mixtures retained 44- 57%. The CAC-based mixture containing hybrid fibers exhibited the highest residual performance, with residual compressive and flexural strengths of 23.4 and 1.8 MPa, respectively. The developed lightweight concrete also achieved approximately 28% lower density and up to 89% lower ambient-temperature thermal conductivity than normal-weight concrete. Although water absorption and weight loss increased with temperature, SEM observations revealed differences in matrix integrity, porosity, and thermal cracking that were consistent with the measured residual mechanical performance.