Non-destructive evaluation of heat-damaged lightweight geopolymer concrete

• CaO addition reduced heat resistance, causing strength loss after 400°C exposure. • UPV and SRH correlated well with strength before heat, but not after 400°C. • UPV and SRH failed to assess damage in heat-exposed LWGC. • SEM and XRD revealed microstructural changes and mineral phase changes post-heating. Assessment of heat-damaged concrete is critical for ensuring the safety and durability of any structure after being subjected to a fire. However, destructive testing (DT) can be costly and risky, especially for damaged structures. The purpose of this study is to evaluate the effect of heat damage on the mechanical properties and microstructural characteristics of lightweight fly ash-based geopolymer concrete (LWGC). Fly ash-based geopolymer concrete was prepared and subjected to heat damage at 400°C for 2 hours. The effect of calcium oxide addition was also investigated. Schmidt rebound hammer (SRH) and ultrasonic pulse velocity (UPV) were employed as non-destructive tests. X-ray diffraction (XRD) technique was conducted on the geopolymer paste samples before and after exposure to determine their mineralogical composition. The Scanning Electron Microscope (SEM) observations were also investigated. Results showed that after exposure to 400°C, the 0% CaO mix exhibited a 77.6% increase in compressive strength at 28 days, attributed to the densification caused by the sintering effects of unreacted fly ash. In contrast, compressive strength decreased by 44% and 16.8% for mixes containing 2.5% and 5% CaO, respectively, due to microcracking degradation. Visible color change and surface cracking were observed after heating. Before heat exposure, UPV and SRH values correlated strongly with compressive strength (R² > 0.99 and R² > 0.96, respectively). After exposure to 400°C, both UPV and SRH values decreased significantly and lost their correlation with compressive strength, indicating deterioration of the internal microstructure and increased porosity. Overall, non-destructive tests were found to be insufficient for reliable strength assessment of LWGC after high-temperature exposure, highlighting the continued need for destructive testing in post-fire evaluations.