Mechanical and microstructural behavior of colloidal nano-silica modified pumice lightweight concrete under freeze-thaw cycles

This study investigated the mechanical and microstructural behavior of colloidal nano-silica modified pumice lightweight concrete under freeze-thaw cycles. In the study, the control mixture (REF) and the mixture containing 5% nano-silica (NS5), which previously exhibited the most balanced performance under normal curing conditions, were comparatively evaluated. Freeze-thaw behavior was investigated using separate specimen series having the same mixture proportions. Compressive strength and ultrasonic pulse velocity (UPV) tests were performed to evaluate the mechanical performance, while microstructural changes were investigated through SEM, EDS, XRD, and FTIR analyses. The results showed that nano-silica contributed to matrix densification, promoted additional C-S-H formation, and improved the mechanical performance under normal curing conditions. However, after freeze-thaw cycles, microcrack development, UPV reduction, and strength loss were observed, particularly in the denser microstructures. The restricted dissipation of internal hydraulic pressure is considered a possible explanation for this behavior rather than a directly demonstrated mechanism. The combined evaluation of FTIR, XRD, SEM, and EDS results indicated that freeze-thaw exposure may cause partial structural disorder and microstructural deterioration within the dense C-S-H network.