Salt-frost scaling resistance characteristics of nano silica-modified recycled aggregate concrete

This paper investigated the effect of different nano-silica (NS) dosages (0, 1%, 2% and 3% of the cement mass) on the salt-frost scaling resistance of recycled aggregate concrete (RAC), aiming at enhancing the microstructures for improving the durability of RAC in cold region. In this experiment, the RCA replacement ratios was designed as 0%, 50%, and 100% (concrete was named R0, R50 and R100 respectively) by a mass substitution method, respectively. The multiscale modification mechanism of NS-modified RAC under salt frost action were investigated by means of macroscopic properties tests, X-ray micro computed tomography (X-CT) and scanning electron microscopy (SEM). The pore distribution and microtopography of mortar matrix and interfacial transition zones (ITZs) inside RAC was further analyzed for exploring the correlation between the macroscopic and microscopic properties of salt frost-damaged RAC. The results show that the addition of NS can effectively improve the compressive strength of RAC and inhibit the freeze-thaw damage deterioration of pore structures, and the salt-frost resistance of RAC was gradually enhanced with the increase of NS dosages at the duration of same freeze-thaw cycles. The total pore volume of NS-modified RAC specimens was significantly reduced, especially for the pores within size of 200μm. In addition, the addition of NS reduces the generation of microcracks to strengthen the microstructure of mortar matrix and multiple ITZ, and further improves the bonding properties between aggregate and the old mortar. After 100 freeze-thaw cycles, less small pores and narrower cracks appeared inside NS-modified RAC specimens. Generally, the appropriate NS dosages can compensate the initial defects of RCA to some extent, thus improving the salt-frost resistance of RAC. This study is of great significance in promoting the rational utilization of RAC in cold regions.