Effect of endogenous ions and nanoparticles on the salt frost resistance and microstructure of SSC

To study the effects of nanoparticles, endogenous and exogenous ions on the durability of concrete during seawater corrosion and freeze-thaw cycles, this paper designed durability tests of ordinary Portland concrete (OPC) and seawater sea sand concrete (SSC) under the coupled action of salt-freezing cycles. The macroscopic results of the tests demonstrate that endogenous ions can decrease the durability of SSC under salt-freezing cycles and altered the shape and composition of the hydration products of SSC. Adding nanoparticles can mitigate the unfavorable effects of endogenous ions on the resistance of SSC to salt-freezing cycles and improve its durability. At N =125 cycles, the mass loss of SSC with the addition of 1% nano-CaCO 3 and 2% nano-SiO 2 is decreased by 29.5% and 33.6%, respectively, compared to SSC. Microscopic tests indicate that the hydration process of cement in SSC is impeded by endogenous ions, leading to a decrease in both the chain length and quantity of C-S-H gels. During salt-freezing cycles, endogenous ions further exacerbate the deterioration of the interface transition zone (ITZ) in SSC. However, the incorporation of nanoparticles enhances the pore structure of SSC, boosts the elastic modulus of ITZ, and increa ses the chain length and amount of C-S-H gels. Moreover, adding nanoparticles helps reduce the breakdown of C-S-H gels, AFt, and Friedel's salt, strengthening SSC's resistance to salt-freeze cycles.