Durability of Concrete with Nanoparticles under the Action of Both Cl⁻ Penetration and Freeze–Thaw Cycles

In the northern region of China, freeze–thaw cycles and chloride ion (Cl⁻) penetration are the primary factors affecting the durability of marine concrete. To investigate the influence of nanoparticles on the Cl⁻ penetration resistance of marine concrete under the action of freeze–thaw cycles, nano-SiO₂ and nano-Fe₃O₄ were added to plain concrete in different amounts. The Cl⁻ penetration resistance of concrete under the action of freeze–thaw cycles was measured in a 5% NaCl solution. The relative dynamic modulus, mass loss rate, and Cl⁻ content at different depths in concrete were measured to evaluate the salt-scaling resistance. The test results show that under salt freeze–thaw cycles, the relative dynamic modulus, bound Cl⁻ content, and Cl⁻ binding capacity of concrete with nanoparticles were higher than those of plain concrete. However, the mass loss, total Cl⁻ content, and free Cl⁻ content in concrete with nanoparticles were lower than those of plain concrete. With increasing nanoparticle content, the relative dynamic modulus, bound Cl⁻ content, and Cl⁻ binding capacity of the concrete first increased and subsequently decreased. However, the mass loss rate, total Cl⁻ content, and free Cl⁻ content first decreased and subsequently increased. The optimum amount of nano-SiO₂ and nano-Fe₃O₄ in the concrete was 2%. The improvement effect of nano-SiO₂ on the salt-scaling resistance of concrete was superior to that of concrete with the same amount of nano-Fe₃O₄. The addition of nanoparticles can accelerate the formation of hydrated products, enhance the chemical binding capacity and physical adsorption capacity of hydrated products to Cl⁻, and reduce the free Cl⁻ content in concrete. This can improve the Cl⁻ penetration resistance of marine concrete under the action of freeze–thaw cycles. This study provides a reference for the durable design of marine concrete in the northern region.