Study on the Mechanical Properties and Chloride Ion Penetration Resistance of a PCCP Fine-Aggregate Concrete Protective Layer Modified with Nanosilica

Abstract Increasing the durability of the prestressed concrete cylinder pipe (PCCP) protective layer effectively protects prestressed steel wires from chloride corrosion. Appropriate nanosilica (NS) dosage strengthens the long-term properties of the fine-aggregate concrete applied as a protective layer. The study investigated the impact of varying amounts of NS on the mechanical performance and chloride penetration resistance of fine-aggregate concrete subjected to both standard and steam curing. The results indicate that NS significantly influences the strength of fine-aggregate concrete under both curing conditions, with an optimal dosage of 1%, showing a 31.28% enhancement. Using chloride ion diffusion tests and water absorption tests, the permeability of the protective layer was evaluated. As the NS content increased, permeability initially decreased and stabilized, with the chloride diffusion coefficient reducing by 18.03%–43.92%. Microscopic tests, including scanning electron microscopy (SEM), X-ray diffraction (XRD), mercury intrusion porosimetry (MIP), and thermogravimetric analysis (TG-DTG), indicated that NS promotes hydration products, effectively increasing the strength of fine-aggregate concrete. Additionally, under steam curing conditions, NS induces a cooperative effect, further enhancing strength. After adding 1% NS, the total porosity decreased by 17.63% and 16.99%, and the chloride diffusion coefficient decreased in proportion to porosity reduction. NS also promotes the consumption of calcium hydroxide (CH), generating more hydration products; under standard curing conditions, the CH content decreased from 8.16% to 7.35% after adding 1% NS, and under steam curing conditions, it decreased from 8.62% to 7.96%.