To address the engineering risk of steel wire corrosion induced by chloride salts in Prestressed Concrete Cylinder Pipe (PCCP) in saline soil environments, which readily triggers wire breakage and pipe bursting, improving the chloride erosion resistance of PCCP protective layer mortar is of critical engineering value. Aiming at the prominent performance gap between laboratory-fabricated specimens and on-site roller-sprayed molded specimens, this study introduces 3% water reducer to optimize the laboratory molding process, realizing the matching of key indicators between the two types of specimens. Through a 180 d chloride dry-wet cyclic erosion test, combined with microscopic characterizations via X-Ray Diffraction (XRD), Scanning Electron Microscope (SEM) and Mercury Intrusion Porosimetry (MIP), as well as correlation analysis of macroscopic indicators including compressive strength and total porosity, we systematically investigate the effects of Nano-Silica (NS), Fly Ash (FA) and Calcined Layered Double Hydroxides (CLDHs) on the chloride erosion resistance of PCCP mortar, and elucidate the corresponding enhancement mechanism. The results show that the L2 group with 2% CLDHs alone exhibits excellent erosion resistance, with compressive strength increased by 56.39%, chloride ion concentration at the same depth decreased by 31.1%, and total porosity reduced by 8.69% at 180 d compared to the reference group; the F20-N0-L2 group with 20% FA and 2% CLDHs presents the optimal comprehensive performance, with flexural strength increased by 51.28%, chloride ion concentration decreased by 41.1%, and total porosity reduced by 24.07%.

The thickness of the protective layer mortar for prestressed concrete cylinder pipe (PCCP) is usually only 25 mm, whereas the laboratory standard mortar specimen sizes are 70.7 mm or 40 mm. To establish an effective correlation between experimental results and the actual performance of components, it is crucial to systematically investigate the influence of size effects. To address this issue, this study produces cubic specimens of three sizes, 70.7 mm, 40 mm, and 25 mm, by varying the contents of fly ash (FA) and nano silica (NS), in order to explore the impact of these supplementary materials on the performance of PCCP protective layer mortar and to reveal the regularity of strength variation with specimen size. The main conclusions are as follows: Firstly, the optimal combined dosage of FA (20%) and NS (3%) is determined, which shows the best 28-day mechanical performance and dimensional stability. Secondly, the mortar strength exhibits a clear size effect: as the specimen size decreases from 70.7 mm to 40 mm, the strength increases, but further reduction to 25 mm led to a decrease in strength. Finally, the established size conversion equation indicates that the conversion coefficient first increases and then decreases with the admixture content and is dependent on the dosage. This provides a theoretical basis for accurately predicting the actual performance of thin-layer PCCP mortar.