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.

With the continuous promotion of cross-regional water transfer projects, prestressed steel cylinder concrete pipe (Prestressed Concrete Cylinder Pipe, PCCP) has been widely used. However, due to the long-term erosion of Cl^- and $\text{S}{\text{O}}_4^{2-}$ in the soil, the external mortar expansion and cracking, internal steel wire corrosion, resulting in PCCP bursting have occurred repeatedly, so it is important to explore the different factors on the basic performance of the PCCP protective layer mortar, and erosion resistance of the law and to improve the anti-erosion performance of the PCCP protective layer mortar have important practical significance of the project. In this paper, the performance requirements and influencing factors of PCCP protective layer mortar are sorted out, and the improvement methods of erosion resistance of PCCP protective layer mortar are summarized. The results of the study show that the PCCP protective layer mortar mixing ratio, production process and maintenance conditions have a greater impact on its mechanical properties, water absorption and erosion resistance. In the PCCP mortar mixed with mineral admixture and mortar compacting agents and other additives can effectively improve the compactness of the mortar. Anti-corrosion coating on the surface of PCCP mortar protection layer can effectively prevent the intrusion of external erosion ions, thus improving the durability and service life of PCCP pipelines.