Deterioration mechanism of coral reef sand concrete under scouring and abrasion environment and its performance modulation

In complex marine environment, coral reef sand concrete structures such as breakwaters are subjected to abrasion by sand-containing currents, which seriously shortens the service life and cycle of structures. Therefore, this paper investigates the health status of on-site breakwaters, moreover, reveals the damage mechanism of coral reef sand concrete (CRSC) under scour and abrasion environments in combination with indoor macroscopic and microscopic experiments. In addition, the effects of fly ash (FA) and silica fume (SF) on the mechanics and durability, hydration and hydration products of CRSC are investigated so that corresponding measures can be proposed to improve the service performance of breakwaters. The results demonstrate the indentation hardness of coral aggregate is greater than that of ITZ and cement stone matrix. Therefore, the ITZ and cement stone matrix with lower hardness are scoured and abraded by water flow containing coral reef blocks with high hardness, resulting in abrasion failure of CRSC. FA and SF prolong the induction period of cement, and reduce the hydration rate and accumulated heat of the accelerated period and second peak. The hydration reaction between the supplementary cementitious material and Ca(OH)2 occurred within 20-40 h, resulting in the third exothermic peak of the system. The hydration process of the admixture-cement system goes through three stages: crystallization nucleation and crystal growth (NG), phase boundary reaction (I) and diffusion (D). FA mainly plays a physical filling role in the early stage, and it reduces the early C-S-H polymerization degree and the total volume of C-S-H phase, but slightly increases C-S-H polymerization degree in later stage. However, SF is involved in hydration reaction in the early stage, and increases the C-S-H polymerization degree and total volume of C-S-H phase.