Chloride transport and pore-structure deterioration of OPC and magnesium phosphate cement concrete under sustained water pressure

Abstract

Estuary ship-lock concrete is exposed to chloride-bearing water and sustained hydraulic pressure, which may accelerate chloride ingress and induce pore-structure deterioration. In this study, the chloride transport behavior and microstructural damage of ordinary Portland cement (OPC) concrete and magnesium phosphate cement (MPC) concrete were investigated under coupled chloride exposure and sustained water pressure, with reference to the service environment of the Qingnian Hub ship-lock along the Pinglu Canal. Multi-stage constant water pressures were applied, and chloride concentration profiles were measured after different exposure durations. X-ray computed tomography (X-CT) and scanning electron microscopy (SEM) were used to characterize pore-structure evolution and microscopic damage. The results showed that sustained water pressure promoted chloride ingress in both concretes by introducing a pressure-induced advective component in addition to diffusion. Within the pressure range of 30–150 kPa, chloride concentration increased with increasing water pressure; the apparent chloride diffusion coefficient ( D a ) and surface chloride concentration ( C s ), derived by fitting the measured chloride concentration profiles, also increased with pressure. Compared with OPC concrete, MPC concrete exhibited lower chloride concentration, smaller penetration depth and lower water-pressure sensitivity. Its chloride penetration depth was approximately 60%–67% of that of OPC concrete, and its apparent diffusion coefficient was approximately 44%–48% of the OPC value. X-CT and SEM results indicated that OPC concrete experienced more evident pore coarsening, increased pore connectivity, microcrack development and interfacial degradation, whereas MPC concrete retained a denser and less connected pore structure. These findings suggest that MPC concrete has potential for improving the durability of estuary ship-lock structures exposed to chloride-bearing water and sustained hydraulic pressure, especially in repair and reinforcement applications.