Experimental investigation of magnesium phosphate cement mortar for the repair of long-term marine-exposed concrete

Chloride ions tend to ingress rapidly in damaged areas of marine concrete structures, such as cracks and spalling zones. Without timely intervention, embedded steel reinforcement in these regions undergoes accelerated corrosion, ultimately compromising structural integrity. To reduce chloride ingress pathways, prompt repair of these localised damage sites is required, which necessitates repair materials that exhibit rapid reactivity and robust interfacial compatibility with the deteriorated substrate. In this study, a novel magnesium phosphate cement (MPC) mortar was designed to form a tightly bonded interface with long-term marine-exposed concrete, thereby minimising chloride penetration and ensuring durable protection. The repair performance was systematically evaluated through experimental investigations covering three aspects: the material properties of the MPC repair system, the long-term durability characteristics of the 38-year marine-exposed concrete substrate and their interfacial compatibility. The results showed that the developed MPC mortar exhibited excellent early-age mechanical properties and formed a tightly bonded interface with the substrate, effectively curtailing chloride ingress pathways. By ensuring interfacial compatibility, the MPC mortar repair layer significantly mitigated chloride penetration, thus achieving long-term protection. By integrating material characterisation, field exposure data and electrochemical assessment, this study provides a systematic experimental basis for the application of the developed MPC mortar in the repair of marine concrete structures.