Abstract: Damaged structures on coral islands have been spalling and cracking due to the dual corrosion of tides and waves. To ensure easy access to aggregate materials, magnesium potassium phosphate cement (MKPC) and coral sand are mixed to repair damaged structures on coral islands. However, coral sand is significantly different from land-sourced sand in mineral composition, particle morphology, and strength. This has a substantial impact on the hydration characteristics and macroscopic properties of MKPC mortar. Therefore, in this study we investigated the compressive strength, interfacial mechanical properties, and corrosion resistance of MKPC coral sand mortar. Changes in the morphology, microstructure and relative contents of hydration products were revealed by scanning electron microscope-energy dispersive spectrometer (SEM-EDS) and X-ray diffraction (XRD). The results indicated that the compressive strength increased linearly with the interfacial micro-hardness, then stabilized after long-term immersion in pure water and Na₂SO₄ solution, showing excellent corrosion resistance. Compared with MKPC river sand mortar, the hydration products of coral sand mortar were an intermediate product 6KPO₂·8H₂O with a relative content of 3.9% at 1 h and 4.1% at 12 h. The hydration product MgKPO₄·6H₂O increased rapidly after 7 d curing, with an increased growth rate of 1100%. Our results showed that coral sand promoted the nucleation and formation of hydration products of MKPC, resulting in better crystallinity, tighter overlapping, and a denser interfacial transition zone. The results of this study provide technical support for applying MKPC mortar as a rapid repair material for damaged structures on coral islands.