Effect of coral sand on the mechanical properties and hydration mechanism of magnesium potassium phosphate cement mortar

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 (CS) are mixed to repair damaged structures on coral islands. However, CS 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 CS 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, and then stabilized after long-term immersion in pure water and Na 2 SO 4 solution, showing excellent corrosion resistance. Compared with MKPC river sand (RS) mortar, the hydration products of CS mortar were an intermediate product 6KPO 2 ·8H 2 O with a relative content of 3.9% at 1 h and 4.1% at 12 h. The hydration product MgKPO 4 ·6H 2 O increased rapidly after 7-d curing, with an increased growth rate of 1100%. Our results showed that CS 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.