Compared with standard sand mortar, the physical and mechanical properties of coral sand mortar(coral mortar, for short), and the effects of mineral admixtures on the coral mortar were investigated. Scanning electron microscope and energy dispersive X ray spectrometer(SEM EDS), X ray diffraction(XRD) and Fourier transform infrared spectrometer(FTIR) were used to explore the microscopic mechanism involved in the mechanical properties, volume stability and hydration of mortar. It is showed that mechanical strength of the coral mortar is lower than that of the standard sand mortar, and the microstructure of cement mortar and interface transition area are improved by the mineral admixture, also the mechanical strength of coral mortar is improved and its 28d compressive strength is increased by 21% as compared to the control. The uneven surface of coral sand can form a tight meshing state with hardened cement mortar, which is helpful to improve the volume stability of mortar. The internal curing effect of coral sand improves the mechanical properties and resistance to shrinkage of the mortar. Furthermore, micro aggregate filling effect and volcanic ash effect of fly ash and slag improve the mechanical properties and the ability to resist chloride ion diffusion of the coral mortar. The continuous release of Ca2+ and Mg2+ from coral sand is involved in the hydration reaction of cement and helps to generate more hydration products.