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 Na2SO4 solution,showing excellent corrosion resistance.Compared with MKPC river sand(RS)mortar,the hydration products of CS 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 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.

Effect of Borax on Hydration and Hardening Properties of Magnesium and Pottassium Phosphate Cement Pastes

Magnesium and potassium phosphate cement （MKPC） sample were prepared by mixing dead burnt MgO powder, potassium phosphate and different dosages of retarder borax to investigate the effect of borax on its hydration and hardening characteristics. The pH value, fluidity, hydration temperature and strength development of MKPC paste were investigated, and the mineralogical composition and microstructural morphology of its hydration products were analyzed. The experimental results indicated that, within a certain dosage, borax caused an endothermal effect for MKPC paste, which decreased the early hydration rate of MKPC paste, increased the fluidity of MKPC paste. Thus, strength and micro-morphology of hardened MKPC are affected. It can be concluded that borax in MKPC paste retards the early hydration rate of MKPC paste by forming a film onto surface of MgO, decreasing the temperature and increasing the pH value of the system. As borax dosage varying, different factors may dorminate the effects.

Stability of K-struvite in High Temperature and Acid-base Environment

K-struvite was prepared by precipitation method,and the stability of K-struvite in high temperature and acid-base environment were investigated by X-ray diffraction(XRD),thermogravimetric analysis(TG/DSC),and infrared spectroscopy(FT-IR).The results show that K-struvite decomposes from 50 to 110℃,and the mass loss begins at 50℃before being completely destroyed at 110℃,then further heating at temperature above 500℃leading to complete loss of the binding water in K-struvite.Moreover,K-struvite is more stable in alkaline environments than acidic environment.