Investigation on the potassium magnesium phosphate cement modified by pretreated red mud: Basic properties, water resistance and hydration heat

Potassium magnesium phosphate cement (MKPC) was modified by red mud pretreated with potassium dihydrogen phosphate (KH2PO4) to improve water resistance, volume stability, and economy. The effects of pretreated red mud on the fluidity, setting time, mechanical properties, water resistance, and hydration heat of MKPC were investigated. The influence mechanism of pretreated red mud on the macro performance of MKPC was clarified by analyzing composition and morphology of the hydrated products, and the hydration thermodynamics. The results showed that with the increase of pretreated red mud content, the water resistance of MKPC was significantly improved, and the peak temperature and hydration heat were significantly reduced. This is because the participation of pretreated red mud refines the crystal structure of MgKPO4·6H2O, and reduces the concentration of dead burned magnesium oxide (MgO) and KH2PO4 in MKPC mixture through filling and dilution effects. In addition, the combined action of KH2PO4 and sodium dihydrogen phosphate rich in pretreated red mud reduced the hydration heat of MKPC. When the content of pretreated red mud is 20 w%, the compressive strength of MKPC cured in water was close to that of the control group, the strength retention rate reached 0.86, and the peak temperature and cumulative hydration heat were significantly reduced, indicating that it is more suitable for emergency repair of hydraulic structures.