Hardening and microstructural properties of red mud modified magnesium ammonium phosphate cements

Magnesium ammonium phosphate cement (MAPC) was modified by red mud pretreated with ammonium dihydrogen phosphate (NH4H2PO4) to improve water resistance, volume stability, and economy. The effects of pretreated red mud on fluidity, setting time, mechanical properties, and water resistance of MAPC were investigated. Combined with the composition and morphology of the hydration products, the mechanism of the influence of pretreated red mud on macroscopic properties was clarified. Furthermore, the analysis of hydration temperature, hy-dration heat, and the thermodynamics of the hydration reaction was to expound the effect of the pretreated red mud and the rich sodium phosphate salt on the hydration hardening process of MAPC. The experimental results showed that the water resistance of MAPC was obviously improved, and hydration peak temperature and hydration heat were significantly reduced with the increase of pretreated red mud content. When the pretreated red mud was blended at 30%, the mechanical properties of MAPC were slightly reduced, the water resistance was increased by 97.4%, and the peak hydration temperature and total heat were reduced by 27.8% and 28.4%, respectively, compared with the control group. This is due to the filling and diluting effects of red mud particles, especially the dilution of dead-burned magnesium oxide (MgO) and NH4H2PO4 concentration in the MAPC mixture. In addition, it was attributed to the compounding effect of sodium dihydrogen phosphate, which was abundant in the pretreated red mud.

Automated summary

Magnesium ammonium phosphate cement (MAPC) was improved in terms of water resistance, volume stability, and economy by modifying it with red mud pretreated with ammonium dihydrogen phosphate (NH4H2PO4). The effects of pretreated red mud on different properties of MAPC were investigated, and the mechanism behind these effects was clarified. The experimental results showed that the water resistance of MAPC was significantly improved, and the mechanical properties, hydration peak temperature, and hydration heat were also affected by the pretreated red mud content.