Influence of supplementary materials on the early age hydration reactions and microstructural progress of magnesium phosphate cement matrices

Abstract

The microstructures of magnesium phosphate cement (MPC) compounds are porous due to rapid speed of hydration reaction and great level heat release at very early ages. Consequently, MPCs lose mass in water region for spongy structure, which limits the application in the water structures. For improving the solidity of microstructures, decrease of high hydration heat discharge and little extension of final setting time (FST) of the magnesium phosphate cement (MPC) composites at early hydration ages, different supplementary cementitious materials (SCMs) such as fly ash (FA), silica fume (SF), bauxite (BX), Phosphogypsum (PG), aluminum silicate (AS) and silica nanoparticles (SNP) were introduced to MPC compositions. It was revealed that pH was not too acidic for adding foreign materials at very early stages (i.e. 4 mins–10 min), where dissolving speed of magnesia (MgO) particles were declined. Thus, FST was delayed and decreased heat release of fresh specimens except for combination with SNP. The research also identified the different intermediate crystals such as mullite, enstatite, berlinite, lizardite, CSH and ettringite in the microstructures of MPC blended with SCMs, which was confirmed by XRD, SEM and FTIR experiments. Furthermore, the structural integrity of samples was significantly improved as compared to control for the existence of secondary rocks, where the specimens having FA, BX and AS presented the mechanical strength properties 30% higher at 28d than the reference matrix. However, the explored results might be a potential solution to design and improve the water resistance properties of MPC compounds for applying in the water structures.