Experimental investigation on the volume stability of magnesium phosphate cement with different types of mineral admixtures

Magnesium phosphate cement (MPC) composited by dead burned magnesia (MgO) and potassium dihydrogen phosphate (KH2PO4, KDP) is a type of excellent repairing material, and its volume stability has great influence on its applications. This paper studied the effects of different types of mineral admixtures on the volume stability of MPC mortar by testing its volume deformation at different curing ages. The main mechanisms to explain how the mineral admixtures affect the volume stability were studied by XRD and ESEM analyses. The experimental results indicate that the influence of ultra-fine fly ash on the volume deformation is different from that of fly ash and silica fume. The volume expansion increases as the content of fly ash in mortar increases while the volume expansion decreases as the content of ultra-fine fly ash increases. For MPC-fly ash system, the use of ultra-fine fly ash can decrease the volume expansion significantly. Moreover, the addition of dipotassium hydrogen phosphate also has significant influence on the volume stability of MPC mortar. The XRD analysis indicates that the amount of sylvite in MPC mortar is the reason for the changes in the volume stability after adding mineral admixtures. The ESEM imaging confirms that the use of ultra-fine fly ash can decrease the quantity of micro cracks in the MPC mortar.