Properties of magnesium phosphate cement containing redispersible polymer powder

• The effects of a redispersible polymer powder (RPP) on MPC mortar are evaluated.
• The addition of RPP can improve the bonding strength and water resistance of MPC mortar.
• The possible mechanism is that RPP may modify the pore structure in set MPC mortar.

Magnesium phosphate cement (MPC) prepared with dead burned magnesia oxide (MgO), potassium dihydrogen phosphate (KH2PO4, KDP) and some retarders at a given proportion was applied as repairing material. In this research, the mechanical properties and durability of MPC modified by a silane-based redispersible polymer powder (RPP) were investigated. The experimental results demonstrate that the compressive and flexural strength at different curing ages increased as increasing RPP content initial, and decreased as the content going beyond 0.35%. The addition of RPP significantly improved the bonding strength between MPC mortar and substrate OPC concrete, and the flexural and tensile bonding strength of MPC with 0.233% RPP at 1 day of curing were 4.6 MPa and 1.65 MPa, respectively. The addition of RPP can improve the pore structure and transfer the hydrophilic surface of some minerals to hydrophobic one, hence improving the water resistance of MPC. For MPC mortar with RPP after freeze-thaw cycles, the mass loss and the freeze-thaw influence coefficient were both smaller due to less water permeating into hardened MPC mortar. Due to more harmless pores in MPC mortar with RPP, the volume stability was improved. The XRD analysis illustrates that small amount of RPP has little impact on the hydration reaction of MPC. The SEM micrographs confirms that the pore structure of set MPC could be improved by adding an appropriate content of RPP.