Effect of polyvinyl alcohol powder on the bonding mechanism of a new magnesium phosphate cement mortar

•PVA can significantly improve the bonding performance between MSPM and OPM.•PVA film can encapsulate the surface of Ca(OH)2 and ettringite crystals at the interface.•The covering effect of PVA film will affect the normal process of hydration. Silica-Magnesium Potassium Phosphate cement mortar (MSPM) which is made up of dead burnt magnesia powder, Dipotassium phosphate K2HPO4·3H2O (P), silica fume and a certain proportion of ISO standard sand, is used as repairing materials helping study the bonding performance between MSPM and ordinary Portland cement mortar (OPM). In this research, the effect of polyvinyl alcohol powder on MSPM bonding strength (flexural bonding strength and flexural two-sided bonding strength) was investigated. The bonding mechanism was studied with X-ray diffraction (XRD), Mercury intrusion porosimetry (MIP) and environmental scanning electron microscopy (ESEM). The experimental results show that PVA powder has good film forming properties. The PVA film formed by PVA powder will encapsulate reactants (silica fume, magnesium oxide) and hydration products (struvite-K) in MSPM system, and form structuring O-Mg-O with Mg2+. In addition, the PVA film encapsulates the surface of Ca(OH)2 and ettringite crystals at the interface, and complexes with Ca2+ to form O-Ca-O molecular chain structure, reducing the number of crystals at the interface. When the PVA content reaches 0.4%, the bonding performance between the OPM and the MSPM is greatly improved due to the good bonding properties of the PVA film and the ionic complexation reaction occurring at the bonding interface. However, when the content of PVA reaches 0.6%, PVA cannot be uniformly dispersed in the reaction system due to agglomeration. In addition, the covering effect of PVA film has affected the normal progress of hydration reaction, so the improvement in bonding performance at the bonding interface is limited.