Performance investigation of magnesium phosphate cement-emulsified asphalt (MPC-EA) as a sustainable repair material for pavement cracks

Magnesium phosphate cement (MPC) is widely recognised as an effective material for pavement crack repair. However, its characteristically high elastic modulus often leads to stress concentrations and secondary pavement distress, particularly in flexible asphalt layers. Built upon a previously developed magnesium phosphate cement-emulsified asphalt (MPC-EA) composite, this study systematically investigated its repair performance under various mix designs. By optimising the ratios of dead-burned magnesium oxide to phosphate (M/P) and borax to magnesium oxide (B/M), the material's suitability for pavement maintenance was evaluated. The experimental results indicated that all groups exhibited slight volume expansion over 30d, a property highly beneficial for ensuring a tight seal within pavement cracks. An increase in borax content significantly enhanced the penetrability of the composite, facilitating the repair of fine cracks. In flexural bond strength tests, the composites achieved maximum 7d strengths of 3.18MPa and 3.47MPa, demonstrating sufficient load-bearing capacity for pavement structures, although higher borax dosages led to strength reduction. Diagonal shear tests further identified that the optimal interfacial bonding was achieved at an 8% borax ratio and lower MgO content. Microstructural analysis revealed that while stoichiometry influenced the hydration degree, it did not alter the fundamental functional group species. Scanning electron microscopy (SEM) observations confirmed that excessive borax or MgO delayed the hydration reaction, leading to the encapsulation of hydration products by an asphalt film. This mechanism, while modifying the bond with the old pavement substrate, shifts the adhesion towards bitumen-dominated behaviour. Finally, an optimised proportioning range was established to balance mechanical strength and deformation compatibility, providing theoretical guidance for the durable repair of asphalt and concrete pavement distresses.