Nano-silica-enhanced high-performance magnesium phosphate cement repair mortars: Optimization of interfacial bonding

This study explored the effects of nano-silica (NS) as a substitute for dead-burned magnesium oxide (MgO) on the performance of magnesium phosphate cement (MPC) and assessed the interfacial bonding between MPC-NS composite mortar and ordinary Portland cement (OPC) substrates. This study scrutinizes the impact of varying NS replacement ratios (ranging from 0 % to 4 % by mass of MgO, R_M/N) on the setting time, compressive strength, flexural bond strength, and tensile bond strength for MPC. The investigation places particular focus on factors such as water resistance, curing age, and substrate surface roughness. The results demonstrate that the inclusion of NS significantly reduces the initial setting time of MPC to 17 min. A 3 % NS replacement ratio was found to optimize mechanical performance and water resistance, achieving a compressive strength of 58.45 MPa after 7 days of water immersion followed by 28 days of curing. Roughening the surface of the OPC substrate was shown to increase interfacial bond strength by 5.5–264 % compared to smooth surfaces. Microstructural analysis using XRD, SEM-EDS, and TGA-DTG revealed that 30-nm NS particles densify the MPC matrix through pore-filling. The dissolution of MgO releases Mg²⁺ and OH⁻, which further promote the dissolution of amorphous silica and the sustained production of M-S-H gels. This process reduces interfacial crystalline phases and strengthens chemical bonds, thereby significantly enhancing the interfacial bond strength and durability between MPC-NS mortar and OPC. However, an excessive level of NS (4 %) can cause nanoparticle agglomeration and increase mortar consistency, potentially compromising its homogeneity and compactness, and thus limiting improvements in both mechanical and bonding performance. These findings provide a robust foundation for the development of high-performance, durable, and rapid-hardening inorganic repair materials.