Magnesium phosphate cement (MPC) has attracted recent attention due to its rapid setting, high early strength, and excellent bonding performance, making it a promising material for rapid repair and strengthening of concrete structures. In composite repair systems, the interfacial behavior between MPC-based repair materials and Portland cement concrete (PCC) substrates plays a decisive role in the overall structural performance, durability, and failure mode. However, the interface between MPC and PCC is often characterized by microstructural heterogeneity, the mismatch of mechanical properties, and the presence of initial defects, significantly reducing interfacial fracture resistance, especially under complex loading conditions. Nano-modification is widely recognized as an effective strategy to enhance the mechanical properties and microstructure of cement-based materials. Nanoparticles of silica (NS) can accelerate hydration, refine pore structure, and improve interfacial bonding through physical filling and chemical coupling effects due to its extremely high specific surface area and high pozzolanic reactivity. Despite extensive studies on the mechanical performance and modification mechanisms of NS-modified MPC (NS-MPC), systematic investigations on the interfacial fracture behavior between NS-MPC and PCC under mixed-mode loading conditions remain limited. It is thus essential to clarify the interfacial fracture mechanism and fracture resistance of NS-MPC/PCC systems under different fracture modes. In this study, asymmetric semi-circular bending beam (ASCB) specimens were employed to comprehensively investigate the mixed-mode interfacial fracture behavior of NS-MPC/PCC interface. The combined effects of initial crack length and fracture mix on the interfacial bonding strength, fracture parameters and nano-modification were investigated.

NS-MPC was prepared with dead-burned magnesia (M), potassium dihydrogen phosphate (P), NS, borax retarder (Bo), and deionized water (W). An orthogonal experimental design L9(3⁴) was adopted to optimize the mix proportion, considering four factors (i.e., Mg/P mass ratio, water-to-cement ratio (W/C), borax content (Bo/M), and NS content (NS/C)). The compressive and flexural strengths at 3 h and 1 d were selected as evaluation indices. According to the standard GB/T 17671—2021, the specimens NS-MPC of 40 mm × 40 mm × 160 mm were prepared to conduct flexural and compressive strength tests. Based on the results of orthogonal analysis, the optimal mix proportion of NS-MPC was determined and further compared with conventional MPC at different curing ages.

The interfacial fracture performance was evaluated by ASCB test, and the pure mode Ⅰ, pure mode Ⅱ and mixed mode Ⅰ/Ⅱfracture were achieved via adjusting the support position. Different crack length ratios (i.e., a/R=0.1, 0.2, and 0.3) and mixed mode parameters were selected to systematically investigate the mechanical response of the NS-MPC/PCC interface under different fracture modes. The geometric factors of the ASCB specimens were calibrated by the finite element method to achieve the quantitative analysis of the interfacial fracture parameters.

The microstructure and elemental composition of the interface were characterized by scanning electron microscopy/energy dispersive spectroscopy (SEM/EDS). After the samples were dried at a low temperature to terminate hydration and treated for conductivity, the interface morphology and the distribution characteristics of hydration products were analyzed under high vacuum conditions.

Based on the results of orthogonal experimental design, the mechanical properties of NS-MPC are systematically investigated. The optimal mix is determined to be M/P of 2.5/1, W/C of 0.14, Bo/M of 0.08, and NS/C of 0.04. NS-MPC exhibits significantly enhanced mechanical properties. The compressive and flexural strengths at 3 h increase to 47.3 MPa and 7.3 MPa, respectively, and increase to 76.5 MPa and 8.6 MPa at 1 d. From 7 to 28 d, the compressive strength increases slowly from 81.3 MPa to 88.7 MPa, and the flexural strength increases from 9.1 MPa to 9.4 MPa, which still maintains a high value. The flexural-to-compressive strength ratio of NS-MPC is 9%, which is higher than that of pure MPC (i.e., 5%). It is indicated that NS can improve the strength and the toughness.

The interface bonding strength of NS-MPC/PCC decreases with the increase of a/R. This is consistent across all the fracture modes (i.e., pure mode Ⅰ, pure mode Ⅱ, and mixed mode Ⅰ/Ⅱ). The interface bonding strength corresponding to pure mode II fracture is the maximum value, while that of pure mode I is the minimum value. The force-displacement response shows that the interface fracture process is dominated by brittle failure. The increase of crack length significantly reduces the peak load-bearing capacity and deformation ability, and promotes the crack to expand dominantly from the interface to bend and branch, especially at larger a/R rates.

The fracture parameter analysis indicates that the interface fracture toughness is highly sensitive to the fracture modes and a/R. As the mix of fracture increases, the effective fracture toughness shows an overall downward trend. The most unfavorable fracture condition occurs in the mixed mode I/II fracture, where the interface crack resistance is the lowest. Pure mode I fracture causes a damage to the interface, and the larger crack length significantly weakens the crack resistance of the pure mode II interface.

The microscopic analysis reveals that the interface of approximately 20 μm wide is formed between NS-MPC and PCC. The hydration products K-struvite can occur in the interface. Moreover, elements Mg, P, and Si appear certain degrees of infiltration and enrichment on the both sides of the interface. The addition of NS effectively improves the interface reaction activity and microstructure densification.

NS-MPC demonstrated significantly enhanced compressive and flexural strength, indicating that NS could effectively promote the hydration reaction and structural densification. The results of the ASCB test indicated that the bonding strength of the NS-MPC/PCC interface decreased as a/R increased. The interface fracture process showed a significant nonlinear characteristic. The pure mode I fracture was the most unfavorable. The mixed fracture analysis revealed the nonlinear relationship between the effective fracture toughness and the mixity of fracture. The SEM images and EDS spectra further confirmed that NS enhanced the chemical bonding and crack resistance at the interface via promoting the formation and filling of Si-P-Mg composite hydration products in the interface.