Physical-chemical synergistic toughening mechanism of wollastonite powder in MgO-K2HPO4-SiO2 cement system

Magnesium Silicon Potassium Phosphate Cement (MSPPC, MgO-SiO₂-K₂HPO₄) exhibits excellent mechanical strength and promising potential for structural repair applications. However, its inherent brittleness severely limits its performance under dynamic loading conditions. To enhance the toughness of MSPPC, this study introduces wollastonite powder (CaSiO₃), which possesses a combination of potential chemical reactivity, micro-filling capacity, and fiber-bridging effects. The effects of partial MgO replacement by wollastonite (0–50 wt%) on the mechanical and microstructural properties of MSPPC were systematically investigated. The toughening efficiency was evaluated through impact resistance tests, while XRD and SEM analyses were conducted to study the underlying mechanisms. The results indicate that when 30 wt% of MgO is replaced by wollastonite, the composite achieves the optimal toughening performance. Compared with the control sample (0 wt% replacement), the initial and final impact counts increased by approximately 4.7 times and 9.2 times, respectively. Microstructural observations revealed that wollastonite enhances toughness through a "physical-chemical synergistic mechanism". Physically, its fine particles fill capillary pores effectively, while its fibrous morphology bridges microcracks and suppresses their propagation. Chemically, wollastonite dissolves in the alkaline environment and participates in hydration reactions, promoting the formation of amorphous or poorly crystalline calcium silicate hydrate (C-S-H), magnesium silicate hydrate (M-S-H), K-rich silicate gel, which collectively strengthen the solid matrix and refine the pore structure. This study is the first to propose and verify the "physical-chemical synergistic toughening mechanism" of wollastonite in MSPPC, providing new theoretical insights and technological guidance for the design and development of high-toughness magnesium silico-phosphate cement materials. Compared with conventional toughening strategies such as fiber reinforcement or polymer modification, the proposed physical-chemical synergistic mechanism enables simultaneous crack-bridging, pore refinement, and solid-phase strengthening without introducing additional organic components or interfacial incompatibility. This feature makes wollastonite-modified MSPPC suitable for impact-resistant and durable repair materials in harsh service environments.