Strengthening Mechanisms and Microstructure Evolution of Magnesium Potassium Phosphate Cement Modified by Nano-Fe2O3 and Nano-SiO2

Abstract

Magnesium potassium phosphate cement (MKPC) exhibits rapid setting and high early strength, but its long-term performance is limited by microstructural heterogeneity and pore structure defects. These microstructural defects can increase pore connectivity and facilitate the ingress of aggressive agents, thereby limiting the long-term durability and service reliability of MKPC-based repair and protective materials. Nanomaterials have been applied to improve MKPC performance; however, the differences between conventional nano-SiO2 (NS) and nano-Fe2O3 (NF), particularly their effects on hydration regulation and microstructure evolution, remain insufficiently understood. In this study, the effects of NF and NS incorporation on the hydration behavior, phase evolution, pore structure, and mechanical properties of MKPC were comparatively investigated. Orthogonal experiments, mechanical testing, calorimetry, XRD, FTIR, Raman mapping, SEM/EDS, MIP, and nanoindentation were employed to establish the relationship between nano-modification, microstructural evolution, and mechanical performance. The results provide a basis for selecting suitable nanomodifiers for MKPC-based materials used in rapid repair, protective applications, and other construction scenarios requiring rapid strength development and improved microstructural compactness. Compared with pure MKPC and previously reported NS-MKPC results, NF-MKPC showed higher strength development, refined pore structure, and improved micromechanical uniformity. The observed performance enhancement of NF-MKPC is consistent with accelerated early hydration, possible heterogeneous nucleation, pore refinement, and matrix densification. In comparison, NS-MKPC exhibited a different hydration and pore-evolution behavior under the investigated conditions. These findings indicate that NF and NS may regulate hydration and microstructure development differently in MKPC and provide guidance for selecting suitable nano-modifiers for high-performance phosphate cement materials. Under the investigated conditions, NF modification shows potential for MKPC applications requiring rapid strength development and improved microstructural compactness, such as rapid pavement repair, concrete surface repair, and protective coating applications.