Feasibility study on magnesium phosphate as an innovative stone-mimicking restoration material for limestone cultural relics

The limestone cultural relics within the West Lake World Cultural Heritage Site in Hangzhou, China, have suffered severe material loss and deep cracks due to prolonged natural and anthropogenic erosion. This necessitates the urgent development of highly efficient and compatible stone-mimicking restoration materials for their repair and preservation. This study systematically investigates the effects of varying metakaolin (MK) additions on the performance and microstructure of magnesium phosphate cement (MPC). Using mercury intrusion porosimetry, X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetry (TG-DSC), and scanning electron microscopy (SEM), it was found that MK significantly enhances the mechanical properties of MPC by filling pores and forming aluminum phosphate gel, while also prolonging the setting time of the paste. Based on these findings, MPC with 30 % MK addition (F-MPC) was selected as the binder, and commonly used materials in cultural heritage restoration—hydraulic lime (F-NHL), Portland cement (F-OPC), and epoxy resin (F-EP)—were employed as comparative materials. Calcite particles and iron oxide pigments were incorporated to create stone-mimicking restoration materials. Results showed that the mechanical strength of F-NHL and F-OPC was significantly lower than that of the limestone substrate, whereas F-EP exhibited much higher strength but severely compromised the substrate's breathability. In contrast, F-MPC demonstrated excellent compatibility with limestone in terms of mechanical properties while maintaining good breathability. This study provides theoretical support and practical guidance for the innovative application of magnesium phosphate in limestone restoration, highlighting its significant value and broad prospects in the field of cultural heritage conservation.