Consolidating stone relics with calcium phosphate oligomers

The invaluable stone cultural relics are suffering from deterioration, leading to the reduction of mechanical strength. Hydroxyapatite (HAP) has been increasingly received attention due its strong consolidation strength and the ability to enhance the substrate’s resistance to weathering. Nevertheless, the generation of HAP needs calcium sources (CaCO₃, CaCl₂, etc.), restricting its application and may bring salts to the stone artifacts. Here, we, for the first time, introduce calcium phosphate oligomers (CPO), a nanoscale amorphous calcium phosphate, to consolidate stone artifacts. This nanomaterial resolves the calcium requirement for HAP-based stone consolidation. We find that CPO (∼100 nm) maintains high kinetic stability in ethanol and can effectively crystallize to be HAP nanosheets in less than 3 days. Application characterizations demonstrate the feasibility of CPO: It leaves stone color unaltered and significantly enhance the consolidation strength and resistance to acid erosion, offering a modest advantage over diammonium phosphate (DAP) and showing comparable consolidation effectiveness with DAP and CaCl₂. Moreover, it has excellent penetration depth through stone sample (centimeter level), with minimal impact on stone’s porosity and breathability. Our work will fuel the development of nanomaterials in the conservation of cultural heritage.