In-situ formation of an aluminum phosphate coating with high calcite-lattice matching for the surface consolidation of limestone relics.

• Aluminum phosphate protection material had high compatibility with limestone. • The mechanical strength of limestone weathering layer was improved by aluminum phosphate. • The wettability of the weathering layer was reduced and the durability was improved. • Aluminum phosphate was nucleated and stacked in situ with the aid of limestone matrix. The surface weathering of open-air limestone caused by long-term acid-rain erosion and thermal stress is an urgent issue to solve. In this study, seven concentrations of diammonium phosphate and aluminum chloride hexahydrate (fixed molar ratio of 1000:1) were used to generate aluminum phosphate protective materials in situ with high lattice matching with calcite for surface consolidation. The compatibility test, mechanical property evaluation, and protective mechanism of the material were elucidated by color change, porosity, permeability, surface hardness, infinite compressive strength, Fourier-transform infrared spectroscopy, and scanning electron microscopy–energy-dispersive X-ray spectroscopy. It was found that aluminum phosphate had good aesthetic compatibility and water–vapor exchange balance with the matrix and surrounding environment. Meanwhile, the surface coating grown by crystal accumulation could play a role in filling pores and agglomerated particles, thereby improving the hardness (83.5 HD), infinite compressive strength (4.05 MPa), and acid-rain erosion resistance of the limestone weathering layer. Aluminum phosphate can play a vital role in the treatment and prevention of limestone surface-weathering diseases and has high research value and broad application prospects. [ABSTRACT FROM AUTHOR]