Silica-based hybrids for adhesive coatings and their anti-salt damage in the protection of ancient sandstone

Hybrid coatings have been used as the protective materials on sandstone monuments for many decades. But the protective performance is strongly depended on the adhered interface between protective coating and matrix. To find out how the hybrid coatings affect the adhesive performances and anti-salt damage behaviors during Na₂SO₄ salt-loaded hygrothermal aging (SLHA) cycles, the surface/mechanical properties of three silica-based hybrid coatings (S1-S3) and their anti-salt damage results (surface weathering appearance, mass change, porosity, water absorption and morphology of pore structures) of the sandstones adhered by these hybrid coatings have been evaluated and discussed. The results reveal that dendritic Na₂SO₄ crystals cover on the sand grains in both natural and hydrophilic silica/PVA (S1) adhered sandstones, which makes the adhered sandstone perform the least damage. Owing to the high adhesive and mechanical strength, S1-adhered sandstones perform the best anti-salt damage during Na₂SO₄ SLHA cycles. While the needle-like and cumulate ellipsoidal Na₂SO₄ crystals precipitate in the pores and interstices result to much severe salt damaged in adhered sandstones by hydrophobic P(GMA-MAPOSS) (S2) and PGMA-g-P(MA-POSS) (S3) during Na₂SO₄ SLHA cycles, although they enhanced the strength of adhesive pore walls. Actually, the macro salt damage behaviors on the sandstones treated by different hybrid coatings are proved to be a competition result of different crystallization patterns. Therefore, the pore interface adjusted by protective coatings should be one of the decisive factors for anti-salt damage during the sandstone protection.