Hydrophobic and hydrophilic SiO₂-based hybrids in the protection of sandstone for anti-salt damage

The anti-salt damage of sandstone protected by hydrophobic and hydrophilic SiO₂-based hybrids is evaluated in NaCl, Na₂SO₄ and NaCl-Na₂SO₄ salt-loaded hydrothermal aging (SLHA) cycles. Although both hydrophobic and hydrophilic SiO₂-based hybrids could prevent the sandstone from salt damage through improving the matrix strength, the hydrophobic hybrid performs much better protection than hydrophilic one. The sandstone protected by hydrophobic SiO₂-based hybrid shows nearly no salt-damage, which is attributed to its excellent water repellence, high adhesive strength and good compatibility with sandstone matrix. However, the hydrophilic SiO₂-based hybrid tends to induce an exterior-to-interior salt-damage behaviour due to the frequent circulation movement of water with salt to result in the formation of surface efflorescence and interior sub-efflorescence in the protected sandstone. Furthermore, the hydrophobic-protective effect is also confirmed by another alternative hydrophobic POSS-based hybrid to offer stronger protection in anti-salt damage than that protected by hydrophilic hybrid. Nevertheless, there lies the difference, the hydrophobic SiO₂-based hybrid penetrates into the inner pores of the sandstone and develops a strong cohesion with sand-grain through the formation of Si-O bonds, but the hydrophobic POSS-based hybrid protects the sandstone with a weaker physical interaction between hybrid and sand-grains resulting in a fractured damage. Therefore, SiO₂-based hybrid is superior to POSS-based hybrid in promoting the anti-salt ability of sandstone. It is believed that these results could contribute much to the future protection of stone monuments by different hybrids.