The salt removal from ancient sandstone using CaCO₃ oligomer- and POSS siloxane-enhanced zwitterionic polymer

The weathering of outdoor sandstone cultural relics was dominantly induced by the circulating crystallization of soluble salt Na₂SO₄. This work proposed to weaken the salt-induced deterioration by a protective material that could reinforce the fragile sandstone and simultaneously remove the salt via continuously transporting the salt towards the air-sandstone interface without damage to the sandstone. Therefore, a new protective material bearing poly(methacryloyloxylethyl phosphorylcholine) (PMPC)-based zwitterionic polymer segment (PMH), hydrophobic polyhedral oligomeric silsesquioxane (POSS) segment (APTES-POSS) and small-size CaCO₃ oligomer was prepared and denoted as POSS-PMH/CaCO₃ composite. The presence of phosphorylcholine zwitterionic groups with water/salt affinity contributed greatly to the salt crystallization resistance of POSS-PMH/CaCO₃ coating. Specifically, Na₂SO₄ crystallization pressure in the (POSS-PMH/CaCO₃)-coated confined medium decreased by 88.1 % and 59.6 % as compared with that in the glass plate confined medium and that in the (APTES-POSS)-coated confined medium, respectively. In addition, the salt in (POSS-PMH/CaCO₃)-treated sandstone could be ingeniously removed by zwitterion-induced outward transport. As a functional additive to traditional paper pulp poultice, POSS-PMH/CaCO₃ could also achieve a high-efficiency extraction of salt from sandstone. More critically, the hydrophobicity and strong interfacial bonding ability of APTES-POSS segment, as well as the small size effect and crosslinking property of CaCO₃ oligomer highly facilitated the interfacial adhesion of POSS-PMH/CaCO₃ towards sandstone, improving the service life of POSS-PMH/CaCO₃ and meanwhile avoiding the potential damage to sandstone during the removal of salt.