Salt damage is one of the most common and serious diseases in silicate cultural relics. In this research, low-field nuclear magnetic resonance (low-field NMR), automatic high-speed X-ray microtomography imaging, polarized light microscopy, and ultra-depth of field microscopy were applied to investigate the migration, distribution, and crystallization of NaCl and Na2SO4on the surface of hydrophilic media, glass capillaries, and porous SiO2materials, respectively. The results show that these two salts have different crystal growth behaviors in the same medium. NaCl grows in a granular form on the surface of hydrophilic medium and generally crystallizes outside the glass capillary tube, whereas Na2SO4grows in a circular ring and always crystallizes inside, and some bubbles can be seen clearly in the hydrophilic medium. Meanwhile, different from NaCl, which is mainly concentrated on the upper surface of SiO2sample, the migration of the Na2SO4solution is distributed in the whole sample, and crystals accumulate on the interior of the sample surface. The different crystallization behaviors of salts are speculated to be related to damage conditions such as efflorescence and mural blisters in silicate cultural relics.Keywords:salt damage;penetration depth;migration;distribution;crystallization
Mengjun Jia,Junyan Liang,Ling He,Xiang Zhao,Stefan Simon
The anti-salt damage of sandstone protected by hydrophobic and hydrophilic SiO 2 -based hybrids is evaluated in NaCl, Na 2 SO 4 and NaCl-Na 2 SO 4 salt-loaded hydrothermal aging (SLHA) cycles. Although both hydrophobic and hydrophilic SiO 2 -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 SiO 2 -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 SiO 2 -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 SiO 2 -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, SiO 2 -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.
Mengjun Jia,Ling He,Aizhao Pan,Xiaoqin Ma,Shengying Huang,Stefan Simon
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 Na 2 SO 4 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 Na 2 SO 4 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 Na 2 SO 4 SLHA cycles. While the needle-like and cumulate ellipsoidal Na 2 SO 4 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 Na 2 SO 4 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.
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