Morphology transformation of Na2SO4 from prism into dendrite enhanced desalination efficiency in sandstone by biodegradable polycarboxylate modifiers

The salt decay on the surface of stone cultural relics is the greatest damage to its loaded information. Poly-carboxylate modifiers can reduce salt crystallization pressure and/or change crystal habits, thereby becoming a new method for controlling salt crystallization damage in pores. However, the lack of precise regulation of crystal morphology by modifier and the unclear relationship between modifier and crystal transformation during salt growth hinder its practical application. Herein, diethylenetriamine pentamethylene phosphonic acid (DTPMP), phosphorus-free, green polyaspartic acid (PASP) and polyepoxysuccinic acid (PESA), as well as three composite modifiers composed of DTPMP and PESA (M-1:9, M-2:8, M-3:7) were systematically investigated. Video mi-croscope, scanning electron microscope, and X-ray diffraction showed that the addition of these modifier could obviously transform the crystal morphology of sodium sulfate(thenardite) from prism into dendrite, which was related to the significant change in the intensity ratios of corresponding peaks on the different crystal planes. Molecular dynamics simulations were conducted to investigate the interactions of PASP and PESA on different surfaces of sodium sulfate, revealing that the crystal transformation of sodium sulfate was mainly influenced by the coordination/adsorption effect of the modifiers on the (010) plane. All modifiers were used for sandstone desalination, and the clean and desalted sandstones were analyzed for desalting efficiency, colorimetry and porosity, which showed that PESA, PASP and composite modifier M-3:7 all promoted the precipitation of sodium sulfate on the surface. Especially, composite modifier M-3:7 prefers to change the crystallization and morphology of sodium sulfate from prism into dendritic flower, which can be used to protect to sandstone cultural relics from salt damage.