多孔材料内含盐水溶液中离子的析出结晶是造成多孔结构破坏的重要因素, 因此在建筑保护和地貌学研究中受到了极大关注. 现有研究主要集中于微孔介质中盐的孔内结晶行为. 本文对比研究了限制于纳孔硅胶颗粒孔隙内的NaCl, NaNO_3, Na_2SO_4三种盐溶液在蒸发过程中盐的孔外结晶行为. 利用扫描电子显微镜对所形成晶体的形貌进行了表征. 实验结果表明: 1)随孔径从2 nm增加至15 nm, NaCl和NaNO_3在硅胶颗粒表面的结晶由晶粒转变为晶须形态, 而Na_2SO_4则由晶须转变为晶粒形态; 2) NaCl和NaNO_3晶须的生长主要沿垂直于颗粒表面的方向, 而Na_2SO_4晶须则在硅胶颗粒表面斜向生长, 后一种生长方式对硅胶颗粒产生横向的应力, 从而对孔结构具有更强的破坏作用; 3) NaNO_3的细长晶须所具有的分支和珠链结构表明其在结晶过程中发生了Plateau-Rayleigh失稳.

Owing to its destructive power to porous structures such as buildings and rocks, salt weathering has attracted considerable attention in the community of civil engineers and geomorphologists, who devote their efforts to conservations of architecture and engineering structures afflicted by salt attack, and to the investigation of natural landscape caused by the same group of processes, respectively. Precipitation of dissolved salts is a direct cause of salt weathering effect. Crystallization phenomena in salt weathering can be crudely categorized under efflorescence and subflorescence with respect to the distinct precipitation sites, and the latter is believed to be able to cause more destructions to porous structure. In contrast to subflorescence for which even models of statistical dynamics have been well-established, efflorescence has drawn less attention, partly because of the complexity of constructing a sound theoretical model to describe the mass transport process there involved. As a serie of sodium salts is the main culprit of salt weathering, the current work deals with experimental study of efflorescences of the aqueous NaCl, NaNO_3 and Na_2SO_4 solutions on the surface of porous silica gel particles. We investigate the influences of salt concentration and pore size on the crystal morphology arising in efflorescence by using scanning electron microscopy. It is found that though Na_2SO_4 effloresces on the specimen surface, its inclination towards subflorescence makes the whiskers appear on specimen with smaller pore radii at low concentrations, which differs obviously from the cases of NaCl and NaNO_3. Moreover, unlike the upright growths of NaCl and NaNO_3 crystals, the whiskers of Na_2SO_4 are always oblique to the specimen surface, and the large lateral stress to the specimen thus induced may become another factor of its destructive power apart from the subflorescing trend. The crystallization behaviors of Na_2SO_4, i.e., both the oblique whiskers and regular crystallites, indicate that mirabilite (Na_2SO_4·10H_2O) is the main precipitation, which is consistent with the high relative humidity employed in this article. Remarkably, the thinnest whiskers of NaNO_3 exhibit the branching and ball-chain structures, indicating that plateau-Rayleigh instablility occurs in the growth process. Our results are expected to inspire more deliberate studies for the full understanding of detailed processes and mechanism involved in efflorescence of aqueous salt solutions.