Salt migration and capillary absorption characteristics of cement mortar partially immersed in NaCl solution

Salt crystallization represents a major reason for the damage observed for porous building materials in humid coastal areas. Despite the several theories developed to explain the process, no unanimous opinion has been proposed for the mechanism underlying the damage. A better understanding of the migration, crystallization, and distribution of salts in porous building materials in the presence of water is thus warranted to improve the durability for cultural heritages and new buildings. This paper reports an experimental study of crystal distribution, micromorphology, chloride ion migration, pore structures, and capillary absorption behaviors of cement-mortar specimens (side-coated and side-uncoated) at three different water-cement ratios (0.4, 0.6, and 0.8) after 120-day partial immersion in 5 wt% NaCl solution. Results showed that the different pore characteristics and side-coating treatment of the specimens governed not only capillary absorption, but also salt migration and distribution. The specimens with a large average pore diameter and greater porosity yielded more external salt crystals, while those with a small average pore diameter and dense surfaces demonstrated the reverse. For vertical migration, the Cl− concentration changed slightly with height of the specimens, except for the top segment. For horizontal migration, the Cl− concentrations in the central samples were higher than the edge samples. Of note, the side-coated specimens were associated with reduced external salt crystals and lower Cl− concentration in their edge samples. Compared with the salt-free specimens, both the capillary absorption coefficient (Acap) and capillary moisture content (Wcap) of the salt-contaminated specimens were lower (water-cement ratio = 0.6, −45.72%, −56.39%; water-cement ratio =0.8, −60.8%, −56.34%), illustrating the inhibitory effect of salt crystallization on the hygroscopic properties of the porous materials. Lastly, both the Acap and Wcap of the side-coated specimens were lower than uncoated ones.