Chloride Diffusion and Wicking in Concrete Exposed to NaCl and MgCl2 Solutions

Abstract

Wicking occurs in concrete in practice when fluid absorption takes place at one surface of the element and drying at another surface. This paper examines chloride diffusion and wicking in concrete with varying water-to-cement ratios (w/c) and varying cement replacement levels by fly ash and slag. The influence of salt type (NaCl and MgCl2) on chloride transport is examined. The chloride binding capacity is similar for the cement paste with different w/c. The replacement of cement with fly ash and slag increases the bound chloride content. When compared to NaCl solutions, a greater bound chloride content is observed for cement paste exposed to MgCl2 solutions due to the formation of the additional phases, such as calcium oxychloride. Samples exposed to wicking had a greater chloride penetration depth than diffusion alone due to the contribution of advection to the chloride transport. The salt type significantly influences the chloride transport. The penetration depth was observed to be lower in concrete exposed to MgCl2 solution because the additional reaction products form in and block pores for chloride transport. When concrete with a higher w/c is exposed to MgCl2 solution during wicking, there is a larger reduction in the penetration depth compared to that in NaCl solution because the greater amount of Ca(OH)2 results in the formation of reaction products that block more pores. The reduction in the penetration depth due to blocking by reaction products is less in concrete with fly ash and slag due to the lower content of Ca(OH)2.