Effect of Magnesium Sulfate in Chloride Solution on Variation in Microstructure, Chloride Diffusion, and Chloride-Binding Behavior of Concrete

In this study, variation in microstructure, diffusion of chloride (Cl-) ions, and binding of chloride in concrete exposed to solutions containing MgSO4 (MS) in NaCl (NC) under the wetting-drying cycle are analyzed and reported. The results reveal that the amount of compounds such as ettringite (E), calcium hydroxide (CH), gypsum (G), calcite (CC), and calcium chloroaluminate (CCA) varies with the depth of concrete from the exposure face. Among exposure solutions, the amount of E was higher in NC + MS solutions compared with NC solutions and with increased MS content. The amount of CCA was higher in NC solutions compared with NC + MS solutions with an increase in NC content. The amount of CH was higher in concrete subjected to NC + MS solutions than to NC solutions and decreased with an increase in NC content. The amount of CCA was mostly higher in ordinary portland cement (OPC) with 20% fly ash concrete (O20FA-C) than in OPC concrete (OPC-C) followed by portland pozzolana cement (PPC) concrete (PPC-C). The amount of E was higher in O20FA-C than in PPC-C followed by OPC-C. Free chloride content (C-f) and apparent chloride diffusion coefficient (D) were reduced in the presence of MgSO4 mainly due to the effect of filling of pores with E. PPC-C exhibited better performance against ingress of chloride (Cl-) ions than that of O20FA-C followed by OPC-C. MS in exposure solutions reduced chloride binding. The concrete made with OPC exhibited higher chloride binding than O20FA followed by PPC.

Automated summary

This study analyzed and reported the variations in microstructure, diffusion of chloride ions, and chloride binding in concrete exposed to solutions containing MgSO4 and NaCl under wetting-drying cycles. The results showed that the amounts of ettringite, calcium hydroxide, gypsum, calcite, and calcium chloroaluminate varied with the depth of the concrete. The presence of MgSO4 reduced the amount of free chloride and apparent chloride diffusion coefficient due to the filling effect of ettringite. Concrete made with ordinary Portland cement exhibited higher chloride binding capacity compared to concrete made with fly ash and Portland pozzolana cement.