New insights into how MgCl2 deteriorates Portland cement concrete

The consensus on mechanisms responsible for how chloride-based salts attack concrete has yet not been achieved. This work exposed laboratory-fabricated concrete samples to MgCl2 solutions of various concentrations along with freeze/thaw and wet/dry cycles, with NaCl solutions as the control. The laboratory investigation revealed a new chemical mechanism during the MgCl2-induced deterioration of concrete, i.e., the formation of multi-phase nano-sized crystals, including CaCl2, Mg(OH)(2), and Mg-3(OH)(5)Cl center dot(H2O)(4). This mechanism was further corroborated by the testing of concrete samples cored from several selected concrete bridge decks, where cumulative exposure to MgCl2 deicer resulted in significantly compromised splitting tensile strength (as high as 50%) as well as reductions in microhardness (up to 60%, often at a depth of 25 to 50 mm). This study provides new insights into the risks of MgCl2 to the concrete-built environment. It also alerts that visual inspection could be misleading for condition assessment of concrete exposed to MgCl2.