New insights into how MgCl2 deteriorates Portland cement concrete.

Abstract 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 MgCl 2 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 MgCl 2 -induced deterioration of concrete, i.e., the formation of multi-phase nano-sized crystals, including CaCl 2 , Mg(OH) 2 , and Mg 3 (OH) 5 Cl·(H 2 O) 4. This mechanism was further corroborated by the testing of concrete samples cored from several selected concrete bridge decks, where cumulative exposure to MgCl 2 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 MgCl 2 to the concrete-built environment. It also alerts that visual inspection could be misleading for condition assessment of concrete exposed to MgCl 2. Highlights • A new chemical mechanism during the MgCl 2 -induced deterioration of concrete. • Formation of nano-sized crystals of CaCl 2 , Mg(OH) 2 , and Mg 3 (OH) 5 Cl·(H 2 O) 4. • This mechanism was further corroborated by the testing of field cores. • Field exposure significantly compromised splitting tensile strength (as high as 50%), and caused a reduction in microhardness (up to 60%). • The degradation often peaked inside concrete, thus elusive to visual inspection. [ABSTRACT FROM AUTHOR]