Cementitious materials exposed to high concentration of sodium chloride solution: Formation of a deleterious chemical phase change

Abstract

This paper investigates deleterious chemical interactions between cementitious materials and high concentrations of sodium chloride (NaCl) solution (i.e., salt concentration >12% by mass). The behavior of mortar specimens made using Type I and Type V ordinary portland cements was investigated as the temperature varied in the presence of NaCl solution with concentrations of 15% and 20% (by mass). A low-temperature longitudinal guarded comparative calorimeter (LGCC) in conjunction with an acousto-ultrasonic technique were used to monitor the damage development in mortar samples. Cooling-heating cycles were applied with a minimum temperature above the freezing temperature of the solution to avoid ice formation. Nevertheless, damage was observed in specimens. Calcium aluminate phases, mainly tricalcium aluminate (C3A), seem to be the source of damage as the magnitude of damage was relatively minor when Type V cement (C3A < 5% by mass) was used. To further investigate the source of the damage, a low-temperature differential scanning calorimeter (LT-DSC) was used to study chemical phase transitions during temperature change in three types of powder specimens exposed to different concetrations of NaCl solution: (1) hydrated cement pastes, made using Type I and Type V cements, (2) cement pure phases, consisting of C3A and gypsum, and (3) cement hydration products, consisting of calcium hydroxide (CH) and calcium sulfoaluminate phases (monosulfate (AFm) and ettringite (AFt)). A formation of a chemical phase at a temperature range between 0 °C and 10 °C was observed that may be the source of damage in cementitious materials exposed to high concentration of NaCl solutions. It was found that the interaction of high concentrations of NaCl solution with C3A and calcium sulfoaluminate phases in the cementitious system are the origin of the chemical phase formation.