Compressive strength reduction of cement pastes exposed to sodium chloride solutions: Secondary ettringite formation

Abstract

Interactions of sodium chloride (NaCl) with cement hydrates can cause chemical and physical changes in cement pastes, leading to premature deterioration, particularly in cold climates. The present study explores the conditions responsible for the damage and investigates potential chemical changes in cement paste exposed to different concentrations of NaCl solutions (0 to 4 M) at temperatures of 25 °C and 5 °C. Thermogravimetric analysis (TGA) was used to investigate the effect of salt concentrations on the leaching of calcium hydroxide (CH) and Friedel’s salts (FS) formation. X-ray diffraction (XRD) was used to study phase transformations and final phase products associated with the chemical changes. Thermodynamic modeling was performed to predict changes in the hydrated phase assemblage and corresponding volumetric changes due to NaCl ingress. A noticeable reduction in the compressive strength was observed with the increase in NaCl concentrations at 5 °C compared to 25 °C. It was found that temperature plays a vital role in reducing the compressive strength in the presence of NaCl. Leaching of CH was not observed, indicating the stability of the CH phase in the presence of NaCl solution. The FS formation was found to increase as salt concentrations increase up to a point (2 M NaCl) where it started to plateau at higher salt concentrations. Observations from XRD revealed that the secondary ettringite formation is the final phase product of chemical changes that occurred in cement paste exposed to high NaCl concentration and lower temperature. Since the TGA confirmed the stability of FS and CH phases at high NaCl concentrations, the reduction in the compressive strength appeared to be mainly due to secondary ettringite formation. Results from thermodynamic calculations were in excellent agreement with the experimental results, indicating that the damage may be attributed to expansions associated with the secondary ettringite formation.