Damage in cement pastes exposed to NaCl solutions

Friedel's salt forms in cementitious materials exposed to NaCl solutions. This paper quantifies the amount of Friedel's salt that forms and relates the formation of Friedel's salt to the damage. Thermogravimetric analysis (TGA) is used to quantify the amount of Friedel's salt. The volume change associated with the formation of Friedel's salt is measured. Flexural strength is measured for cement pastes exposed to NaCl solutions using the ball-on-three-ball test. While Friedel's salt was observed, no calcium oxychloride was detected in cement pastes exposed to NaCl solutions. As the NaCl concentration increases, the amount of Friedel's salt formed in the cement pastes increases before remaining constant at high concentrations. For the same NaCl concentration, similar amounts of Friedel's salt are obtained in cement pastes with varying water-to-cementitious materials ratios (w/cm), while a greater amount of Friedel's salt exists in cement pastes with a higher fly ash replacement level. As the NaCl solution concentration increases, the flexural strength of the cement paste decreases. There is a greater flexural strength reduction in the cement pastes with higher w/cm. For cement pastes with a higher fly ash replacement, the flexural strength shows a smaller decrease when exposed to NaCl. Since TGA confirmed little leaching of Ca(OH)2 in the cement paste, the strength reduction appears to be mainly due to the formation of Friedel's salt. Little volume change is associated with the formation of Friedel's salt, which indicates that the damage may primarily be attributed to the crystallization pressure associated with the formation of Friedel's salt in pores.