${\mathrm{Na}}_2{\mathrm{SO}}_4$ Salt Weathering of Calcium Sulfoaluminate Cement Paste Partially Immersed in a ${\mathrm{Na}}_2{\mathrm{CO}}_3$ Solution

${\mathrm{Na}}_2{\mathrm{CO}}_3$ is considered to be a salt that causes destructive weathering distress on cementitious materials. In this paper, calcium sulfoaluminate (CSA) cement paste was partially immersed in a 10% ${\mathrm{Na}}_2{\mathrm{CO}}_3$ solution under constant conditions [$20\pm 2°\mathrm{C}$, and $60\pm 5\%$ relative humidity (RH)] and then under alternate sets of conditions ($30\pm 2°\mathrm{C}$ and $80\pm 5\%$ RH for 24 h, then $20\pm 2°\mathrm{C}$, and $60\pm 5\%$ RH for 24 h). The results showed that the paste remained relatively sound after 72 days of exposure under the constant conditions, but the paste was severely damaged after only four cycles under the alternate conditions. The microanalysis results of X-ray diffraction (XRD), environment scanning electron microscope (ESEM), energy dispersive spectrum (EDS), and ion chromatography (IC) indicated that ${\mathrm{Na}}_2{\mathrm{CO}}_3$ could not be identified in the damaged paste, and the mechanism was that ${\mathrm{Na}}_2{\mathrm{SO}}_4$ caused crystallization distress. ${\mathrm{Na}}_2{\mathrm{SO}}_4$ is the product of the chemical reaction between ettringite and a ${\mathrm{Na}}_2{\mathrm{CO}}_3$ solution.