Simulation of Thaumasite Sulfate Attack on Portland Cement Mixtures Using Synthesized Cement Phases

In this work, a simulation of thaumasite sulfate attack on portland cement mixtures was performed, using synthesized cement phases. Three model systems containing ${\mathrm{C}}_3\mathrm{S}$, ${\mathrm{C}}_3\mathrm{A}$, and alkali sulfate (${\mathrm{CaSO}}_4$, ${\mathrm{Na}}_2{\mathrm{SO}}_4$, and ${\mathrm{K}}_2{\mathrm{SO}}_4$, respectively) and a fourth one consisting of ettringite and C−S−H, were designed, and several mixtures were prepared. $\mathrm{Ca}(\mathrm{OH}{)}_2$ and ${\mathrm{CaCO}}_3$ were optionally added. Specimens stored at 5°C and 20°C in an atmosphere saturated in water vapor for 75 months were investigated with X-ray diffraction analysis and scanning electron microscopy. Thaumasite formed in all samples without ${\mathrm{C}}_3\mathrm{A}$ at 5°C, and at 20°C when sulfates were derived from ${\mathrm{CaSO}}_4$. Solid solution of thaumasite and ettringite was detected in samples incorporating ${\mathrm{C}}_3\mathrm{A}$ and ${\mathrm{CaSO}}_4$ or ${\mathrm{Na}}_2{\mathrm{SO}}_4$ at 5°C. This was also observed at 20°C when ${\mathrm{CaSO}}_4$ was the source of sulfates, whereas ettringite formed in the case of ${\mathrm{Na}}_2{\mathrm{SO}}_4$. The absence of thaumasite and ettringite in specimens containing ${\mathrm{C}}_3\mathrm{A}$ and ${\mathrm{K}}_2{\mathrm{SO}}_4$ was attributed to the alkali carbonation process. In the fourth system, decomposition of ettringite occurred in the specimens exempt of ${\mathrm{Ca}(\mathrm{OH})}_2$. The work is supposed to contribute to understanding the primary causes of concrete damage when the concrete is subjected to long-term sulfate attack.