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 C3S, C3A, and alkali sulfate (CaSO4, Na2SO4, and K2SO4, respectively) and a fourth one consisting of ettringite and C-S-H, were designed, and several mixtures were prepared. Ca(OH)2 and CaCO3 were optionally added. Specimens stored at 5 degrees C and 20 degrees C in an atmosphere saturated in water vapor for 75months were investigated with X-ray diffraction analysis and scanning electron microscopy. Thaumasite formed in all samples without C3A at 5 degrees C, and at 20 degrees C when sulfates were derived from CaSO4. Solid solution of thaumasite and ettringite was detected in samples incorporating C3A and CaSO4 or Na2SO4 at 5 degrees C. This was also observed at 20 degrees C when CaSO4 was the source of sulfates, whereas ettringite formed in the case of Na2SO4. The absence of thaumasite and ettringite in specimens containing C3A and K2SO4 was attributed to the alkali carbonation process. In the fourth system, decomposition of ettringite occurred in the specimens exempt of Ca(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.