Improved volume stability of mortar bars exposed to magnesium sulfate in the presence of bicarbonate ions

The effect of the simultaneous occurrence of sulfate and bicarbonate ions in magnesium containing solutions was investigated for a CEM I and CEM III/B mortar under laboratory conditions. The presence of bicarbonate ions, which is common in natural waters, prevents the typical expansion and surface erosion of CEM I mortars and the surface deterioration of CEM III/B mortars. The chemical effect of the bicarbonate ions is twofold. The precipitation of CaCO3 lowers the availability of CaO for the formation of additional sulfate bearing phases like ettringite and gypsum. The second effect is the reduction of supersaturation of the pore solution with respect to ettringite. The absence of deterioration is linked to the formation of only small amounts of ettringite and to the absence of gypsum in the affected zones. Thermodynamic modeling suggests that relatively small bicarbonate concentrations suffice to prevent damage, contribute to the scarcity of damage under natural exposure conditions and to the diversity of results under laboratory exposure conditions.