Accelerated carbonation of reactive MgO cements

Experiments were carried out to determine the conditions for the accelerated carbonation of reactive MgO cements, which are blends of MgO, Portland cement (PC) and pulverised fuel ash (pfa). The samples were exposed to two levels of CO₂ (5 and 20%) in their curing environment at two levels of relative humidity (RH), namely 65 and 98%, RH, and the resulting changes in stiffness and toughness were contrasted with reference samples, which were cured in 98% RH and ambient CO₂ levels. Carbonation of MgO, following its hydration into brucite, yields nesquehonite, whereas that of Portland cement resulted in calcite. The conversion of brucite into nesquehonite increased the strength and toughness of the blends without PC, and this was attributed to the large increase in solid volume due to the carbonation reactions. The accelerated carbonation treatment damaged the blends without MgO, although it did not alter significantly the dynamic stiffness and toughness of the blends containing equal quantities of MgO and PC. Contrary to PC-based cements, in which the highest carbonation rates are found for intermediate humidity levels, the accelerated carbonation of MgO cements requires high levels of humidity and this is attributed to the high water consumption of the carbonation reaction of brucite, whereas carbonation of the PC liberates water