Influence of Slag Chemistry on the Carbonation of Sodium Sulfate-Activated Slag Cements

In this study, the carbonation resistance of sodium sulfate-activated slag cements was determined. Three slags with different compositions were evaluated. Solid specimens were exposed to natural carbonation for a period of up to 60 days under controlled environmental conditions (23 oC and 65% relative humidity). Specimens were also subjected to accelerated carbonation at 1% CO2 concentration for a period of 28 days, under similar environmental conditions to those adopted for natural carbonation. The phase assemblage evolution post-carbonation was studied using X-ray diffraction and scanning electron microscopy. Results confirmed that the slag chemistry and CO2 concentration influence the type of CaCO3 polymorphs forming in these systems upon carbonation. These materials also present different carbonation rates as a function of the exposure conditions, and binders produced with the slag with the higher Mg/Al ratio seem to have a higher carbonation resistance compared with other cements evaluated. Although very rapid carbonation was observed under accelerated conditions, naturally carbonated conditions did not present a visible carbonation front after 60 days, when measured using the phenolphthalein indicator method. Results suggest that the accelerated carbonation conditions tested in this study are too aggressive to replicate in a meaningful way what is observed in natural carbonated specimens for these binder systems.