Structural alterations in alkali-sulfate-activated slag cement pastes induced by natural and accelerated carbonation

The impact of carbonation, induced at different CO₂ concentrations (0.04 or 1 %), in the phase assemblages and compressive strength of Na₂SO₄-activated slag materials was determined. Carbonation led to Ca-bearing phases' decalcification (mainly C-(A)-S-H type gel and ettringite) forming different CaCO₃ polymorphs, independent of the slag composition or carbonation conditions adopted. In specimens exposed to 0.04 % CO₂, a negligible carbonation front was observed, along with a continued phase assemblage evolution and compressive strength gain after 500 days of exposure. Conversely, exposure to 1 % CO₂ led to complete carbonation after 28 days, and a significant compressive strength reduction. Accelerated carbonation does not lead to the development of comparable microstructures to those observed in naturally carbonated pastes. The accelerated carbonation rates were ~ 33 times higher than those determined under natural carbonation exposure. Therefore, accelerated tests are considered unsuitable for predicting the long-term carbonation performance of Na₂SO₄-activated slag cements.