MgO content of slag controls phase evolution and structural changes induced by accelerated carbonation in alkali-activated binders

The structural development and carbonation resistance of three silicate-activated slags (AAS) with varying MgO contents (< 7.5 wt.%) are reported. AAS with lower MgO content reacts faster at early age, forming gismondine and C-A-S-H type gels, while in slags with higher MgO content (> 5%), hydrotalcite is identified as the main secondary product in addition to C–A–S–H. Higher extent of reaction and reduced Al incorporation in the C–S–H product are observed with higher MgO content in the slag. These gel chemistry effects, and particularly the formation of hydrotalcite, seem to reduce the susceptibility to carbonation of AAS produced with higher MgO contents, as hydrotalcite appears to act as an internal CO2 sorbent. This is evidenced by an inverse relationship between natural carbonation depth and slag MgO content, for paste samples formulated at constant water/binder ratio. Thus, the carbonation performance of AAS can be enhanced by controlling the chemistry of the precursors.