Effect of MgO content on the quantitative role of hydrotalcite-like phase in a cement-slag system during carbonation

This paper reports the carbonation characteristics of a cement-slag system exposed to accelerated carbonation testing, and its improved carbonation resistance with the increasing MgO content in blast furnace slag, in which hydrotalcite-like phase plays a key role. Our research showed that the hydrotalcite-like phase started to carbonate upon contacting with the carbonate ions and bound more than 15 wt% CO₂⁻³ in the mildly carbonated and transition areas. This value was positively associated with the magnesia content of slag. Additionally, the proportion shared by hydrotalcite-like phase decreased in the fully carbonated area, and more CO₂ was fixed in the form of calcium carbonate. Consistent with the thermodynamic modelling, the ratio of CO₂ bound in carbonated hydrotalcite-like phase to the total CaCO₃ continued to decrease as the CO₂ ingress progressed. On the other hand, the reaction between hydrotalcite-like phase and CO₂ was found to be volumetrically stable due to binding CO₂ in the interlayer space, and Mg was still distributed within the original slag grain region. Mg/Al atomic ratio of hydrotalcite-like phase remained nearly the same before and after carbonation. Results of this study quantitatively emphasized the favorable effect of hydrotalcite-like phase to improve the carbonation resistance of slag-rich cementitious systems.