Role of Magnesium-Stabilized Amorphous Calcium Carbonate in Mitigating the Extent of Carbonation in Alkali-Activated Slag

Oil well cements have received a significant amount of attention in recent years due to their use in high-risk conditions combined with their exposure to extremely aggressive environments. Adequate resistance to temperature, pressure, and carbonation is necessary to ensure the integrity of the well, with conventional cements prone to chemical degradation when exposed to CO2 molecules. Here, the local atomic structural changes occurring during the accelerated carbonation (100% CO2) of a sustainable cement, alkali-activated slag (AAS) have been investigated using in situ X-ray diffraction and pair distribution function analysis. The results reveal that the extent of carbonation-induced chemical degradation, which is governed by the removal of calcium from the calcium-alumino-silicate-hydrate (C-A-S-H) gel, can be reduced by tailoring the precursor chemistry; specifically the magnesium content. High-magnesium AAS pastes are seen to form stable magnesium-containing amorphous calcium carbonate phases, which prevents the removal of additional calcium from the C-A-S-H gel, thereby halting the progress of the carbonation reaction. On the other hand, lower-magnesium AAS pastes form amorphous calcium carbonate which is seen to quickly crystallize into calcite/vaterite, along with additional decalcification of the C-A-S-H gel. Hence, this behavior can be explained by considering (i) the solubility products of the various carbonate polymorphs and (ii) the stability of amorphous calcium/magnesium carbonate, where because of the higher solubility of amorphous calcium carbonate and associated saturation of solution with respect to calcium, additional C-A-S-H gel decalcification cannot proceed when this amorphous phase is present. These results may have important implications for the use of new cementitious materials in extremely aggressive conditions involving CO2 (e.g., enhanced oil recovery and geological storage of CO2), particularly because of the ability to optimize the durability of these materials by controlling the precursor (slag) chemistry.