Sulfate Promotes Compact CaCO Formation and Protects Portland Cement from Supercritical CO Attack.

Supercritical (sc) CO in geologic carbon sequestration (GCS) can chemically and mechanically deteriorate wellbore cement, raising concerns for long-term operations. In contrast to the conventional view of "sulfate attack" on cement, we found that adding 0.15 M sulfate to the acidic brine can significantly reduce the impact of scCO attack on Portland cement, resulting in stronger cement than that found in a sulfate-free system. Scanning electron microscopy revealed a decreased total attack depth in reacted cement in the presence of sulfate. With a newly defined minimum porosity term in reactive transport modeling, our model suggests that sulfate caused CaCO to fill more nanopore spaces in the cement. Small angle X-ray scattering experiments also showed that sulfate can decrease the pore sizes of the carbonate layer. The results suggest that the interactions between sulfate and cement can generate a less porous CaCO layer, which better resists acidic brine. Using this mechanism as a proof-of-concept, we tested the incorporation of sodium sulfate into Portland cement and synthesized new cement composites that show stronger resistance against scCO attacks. These newly discovered interfacial interactions between CaCO and sulfate provide new insights into engineering mechanically strong and green materials for safer GCS.