COUPLING THE CHEMICAL AND MECHANICAL ALTERATIONS OF PORTLAND CEMENT REACTED IN THE PRESENCE OF SULFATE IONS UNDER GEOLOGIC CO2 SEQUESTRATION CONDITIONS

Portland cement is used in building CO₂ injection wells for geologic CO₂ sequestration (GCS), and it can be mechanically deteriorated by chemical reactions with CO₂. Complicating the picture, sulfate ions are usually abundant in brine in subsurface sites. To understand the effect of sulfate ions in CO₂ attack on cement, and to quantitatively relate the resulting chemical and mechanical alterations, we analyzed cement paste samples reacted under GCS relevant conditions. The 10-day experiments were conducted at 95 ^oC under 100 bar of either N₂ or CO₂ in the headspace of the reactor, and in a brine solution with 0.5 M ionic strength, tuned with NaCl. The effects of 0.05 M of sulfate ions were examined. When cement samples were attacked by CO₂, a layered structure developed. From the core to the surface of the cement, there were an intact core, a weak portlandite-depleted zone with microcracks, a dense carbonated layer, and a surface layer. The presence of sulfate ions was found to mitigate CO₂ attack, and to produce a thinner portlandite-depleted zone in the cement matrix. As a consequence, the strength deterioration in the presence of sulfate was only ~50%, compared to the ~80% decrease in cement samples reacted with CO₂ in the absence of sulfate. We hypothesize that the protective effect of sulfate ions resulted from the passivating effects of sulfate adsorption/precipitation on CaCO₃ grains in the carbonated layer, which produced a denser carbonated layer. This result highlights the importance of understanding the role of molecular and nano-scale interfacial physical interactions in mechanical properties of bulk materials, and has applications to subsurface or surface engineered operations.