Paper No. 22-5
Presentation Time: 9:20 AM
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.