Carbonization of Class G well cement containing metakaolin under supercritical and saturated environments

[Display omitted] •Well cement alteration under relevant subsurface conditions at high pressures and temperatures.•The presence of CaCO3 in well cement samples shows that a carbonization reaction has occurred.•The more extensive carbonization was under the CO2 saturated solution than under supercritical CO2.•Larger CO2 ingress was observed when cement slurries were mixed with MK. A well-cement alteration in a CO2-rich environment can affect the well integrity, and, consequently, compromise the overall safety of an external ecosystem. The objective of this study was to evaluate the carbonization of Class G well cement and metakaolin modified Class G cement samples exposed to CO2 at 323 K and 15 MPa under supercritical state and in the saturated solution. Cement alternation was more rapid and extensive under a CO2 saturated solution than under supercritical CO2. Metakaolin amended Class G cement samples showed improved mechanical properties, while their carbonation resistance was impaired and reduced. X-ray Powder Diffraction confirmed that CaCO3 polymorphs were present in greater proportions in the metakaolin-modified samples and when exposed to a CO2-saturated salt solution. Moreover, samples containing metakaolin have a larger specific surface area, and, consequently, can absorb more CO2. The micrographs showed that the cement samples exposed to a CO2-rich environment have rough and porous surfaces containing CaCO3 polymorphic crystalline forms.