The effect of wet supercritical CO2 (90 °C and 20 MPa) on the performance of cement paste (PC) modified with bacterial nanocellulose (BNC) was investigated. The pore structure of carbonated cement shows a clogging over the outer rim of the samples. In contrast, near the sample core, the characteristic peak of pore size distribution shifted towards smaller pores analyzed by mercury intrusion porosimetry. The effect of the carbonation overtime on mechanical properties shows increasing alteration. XRD results show more crystalline phases of hydrated cement in the BNC samples before carbonation. Cement reinforced with BNC shows lower density, a reduction in its porosity, and experiences fewer porosity changes at the cement core. Furthermore, its mechanical performance was less affected by the carbonation process.