By invoking analogies to lime mortars of times past, this study examines the carbonation of portlandite (Ca(OH)₂) by carbon dioxide (CO₂) in the liquid and supercritical states as a potential route toward CO₂-neutral cementation. Portlandite carbonation is noted to be rapid; e.g., >80% carbonation of Ca(OH)₂ is achieved in 2 h upon contact with liquid CO₂ at ambient temperatures, and it is only slightly sensitive to the effects of temperature, pressure, and the state of CO₂ over the range of 6 MPa ≤ p ≤ 10 MPa and 8 °C ≤ T ≤ 42 °C. Additional studies suggest that the carbonation of anhydrous ordinary portland cement is slower and far less reliable than that of portlandite. Although cementation is not directly assessed, detailed scanning electron microscopy (SEM) examinations of carbonated microstructures indicate that the carbonation products formed encircle and embed sand grains similar to that observed in lime mortars. The outcomes suggest innovative directions for “carbon-neutral cementation.”