Production of precipitated calcium carbonate (PCC) via carbon dioxide (CO2) pressure-swing mineral carbonation is a potential way to utilise calcium-rich steel slag and carbon dioxide. Calcium supersaturation and slag surface passivation are two aspects of the calcium extraction step that strongly influence the choice of operating conditions necessary for rapid and complete calcium leaching, which are investigated in the present work. To investigate these aspects, slag dissolution characteristics were studied in a closed high-pressure batch-reactor taking care to eliminate gas-liquid mass transport limitations. This experimental design has two distinct advantages: (1) rapid CO2 absorption necessary for dissolution under acidic conditions and to gain insights into calcium dissolution kinetics and (2) the closed system allowing measurement of the drop in reactor pressure, which along with elemental analysis of leachate is sufficient to determine the ionic species concentration and solution saturation state. The results provide evidence against surface passivation of residual slag by silica or calcium carbonate layers. Further, the experiments confirm high supersaturation with respect to calcite during the dissolution step, which we hypothesise to be a consequence of unfavourable calcite precipitation kinetics due to the low pH and high calcium to carbonate ion ratio. The results show scope for further enhancement in calcium solubility, up to the solubility limit of amorphous calcium carbonate, which can substantially reduce the water volume and CO2 pressure required for dissolution. Pressure-swing to atmospheric pressure led to spontaneous co-precipitation of rhombohedral calcite and amorphous silica, also a paper-filler with similar optical properties to PCC, with impurities less than 1.5 wt%.