The calcite/portlandite phase boundary: enhanced calcite solubility at high pH

The calcite/portlandite phase boundary is an important chemical buffer in environmental and engineering applications where cementitious material is used. For a given pH or alkali concentration, the solution composition at this phase boundary is uniquely defined. Most chemical equilibria computer models predict a marked increase in the solubility of calcite with increasing pH along this phase boundary. This has implications in radioactive waste management because of the potential mobility of 14C-carbonate in high pH water systems. In this study, the effect of KOH concentration on the solution composition of calcite/portlandite saturated solutions was determined experimentally. Solid/water equilibration was approached from both undersaturation and supersaturation conditions and X-ray diffraction analysis was performed on all equilibrated solids. The experimental measurements of calcite solubility confirm the prediction of a substantive increase in carbonate concentration at high pH. Dissolved inorganic C (DIC) increases steadily with KOH concentration. At 5 molal KOH, the DIC concentration is 0.4 molal, which corresponds to 10 l of dissolved CO2 (measured at 25°C and 100 KPa) for each liter of solution. The triple point in the K2O–CaO–CO2–H2O system, where calcite and portlandite are joined by the precipitation of a third K-containing phase, was also delineated in a separate experiment. The third solid was identified to be buetschliite — a rare and highly-soluble polymorph of K2Ca(CO3)2. The triple or invariant point occurs somewhere between 8 and 10 molal KOH and the dissolved carbonate concentration here is equivalent to over 50 l CO2 gas per liter of solution.