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

The calcite/portlandite phase boundary is an important chemical buffer in e nvironmental and engineering applications where cementitious material is us ed. For a given pH or alkali concentration, the solution composition at thi s phase boundary is uniquely defined. Most chemical equilibria computer mod els predict a marked increase in the solubility of calcite with increasing pH along this phase boundary. This has implications in radioactive waste ma nagement because of the potential mobility of C-14-carbonate in high pH wat er systems. In this study, the effect of KOH concentration on the solution composition of calcite/portlandite saturated solutions was determined exper imentally. Solid/water equilibration was approached from both undersaturati on and supersaturation conditions and X-ray diffraction analysis was perfor med on all equilibrated solids. The experimental measurements of calcite so lubility confirm the prediction of a substantive increase in carbonate conc entration at high pH. Dissolved inorganic C (DIC) increases steadily with K OH concentration. At 5 molal KOH, the DIC concentration is 0.4 molal, which corresponds to 10 1 of dissolved CO2 (measured at 25 degrees C and 100 KPa ) for each liter of solution. The triple point in the K2O-CaO-CO2-H2O syste m, where calcite and portlandite are joined by the precipitation of a third K-containing phase, was also delineated in a separate experiment. The thir d solid was identified to be buetschliite - a rare and highly-soluble polym orph of K2Ca(CO3)(2). The triple or invariant point occurs somewhere betwee n 8 and 10 molal KOH and the dissolved carbonate concentration here is equi valent to over 50 1 CO2 gas per liter of solution. (C) 1999 Elsevier Scienc e Ltd. All rights reserved.