| Summary: | Thermodynamic modelling is a relatively new technique with which to study the interactions of ionic species in solution. Using the computer program PHREEQE, and the modified version PHRQPITZ, the effects of a range of reactive anions and cations on the components of cementitious systems are studied with respect to cement barrier performance in a nuclear waste repository. A review is made of the underlying thermodynamic theory, and of the development of the technique of thermodynamic modelling, before concentrating on the program PHREEQE and the method by which it performs calculations. The technique is then applied to the CaO-SiO<sub>2</sub>-H<sub>2</sub>O system under the influence of sulfate, carbonate, chloride, arsenite, Na<sub>2</sub>O, K<sub>2</sub>O, NaCl and MgSO<sub>4</sub>. The effects of NaCl and MgSO<sub>4</sub> on certain sub-systems within the CaO-Al<sub>2</sub>O<sub>3</sub>-H<sub>2</sub>O system are also studied. It is calculated that Ca(OH)<sub>2</sub> and CSH are the main pH maintaining phases, of which CSH has the greater stability under the influence of the species considered. AFm and AFt phases also act as pH buffers, but they are quantitatively less important. A review is made of solid solutions between members of the AFm and AFt groups of minerals. Experimental methods are used to study the extent of solid solution between monosulfoaluminate and Friedel's salt, and between monosulfoaluminate and C<sub>4</sub>AH<sub>13</sub>. In the case of the former, no solid solution is observed, but an intermediate phase, designated Kuzel's salt, is observed. In the latter case, two areas of solid solution at low and high sulfate content, separated by a miscibility gap, are detected. No evidence is found to support the existence of calcium hemisulfoaluminate.
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