Some of the physical, chemical and mineralogical characteristics of Portland cement and related materials relevant to nuclear waste immobilization are defined. The ability to condition and maintain a high aqueous pH is undoubtedly the most important factor: it precipitates many species as hydrous oxides or hydroxides. However, in the longer term, many species – cationic as well as anionic – react with one or more cement components forming solubility-limiting phases. Progress on characterization of these phases is outlined. Many of the host phases have natural equivalents and this gives comfort in respect of their likely persistence over geological time. The emerging picture of immobilization in cement suggests that cement compositions can be tailored in terms of pH, Eh and internal chemistry so as to maximize immobilization potential. Nickel, uranium and chromium and chloride are used as examples.
