Some of the physical, chemical and mineralogical characteristics of Portlan
d cement and related materials relevant to nuclear waste immobilization are
defined. The ability to condition and maintain a high aqueous pH is undoub
tedly the most important factor: it precipitates many species as hydrous ox
ides or hydroxides. However, in the longer term, many species - cationic as
well as anionic - react with one or more cement components forming solubil
ity-limiting phases. Progress on characterization of these phases is outlin
ed. 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 b
e tailored in terms of pH, Eh and internal chemistry so as to maximize immo
bilization potential. Nickel, uranium and chromium and chloride are used as
examples.