In the concept of radioactive waste disposal developed in the UK, OPC blended with pulverised fuel ash or ground granulated blastfurnace slag is being considered for encapsulation of waste forms, as a material for backfilling and sealing a repository, and for concrete in repository construction.

This paper describes a laboratory study of the long term durability of such cements in contact with sulphate-bearing ground water under accelerated exposure conditions. Mineralogical analysis of the cements over the exposure period, carried out with the aid of scanning electron microscope observations and x-ray diffraction studies, provides an indication of the stability of cementitious phases exposed to an aggressive environment.

It is shown that for intact cement blocks there is minimal interaction between cement and sulphate-bearing ground water. Sulphate minerals produced by the reaction are accommodated in voids in the cement with no adverse effect on the cement structure. However, crystallisation of C-S-H and sulphate minerals along cracks in hardened cement specimens causes expansion of fracture surfaces resulting in a more accessible route for ground water intrusion and radionucleide migration.

The reaction of cement with ground water is greatly accelerated by the use of powdered material. Ettringite formed in the reaction is found to be unstable under these conditions. The mineralogical assemblage after exposure for 1 year is calcite, hydrotalcite, C-S-H and quartz.