Reactions that occur between a radioactive waste and a cement matrix being used for encapsulation can compromise the integrity of the resulting wasteform. The present paper describes some of the interactions that can occur within Portland cement based systems. It highlights the fact that to achieve successful encapsulation of the wide range of waste types that make up the legacy or historic wastes present in the UK, a toolbox or formulary of different cement systems with differing chemistries is needed. Some potential alternative cement systems are discussed.