The invention provides a cementitious composition comprising at least one sulphoaluminate cement which comprises a sulphoaluminate salt of an alkaline earth metal, the cementitious composition being essentially free of other cementitious components. Preferably the at least one sulphoaluminate salt of an alkaline earth metal comprises calcium sulphoaluminate and the cementitious composition additionally comprises at least one further salt of an alkaline earth metal, preferably calcium sulphate. The invention also provides a method for the encapsulation of materials, the method comprising treating the materials with the said at least one cementitious composition. Preferably, the materials which are encapsulated according to the method of the invention comprise waste materials generated in the nuclear processing industry comprising amphoteric reactive metals.