A mixer for mixing fluid or particulate materials is disclosed. The mixer comprises a mixing drum, within which the material can be mixed, said mixing drum including one or more openings through which material to be mixed can be charged into the drum and through which the mixed material can be discharged from the drum, characterised in that the mixer includes an outer structure of dimensions which enable the mixer to be stackable, detachable and transportable. A number of the mixers can be stacked one on top of the other so as to minimise the floor space or ground area taken up by the mixers. If the dimensions of the outer structure are to conform to the conventional containers, it will enable the mixers to be stackable with these conventional containers, thus resulting in more efficient and economic logistic handling of such mixers when delivered by international freight companies.