A range of cements based on calcium sulphoaluminate, 4CaO·3Al2O3·SO3. ‘C4A3S’ are commercially available. The phase contents of some commercial clinkers have been determined. Their hydration behaviour, including calorimetric heat evolutions, are reported. The clinker mineralogies control reactivity and dimensional stability; typically, sulphoaluminate formulations with added lime, gypsum and calcium aluminates are expansive, but those consisting of C4A3S and belite (Ca2SiO4) hydrate rapidly but give rise to dimensionally stable products. The role of accelerating and retarding admixtures is explored. The presence of admixes has a secondary role in controlling the dimensional stability of belite-sulphoaluminate formulations.