Calcium sulfoaluminate, C4A3S or CSA, often known as Klein s compound, can be readily synthesized as a single phase at about 1300°C. It is also formed along with belite and a ferrite phase when limestone is reacted with various minerals or waste products at 1200–1300°C. The presence of CSA can lead to early strength development, and such systems may be regarded as low-energy cements, providing environmental benefits, notably emission of less carbon dioxide than in the production of Portland cement. Under controlled, specified conditions, usually including addition of gypsum, the hydration of CSA gives acceptable setting times and good strength development. These properties are based on the rapid formation of ettringite and variable quantities of an amorphous gel phase. These characteristics have already led to the formulation of various commercial products taking advantage of these special properties for applications in building chemistry and mining. Further systematic research is required leading to greater application of these novel cements.