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.

Portland cement mortars were made from natural quartz and crushed limestone sands, and combinations of thetwo. Three types of concrete were made from granite and quartz sand, coarse and fine flint, and coarse and finelimestone. Mortar and concrete prisms were cast and cured at 95–100°C and subsequently stored at roomtemperature under water. The quartz sand mortars exhibited large expansions within the first year but thelimestone mortars remained stable even after 6 years. The ultimate expansion of those mortars with combinationsof limestone and quartz aggregate had intermediate values. The granite aggregate concretes expanded readily butthe limestone and the flint concretes started to expand only after very long induction periods. X ray diffraction,scanning electron microscopy with back-scattered electron imaging and X ray microanalysis were used to examinethe composition and microstructure of the hydration products. Differences in the features of ettringite bands thatdeveloped in the various heat-cured mortars and concretes were observed. It was shown that the rough surface ofthe limestone sand and aggregate particles was able to interrupt the development of an ettringite band in thepaste-aggregate transition zone. Microstructural observations give support to the growth of ettringite bands asthe mechanism responsible for the observed expansion.

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