Quick setting admixtures

Many substances are known to act as accelerators for concrete. These include soluble inorganic chlorides, bromides, fluorides, carbonates, thiocyanates, nitrites, nitrates, thiosulfates, silicates, aliuninates, alkali hydroxides, and soluble organic compounds such as triethanolamine, calcium formate, calcium acetate, calcium propionate, and calcium butyrate. Some of them are used in combination with water reducers. Quick setting admixture s used in shotcrete applications and which promote setting in a few minutes may contain sodium silicate, sodium aluminate, aluminum chloride, sodium fluoride, strong alkalis, and calcium chloride. Others are solid admixtures such as calcium aluminate, seeds of finely divided Portland cement, silicate minerals, finely divided magnesium carbonate, and calcium carbonate. Of these, calcium chloride has been the most widely used because of its ready availability, low cost, predictable performance characteristics, and successful application over several decades.In some countries the use of calcium chloride is prohibited, in some others, such as Canada and the USA, the use of calcium chloride is permitted provided certain precautions are taken. Attempts have continued to find an effective alternative to calcium chloride because of some of the problems associated with its use. 

The soluble sulfate versus time curve for the cement containing the natural anhydrite is radically changed when CLS is present [130, 131]. The rate of solution of natural anhydrite, which is much slower than that of gypsum or calcium sulfate hemihydrate, is further retarded in the presence of chemical admixtures, which leads to a sulfate-starved system in the concrete, often producing rapid set and an increase in rate of concrete slump loss (Fig. 7.42). Apparently the adsorption of the lignosulfonate by the natural anhydride plus the rapid reaction between the soluble SO and the tricalcium aluminate keeps the SO level near zero concentration for the first 10-20 min causing rapid stiffening and quick set (approximately 15 min). It is also reported that chemical... 

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