Precipitation effects inorganic retarders and setting accelerators

4 Precipitation effects inorganic retarders and setting accelerators 

Many salts precipitate additional phases when added to CjS or cement pastes. If the hydroxide of an added cation is less soluble than CH, either it or a basic or complex salt is precipitated. Examples of such precipitates are Mg(OH)2 or AFm phases (K57), Zn(OH)2 or CaZn2(0H) -2H20 (A27) and basic lead nitrate or sulphate (T54). Precipitation will leave in solution all or part of the added anion and an equivalent amount of Ca. The calcium salt of an added anion, if of sufficiently low solubility, is similarly precipitated examples are provided by SO , COj , PO4 , F, silicate, aluminate and borate. In this case, the added cation remains in solution with an equivalent amount of OH 

Retardation is probably caused by the formation of protective layers over the cement grains this has been demonstrated for lead (T54) and zinc (A27) salts and carbonates (U24). In the latter case, it was shown that the layer formed at low concentrations, which caused retardation, was more compact than that formed at higher concentrations, when acceleration occurred. 

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