Insights into the retarding mechanism of linear sodium polyphosphates on the hydration of α-calcium [alpha-calcium] sulfate hemihydrate

α-calcium sulfate hemihydrate is commonly used as machine applied plaster for interior walls. To ensure that the setting of the binder does not occur within or before the application in general retarders are added. In this work the retarding effect of linear sodium polyphosphates of different average chain length was investigated with regard to their efficiency and working mechanism. It was shown that the effect relies on various interactions and depends on the chain length of the polyphosphates. But, in general, linear sodium polyphosphates react with calcium ions in solution and a voluminous precipitate of calcium polyphosphate is formed. The precipitate adsorbs on the binder particle and strongly decreases its dissolution rate. This effect presents the key process in the retarding mechanism of linear polyphosphates