Study of the retarding mechanism of linear sodium polyphosphates on α-calcium sulfate hemihydrate

Linear sodium polyphosphates possessing different average chain lengths were characterized and studied as retarder for α-calcium sulfate hemihydrate. Their effect on the hydration kinetics of aqueous α-CaSO4·0.5H2O pastes was quantified by heat flow calorimetry, mini slump test and determination of time-dependent degree of hydration. Already at very low dosages (0.025% bwob) polyphosphates strongly retard hydration of hemihydrate. The retarding effect decreases with increasing chain length. Interaction between polyphosphates and calcium ions and Ca2 + concentration dependent formation of insoluble precipitates were studied via pH and turbidity measurements. Additionally, the influence of polyphosphates on individual hydration steps (binder dissolution and gypsum precipitation) was determined. It was found that the working mechanism of polyphosphates mainly relies on a strong decrease of binder dissolution rate while delayed gypsum precipitation from the solution and inhibition of dihydrate crystal growth by adsorption of polyphosphate play a minor role.