Retardation mechanism of cement hydration by a comb polyphosphate superplasticizer

The retardation mechanism of cement hydration as imparted by the addition of polyphosphate comb superplasticizer to model cement containing C₃S, C₃A and calcium sulfate hemihydrate is studied using XRD, ss NMR and calorimetry. Our findings show the retardation effect caused by the direct addition of polyphosphate comb superplasticizer differs significantly to that of conventional polycarboxylate superplasticizers. Conversely to polycarboxylates, polyphosphates, at a low dosage, inhibits the silicate reaction without affecting the aluminate reaction and formation of ettringite. Yet, at high doses, both reactions are hampered, and the induction period extended, followed by accelerated aluminate and silicate reactions.