Influence of a Long-Term Retarder on the Hydration of Clinker and Cement

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Abstract
This paper summarizes the results of investigations on the effects of a long-term retarder (LTR) based on phosphonic acid (PBTC) on the hydration of several clinker phases (C3S, C3A, C4AF), Portland cement clinker, and Portland cement. It could be shown that long-term retardation is due to a thin sparingly soluble layer of calcium phosphonate on the particle surface. The formation of sparingly soluble calcium phosphonate requires 2.5 moles Ca2+ per mole PBTC. If enough Ca2+ is available, sparingly soluble calcium phosphonate precipitates and forms a layer around the cement particles, which retards further hydration immediately. In the contrary a lack of Ca2+ causes a short-term acceleration in hydration of the reactive clinker phase C3A. The investigations show that a controlled long-term retardation can only be achieved if the cement shows an optimal “natural” setting retardation; hence, when the sulphate ingredient is fully compatible with the reactivity of C3A.

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