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Influence of a phosphonic-acid-based long-term retarder
on the hydration of clinker and cement; part 1
Dr. J. Rickert, Prof. Dr. G. Thielen, Forschungsinstitut der Zementindustrie, Düsseldorf, Germany
Long-term retarders (LTR) are concrete admixtures that can have a very strong inhibiting effect on the hydration of cement. When used as recycling aids, for example, they enable wash water and concrete residues to be reused directly when fresh concrete is being recycled. The effects of such concrete admixtures, which interfere strongly in the hydration process, on the hydration of clinker and cement are usually only determined empirically and give rise to conflicting opinions. There is still a substantial lack of scientifically based knowledge about the action mechanisms of long-term retarders. The effects of long-term retarders based on phosphonic acid (PBTC) on the hydration of -different clinker phases (C3S, C3A, C4AF), Portland cement clinker and Portland cements were investigated systematically in several research projects so that concretes could be retarded reliably and economically for long periods. The investigations on pure clinker phases showed that the long-term retardation is based essentially on blocking the hydration of the calcium silicates. This blocking is attributed to the formation of a sparingly soluble layer of calcium phosphonate that needs dissolved calcium for its formation. The results of the investigations on pure clinker phases have been confirmed during the investigations on Portland cements. The initial reactions of the aluminates with dissolved sulfate to form fine-grained primary ettringite were not adversely affected by the long-term retarders. Planned long-term retardation of the hydration of the calcium aluminates and silicates was always possible with sulfate-optimized Portland cements. When the amount of available calcium sulfate was too small the hydration of the C3A was strongly accelerated by the addition of long-term retarders, resulting in particularly rapid setting and stiffening. However, the further strength devel-opment was strongly inhibited because, at the same time, the less reactive calcium silicates were retarded. It became apparent that sulfate optimization of the cement cannot be replaced by long-term retarders and that the formation of primary ettringite initiated by calcium sulfate as well as the provision of calcium ions are essential pre-conditions for planned functioning of long-term retarders.
Summary CEMENT INTERNATIONAL 01/2006 pp 102-108
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