Interactions between Cement and Combined Concrete Admixtures. The Influence on Cement Paste Rheology

摘要

During the last decades, innovative concrete applications have been theresult of technological developments as new processing techniques, the use of concrete admixtures and advanced materials. All these improvements increase the demand for better performing concrete with enhanced workability. However, the lack of reliable engineering tools in traditional concrete production methods has led to some inconsistent control of the workability. Therefore, today’s research is increasingly investigating advanced models that capture the flow behavior of fresh concrete as accurately as possible. These models are continuously improving butgenerally keep facing problems which are linked to the agglomeration ofthe cement particles and to the implementation of combined concrete admixtures. The objective of this work is to investigate the rheological mechanism in cement paste for the combined use of superplasticizer, retarder and accelerator. In particular, an effect on the particle agglomeration is aimed for in order to contribute to a more fundamental understanding of the concrete’s flow behavior. Concerning the superplasticizer impact on cement paste, thermodynamic modelling and mineralogy studies led to the conclusion that the superplasticizer can change the hydrate morphology to such an extent that the interparticle contact forces are modified. The extra addition of retarder led to a densified polymer layer at the cement grain surface which contributed to the steric stabilization of the cement paste. An additional electrostatic effect and the induced changes in hydrate morphology are also demonstratedto contribute to the low paste viscosity. With the extra addition of a calcium salt accelerator to the cement paste, an interstitial structure was assumed to diminish the rheological effect of the polymer layer.All the developed conceptual mechanisms were implemented in a coherent agglomeration model, based on measurable parameters. In this model, the internal and external hydrates in a cement agglomerate were defined and quantified. Generally, the cumulative amount of hydrates within the agglomerates influenced the agglomerate stability and the amount of external hydrates determined the reagglomeration rate. For the superplasticizer only, a proportional relation was found between the change in agglomeration rate and the external agglomerate connectivity while, for the extra addition of retarder and accelerator, a reverse relation was found. The latter was attributed to a mechanical contribution of the interstitial volume. On the one hand, the retarder creates a source of slowly hydratingnuclei and particles in that volume and, on the other hand, these particles are expected to coagulate due to the accelerator addition. In the second case, the interstitial volume also delivered a contribution to thestress resistance of the cement paste at rest.This fundamental research combines dedicated analytical methods and conceptual models to improve the understanding of particle agglomeration and contributes to a more extensive insight into the concrete rheology.