Adsorption de polyélectrolytes en relation avec la rhéologie d'une suspension de carbonate de calcium (original) (raw)
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Adsorption of polyelectrolytes and rheology of suspensions of calcium carbonate
This study focuses on the superplasticizers which allow to increase the concentration of the particles (cement) of a concrete while maintaining good handling. The superplasticizers analyzed here are polyelectrolytic: polymethacrylates grafted with side polyethyleneoxide chains (PEO) of different lengths. We present a study of these molecules in solution, including the influence of OH -ions on the PEO chains. We analyse the effect of the superplasticizers on the rheology of suspensions of CaCO3 particles of micron and nanometer size ranges..
Cement Concrete Composites, 2009
This parametric study links the molecular structure of a carboxylate-type of superplasticizer with their performance in cement pastes with different C3A-contents. Beside the variation of the C3A-content, the experimental synthesized superplasticizers have been varied by polyethylene-oxide side chain density and length. The connection between the superplasticizers, their effect on workability properties and retardation phenomenon and the dependency of C3A-content in the cement paste has been investigated.The characteristic interaction phenomenons between different PCE-architectures and different C3A-contents have been examined by calorimetric, rheological, adsorption, and zeta potential measurements. This study shows that with decreasing side chain density the PCE molecules adsorb stronger and thus, lower the yield stress of a cement paste by steric stabilization. It is also shown that PCE molecules with long side chains delaying the setting of the cement paste to less extend than PCE molecules with shorter side chains. Consequently, in terms of optimization of the molecular architecture, good workability can be achieved by addition of highly charged PCE with long side chains. The latter minimizes undesired retardation phenomena.
Multi-method approach to study influence of superplasticizers on cement suspensions
Cement and Concrete Research, 2011
Superplasticizers are widely used in concrete processing to increase the rheological properties of hardening pastes. In this study, different techniques (rheology, adsorption, atomic force microscopy—AFM, and ζ-potential) are used to characterize the impact of polycarboxylate-ether based superplasticizer (PCE) on particle suspensions. Results obtained with two cements and two inert powders (MgO and calcite) show that superplasticizer efficiency is strongly influenced by polymer architecture and by the ionic species present in solution. Additionally, experiments performed with AFM and ζ-potential contributed to characterize dispersion forces exerted by superplasticizers at the solid–liquid interface. The application of plateau AFM-tips coated with platinum reveals that dispersion forces depends on the presence of ions in solution, and that multilayer formation occurs with certain superplasticizer types. A further conclusion includes the idea that the PCE has a lubricating effect between adjacent particles and PCE increases surface wettability.
Superplasticizers are commonly used in modern cementitious building materials like self-compacting concrete. However, there are still many unsolved problems concerning cement/superplasticizer compatibility like poor flow behaviour, early slump loss or strong retardation. The objective of this study is to link the molecular structure of polycarboxy-late-ether type superplasticizers with their performance in cementitious systems in order to develop new enhanced products. Different experimental superplasticizers have been synthesized and applied in cement pastes. The examined cements differed mainly in their C 3 A-content. Various methods have been used to characterize the cement -superplasticizer interactions, like rheological measurements, conduction calorimetry, and determination of the adsorption isotherm by TOC analysis, as well as XRD and TGA measurements. As result of this study, characteristic correlations between molecular structure of the superplasticizer and C 3 A-content of the cement with respect to adsorption behaviour, rheology and retardation could be determined.
Adsorption of polyelectrolytes and its influence on the rheology.pdf
In this study the influence of polycarboxylate-based polyelectrolytes on the particle interaction among tricalcium silicate (C3S, main clinker phase), calcium silicate hydrates (C–S–H), and calcium aluminate sulfate hydrates (ettringite) (main hydration phases) has been examined. These phases are the constituents of major concern during early hydration of cement suspensions. The results of zeta potential measurements on single mineral phase experiments show that the phases C3S and C–S–H are positively charged in synthetic pore solution (liquid phase of hydrating cement suspension), whereas the ettringite is negatively charged. Due to these opposite charges, ettringite crystals should coagulate with C–S–H phases and/or deposit on surfaces of the much larger C3S clinker particles. This behavior was proven by cryo-microscopic analysis of high-pressure frozen cement suspensions, which illustrates the consequences of colloidal mechanisms on the microstructure of early cement suspensions. Furthermore, it is shown that the polyelectrolytes have a much higher adsorption affinity to ettringite surfaces (hydrate phase) compared to silicate surfaces. However, the results from rheology experiments reveal that the presence of polyelectrolytes has a strong impact on the suspension properties of all investigated mineral phases by decreasing yield stress and plastic viscosity. From the results it can be concluded that the ettringite is the dominant mineral phase in terms of the state of dispersion which includes particle–particle and particle–polyelectrolyte interaction in the bulk cement system.
Influence of superplasticizer adsorption on the rheology of cement paste
The production of high performance concrete requires the development of new superplasticizers with increased efficiencies. The knowledge of the mechanisms by which these admixtures act is of primary importance for the design of new polymers. They are believed to be linked to the dispersion cement agglomerates by electrostatic, steric or entropic repulsion, or improved wetting (capillary effect). The two first are directly linked to adsorbed polymers. The third is linked to both adsorbed and solvated polymers, while the last is mainly linked to polymers remaining in solution.
2013
In this thesis, the impact of sulfate ions on the sorption behavior (adsorption and/or chemisorption) of polycarboxylates on different cementitious materials was investigated. The study demonstrates that in general, the impact of sulfate on the adsorption behavior of PCEs is dependent on the contents of C 3 A and alkali sulfate present in a cement sample, the concentration of sulfate ions in the pore solution, the dosage of polymer added to the slurry, the chemical structure of the PCE used (type and modification), the anionic charge amount of the PCE, the packing density of the Ca 2+ ion layer adsorbed onto negatively charged sites occurring on the mineral surface of the particle, and early and delayed addition of the superplasticizer.
Analysis of superplasticizers used in concrete
Analusis, 1998
O rganic poly m e rs are large ly used in the processing of ceramic powders in order to improve rheological properties. The industry of hydraulic concrete (initially concretes are also pow d e rs ) currently uses admixtures called superplasticizers, which allow the production of high performance concrete. Although these admixtures are widely used, their mechanisms of action remain unclear. Measuring the adsorption of these polym e rs onto the surface of cement part i cles in aqueous suspensions is usually the first step in related studies. It is often based on a global analysis of the admixture in the aqueous phase, u s u a l ly by colorimetry or total organic carbon a n a ly s i s . This traditional appro a ch totally neglects effects linked to molecular mass distribution.
Interaction of polycarboxylate-based superplasticizers.pdf
This parametric study links the molecular structure of a carboxylate-type of superplasticizer with their performance in cement pastes with different C 3 A-contents. Beside the variation of the C 3 A-content, the experimental synthesized superplasticizers have been varied by polyethylene-oxide side chain density and length. The connection between the superplasticizers, their effect on workability properties and retardation phenomenon and the dependency of C 3 A-content in the cement paste has been investigated.
Interaction of superplasticizers with model powders in a highly alkaline medium
It is broadly recognized that the adsorption of superplasticizers on cement particles is a key factor in determining the rheology of concrete. In order to avoid the problems linked to the hydration of cement, the adsorption of superplasticizers is often studied on unreactive model powders. However, in order for the model system to remain as close as possible to cement, the surface should have a similar charge and a similar chemical nature. Furthermore, the pH of the solution should be close to that of the hydrating cement (about 12.5). Under these conditions, cement has been shown to have a positively charged surface. The model powders used in this study were Mg(OH) 2 and dead burnt MgO, which have nominal isoelectric points of 12.0 and 12.4 respectively, and which are chemically similar to Ca(OH) 2 and CaO.