Changes in C3S hydration in presence of cellulose ethers

International audienceThe influence of cellulose ethers (CE) on C3S hydration processes was examined in order to improve our knowledge of the retarding effect of cellulose ethers on the cement hydration kinetics. In this frame, the impacts of various cellulose ethers on C3S dissolution, C-S-H nucleation-growth process and portlandite precipitation were investigated. A weak influence of cellulose ethers on the dissolution kinetics of pure C3S phase was observed. In contrast, a significant decrease of the initial amount of C-S-H nuclei and a strong modification of the growth rate of C-S-H were noticed. A slowing down of the portlandite precipitation was also demonstrated in the case of both cement and C3S hydration. CE adsorption behavior clearly highlighted a chemical structure dependence as well as a cement phase dependence. Finally, we supported the conclusion that CE adsorption is doubtless responsible for the various retarding effect observed as a function of CE types

Impact des éthers de cellulose sur l'hydratation du silicate tricalcique

National audienceLes mécanismes d'hydratation du ciment en présence de molécules organiques restent assez mal élucidés. Les éthers de cellulose sont systématiquement introduits dans les formulations des mortiers industriels. Leurs conséquences macroscopiques sur la réaction d'hydratation du ciment sont parfaitement connues, notamment en terme de ralentissement de la cinétique d'hydratation, mais elles n'en restent pas moins inexpliquées. Par conséquent, l'interaction des éthers de cellulose avec la matrice cimentaire a été examinée. Les résultats tendent à montrer que les éthers de cellulose ont un impact mineur sur la dissolution des différentes phases anhydres. Au contraire, ces adjuvants présentent un effet important sur la germination-croissance de certains hydrates comme l'hydrosilicate de calcium et la portlandite

Effect of cellulose ethers on water retention in freshly-mixed mortars

International audienceCellulose ethers are polymers frequently introduced into mortar formulations in order to improve water retention capacity and workability of the freshly-mixed materials. Physico-chemical parameters of these admixtures (molecular weight, granulometry, substitution degrees, etc) seem to have a strong influence on mortar water retention capacity. In this paper, the influence of cellulose ether molecular weight on mortar water retention and its consistency was studied. Moreover, a new method was used to evaluate mortar consistency, named Consistor Baronnie method. This method was confirmed with rheological measurements

Influence of Cellulose Ether Particle Size on Water Retention of Freshly-Mixed Mortars

International audienceCellulose ethers are polymers frequently introduced into mortar formulations in order to improve water retention capacity and workability of the freshly-mixed materials. Physico-chemical parameters of these admixtures (molecular weight, granulometry, substitution degrees, etc) seem to have a strong influence on mortar water retention capacity. In this paper, the influence of cellulose ether particle size was studied. Two behaviors were highlighted regarding the particle size effect on mortar water retention. On the one hand, for cellulose ethers providing intermediate water retention, this parameter is fundamental: the thinner the particles, the better the water retention. The increase in water retention was explained by the rate of dissolution of every fraction which was faster for the thinnest particles. On the other hand, for admixtures providing strong water retention, the effect of this parameter was weaker or not relevant. Indeed, a cellulose ether concentration threshold was noticed, justifying this behavior

Water transport in freshly-mixed mortars containing cellulose ethers

Groupe de travail n°2 : Interactions éther de cellulose-ciment du CEReM (Consortium d'Etudes et de Recherches sur les Mortiers)Polysaccharides are polymers frequently introduced into mortar formulations in order to improve the workability and water retention capacity of the fresh materials [1]. Among all the polysaccharides, cellulose ethers seem to be the most suitable molecules to obtain a mortar with pretty good water retention ability (i.e. higher than 94%). On the other hand, polysaccharides such as starch ethers or starches generally induce a low increase in water retention (i.e. up to 85%). Mortar consistency may contribute to its water retention capacity but this hypothesis should be checked by further investigations. Indeed, cellulose ethers could induce excellent water retention thanks to the possible superposition of two phenomena [2]: - a rheological effect similar to the one caused by other polysaccharides; - an effect that could be specific to cellulose ethers, which remains to be defined. It could be caused by a modification of the porous network in the fresh state, osmotic pressure or the presence of a cellulose ether film playing the role of diffusion barrier. The molar mass distribution of polysaccharides has been determined by size exclusion chromatography. Using such well-known molecules, it should be possible to understand the functioning of three types of cellulose ethers (HEC, HPMC and HEMC) on mortar water retention. Then, the influence of mortar consistency on water retention has been studied in order to verify if there is a relationship between these two parameters. To complete the panel, the impact of starch ethers on water retention and mortar consistency has also been investigated

Some aspects of cellulose ethers influence on water transport and porous structure of cement-based materials

International audienceThis paper evaluates and compares the impact of cellulose ethers (CE) on water transport and porous structure of cement-based materials in both fresh and hardened state. Investigations of the porous network (mercury intrusion porosimetry, apparent density, 2D and 3D observations) emphasize an air-entrained stabilisation depending on CE chemistry. We also highlight that CE chemistry leads to a gradual effect on characteristics of the water transport. The global tendencies brought by CE are: higher water retention, lower capillarity water absorption, lower liquid water permeability as well as higher water vapour permeability. All things considered, we demonstrate that CE chemistry is an important controlling factor on water transport and porous structure evolution

Nuclear magnetic relaxation dispersion investigations of water retention mechanism by cellulose ethers in mortars

International audienceWe show how nuclear magnetic spin-lattice relaxation dispersion of proton-water (NMRD) can be used to elucidate the effect of cellulose ethers on water retention and hydration delay of freshly-mixed white cement pastes. NMRD is useful to determine the surface diffusion coefficient of water, the specific area and the hydration kinetics of the cement-based material. In spite of modifications of the solution's viscosity, we show that the cellulosic derivatives do not modify the surface diffusion coefficient of water. Thus, the mobility of water present inside the medium is not affected by the presence of polymer. However, these admixtures modify significantly the surface fraction of mobile water molecules transiently present at solid surfaces. This quantity measured, for the first time, for all admixed cement pastes is thus relevant to explain the water retention mechanism

HEC influence on cement hydration measured by conductometry

International audienceCellulose ethers are of universal use in factory-made mortars, though their influences on mortar properties at a molecular scale are poorly understood. Recent studies dealt with the influence of hydroxyethylmethyl cellulose (HEMC) and hydroxypropylmethyl cellulose (HPMC) molecular parameters on cement hydration. It was concluded that the degree of substitution is the most relevant factor on cement hydration kinetics, contrary to the molecular weight. Nevertheless, the major role played by the substitution degree has not been verified for other types of cellulose ethers such as hydroxyethyl cellulose (HEC), which generally possesses a higher hydration retarding capacity compared to HPMC and HEMC. In this frame, a study of the impact of HEC molecular parameters on cement hydration was performed. A negligible influence of the molecular weight was observed. Moreover, the results emphasize that the hydroxyethyl group content mainly determines the delay of cement hydration

Influence of the intrinsic characteristics of mortars on their biofouling by pigmented organisms: Comparison between laboratory and field-scale experiments.

International audienceBiodeterioration of mortars by the photosynthetic microorganisms is affected by their intrinsic properties such as porosity, roughness and surface pH. The influence of these parameters was examined using an accelerated fouling test in laboratory and a natural fouling test in the real-world ( in situ). Based on color measurement and image analysis, the impact of each intrinsic parameter was evaluated. The results differed from a scale to the other one. No influence of porosity was measured on the algal colonization rate in the laboratory test whereas, a high porosity seemed to increase slightly the bioreceptivity of the mortars exposed outdoor. The roughness, in both tests, promoted the microbial colonization. However, the discrimination of roughness grades was better in the laboratory test than in the in situ one. The surface pH influenced remarkably on the accelerated biofouling test but not on the in situ one. These dissimilarities resulted from the differences in experimental configurations of the two tests