Influence of hydroxypropylguars on rheological behavior of cement-based mortars

International audienceHydroxypropylguars (HPGs) are used as water retention agents in modern factory-made mortars. Nevertheless, these molecules can also impact the rheological behavior of cement-based materials. The influence of HPG and its dosage on mortar rheological properties was thus investigated thanks to a suitable measurement procedure. HPG allows keeping a positive yield stress value while the yield stress of hydroxypropyl methyl cellulose (HPMC) mortars was found to decrease with an increase in dosage. HPG increases the shear-thinning behavior and the consistency of mortars. The study of pore solution viscosity suggests that the entanglement of HPG coils beyond a threshold dosage is crucial to understand the rheological macroscopic behavior of HPG-admixed mortars. Nevertheless, the increase in mortar viscosity induced by HPG was lower than expected which reveals additional and specific repulsive forces induced by polysaccharides

Modification of cement hydration at early ages by natural and heated wood

International audienceHeat treatments of wood are widely used for the reduction of wood swelling and dimensional instability of wood-cement composites. The effect of natural and heated wood on the hydration of cement at early ages was investigated by isothermal calorimetry, thermogravimetry (TGA) and Fourier transform infrared (FTIR) spectroscopy. The addition of wood strongly delays and inhibits the hydration of the silicate phases. Consequently, the amount of portlandite is lower in composites than in neat cement. Approximately 30% of the inhibition of portlandite precipitation is due to an increase of calcium carbonate content in cement paste. The absorption of a part of water by wood produces a decrease in gypsum consumption. Nevertheless, the ettringite formation is not significantly affected since a diffusion of sulfate ions from wood occurs

Thermo-gravimetric analysis as a tool for the optimisation of wood heat treatment parameters

9 pagesRetification is a heat treatment that decreases the swelling of wood and increases its resistance to fungal attack. It consists in a mild pyrolysis of wood (180°C-260°C) that takes place in a non oxidative atmosphere (nitrogen). The industrial development of retification requires optimisation of the treatment temperature and duration. In order to enhance the homogeneity of temperature in the furnace, and to avoid exothermic reaction, low temperatures seem to be preferable to high temperature. On the contrary, duration and temperature of treatment have to be high enough to provide good biological resistance and stabilization to the wood. However, high temperatures lead to a loss of mechanical strength. A question arises from these previous observations: is there any equivalence between a treatment of short time carried out at high temperature and a treatment of longer time at lower temperature? Answering this question can help to optimise rétification temperature and duration. The purpose of this study is to evaluate the relevance of a “time temperature equivalence” (TTE) for wood pyrolysis in the temperature range of retification. The principle of TTE is adapted from the study of wood viscoelastic properties. In this study, it is applied to the rate of anhydrous weight loss during wood pyrolysis. Thermo-gravimetric analysis (TGA) were performed on maritime pine (Pinus pinaster Ait.-) and beech (Fagus sylvatica) wood powder. Isothermal degradations were carried out at different temperatures ranging from 160°C to 260°C. A specific data analyse was carried out on the TGA derivative (DTG) in order to assess the relevance of the TTE in the temperature range of retification. It gave interpretable results for maritime pine, but not for beech. It showed that for maritime pine wood the TTE is confirmed from 200°C to 220°C, and not confirmed for temperatures superior to 230°C. An optimization of the temperature and time of treatment is thus possible

Importance of coil-overlapping for the effectiveness of hydroxypropylguars as water retention agent in cement-based mortars

International audienceThis paper is devoted to the role of hydroxypropylguars (HPG) on water retention (WR) capacities of cement-based mortars. WR and pore solution viscosity of mortars formulated with different dosage and kind of HPG were investigated. The results allowed us to rule-out that WR is only governed by pore solution viscosity. A sharp change in slope was observed on the curve WR vs. polymer dosage. This was attributed to the threshold dosage corresponding to the network formation of hydrocolloidal associated HPG molecules in pore solution. Below this dosage, no effect of HPG on WR was obtained. Above this dosage, HPG aggregates may stop the water flow by plugging the porous network of a thin HPG-enriched filter cake at the interface mortar-substrate. This study showed moreover that additional substitution of alkyl chain on HPG improves its effectiveness at low dosage. This was attributed to an enhancement of overlapping

Hydration and rheology of sulfoaluminate cements (CSA) in presence of polycarboxylate superplasticizers (PCE) and citric acid

ISSN: 2523-935XInternational audienceIt is admitted that the cement industry contributes for about 5-7% of the total man-made CO2 emissions. A possible way for decreasing these CO2 emissions is the development of alternative clinkers with lower proportion of limestone in the raw materials. Calcium sulfoaluminate cements (CSA) are regarded as a promising low CO2 alternative to Portland cements. Generally, with a sufficient amount of calcium sulfates, the main hydration product of these cements is ettringite, which strongly affects the workability of CSA pastes. Polycarboxylate superplasticizers (PCE), combined with retarders, can be used with the aim to decrease the yield stress and the viscosity of this type of pastes. However, the multitude of components of these cements makes it difficult to understand the effect of these additives on the hydration process and on the properties of pastes. The aim of this work is to provide a better understanding of the: (i) effect of the molecular structure and the dosage of PCE on the hydration and rheology of CSA (ii) effect of citric acid on the dispersing effectiveness of PCE . Three PCEs, with the same chemical structure but different molecular structure, were investigated. Vicat tests and isothermal calorimetry were used to describe the hydration process. While, Rheological properties were characterized with mini-cone tests. Adsorption measurements were carried out with Total Organic Carbon analyzer and ion chromatography. The results reveal a competitive adsorption between citric acid and PCEs, which decreases the dispersing effectiveness of the latter

Study of efflorescence forming process on cementitious materials // Etude du processus de formation des efflorescences sur des matériaux cimentaires

International audienceEfflorescence is a white deposit of calcium carbonate on the surface of cementitious materials. It appears under specific climatic conditions. Besides the aesthetic problems, an important economical problem also exists, especially for colored materials. Indeed, this phenomenon is a cause for rejection of products by customers. The aims of the study presented herein are to understand the mechanisms involved and identify the main parameters which control efflorescence formation. A procedure was previously performed to produce primary efflorescence in a climatic chamber. As pozzolans are frequently used in order to decrease the intensity of efflorescence, substitution of cement by different pozzolans was studied. This latest research allowed the Authors to underline which parameters were modified by the substitutions and to link decreases in the intensity of efflorescence with those modifications. // Les efflorescences forment un dépôt blanc de carbonate de calcium à la surface de matériaux cimentaires. Elles apparaissent pour des conditions climatiques spécifiques. Au delà du problème esthétique, un important problème économique intervient car ce phénomène peut être la cause du rejet d'un produit par les clients. Les objectifs de l'étude sont la compréhension des mécanismes et l'identification des paramètres principaux qui contrôlent la formation d'efflorescences. Au préalable, un test a été mis au point dans une chambre climatique pour reproduire des efflorescences. Comme les pouzzolanes sont souvent utilisées pour atténuer les efflorescences, la substitution du ciment par différentes pouzzolanes a été étudiée. Cette étude a permis d'identifier les paramètres affectés par ces substitutions et de relier ces modifications à l'atténuation des efflorescences

Etude physico-chimique d'un composite ciment-bois durant les premières heures d'hydratation.

3 pagesDans le domaine des nouveaux matériaux de construction, l'utilisation de composites à base ciment et de matériaux organiques devient essentiel. Elle permet d'améliorer ou de modifier les propriétés d'un ciment. Dans ce contexte, le matériau composite ciment-fibre de bois est extrêmement intéressant de par ses propriétés d'isolation phonique et acoustique ainsi que par sa faible masse spécifique. Le but de ce travail est de mieux comprendre les interactions chimiques entre le bois et le ciment ainsi que ces conséquences sur l'hydratation du ciment

Energy requirement for fine grinding of torrefied wood

International audienceThe purpose of this study is to investigate the influence of torrefaction on wood grinding energy. Wood chips were torrefied at different temperatures and durations. The energy required to obtain fine powder was measured. Particle size analyses were carried out on each powder sample. It is showed that torrefaction decreases both grinding energy and particle size distribution. A criterion to compare grindability of natural and torrefied wood is proposed. It takes into account both grinding energy and particle size distribution. It accounts the energy required for grinding particles to sizes inferior to 200 μm, for given grinding conditions. Torrefaction is characterised by the anhydrous weight loss (AWL) of wood. For AWL inferior to around 8%, grinding energy decreases fast. Over 8%, grinding energy decreases at a slow rate. Particle size distribution decreases linearly as the AWL increases. Both for spruce and beech, the grinding criterion is decreased of 93% when the AWL is around 28%

Modelling anhydrous weight loss of wood chips during torrefaction in a pilot kiln

International audienceBeech and spruce chips were torrefied in a batch rotating pilot kiln. For each torrefaction the temperature curve of the moving chips bed was recorded. The anhydrous weight loss (AWL) of each torrefaction was measured. Effect of torrefaction temperature and duration on the AWL was studied. In order to optimise short time torrefaction, models that can estimate the AWL from the chips temperature curve are required. Three phenomenological models were successfully applied. They all gave good correlations between experimental and calculated AWL. These three models can be employed to optimise industrial torrefaction. However, the more complex they are, the more difficult it is to understand their physical meaning. It is thus preferable to use simple model for the industrial control of torrefaction

Combination of superplasticizers with hydroxypropyl guar, effect on cement-paste properties

International audienceViscosity-modifying agent (VMA) are used in several applications of concrete, such as underwater con-crete, Self-Compacting Concrete (SCC) or Self-Levelling Underlayments (SLU) in order to improve thewashout resistance and the stability. The study focuses on the modifications of cement pastes propertiesimplied when a VMA (hydroxypropyl guars or HPG) and a superplasticizer are introduced together. Forreaching this objective, two chemically different polycarboxylate-based superplasticizers (PCE) andtwo HPGs exhibiting different molar substitution ratios (MSHP), were studied. A method, combining totalorganic carbon and size exclusion chromatography, was developed in order to quantify the adsorption ofthe both admixtures. The adsorption of HPGs appears being significantly lowered by the presence of PCE,while only the adsorption of the less charged PCE is slightly affected by the HPG. As consequence, strongmodifications of the rheological properties of cement pastes were noticed when HPG and PCE are com-bined. The desorption of HPG leads to higher yield stress and residual viscosity than with PCE alone