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

Procédé d’agglomération et de recyclage en compactés de poudres de bauxites

National audienceLa bauxite est un minerai utilisé pour l’élaboration de l’aluminium ou directement sous forme de matière première pour la fabrication d’aluminates de calcium pour les ciments techniques. Elle est, dans ce dernier cas, essentiellement utilisée sous forme de bloc. On assiste depuis quelques années, à une raréfaction des blocs de bauxite alors que leur manipulation conduit souvent à des déchets sous forme de poussières qui ne sont pas valorisées à l’heure actuelle. L’objectif de ces travaux est de créer une nouvelle filière de recyclage de fines particules minérales pour pouvoir réutiliser cette matière première en remplacement de ressources naturelles qui s’épuisent. Ces produits sont mis en forme sous forme de compactés par compression directe. La bauxite est mélangée à du ciment et de l’eau puis introduite dans une matrice afin de subir un cycle de compression. Le compacté obtenu est ensuite stocké à température et hygrométrie contrôlées pour obtenir une hydratation maitrisée du ciment. Le compacté ainsi obtenu doit répondre à des spécifications bien précises en termes de densité, de porosité et de résistance mécanique grâce notamment aux hydrates spécifiques formés.L’outil principal utilisé pour caractériser les compactés est la micro tomographie par rayons X, qui est non destructive et permet d’observer en leur cœur même le comportement des grains et l’homogénéité en densité dans tout le volume. Les propriétés importantes des blocs telles que la porosité ou la distribution de taille des pores sont alors caractérisées grâce à l’analyse d’image des clichés de tomographie et comparé à la porosimétrie par intrusion de mercure. Cette étude de l’évolution des paramètres texturaux des compactés de bauxite doit permettre de comprendre et d’appréhender les phénomènes se déroulant pendant l’étape de compression. Il en suivra une optimisation des paramètres de compression et de mélange pour obtenir des compactés possédant les propriétés souhaitées

Agglomeration of powders with a new-coupled vibration-compaction device

Inorganic powder recycling should be a crucial process for the “smart factories” in the future. A complex three-phase system (bauxite mixed with ordinary Portland cement and water) with a new-coupled vibration-compaction device is studied. The compressive stress of compacts seems to be improved by using this device at low compaction pressure leaving the other characteristics unchanged. The tomographic study of macroscopic porosities shows differences in the pores repartitions inside vibrated and untreated compacts. Classic porosity repartition is shown in the classic compacted bauxite compacts whereas in the vibrated-compacted bauxite exhibits inhomogeneities. Despite this, we find these results quite promising for further investigations

Effect of different retarders and their combination with superplasticizer on the properties of CSA

International audienceThe aim of this work was to investigate the effect of polycarboxylate type superplasticizers (PCE), and three different retarders (citric acid, tartaric acid, potassium gluconate) and their combination on hydration process and rheological properties of CSA at early age. The hydration process was described through isothermal calorimetry, while the rheological properties were characterized through yield stress of cement pastes determined by rheometry. The results show that gluconate is the most powerful retarder, followed by tartaric acid, then citric acid. PCE strongly improves the fluidity of the pastes but a rapid loss of workability is noticed. On the contrary, retarders slightly decrease the yield stress of the pastes but slowdown the loss of fluidity. The combination of the both admixtures allows to maintain high fluidity for longer time. However, for citric acid, and even more for tartaric acid, a rheological instability, characterized by a decrease followed by an increase in the yield stress over time, is noticed. Potassium gluconate seems to be the best retarder to combine with PCE since no instability and longer maintenance of workability were observed. An optimum dosage, leading to the best initial fluidity to the pastes, was also determined for the three retarders. The rheological results are related to the delay induced by the retarder on the hydration products formation for the first hours of hydration and to competitive adsorption between retarder and PCE

Agglomeration of powders with a new-coupled vibration-compaction device

Inorganic powder recycling should be a crucial process for the “smart factories” in the future. A complex three-phase system (bauxite mixed with ordinary Portland cement and water) with a new-coupled vibration-compaction device is studied. The compressive stress of compacts seems to be improved by using this device at low compaction pressure leaving the other characteristics unchanged. The tomographic study of macroscopic porosities shows differences in the pores repartitions inside vibrated and untreated compacts. Classic porosity repartition is shown in the classic compacted bauxite compacts whereas in the vibrated-compacted bauxite exhibits inhomogeneities. Despite this, we find these results quite promising for further investigations

Effect of guar gum derivatives combined with superplasticizers on properties of portland cement-pastes

ICBBM: International Conference on Bio-based Building Materials (3BM) /ECOGRAFI : ECOlogical valorisation of GRAnular and FIbrous materials / Proceedings PRO 119 / e-ISBN: 978-2-35158-192-6International audienceChemical admixtures allow to create a variety of fresh and hardened state properties in cementitious materials. In the case of self-compacting concrete, plasticizers or superplasticizers are introduced with the aim to decrease the yield stress and the viscosity of the materials. However, in order to prevent segregation and bleeding, and to improve the water retention of cement-based system, stabilizing agents or viscosity agents are often introduced in addition. Among these admixtures, polysaccharides are most commonly used. The aim of this study is to provide an understanding of competitive or synergetic effects induced by the combination of hydroxypropyl guar stabilizing agents (HPG) and superplasticizers on cement pastes. Two polycarboxylate superplasticizers (PCE) exhibiting different charge densities and one HPG were studied. It was found that the combination of HPG with PCE superplasticizer strongly affects the rheological behavior of cement pastes. Despite the presence of HPG, the viscosity of the pastes strongly decreased with increasing dosage of PCE until it became close to the viscosity of a cement paste with PCE only. However, the use of HPG in combination with PCE allows maintaining a significant yield stress in the cement paste compared to PCE alone. The increase in the charge density of the PCE seemed to amplify the drop of the viscosity and to reduce the gain on the yield stress induced by HPG. The results also highlight a delay in the setting-time of the cement paste by adding HPG and PCE. The delay induced by HPG is significantly lower than that generated by PCEs. However, the combination of the both kind of admixtures leads to a slightly shorter setting-time compared to the PCE alone

Development of treatment to prevent the algal biofouling

International audienceColonization by microorganisms is often observed on building materials and the aesthetic problems caused by this proliferation lead to high maintenance and repair costs.Curative and preventive methods have been developed. Nowadays, biocides are the most common treatment used against biocolonization. Their efficiency is, however, highly dependent on concentrations of active substances and therefore goes hand in hand with their environmental toxicity. It appears necessary to develop more environmentally friendly methods.Previous studies in our laboratory were dedicated to the study of the colonization of mortar surface by algae both in accelerated and in situ tests. The results showed that intrinsic characteristics of cement based mortars, such as porosity, roughness and surface pH, as well as chemical composition of the cement, affected algal fouling. A slowdown in microbial growth on calcium aluminate cement (CAC) based mortars was notably observed.Taking these results into account, the aim of the present study is the development of preventive treatments based on CAC without adverse effects on the environment. We are working on a CAC based coating applied in a thin layer of 1mm on the surface of a cementitious substrate. The objective is to maintain the slowing effect of CAC on microbial growth while reducing costs by decreasing the amount of CAC used, since it is only applied at the mortars surface. In order to remain free of the variables linked to their intrinsic properties, a single formulation of substrate (mortar) based on Portland cement (CEMI, 52,5N, Holcim) was used as reference. The bioreceptivity of the mortars with the CAC based coating is characterized by image analysis during accelerated tests and compared with reference mortars. The experiment consists in periodically sprinkling an algal suspension on top of mortar samples inclined at 45° in a glass chamber. The green algae used in this study, which have been selected thanks to their representativeness, are Klebsormidium flaccidum.The addition of nanoparticles in the coating will be considered. The aim would be to insert antimicrobial substances (such as TiO2, silver or copper nanoparticles) on the surface of mortars while maintaining the CAC efficiency. One of the main issues of this process would be to protect the nanoparticles from leaching

Développement d’un traitement préventif contre la biocolonisation algale des matériaux cimentaires

National audienceLa biodétérioration des matériaux cimentaires est un phénomène couramment observé sur les façades d’immeubles, les maisons et les ouvrages d’art. Les altérations indésirables, causées par l’activité d’organismes vivants, sont principalement d’ordre esthétique bien que les propriétés physiques des matériaux puissent aussi être impactées. Ce phénomène implique des coûts d’entretien et de réparation élevés. Des méthodes préventives et curatives sont ainsi mises en place pour lutter contre cette biocolonisation. Les biocides sont les traitements les plus utilisés. Ils présentent cependant l’inconvénient d’avoir un impact environnemental qu’il ne faut pas négliger. Leur efficacité dépend de la concentration en matières chimiques actives utilisées et donc de la toxicité associée à ces substances. Il apparait ainsi nécessaire de développer des méthodes préventives plus respectueuses de l’environnement. Dans un contexte où l’engouement pour les nanoparticules ne cesse d’augmenter, leur utilisation dans les matériaux de construction constitue une piste prometteuse pour le développement de surfaces antimicrobiennes