3D simulation of the leaching of cement-based materials in order to compare different leaching tests

International audienceLeaching of cement-based materials is a complex process that depends on both the material intrinsic properties and the leaching test. The microstructure and cement-type of the material are typical examples of the former whereas the later consist of solution pH and composition, solution renewal rate and the liquid/solid volume ratio. As most of the tests are not normalized, this leads to a diversity of results in terms of alteration layer thicknesses and leaching kinetics. As a consequence, it is often difficult to compare and transpose experimental data from one experience to the other. 1D reactive transport modeling has proven to be adequate to simulate the coupling between diffusion, dissolution / precipitation and sorption processes that take place during leaching. The aim of this study is to demonstrate the usefulness of 3D reactive transport modeling to accurately simulate the features of the leaching device features, which is essential to compare results obtained by different leaching tests. Such simulations also enable to assess the mutual interaction between samples if the device contains several of them. Additionally, these simulations represent an interesting numerical tool in order to design new or complementary leaching tests

Improving properties of recycled concrete aggregates by accelerated carbonation

peer reviewedDesign of concrete with recycled concrete aggregates by means of Packing Density Metho

In situ assessment of the setting of tricalcium silicate–based sealers using a Dentin pressure model

Introduction EndoSequence BC Sealer (Brasseler, Savannah, GA) is a premixed tricalcium silicate–based root canal sealer that requires moisture from the root dentin to hydrate. The aim of this study was to investigate the setting of EndoSequence BC Sealer and other sealers in contact with human dentin in a simulated clinical environment. Methods EndoSequence BC Sealer, MTA Fillapex (Angelus, Londrina, Brazil), Septodont Sealer (Septodont, Saint Maur-des-Fosses, France), and Apexit Plus (Ivoclar, Schaan, Lichtenstein) were assessed. Caries-free lower premolars extracted for orthodontic purposes in patients aged 13–16 years were standardized to a 10-mm root length and were filled with test sealers and set up in a dentin pressure model for 14 days. In addition, set sealers immersed in physiologic solution for 14 days were also assessed. The set materials in solution and materials retrieved from the dentin pressure setup were characterized by scanning electron microscopy and X-ray diffraction analysis. The setting time and radiopacity were assessed using ISO 6876:2002 specifications. Furthermore, mineral ion leaching was evaluated by inductively coupled plasma mass spectrometry. Results All the sealers tested exhibited formation of a calcium phosphate phase when in contact with physiologic solution. Septodont Sealer and Apexit Plus did not exhibit the formation of a calcium phosphate phase in the dentin pressure setup. The fluid in the system was enough to allow the setting of EndoSequence BC Sealer, which did not set in a dry environment. All materials leached calcium with the Septodont Sealer, exhibiting double the calcium ion leaching compared with EndoSequence BC Sealer. Conclusions Using the dentinal fluid pressure system resulted in an adequate flow of dentinal fluid that allowed EndoSequence BC Sealer to set inside the root canal. Although the sealers tested were tricalcium silicate based, the hydration reaction and bioactivity in the presence of dentinal fluid were different to hydration in vitro. Thus, clinically, material bioactivity cannot be assumed.peer-reviewe

Use of phytostabilisation to remediate metal polluted dredged sediment

International audiencePhytostabilisation (combined use of tolerant plants and soil amendments) experiments were conducted at the field scale on dredged sediments polluted with metals. A sediment deposit contaminated with metals and metalloids (Cd, Zn, Pb, Cu and As) was experimentally established in 2002 and monitored until 2004 as part of the European project PHYTODEC (5th Framework Program; EVK1-CT-1999-2004). Six out of nine plots were treated by adding amendments to immobilise metals (Thomas Basic Slags or Hydroxylapatite). Two grass species (Festuca rubra and Deschampsia cespitosa) were sown on six plots previously treated or not. The three unvegetated plots left were taken as controls. After two years of monitoring, the couple Thomas Basic Slags/D. cespitosa was the most efficient additive/plant couple for phytostabilisation purpose. In this 3 years project, we continue to work on the sediment deposit site previously described and propose to demonstrate the long-term sustainability of phytostabilisation by addressing the following points: (i) sustainability and maintenance of the vegetation cover and amendment action, (ii) reduction of metals mobility and bioavailability, (iii) study of metal mobilization and immobilization mechanisms related to specific bacterial microflora

Effect of the Chemical Composition of Building Materials on Algal Biofouling

International audienceThe main cause of aesthetical deterioration of outdoor exposed building materials is the colonization by microorganisms. This phenomenon depends on factors such as geographical situation, environmental conditions and surface state of the substrate. Several researches have been devoted to the study of the effect of porosity, roughness and surface treatment on the biofouling of building materials. However, none of them has addressed the influence of cement composition. The main objective of this study is thus to highlight the influence of the composition of the material on its biocolonization by algae. The green alga Klebsormidium flaccidum was chosen because of its representativeness in France. It is indeed the species the most frequently identified and isolated from samples taken on sites. In order to characterize the influence of the composition of building materials on their biofouling, the behavior of mortars prepared with two types of Portland cement and two types of calcium aluminates cement is studied. The biofouling is followed by measuring the covering rate thanks to image analysis. This work is realized both on samples exposed outdoor and on samples tested in a laboratory bench. Obtained results prove that the composition of cementitious materials is a determining factor

An in-situ and laboratory study of the effect of the intrinsic properties of mortars on their potential bioreceptivity

International audienceThis study aims to clarify the effect of mortar intrinsic properties (porosity, roughness and carbonation level) on its ability to biofouling. Two scales experimental tests, an accelerated fouling in laboratory and a natural fouling in the real-world, were set-up. The first one was conducted in a closed device allowing a periodic sprinkling of an algal suspension on the samples surface. The outdoor test samples were exposed in a park at Grenoble (France). The colonization rate of the sample surface was evaluated by image analysis. The results show that the impact of each intrinsic parameter is quite different as function of the test. The porosity has no influence on the algal colonization of the samples exposed in indoor whereas a high porosity seems to increase slightly the bioreceptivity of ones exposed outdoor. The roughness, in both tests, promoted the microorganisms attachment and so their colonization. However, the discrimination of roughness grades is higher in the laboratory test than in the in-situ one. The surface pH significantly influences on the accelerated biofouling but not on the in-situ one. These dissimilarities result from the difference in experimental configurations of the two tests. Thus the laboratory test should be adjusted to be more suitable and to allow an extrapolation of results

On the global hydration kinetics of tricalcium silicate cement

We reconsider a number of measurements for the overall hydration kinetics of tricalcium silicate pastes having an initial water to cement weight ratio close to 0.5. We find that the time dependent ratio of hydrated and unhydrated silica mole numbers can be well characterized by two power-laws in time, $x/(1-x)\sim (t/t_x)^\psi$. For early times $t < t_x$ we find an `accelerated' hydration ($\psi = 5/2$) and for later times $t > t_x$ a `deaccelerated' behavior ($\psi = 1/2$). The crossover time is estimated as $t_x \approx 16 hours$. We interpret these results in terms of a global second order rate equation indicating that (a) hydrates catalyse the hydration process for $t<t_x$, (b) they inhibit further hydration for $t > t_x$ and (c) the value of the associated second order rate constant is of magnitude 6x10^{-7} - 7x10^{-6} liter mol^{-1} s^{-1}. We argue, by considering the hydration process actually being furnished as a diffusion limited precipitation that the exponents $\psi = 5/2$ and $\psi = 1/2$ directly indicate a preferentially `plate' like hydrate microstructure. This is essentially in agreement with experimental observations of cellular hydrate microstructures for this class of materials.Comment: RevTeX macros, 6 pages, 4 postscript figure

Phytostabilisation d'un site pollué par les éléments traces : opération pilote et pérennité du traitement

National audienceGrâce à l'utilisation combinée de plantes et d'agents immobilisants, la phytostabilisation assistée limite les risques associés à un sol contaminé en réduisant la biodisponibilité des polluants. Le rôle des plantes est de réduire le lessivage et l'érosion. En Europe et dans le monde, de nombreux sites ont déjà été phytostabilisés avec succès. Cependant, peu de projets de phytostabilisation associent une surface et une durée d'expérimentation suffisantes pour établir l'efficacité sur le long terme et en conditions réelles d'une telle pratique pour la gestion d'un site pollué. La nécessité de conduire des opérations pilotes sur une longue durée a été ainsi mise en avant par la communauté scientifique. L'objectif de ce projet mené sur un dispositif pilote mis en place en 2002 est d'estimer l'efficacité dans le temps d'une phytostabilisation assistée appliquée à un sédiment pollué par les éléments traces (Cd, Zn, Pb, Cu, As). Les paramètres suivis dans cette étude sont liés aux plantes initialement semées, aux espèces végétales venues spontanément coloniser le dispositif, aux agents immobilisants utilisés et aux caractéristiques physico-agronomiques de la matrice polluée. Une synthèse des résultats obtenus sur 7 années de suivi sera présentée lors du colloque

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