Impact of carbonation on the durability of cementitious materials: water transport properties characterization

International audienceWithin the context of long-lived intermediate level radioactive waste geological disposal, reinforced concrete would be used. In service life conditions, the concrete structures would be subjected to drying and carbonation. Carbonation relates to the reaction between carbon dioxide (CO 2) and the main hydrates of the cement paste (portlandite and C-S-H). Beyond the fall of the pore solution pH, indicative of steel depassivation, carbonation induces mineralogical and microstructural changes (due to portlandite and C-S-H dissolution and calcium carbonate precipitation). This results in the modification of the transport properties, which can impact the structure durability. Because concrete durability depends on water transport, this study focuses on the influence of carbonation on water transport properties. In fact, the transport properties of sound materials are known but they still remain to be assessed for carbonated ones. An experimental program has been designed to investigate the transport properties in carbonated materials. Four hardened cement pastes, differing in mineralogy, are carbonated in an accelerated carbonation device (in controlled environmental conditions) at CO2 partial pressure of about 3%. Once fully carbonated, all the data needed to describe water transport, using a simplified approach, will be evaluated

Corrosion behavior of iron in cementitious solution at 80°C in anoxic condition

International audienceThe Belgian reference concept for high-level radioactive waste packaging proposed by ONDRAF/NIRAS is called Supercontainer. The design of the Supercontainer includes a stainless-steel liner, a concrete buffer and a carbon steel overpack containing one or several waste canisters. An objective of the concrete buffer is to maintain high alkaline conditions on the surface of the overpack during underground disposal of these wastes. In these conditions (alkaline and anoxic conditions), a passive film forms on the surface of the steel, resulting in very low and almost negligible uniform corrosion rates (passive dissolution). This film is generally composed of a double layer an inner layer, accounting for passivity, and a more or less porous outer layer. The latter is in contact with the environment and is formed by precipitation of elements in solution. The presence of silicon and cations from the cementitious environment could lead to neo-formed phases such as ferrous-silicates or phyllosilicates possessing corrosion inhibiting properties. The aim of this work is to study the impact of the cementitious materials during the corrosion at 80DC in anoxic media by characterizing the layer formed during the corrosion process. To ensure these measurements, iron plates were corroded in solution representative of a cementitious environment (at 80DC and under anoxic conditions). Evolution of the nature and growth of the corrosion layer is studied on samples after several duration tests. Corrosion products are characterized by means of several analytical techniques such as scanning electron microscopy, Raman micro-spectroscopy, X-ray diffraction, X-ray photoelectron spectrometry and transmission electronic microscopy. Results have revealed evolution of composition as a function of time. Layers are mainly composed of magnetite and others phases containing iron, calcium and silicon

Impact of carbonation on unsaturated water transport properties of cement-based materials

International audienceIn unsaturated conditions, the durability of concrete structures is strongly dependent on the evolution of the amount of free water within concrete porosity. Reliable durability assessment of concrete structures in relation to their environment thus requires accurate unsaturated water transport description as well as reliable input data. The effect of carbonation on water transport remains poorly studied and data are lacking. It was then the purpose of this article to acquire all the data needed to describe unsaturated water transport in carbonated cementitious materials (porosity, water retention and unsaturated permeability). Four hardened pastes made with four different binders were carbonated at 3% CO2 to ensure representativeness with natural carbonation. Beyond the modification of the water retention curve and porosity clogging, significant microcracking due to carbonation shrinkage was observed. The consequence on permeability highlighted a competition between porosity clogging and microcracking that was dependent on the initial mineralogical composition

: Exemple de la dégradation d'une pâte de ciment Portland dans de l'acide borique.

National audienceLa méthodologie mise au point au Laboratoire d'Etude du comportement des bétons et des Argiles (LECBA) du CEA et éprouvée sur de nombreux cas de dégradation des matériaux cimentaires (eau pure, attaque sulfatique) sera presentée. L'exemple de la dégradation d'une pâte de ciment en acide borique sera utilisé pour illustrer la démarche adoptée. Les limites de la modélisation de ce cas avec le code HYTEC seront soulignées

The validity of the formation factor concept from through-out diffusion tests on Portland cement mortars

International audienceThe diffusion coefficients of ions and radionuclides in cementitious materials are the basic parameters to evaluate the state of the degradation of structures. In this article, three different tracers (two ions, and a radionuclide) were tested on the same formulations of mortars (sand volume fractions from 0 to 60 per cent) in terms of the through-out diffusion, to determine the effective diffusion coefficients of each tracer and each formulation. The aim of this study is to prove the validity of the formation factor equation relating the effective diffusivity of a tracer in cementitious material to its diffusion coefficient in pure water. This result is extremely interesting because once the geometric formation factor of a material is known, it is possible to determine the values of the effective diffusion coefficients of any other diffusing species in this material

Coupled chemo-transport-mechanical modelling and numerical simulation of external sulfate attack in mortar

International audienceWe develop and apply in this study a chemo-transport-mechanical model for simulating the external sulfate attacks in Portland (CEM I) cement pastes and mortars. Basically, this degradation consists in the simultaneous decalcification of the hydrated phases resulting from leaching processes, and the migration of sulfate ions within the material and its subsequent interactions with these phases. The sulfate uptake leads generally to ettringite precipitation mainly from monosulfate, which in turn may produce intense macroscopic expansions and cracking. In our approach, crystallization pressures arising from the restrained growth of monosulfate crystals due to the confinement of the surrounding C–S–H matrix are assumed to initiate the observed macroscopic expansions. A macroscopic strain tensor evaluated from the volume fraction of supplementary precipitated ettringite is further introduced in the mechanical behavior law for explicitly reproducing the macroscopic expansions. Analytical homogenization schemes are applied to estimate both mechanical and diffusive properties from the local volume fraction of solid phases. The numerical platform Alliances is then used for solving both reactive transport and mechanical coupled problems, and is applied to the simulation of laboratory tests consisting in prismatic mortar specimens immersed in solutions containing sodium sulfate and subjected to free expansions. Comparison of the numerical results with experimental ones in terms of phase assemblage profiles, evolutions of mass changes and expansions shows a correct agreement. Finally, the extension of the model towards cases of restrained displacement conditions is discussed and some modifications regarding the kinetics of ettringite precipitation are proposed for such situations

Boric acid attack of the reinforced concrete used in Spent Fuel Pool

International audienceIn the framework of Spent Fuel Pools (SFP) lifetime studies, an investigation of the concrete degradation inaqueous boric acid has been requested by the Electric Power Research Institute. The main goal of this study is to identify thephysico-chemical degradation mechanisms involved in a boric acid medium. A well-tested methodology for testingcementitious materials degradation in other solutions (water, sulfate solution…) was applied. This methodology involved anexperimental study and computational modeling. For the particular case of boric acid attack, a multi-scale approach wasused; concrete as well as its main components (cement paste and aggregates) were studied. The degradation experimentswere carried out for three to eight months in 2400ppm boric acid solution. Aggressive solution conditions were maintained bypH regulation and periodical renewal. Characterization concerned the composition of the degradation solution during theexperiments, as well as the mineralogical evolution of the degraded cementitious materials at the end of the experiments.Solution analysis was performed by ionic-chromatography and solid characterizations were carried out by the means of XRDand SEM observations. The study of the Portland cement paste degradation shows that the leaching mechanism is driven bydiffusion. The degradation kinetics in boric acid is higher than the one in pure water. The process of concrete degradation ismore complex; a nonlinear behavior of the calcium leaching over square root of degradation duration can be noticed.Besides, an additive contribution of cement paste and aggregates on the leached elements from concrete is suggested.Nevertheless, the degraded thickness ranges from 2400 $\mu$m to 2800 $\mu$m, which is significantly lower than the one obtained in cement paste at the same duration (3400 $\mu$m). This observation is quite unexpected and could indicate a possible surface dissolution. Finally, reactive transport numerical calculations are carried out with HYTEC platform to be confronted to experimental results. The first results on cement paste degradation are encouraging; the four zones composing the degraded Portland cement paste profile pattern that were identified experimentally are obtained from numerical simulation. This modeling work must be continued to improve the description of the degradation kinetisc on cement paste and to include themodeling of concrete degradation

On the relationship between the formation factor and diffusion coefficients of Portland cement mortars

International audienceConcrete durability continues to be a subject of considerable interest, especially with the use of cement based materials on structures requiring improved sustainability and resistance to aggressive ions penetration or radionuclide release. Diffusion is considered as one of the main transport phenomena that causes migration of aggressive solutes and radionuclide in a porous media according to most studies.In this work, two different tracers (an ion, and a radionuclide) were tested on the same formulations of mortars (water / cement = 0.4 and sand volume fractions from 0 to 60%) by the through-out diffusion, in order to determine the effective diffusion coefficients of each tracer and each formulation. The obtained results have proven the validity of the formation factor equation relating the effective diffusivity of a tracer to its diffusion coefficient in pure water

Swelling behavior of ion exchange resins incorporated in tri-calcium silicate cement matrix: I. Chemical analysis

International audienceThis paper presents the first part of a theoretical and experimental work aiming at modeling the chemo-mechanical behavior of composites made up of ion exchange resins (IER) solidified in a tri-calcium silicate cement paste (C3S). Because of ion exchange processes, the volume change of the IER may cause internal pressures leading to the degradation of the material. In this study, a predictive modeling is developed for describing the chemical behavior of such material. It is based on thermodynamic equilibria to determine the evolution of the ion exchange processes, and the potential precipitation of portlandite in the composite. In parallel, a phenomenological study has been set up to understand chemical phenomena related to the swelling mechanisms. The model created has been finally implemented in a finite elements software; the simulation of a laboratory test has been performed and the results compared to experimental data