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

: 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

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

Link between microstructure and tritiated water diffusivity in mortars

Ions and radionuclide diffusivity in concrete is one of the most important factors that determine service life and safety assessment of cement based structures in nuclear power plants and radioactive-waste repositories. Apart from the influence of cement paste microstructure, the presence of aggregates may have an impact on transport properties of the material. The well-known interfacial transition zone, denoted by ITZ, is created near the aggregates and characterized by a greater porosity. The goal of this study is to investigate the competition between the more diffusing ITZ zone and the less diffusing aggregates. To this end, several series of tritiated water diffusion tests are conducted on mortars characterized by different water-to-cement ratios and sand volume fractions. In parallel, microstructure of these materials is explored by mercury and water porosimetry. It was observed that at low sand content (0% – 50%), diffusion properties of mortars are dominated by aggregates dilution effect. At 60% sand, diffusion increases significantly suggesting that percolation’s pores threshold has been reached. Results indicate also that sand particle size distribution has a great impact on the diffusivity of mortars

Reinforced Concrete structures degradation issues impact of Chlorides and of Thermal loadings on Corrosion development

International audienceReinforced concrete structures of Fukushima Daiichi Nuclear Power Station have been submitted temporarily to specific environmental conditions such as thermal loading (up to 200DC) and chloride ingress (from seawater) [1]. These conditions are susceptible to lead to corrosion of reinforced concrete components of the NPP. This paper aims at studying environmental conditions encountered by concrete structures, and to propose pertinent parameters for reinforcement corrosion monitoring. The first step of the scenario consists in considering that concrete has been in contact with seawater during several days (we estimate a duration of 15 days in this paper) then pure water replace seawater for cooling fuel. A coupled chemistry-transport software (Hytec) has been used to simulate a CEM V type concrete submitted to such scenario (at ambient T DC). Results show that chloride penetrate into concrete until several millimeters during the 15 days in contact with seawater. As soon as pure water replace seawater, chloride species are leached out from the material. After 2 years, calculations estimate that almost all chloride species disappeared from concrete cover. This result show that reinforcement bars located few centimeters inside concrete have not seen chloride species and should not developed chloride-induced-corrosion. Concrete structures have been submitted to thermal loadings as temperatures were above 200 DC just after the accident, and they stayed above 100 DC for about a half year [1]. Then seawater has been injected inside concrete structures to cool fuel. Previous studies [2] determinate that in such conditions, cold part of the structure is damaged and cracked. Thus we can propose that in a first step, external part of concrete structures have been cracked, then internal parts at high temperature put in contact with seawater at ambient temperature would also cracked. In this context, chlorides from seawater can penetrate through cracks and the degradation issue is linked to corrosion of rebars in cracked concrete. In such conditions, field and laboratory experiments [3, 4] show that quality of steel/concrete interface is the main parameter that has to be considered, more than chloride threshold and crack width as it is commonly specified in codes

Physico-chemical investigation of clayey/cement-based materials interaction in the context of geological waste disposal: Experimental approach and results

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The link between gas diffusion and carbonation in hardened cement pastes

International audienceThe influence of atmospheric carbonation on gas diffusion was investigated using four hardened cement pastes (CEM I, CEM III-A, CEM V-A and a low-alkalinity binder) with common water-to-binder ratio (0.4). The diffusivity to helium and nitrogen of the non-carbonated and carbonated pastes was measured at different relative humidities. Carbonation decreased the diffusivity of the CEM I paste whereas that of the other binders significantly increased after carbonation. These results show the competition between porosity clogging and cracking induced by carbonation. The consequence of carbonation are then believed to depend on the considered binder. For OPC, clogging dominates leading to the decrease in transport properties after carbonation whereas cracking dominates for blended cements leading to significant increase in transport properties after carbonation