Performance of plain and slag-blended cements and mortars exposed to combined chloride-sulphate solution

The durability of reinforced concrete structures exposed to aggressive environments remains a challenge to both researchers and the construction industry. This study investigates the hydration, mechanical properties and durability characteristics of ground granulated blast-furnace slag (GGBS) - blended cements and mortars exposed to a combined sodium chloride - sulphate environment, at temperatures of 20°C and 38°C. The conditions were chosen so as to assess the performance of slag blends under typical temperate and warm tropical marine climatic conditions. Slags, having CaO/SiO2 ratios of 1.05 and 0.94, were blended with CEM I 52.5R at 30% replacement level to study the influence of slag composition and temperature. Parallel control tests were carried out with CEM I 42.5R. Pastes and mortar samples were cast using 0.5 water to binder ratio, pre-cured for 7 days in water before exposure. Flexural strengths were determined once the samples were 7, 28 or 90 days old. Hydration was followed using x-ray diffraction (XRD), thermal analysis, and calorimetry. Also, sorptivity, gas permeability and chloride diffusion tests were carried out on mortar samples to measure transport and durability characteristics. The results show improved mechanical and transport properties for slag blended cements exposed to environments rich in sodium chloride and sulphate

Modeling the long-term durability of concrete barriers in the context of low-activity waste storage

The paper investigates the long-term durability of concrete barriers in contact with a cementitious wasteform designed to immobilize low-activity nuclear waste. The high-pH pore solution of the wasteform contains high concentration level of sulfate, nitrate, nitrite and alkalis. The multilayer concrete/wasteform system was modeled using a multiionic reactive transport model accounting for coupling between species, dissolution/ precipitation reactions, and feedback effect. One of the primary objectives was to investigate the risk associated with the presence of sulfate in the wasteform on the durability of concrete. Simulation results showed that formation of expansive phases, such as gypsum and ettringite, into the concrete barrier was not extensive. Based on those results, it was not possible to conclude that concrete would be severely damaged, even after 5,000 years. Lab work was performed to provide data to validate the modeling results. Paste samples were immersed in sulfate contact solutions and analyzed to measure the impact of the aggressive environment on the material. The results obtained so far tend to confirm the numerical simulations

THERMODYNAMIC MODELING IN THE Na-NO3-SO4-Cl-OH-H2O CHEMICAL SYSTEM AT 25 °C

International audienceIn the French concept for radioactive waste disposal, long-lived intermediate-level waste (bituminous waste, for instance) and radiferous waste contain large amounts of soluble salts which are essentially composed of nitrate and sulfate. Because of their high solubility, these salts will dissolve into the pore solution coming from the concrete barrier after the closure of the disposal. Then, under repository conditions, the resulting increased ionic strength brine could migrate by diffusion through the cement barrier, potentially reach the surrounding rock and impact the physical and chemical behavior of the constituents of the host material. The extent of this impact, including the spatial extent of the saline plume, is unknown. To determine how much of an issue this is, a first step consists of correctly describing the aqueous solutions' properties and mineral solubility in the nitrate-sulfate system. This study focuses on the Na-NO 3-SO 4-Cl-OH-H 2 O system and, in particular, on the computation of its thermodynamic properties (e.g., water activity or osmotic coefficient, and ion activity coefficients). To this end, we use the semi-empirical thermodynamic Pitzer model [1], which was developed to extend the field of application of the Debye-Hückel equations [2] only valid for a low range of molalities. The Pitzer model relies on the description of specific interactions between aqueous species that become dominant over ionic strength as concentrations increase. For one electrolyte, hypothetically totally dissociated in water, the model involves three adjustable interaction parameters (β (0) c/a , β (1) c/a and C φ c/a). In the quaternary system, two additional specific interactions are involved and the related interaction parameters can be determined: θ c/c' or a/a' and ψ c/c'/a or a/a'/c. For more complex systems, no supplementary parameters are necessary. In case the electrolyte is considered partially dissociated, neutral species, n, can be present in the aqueous solution, which implies new additional specific interactions. Thus new binary and ternary interaction parameters can be determined (λ nc , ζ nca …). Consequently a step-by-step approach is necessary to study a complex chemical system. First, all the binary subsystems are studied and binary interaction parameters are optimized, mainly on the basis of experimental osmotic coefficient data. Then, the ternary interaction parameters are determined from solubility data. Finally, quaternary systems or more can be studied. In the case of the system of interest in this study, NaNO 3 , Na 2 SO 4 and NaCl are considered totally dissociated whereas the partial dissociation of NaOH must be taken into account, due to its high solubility (28.3 mol • kg-1 at 25 °C) [3]. So in addition to interaction parameters for NaOH-H 2 O, the dissociation constant of NaOH 0 (aq) is required. The binary interaction parameters relative to the aforementioned binary systems are provided by previous studies [3–6], while ternary interaction parameters are determined in this study. Without supplementary data the phase diagram of the quaternary system Na-NO 3-SO 4-OH-H 2 O is determined (Figure 1). The comparison of numerical results with experimental observations is tricky since few data exist on this specific system [7]. Consequently, to show the coherence of the proposed parametrization on the Na-NO 3-SO 4-OH-H 2 O system, the model is extended to study the quaternary Na-NO 3-SO 4-Cl-H 2 O system. Finally, after checking binary and ternary parameters of this last system the model can correctly represent the experimental data of the Na-NO 3-SO 4-Cl-H 2 O system. This check confirms the coherence of the proposed parametrization and the accuracy of the calculations for the Na-NO 3-SO 4-OH-H 2 O system. This chemical system of interest for the waste radioactive storage was recently published [8]

Vertical variability of Near-Surface Salinity in the Tropics : Consequences for SMOS Calibration and Validation

International audienceThe ESA/SMOS (European Space Agency/Soil Moisture and Ocean Salinity) satellite mission aims to detect Sea Surface Salinity (SSS) using L-band radiometry. At that frequency, the skin depth is 1 centimeter. However, the calibration and validation of SMOS measurements will be done with in situ measurements, mainly taken at 5 m depth. In order to anticipate and understand vertical salinity differences in the first 10 m of the ocean surface layer, in situ vertical profiles are analyzed. Measurements come from autonomous drifter and Tropical Atmosphere Ocean (TAO) moorings for observations on local scale and from thermosalinographs (TSG), floats, eXpendable Conductivity-Temperature-Depth (XCTD) and CTD on the entire tropical band (from 30°S to 30°N). For the first time, vertical salinity differences, classified according to their vertical position, are collocated with precipitation computed by satellite. A rain parameter, 3D max rain rate, is defined to take into account the history of rain events. Vertical salinity differences higher than 0.1 pss-78 are observed in the 3 oceans, mainly between 0° and 15°N, coinciding with the average position of the InterTropical Convergence Zone. The highest differences are mainly located near river mouth. Some differences exceed 0.5 pss-78 locally and persist for more than 10 days, unlike the spatial average of salinity differences between 10 m and 1 m, which stay close to 0. A statistical approach is developed to be used for predicting large vertical salinity differences

Steady-state polarization response of chloride-induced macrocell corrosion systems in steel reinforced concrete - numerical and experimental investigations

WOS:000367483000024International audienceChloride-induced macrocell corrosion of steel is recognized as a frequent and harmful problem affecting reinforced concrete structures. For this specific corrosion mechanism, anodic and cathodic areas at steel-concrete interface are spatially separated. Thereby, at equilibrium, anodic and cathodic potentials are different due to electrical resistivity of concrete, which means there is a potential gradient and a macrocell corrosion current flowing in the concrete volume. Despite some available works on electrochemical modelling of free macrocell corrosion systems in reinforced concrete, the literature is very scarce regarding the response of such systems under polarization. Due to this lack of knowledge, the assessment of steel corrosion rate in reinforced concrete is still commonly based on the uniform corrosion assumption. In this paper, the detailed polarization response of the different components of macrocell systems is studied in steady-state condition on the basis of numerical simulations and the qualitative conclusions are validated by original experiments. Some important results are achieved, with potentially major consequences regarding the non-destructive evaluation of steel corrosion in reinforced concrete. In particular, the distribution of the polarizing current in the macrocell system is found to be dependent on the polarization nature (anodic or cathodic) and magnitude. Moreover, in case of chloride-induced macrocell systems, the apparent linear polarization range is actually much higher than usually assumed and significant differences are observed between anodic and cathodic responses. (C) 2015 Elsevier Ltd. All rights reserved

Use of quantitative digital autoradiography technique to investigate the chlorine-36-labelled radiotracer transport in concrete

International audienceThis work presents the capability of digital autoradiography, a non-destructive technique, to improve diffusion parameters determination by the mean of high resolution radionuclide mapping. Results show a good agreement between Cl-36 diffusion profiles acquired by digital autoradiography compared to the classic protocol based onto multi-step-abrasive-surface-peeling procedure. The digital autoradiography technique has been successfully used to localize and quantify Cl-36-labelled radiotracer on the surface of a concrete sample and into the contaminated depth. This study shows how optimized in-diffusion investigations can be performed in only 100 days to assess the diffusion parameters of Cl-36 in a low diffusive concrete sample, while a classical diffusion experiment would take several years to reach diffusion steady state for tritiated water, consider as the reference tracer, and consequently, to allow the estimation of the diffusion parameters for Cl-36

Cement-Based Matrix for tritium trapping research project: MACH3 (PIA2/ANDRA)

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