Ionic aqueous diffusion through unsaturated cementitious materials – A comparative study
Introduction
The mechanism of diffusion through porous media is the ability of a species, ionic or molecular, to move across the porous material when a concentration difference is applied as driving force. This ability can be quantified by a diffusion coefficient, which is a parameter of great importance in several scientific domains. It may allow for instance to assess the suitability of a determined storage solution for nuclear waste disposal. It can also be used to evaluate the durability of a concrete structure vis-à-vis either the corrosion of its reinforcement bars by chloride ion penetration through the concrete cover, or the expansive products generated by sulphate ions entering the material.
The diffusion coefficient is a macroscopic parameter that depends on the characteristics of both the diffusing species and the porous material involved in the diffusion process [1], [2], [3]. Thus, the material microstructure, which characteristics ensue both from the properties of the material itself as well as from its history and environmental conditions, plays a main part when dealing with the diffusion coefficient.
In a recent paper [4], we documented the lack of research works on molecular transport through unsaturated systems (cf. references ibid.) and the lack of data establishing a relationship between the diffusion coefficient and the saturation level of porous materials. Moreover, the techniques widely used in civil engineering to measure diffusivity through cementitious materials are suited for fully saturated conditions. These techniques cannot be used when it comes to partially saturated materials for the very reason that they request to place the samples of material between compartments containing solutions at various concentrations. The water content gradients thus created prevent from maintaining the initial saturation level inside the material.
Climent et al. [5], followed by De Vera et al. [6], published a method for measuring the diffusion coefficient of chloride ions through partially saturated concrete, from a natural diffusion test and by means of Fick’s second law of diffusion. Guimarães et al. [7] presented another experimental set-up for the natural diffusion test allowing also to calculate the chloride diffusion coefficient through unsaturated concrete using Fick’s second law. Ben Fraj et al. [8] proposed a new experimental set-up aiming at characterising chloride ingress in non-saturated concrete. Chloride profiles measured when concrete is partially saturated can also be found in [9].
Our group proposed recently an approach to determine the diffusion coefficient of any ionic species through a partially saturated porous medium [4]. This approach was proven effective in the case of a Portland-cement concrete. We could determine experimentally the diffusion coefficient for different saturation levels of the material. This allowed to show the correlation between the diffusion coefficient and both the porous network characteristics (porosity, porosimetry) and the hydric state of the material.
On that basis, the approach is extended in this paper to different types of cementitious materials: a blended-cement (with mineral additions) concrete and the two corresponding cement pastes (Portland and blended). Once the diffusion coefficient in non-saturated conditions is measured, a comparative analysis allows to identify the leading paths for diffusion through the different kinds of material. Besides, a brief review is first made on the validation of the present method, carried out on saturated materials and detailed elsewhere [10], by means of an electrokinetic technique. This review permits to verify the coherence with respect to the non-saturated sample results presented here, and to make some scale effect considerations.
Section snippets
Materials, characterisation and sampling
Two different kinds of cementitious materials were tested, namely cement paste and concrete. Each of them was prepared using two different varieties of cement: a common Portland one and a blended one (containing mineral additions: flying ashes and blast furnace slag). The composition of both types of concrete was presented before [10]. A water-to-cement ratio of 0.43 and 0.41 was used, respectively. The same ratio was adopted to prepare the two corresponding types of cement pastes. At the end
Experiments – electrochemical impedance spectroscopy (EIS)
The approach used involves the determination of the electrical ohmic resistance of a given porous medium by means of its impedance (AC) response. The technique is based on the formation factor Ff concept [12], [13], i.e. the ratio between the electrical conductivity of the pore solution σo (S/m) contained in a porous material at a given state, and the conductivity of the material itself σmat (S/m) at that state. The results obtained in this way needed to be corroborated by means of a well-known
Results
The characteristic impedance spectra measured by EIS in the case of Portland-cement concrete were presented in a previous paper [4]. In order to make a comparative analysis, regarding specially the main features of these spectra and their evolution with the hydric state of the material, Fig. 5, Fig. 6, Fig. 7 show them together with the different experimental Nyquist plots (Zexp) measured for a number of saturation levels Sl of blended-cement concrete samples. These saturation levels are
Discussion
The diffusion coefficient of chloride ions was determined from the formation factor values calculated for each material at different saturation levels. The results are shown in Fig. 8 for Portland-cement and blended-cement concretes and in Fig. 9, Fig. 10 for Portland and blended cement pastes, respectively.
The discrepancy range within measured values, as a result of the experimental characteristics, was assessed using the ratio between the standard deviation of the formation factor obtained
Conclusion
The main objective of this work was to report the comparative analysis of the diffusion coefficient of ionic species through four different non saturated cementitious materials (two cement pastes and two concretes). In order to measure the diffusion coefficient, an approach based on the formation factor concept and the Nernst–Einstein relationship was used. The validity of the pertaining experimental protocol (impedance-spectroscopy-based) and data processing (by means of a physics-based
Acknowledgement
This work was supported by a contract from ANDRA (Agence Nationale pour la Gestion des Déchets Radioactifs).
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