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Non-Saturated Ion Diffustion in Concrete – New Laboratory Measurements

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Non-Saturated Ion Diffustion in Concrete – New Laboratory Measurements NON-SATURATED ION DIFFUSTION IN CONCRETE – NEW LABORATORY MEASUREMENTS Nilla Olsson1,2, Véronique Baroghel-Bouny2, Lars-Olof Nilsson1 and Mickaël Thiery2 1Lund University, Lund, Sweden 2Paris-Est University, IFSTTAR, Materials Department, Paris, France ABSTRACT Non-saturated ion diffusion properties of cementitios materials were evaluated in an experimental study. The chloride diffusion coefficient was calculated by using the Nernst- Einstein equation. To assess this, resistivity measurements has been performed on mortars with different binders (OPC, OPC with 5% silica fume, 40% slag and 70% slag, respectively) and different w/b ratios (0.38 and 0.53). These have been conditioned to eight climates with RH from 100% to 33% RH. The results from the measurements have been corrected for the conductivity of the pore solution at the different degrees of saturation. The results for the Portland cement binders within the range 100% to 59% RH are presented. They showed that the diffusion coefficient of the mortar with high w/b ratio was higher at high RH. At low RH, the opposite was found and the material with low w/b ratio has higher diffusive transport. By comparing these results with the desorption isotherm, it is shown that the diffusive transport is dependent on the amount of evaporable moisture in the material. Key-words: Ion transport, non-saturation, chloride, alkali. INTRODUCTION Corrosion of the reinforcement is the major degradation mechanism for concrete structures and causes serious damage all over the world. The most common reasons for initiation of corrosion in concrete structures are carbonation and chloride ingress into the structure. Both these processes are strongly dependent on the transport properties of the material. Moisture conditions have a decisive effect on these transport properties [1]. All structures undergo variations in moisture distribution over time. Climate, with precipitation and often large temperature differences, is a major driving force for moisture transport which causes differences in the moisture distribution. The fact that many structures are built with concrete with a low w/b ratio, also makes self-desiccation a major contributor to moisture distribution variations. Moreover, self-desiccation is the reason why also submerged structures may undergo variations in moisture distribution over time [2, 3]. No generally accepted method is available for estimating or for measuring ion transport properties in non-saturated conditions [4, 5]. This is a problem when models are used for service life design of concrete structures. For getting accurate results, these models need to take into account the effects moisture variations have on chloride ingress and other ion transport such as leaching. In this area, there is not enough knowledge today to make accurate predictions of structures’ service life. There is also a lot of work to be done for understanding the link between transport properties and microstructure [6]. The phenomenon of non-saturated ion transport has been investigated in different ways. Several studies, for example Guimaraes et al. [7] and Climent et al. [8], have used Fick’s second law and determined chloride diffusion coefficients from obtained profiles in non-saturated specimens. Guimaraes et al. have used a powder of NaCl for applying the chlorides. Climent et al. have used a gaseous source of chloride to avoid disturbing the conditioning. Vera et al. [9] and Nielsen and Geiker [10] have in the same way used Fick’s second law, but these studies have also taken the binding of chlorides to the paste into consideration. Vera et al. have used the same procedure as Climent et al. Nielsen and Geiker used a NaCl solution and then quickly dried the samples to the same weight as before exposure. Buchwald [11] and Francy [12] have both used impedance spectroscopy and Nernst-Einstein’s equation for evaluating the chloride diffusion coefficient. Buchwald discusses how different masonry materials show a different behavior with a decreasing degree of saturation. Francy’s aim is to model transport, and he concludes that it is crucial to divide the moisture accurately into one part that contributes to ion transport and one part that does not. Otherwise the results can be very misleading. What is also shown in a modeling study by Nilsson [13]. In the present work, the effect of moisture content on the diffusion of ions in non-saturated cementitious systems is investigated. The electrical resistivities of mortar samples conditioned to relative humilities from 100% to 33% RH are measured. The Nernst-Einstein equation is used to calculate the chloride diffusion coefficient from these measurements, after correcting the resistivity for changing ion concentrations in pore solution after drying. The effects of moisture content on the convective transport of ions are not included in this study. THEORY Ionic diffusion may be compared with electrolytic conduction, which are both a random walk of charged species. The movement is caused by different mechanisms, concentration gradient and electrical potential, respectively. This is the basis for the Nernst-Einstein equation, which was used in this study for evaluating the diffusion transport properties of cementitious materials. The Nernst-Einstein equation The Nernst-Einstein equation relates the conductivity of a bulk material σ (S/m) to the diffusion 2 coefficient, D (m /s). The conductivity of the liquid phase σl, i.e. the pore solution, in porous materials is much greater than the conductivity of the solid and vapor phases, σs and σa respectively. Rajabipour [14], referred to by [15], showed that the contribution to the overall conductivity from the solid and vapor phases, is negligible. Pore solutions are typically in the -9 -15 range of σl≈1-20 S/m, the solid phase σs≈10 S/m and air σa≈10 S/m. The Nernst-Einstein equation is given in Equation 1: (1) 2 where σ0 (S/m) and D0 (m /s) are the conductivity and the diffusion coefficient of the substance -9 2 through the liquid phase of the material. For chloride ions D0 equals 1.483-1.484 x 10 m /s in electrolytes within the range 0.1 mol/l to 1.0 mol/l [16]. The calculated diffusion coefficient does not include any interaction of the ions with the binder. Atkinson and Nickerson [17] have compared the results on the ionic transport properties from conductivity and diffusion measurements and found good agreement at saturated conditions. The ionic diffusion takes place in the liquid phase in the material. Therefore is it is interesting to compare ionic transport properties, how moisture is fixed to the material and what characteristics there are of moisture fixation at different RH ranges. During drying part of the liquid phase is lost. Due to the pore connectivity and shape of the pores, the connectivity of the liquid phase is decreased. The connectivity of the liquid phase i.e. pore solution has been shown to be the factor that has the most significant effect on the conductivity of a porous material [15, 18]. Conductivity of the pore solution A key point contributing to getting accurate results of these measurements is having an accurate value for the conductivity of the liquid phase in the material [19]. The conductivity has been calculated from Equation 2 and the chemical composition of the pore solution. (2) 2 where ci (mol/l) is the concentration, zi (-) is the charge and λi (cm S/mol) is the equivalent conductivity for ion i. This last parameter is a function of the equivalent conductivity at 0 indefinite solution, λi , and the ion mobility, which is also dependent on the ionic strength of the solution [20]. The main contributors to the ionic strength of a pore solution in an OPC binder are the hydroxide and alkali ions. This have been shown both by measurements and modelling by Lothenbach and Winnefeld [21] and Snyder et al. [22]. Snyder et al. have also shown that the conductivity of the pore solution can be estimated from these ions. They have developed a single parameter method to calculate the pore solution conductivity for cementitious materials for concentrations up to 2 mol/l, but then with fairly high errors at concentrations above 1 mol/l. Their method has also been used, and verified with measurements, by Rajabipour and Weiss [15]. According to Snyder et al. the equivalent conductivity is calculated by using Equation 3: (3) -0.5 where Gi (mol/l) is an empirical coefficient for the electrical conductivity of solutions at 0 2 various concentrations. The values of Gi are given in Table 1 [22]. In this table values of λi (cm S/mol) [16] are also given. Im is the effective molar ionic strength (mol/l water) and is calculated by Equation 4: (4) where mi is the molarity (mol/l water) and zi as in Equation 2. 0 Table 1 –Equivalent conductivity at infinite solution, λi and conductivity coefficients Gi at 25°C. 0 Ionic species λi Gi (cm2 S/mol) (mol/l)-0.5 OH- 198.0 0.353 Na+ 50.1 0.733 K+ 73.5 0.584 Concentration of hydroxides and alkalis The concentration of hydroxide and alkali ions were calculated by using a method by Taylor [23]. This requires the chemical composition of the OPC, the mix proportions, the porosity and the degree of hydration for the different minerals in the cement. Then the total amount of alkali, nNa,total and nK, total (mol) is calculated by Equation 5; (5) where C (g) is the mass of cement, φNa2O (-) is the mass fraction of Na2O (equivalent for K2O) in the OPC and MNa2O (g/mol) is the molar mass of Na2O (equivalent for K2O). The released alkali, nNa,released and nK, released (mol), is then calculated from the alkalis immediately released to the pore solution from highly soluble sulphate salts, and the alkalis continuously released during hydration from the different clinker phases, see Equation 6; (6) where fNa,sulphate (-) is the fraction of Na in the form of sulphates (fNa,sulfates=0.35 and fK,sulphate=0.55 according to Pollitt and Brown [24]), gNa,i is the fraction of the non-sulfate alkalis in clinker phase, αi (-) and φi (-) are the degree of hydration and the mass fraction of the same clinker phase.
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