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This paper investigates the moisture dependency of ion diffusion in binary cementitious binders with Portland cement (OPC) in combination with slag or silica fume. Resistivity measurements and calculations with a thermodynamic model, GEMS, were used to assess the unsaturated chloride diffusion coefficient. The results show that the relation between the relative diffusion coefficient and the degree of saturation is not significantly affected by w/b (in the investigated range of w/b from 0.38 to 0.53), while the investigated binder compositions seem to affect the relation. These results are of value for service-life models, where they potentially represent important input data. The results also highlight the need for considering the pore solution conductivity when using resistivity measurements for evaluation of ion transport properties. [ABSTRACT FROM AUTHOR]

Abstract: The apparent (Dapp ) and effective (Deff ) migration coefficients obtained in chloride migration tests are investigated in this study. The presented Dapp profiles in concrete show that the apparent migration coefficient is strongly concentration-dependent. As demonstrated, the binding of chlorides during the migration tests is very low at low free-chloride concentrations and therefore the chloride penetration front progresses throughout the concrete only slightly retarded by the binding. The diffusion flux during migration tests is shown to be insignificant compared to the migration flux. The DRCM obtained from the Rapid Chloride Migration (RCM) tests are found to be equal to the computed Dapp at the locations of the chloride penetration fronts, which gives an indication that the DRCM represents only the migration coefficient at the front. A linear correlation is found between the DRCM obtained from the traditional RCM model and the Deff obtained from the chloride transport model which includes non-linear chloride binding and concentrations in non-equilibrium. [Copyright &y& Elsevier]

Abstract: The ionic diffusivity in non-saturated cement-based materials is quantitatively studied using lattice Boltzmann method. The Shan–Chen multi-phase lattice Boltzmann model modified by incorporating the Carnahan–Starling equation of state into the model is used to simulate the equilibrium distribution of water and gas phases with density ratio as high as 775 in the three-dimensional microstructure of cement paste at various degrees of water saturation. The lattice Boltzmann diffusion model is subsequently applied to simulate the diffusion process of the ionic species through the partially saturated cement paste. Before the application of the modified Shan–Chen model, the benchmark tests including bubble test and contact angle test are carried out to validate it. It is shown that the ionic diffusivity is greatly influenced by the degree of water saturation. The simulated relative ionic diffusivity as a function degree of water saturation is in good agreement with the experimental data obtained from literature. In addition, the effect of w/c ratio on the ionic diffusivity through cement paste under non-saturated conditions is investigated. It indicates that there is a significant difference between the relative diffusivity for cement paste with w/c ratios of 0.4 and 0.5 over the whole range of water saturation degree. However, the difference of relative diffusivity for cement paste with w/c ratios of 0.5 and 0.6 is not obvious. [Copyright &y& Elsevier]

Abstract: This paper presents the results of a C3S paste characterization study. The objective was to determine the parameters needed to model the process of degradation. The experimental study focused on determining the portlandite content and the ionic diffusion coefficients of C3S paste. The molar C/S ratio of C–S–H in hydrated C3S pastes was also investigated. The portlandite content was determined with an experimental method based on an electron microprobe analysis. This method leads to a portlandite mass content of 24.4±2.3%. The diffusion coefficient of each ionic species was determined by inverse analysis of diffusion test data performed on hydrated C3S samples using a multiionic transport model. [Copyright &y& Elsevier]

Abstract: The interaction between concrete/cement and swelling clay (bentonite) has been modeled in the context of engineered barrier systems for deep geological disposal of high-level radioactive waste. The geochemical transformations observed in laboratory diffusion experiments at 60 and 90°C between bentonite and different high-pH solutions (K–Na–OH and Ca(OH)2-saturated) were reconciled with the reactive transport code CrunchFlow. For K–Na–OH solutions (pH=13.5 at 25°C) partial dissolution of montmorillonite and precipitation of Mg-silicates (talc-like), hydrotalcite and brucite at the interface are predicted at 60°C, while at 90°C the alteration is wider. Alkaline cations diffused beyond the mineralogical alteration zone by means of exchange with Mg2+ in the interlayer region of montmorillonite. Very slow reactivity and minor alteration of the clay are predicted in the Ca(OH)2-bentonite system. The model is a reasonable description of the experiments but also demonstrates the difficulties in modeling processes operating at a small scale under a diffusive regime. [Copyright &y& Elsevier]

Abstract: The assessment of the durability of cement-based materials, which could be employed in underground structures for nuclear waste disposal, requires accounting for deterioration factors, such as chemical attacks and damage, and for the interactions between these phenomena. The objective of the present paper consists in investigating the long-term behaviour of cementitious materials by simulating their response to chemical and mechanical solicitations. In a companion paper (Stora et al., submitted to Cem. Concr. Res. 2008), the implementation of a multi-scale homogenization model into an integration platform has allowed for evaluating the evolution of the mineral composition, diffusive and elastic properties inside a concrete material subjected to leaching. To complete this previous work, an orthotropic micromechanical damage model is presently developed and incorporated in this numerical platform to estimate the mechanical and diffusive properties of damaged cement-based materials. Simulations of the chemo-mechanical behaviour of leached cementitious materials are performed with the tool thus obtained and compared with available experiments. The numerical results are insightful about the interactions between damage and chemical deteriorations. [Copyright &y& Elsevier]

Abstract: In order to predict long-term leaching behavior of cement constituents for safety assessments of radioactive waste disposal, we modeled diffusive mass transport in micropores in cement based materials. Based on available knowledge on the pore structure, we developed a transport porosity model that enables us to estimate effective porosity available for diffusion (transport porosity) in cement based materials. We microscopically examined the pore structure of hardened cement pastes to partially verify the model. Effective diffusivities of tritiated water in hardened cement pastes were also obtained experimentally, and were shown to be proportional to the estimated transport porosity. [Copyright &y& Elsevier]

Abstract: This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick''s second law of diffusion. In the paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years'' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years'' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash. [Copyright &y& Elsevier]