Calibration of Tang’s Model for Concentration Dependence of Diffusion in Cementitious Materials

Abstract

In this paper the Tang’s model (Cem. Concr. Res. 1999), updated by introducing the pore geometry factor, was calibrated on through diffusion (steady-state) test results obtained from literature. This enabled to propose a new improved approach for estimating the intrinsic friction parameter ψ, that accounts for concentration dependent ionic interactions in cementitious materials pore solution. Uncertainties in multi-parameter estimation from the bulk diffusion (transient) tests are discussed. In the steady state diffusion test, the chloride binding effects can be eliminated, enabling to estimate the instantaneous (Fick 1^st law) diffusion coefficient (D_Fick1). The measured dependency of D_Fick1 as a function of (upstream) concentration, enabled first to calibrate the pore geometry factor (comprising apparent tortuosity and open porosity) at infinitive dilution extrapolation. Finally, the calibrated friction parameter ψ was found to be 1.9 times higher than for NaCl pure solution, in contrast to previously reported values that were factor 21 to 32 (calibrated on bulk diffusion test) or even 100 to 1000 times higher, which are related to uncontrolled interferences with the other parameters. The contribution of individual factors that affect the D_Fick1 dependency on chloride concentration is quantitatively discussed.