A new test method to determine the gaseous oxygen diffusion coefficient of cement pastes as a function of hydration duration, microstructure, and relative humidity

A new test method is developed to determine the gaseous oxygen effective diffusion coefficient through hardened cement pastes conditioned at different relative humidity. The method relies on the measurement of gaseous oxygen accumulation in the downstream compartment of a diffusion cell and on the numerical fitting of a classical diffusion equation (Fick’s second law) on experimental results. Oxygen-effective diffusion coefficients in the range of 10⁻⁶–10⁻¹¹ m²/s can be determined using this test method. The present paper gives a detailed description of the experimental setup, the numerical procedure and presents results obtained on different Portland-based cement pastes. Cement pastes containing silica fume and slag are also tested. Samples are cast at two different volumetric water-per-cement ratios (1.6 and 1.9 m³/m³), tested at three different ages (from 1 day to 8 months) and preconditioned at different relative humidity (3–93%). Hence, the influence of cement composition, hydration duration, relative humidity and the water-per-binder ratio on the oxygen-effective diffusion coefficient De,O2 is investigated. Four microstructural properties: total porosity, pore-size distribution, hydration degree and the degree of water saturation of the tested samples are assessed as intermediate parameters to model oxygen diffusivity as a function of the mix design. Results show that well hydrated blended cement pastes have lower diffusivity than Portland pastes (over one order of magnitude for RH within [33–76] %), even though their total porosity was higher than Portland pastes. For all cement pastes diffusivity is found to be well correlated to the mean pore diameter of samples, at different degrees of water saturation.