Gas permeability of unsaturated cement-based materials: application of a multi-scale network model

Abstract

A multi-scale network model based on the percolation theory and using a renormalization method was applied to describe the porous space of cement-based materials and to determine their gas transport properties. Gas permeability was measured on hardened cement pastes prepared with French OPC (CEM I) and BFS±PFA (CEM V) cements currently used in the nuclear waste industry. These experiments were carried out to study the influence of the degree of water saturation and of the microstructure on materials gas transport properties. Between 10 and 100% relative humidity, the cement pastes' average gas permeability varies over five orders of magnitude (10 À16 ±10 À21 m 2 ). Results show that in the range of water saturation considered, there is no linear relationship between the gas permeability and the degree of water saturation in percentage terms. Permeability change was well observed for cement pastes characterized by w/c ratios of 0´30, 0´40 and 0´50, in the low water saturation domain (,60%). CEM V pastes are globally more permeable to gas than CEM I pastes in the range of one order of magnitude. For the modelling aspect, the application of the XDQ multi-scale model made it possible to model a porous network equivalent to the one obtained by the mercury intrusion porosimetry test. Each class of pores is associated with an elementary cubic random network. The whole porous system is rebuilt by a recurrent process involving superposition and rescaling of all the elementary networks. Then, an equivalent conductivity of the multiscale network is computed and extended to gas permeability. Modelling and experimental results are in agreement and show that mercury intrusion is a relevant method to characterize a porous space in order to estimate the gas permeability of cement-based materials with a conceptual model.

Pore-scale modelling of relative permeability of cementitious materials using X-ray computed microtomography images

“…The measured value of oxygen permeability of cementitious materials not only depended strongly on its degree of saturation but varied with the applied pressure. Galle and Daian [4] carried out gas permeability tests on unsaturated hardened cement pastes prepared with CEM I and CEM V cement with water-to-cement (w/c) ratios of 0.3, 0.4 and 0.5 and reported that the gas permeability of cement pastes was significantly influenced by degree of water saturation and microstructural characterisations of the specimens, such as porosity and pore size distribution. The relationship between gas permeability and degree of water saturation followed a nonlinear regression.…”

Use of Two-Pressure-Head Method to Assess Water Permeability of Structural Concrete

“…These observations may be due to the moisture distribution. As indicated previously, water absorption is able to remove the effect of capillary suction, but only the top surface layer is saturated by this treatment (13,28). If the water flow remained in the top surface region during the 1 bar [14.5 psi] test, where capillary effect was insignificant, a low flow rate was generally observed.…”

“…It is clear that in all cases (C1-C4), the flow rates of AC are much higher than those of MC and SC. This links in the fact that the three curing regimes resulted in different permeation properties and initial moisture contents at near surface concrete (28,29). MC refers to the moisture cured concrete, under which condition additional moisture was present in the concrete and no significant drying occurred from the surface in the whole curing period.…”

Characterizing the Gas Permeability of Natural and Synthetic Materials

“…Tests are generally conducted in a steady state on unsaturated soils (Eischens and Swanson 1996;Samingam et al 2003), rocks (Cosse 1996), geosynthetic clay liners (Didier et al 2000;Shan and Yao 2000;Bouazza and Vangpaisal 2003;Vangpaisal and Bouazza 2004), or cement-based materials (Gallé and Daian 2000;Loosveldt et al 2002). Results show that when the water content increases, corresponding to a lower volume available in the material for gas migration, measurement becomes more and more difficult, leading to relative uncertainties of up to 100% when the water content is over 100% in geosynthetic clay liners (Shan and Yao 2000;Bouazza and Vangpaisal 2003).…”

“…Results show that when the water content increases, corresponding to a lower volume available in the material for gas migration, measurement becomes more and more difficult, leading to relative uncertainties of up to 100% when the water content is over 100% in geosynthetic clay liners (Shan and Yao 2000;Bouazza and Vangpaisal 2003). Gallé and Daian (2000) obtain an important dispersion of permeability values (four orders of magnitude) at 90% of saturation in cement-based materials.…”