Neutron radiography study of water absorption in porous building materials: anomalous diffusion analysis

Abstract

Dynamic neutron radiography was applied to obtain the experimental data concerning the kinetics of the wetting process in two different types (fired-clay and siliceous brick) of porous construction materials. The technique provides accurate experimental data concerning the first instants as well as the advanced stages of the imbibing process. A significant difference in water absorption parameters, contrasting the materials studied, is found. Both the advance of the wetting front and the time evolution of water content distribution within the sample are analysed. It is shown that for both materials the time dependence of the wetting front position d deviates from the classical square root of time t behaviour. The deviation is analysed in terms of the intercept b in the approximation and the exponent n in the power law d ∼ tn/2. Both materials differ in the sign of the intercept parameter: b > 0 for siliceous brick and b < 0 for fired-clay brick. Two stages of the process with different values of the power law exponent were observed. The advanced period exponents are different for each type of brick: n < 1 for siliceous brick and n > 1 for fired-clay brick. At the initial stages (t < 1000 s) the exponents are substantially less than 1 and smaller than those describing the advanced process. The initial stage exponents are lower for the siliceous brick samples. It is shown that the experimental water content profiles collapse to a single curve when plotted against the variable xt−n/2. The evolution of the distribution of liquid in the porous medium was analysed in terms of the anomalous diffusion equation of Küntz and Lavallée (2001 Experimental evidence and theoretical analysis of anomalous diffusion during water infiltration in porous building materials J. Phys. D: Appl. Phys. 34 2547–54) based on the non-linear generalization of the Fick's law. This approach makes a detailed analysis of the generalized hydraulic diffusivity in the low water content region possible. Successful fitting of experimental data with the diffusivity vanishing at null water content proved the importance of the hypodiffusive component of the process.