Definition and variation of diffusion coefficients when drying capillary-porous materials

Abstract

For shrinking materials the variation of the isothermal moisture diffusion coefficient with moisture content depends upon the way in which the diffusion coefficient is defined. However, only the definition based on constant mass dimensions is satisfactory for these materials. The differences between coefficients and between materials are illustrated with three kinds of substance which all exhibit different shrinkage behaviour. For materials like clay, the diffusion coefficient based on volume fraction and the moisture content based on weight ratio have theoretically an infinite range in magnitude, whereas they have a finite range for wood and sand. The diffusion coefficient based on mass flow and the diffusion coefficient based on constant mass dimensions both have a finite range in magnitude for all materials. For clays the latter coefficient tends to zero as the moisture content (weight ratio) tends to infinity. A sand has much larger pores than a clay or wood and consequently much larger diffusion coefficients at corresponding moisture contents. Even in a clay slurry, the volumetric diffusion coefficient may not be as high as in a saturated sand because of the strong forces of attraction between water and clay. The general behaviour of the materials at low moisture contents can be predicted from their relative humidity data.