Moisture Transfer Between Unsaturated Cement Mortar and Ambient Air

Abstract

The objective of this article is to provide a description of moisture transport in building materials by application of a non-linear diffusion model, in which the effective coefficient of diffusion D eff , being a function of moisture content, is implemented. This article proposes the method of determination of this coefficient through correlation of the theoretical desorption isotherms with the experimental ones. To this aim, a number of desorption isotherms at temperature T = 20 • C for several narrow ranges, being adjacent to each other, and for broad ranges of the air relative humidity ϕ were established experimentally for cement mortar samples with water/cement ratio w/c = 0.50. The new knowledge contained in this article concerns some long-lasting measurements carried out in desiccators, for both narrow and broad ranges of the air humidity changes, and evaluation of the diffusion coefficient by applying an advanced optimization algorithm. The final conclusion is that the non-linear diffusion theory well reflects the moisture transport in hygroscopic porous materials like cement mortar.Keywords Building materials · Moisture transport · Non-linear diffusion · Modeling · Model validation Electronic supplementary material The online version of this article (A Area of the disc face (m 2 ) C Moisture concentration in air (kg/m 3 ) C s Moisture concentration adsorbed by pore walls (kg/m 3 ) D Coefficient of diffusion in gas phase (m 2 /s) D s Coefficient of diffusion in adsorbed phase (m 2 /s) D eff Effective coefficient of diffusion (m 2 /s) d Diameter of the sample (m) k Coefficient of convective exchange of vapor (m/s) l Half height of the sample (m) m w Mass of water in the sample (g) m Actual total mass of the sample (g) m ∞ Mass of the sample at the end of desorption (g) R Capillary radius (m) t Time (s) x Spatial coordinate (m) X Two-dimensional vector (-)Greek symbols α Reflection coefficient (-) β Expansion coefficient (-) δ Thickness of absorption layer (m) ϕ Air relative humidity (%)

Water Vapor Adsorption in Porous Building Materials: Experimental Measurement and Theoretical Analysis

“…Therefore, the adsorption isotherms of porous building materials have been measured very frequently so far by various investigators, for instance (Richards et al 1992;Xi et al 1994;Carmeliet and Roals 2001;Miniotaite 2003;Peuhkuri 2003;Häupl and Fechner 2003;Kumaran 2006;Jiřičková andČerný 2006;Pavlík et al 2008;Garbalińska et al 2010). However, the measured adsorption isotherms were mostly presented in the form of raw data only.…”

Sorptive testing of mortars with varying water-cement ratios

“…The sorption of pastes examined in the present paper was also the subject of articles by the authors of [14][15][16][17]. In turn, works [18][19][20][21][22][23] studied cement mortars with w/c ratio = 0.50, 0.65, 0.80 in the processes of sorption and desorption. Different methods of calculation were used based on the inverse problem so that, having recorded the kinetics of the sorption and desorption processes, the diffusion coefficient variability of the tested mortars in a wide range of humidity could be determined.…”

Superficial transportation model using finite volume method

“…Unlike the bulk mass transfer, the mass flux near the interface is induced mainly by adsorption of surface active component. Applying appropriate mass transfer approach, expressed in modification either in diffusion coefficients or model equations themselves, is typical to describe the mass transfer in any media, where concentration gradients do not generate dominant driving force (e.g., ions flux in presence of electric field) or in cases where molecular diffusion coefficients are not known and are substituted by effective diffusivities [69][70][71]. Calculating adsorptive near-interface mass transfer creates the need for effective diffusion coefficient estimation.…”