Method for simultaneous determination of sorption isotherms and diffusivity of cement-based materials Anders Anderberg a,b, ⁎ , Lars Wadsö a a Lund University, Div. Building Materials, Box 118, 221 00 Lund, Sweden b Maxit Group, PO Box 415, 191 24 Sollentuna, Sweden Received 23 November 2006; accepted 23 August 2007 Abstract A method for simultaneous determination of the diffusion and sorption properties of cement-based materials is presented. It is a gravimetric method where one small specimen is exposed to stepwise changes in relative humidity while its mass is being measured. As sorption in cement-based materials is slow, the change in relative humidity to the next level is made before final equilibrium has been reached on the previous level. Approximate final (equilibrium) levels are found by extrapolation using an exponential equation, and a factor is applied to correct for the fact that the sorption step does not start at equilibrium conditions. A correction for external mass transfer resistance is also included. The method can be used in desorption as well as absorption mode. Measurements of two materials are presented and compared with the results obtained using conventional methods. © 2007 Elsevier Ltd. All rights reserved. Keywords: Sorption isotherm; Diffusion; Humidity; Adsorption 1. Introduction Moisture is an important issue in building science as it influences several important material properties. Moist buildings have also been related to negative health effects in several studies [1]. Moisture related processes that may lead to poor indoor air quality are, for example, microbiological growth [2,3] and alkaline degradation of the polymers in flooring adhesives and floor coverings [4]. Most cementitious materials contain excess water when mixed and this water must dry out to prevent future damage to the building and deterioration of the indoor air quality. Knowing the moisture properties of materials is essential for calculating drying times and moisture loads as well as for predicting biological growth, chemical reactions and transport of hydroxyl and chloride ions, for example. The two most commonly measured moisture properties are sorption isotherms (equilibrium moisture content as a function of relative humidity) and transport (diffusion) coefficients. These are measured by different techniques: for example, sorption isotherms by equilibrium over saturated salt solutions [5] and diffusion coefficients by cup measurements [6]. These techniques are time consuming and require many parallel specimens as measure- ments are usually performed in several different RH-intervals. There are also ways to determine diffusion and sorption properties in one measurement. In the time-lag method, often used for membranes [7], the partial pressure is kept stepwise constant on one side of a specimen and the sorption and diffusion properties are calculated from the measured change in vapour pressure on the other side. In gravimetric methods [8,9], a material with known geometry is exposed to stepwise or continuous changes in vapour pressure. The change in mass of the specimen is continuously measured, giving the sorption isotherm at equilibrium and the diffusion coefficient from the kinetics and the sample geometry. A gravimetric method for evaluation of sorption isotherms and diffusion coefficients in absorption in cement-based materials was presented by Tada and Watanabe [10,11]. In this method absorption steps are made from dry conditions to different relative humidities. Thus, several test specimens are required or a single test specimen has to be dried before each measurement step. Each sample consists of 10 to 15 mm sized particles made from crushed material; the present method uses smaller specimens with a more well-defined Available online at www.sciencedirect.com Cement and Concrete Research 38 (2008) 89 – 94 ⁎ Corresponding author. Lund University, Div. Building Materials, Box 118, 221 00 Lund, Sweden. Tel.: +46 46 2227412; fax: +46 46 2224427. E-mail address: anders.anderberg@byggtek.lth.se (A. Anderberg). 0008-8846/$ - see front matter © 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2007.08.023