X-ray absorption studies of drying of cementitious tile adhesive mortars Dale P. Bentz a, * , Claus-Jochen Haecker b , Max A. Peltz a , Kenneth A. Snyder a a Materials and Construction Research Division, Building and Fire Research Laboratory, National Institute of Standards and Technology, Gaithersburg, MD 20899-8615, United States b SE Tylose GmbH & Co. KG (Shin-Etsu Chemical), Rheingaustrasse 190-196, Building H346, 65203 Wiesbaden, Germany Received 24 July 2007; received in revised form 30 October 2007; accepted 30 October 2007 Available online 7 November 2007 Abstract X-ray absorption measurements have been applied to studying film formation at the exposed surface during the drying of cementi- tious tile adhesive mortars as a function of ingredients and mixture proportions. Preliminary observations suggested that in addition to a drying front, concurrently, a densification front is observed proceeding from the exterior of the specimens inward. Due to the extremely high viscosity of the pore ‘‘solution’’ in these mortars, an analysis based on Stokes equation actually suggests that some of the smaller cement particles will be ‘‘carried’’ along with the drying pore solution to the top surface of the specimen where they are sequentially deposited. To verify this hypothesis, further experiments were conducted using either a coarse cement or a fine limestone as a total replacement for the commonly used cement. The X-ray absorption results are further supported by concurrent mass loss measurements on equivalent specimens for each experiment and by particle size distribution analysis of the final dried limestone/silica sand system as a function of depth. Published by Elsevier Ltd. Keywords: Building technology; Drying; Film formation; Mortar; Tile adhesive; Viscosity; X-ray absorption 1. Introduction Specialty thin layer mortars are often formulated for usage as tile adhesives or renderings for external walls. Two common additives to such mortars are cellulose ethers (CE) and redispersible polymer powders (RPP) [1,2]. The most important property of a CE is its water retention capability, which is especially needed when a fresh mortar is applied to an absorbing substrate. CE retains the water in the mortar for a sufficient time to allow proper cement hydration. Furthermore, CE are used to control the work- ability of fresh mortars, first by acting as a thickener and second by entraining air voids (about 25% by volume). RPP are often added to improve mechanical properties. A key property of these mortars is their open time, defined as the time during which tiles may be applied to the mortar surface and achieve adequate adhesion (i.e., the time dur- ing which the mortar surface remains ‘‘tacky’’ to the touch) [2]. Ultimately, this tackiness is lost due to the formation of a film at the top surface of the mortars. The film may be composed of both polymeric materials and (carbonated) hydration products. Jenni et al. [1] have used a variety of analytical techniques to study the polymer–mortar interac- tions in these materials. They observed, for example, that both the CE and the RPP are dissolved/redispersed in the pore solution and can form isolated films during drying. They also identified a fractionation process in which poly- mer components (and calcium hydroxide) were enriched at the top drying surface of the mortars. In this paper, the influence of drying on microstructural development of these mortars will be further studied utilizing X-ray absorption measurements. Previously, X-ray absorption measurements have been applied successfully to studying drying of fresh 0958-9465/$ - see front matter Published by Elsevier Ltd. doi:10.1016/j.cemconcomp.2007.10.007 * Corresponding author. Tel.: +1 301 975 5865; fax: +1 301 990 6891. E-mail addresses: dale.bentz@nist.gov (D.P. Bentz), clausjochen. haecker@SETylose.de (C.-J. Haecker), max.peltz@nist.gov (M.A. Peltz), kenneth.snyder@nist.gov (K.A. Snyder). www.elsevier.com/locate/cemconcomp Available online at www.sciencedirect.com Cement & Concrete Composites 30 (2008) 361–373