Changes in microstructures and physical properties of polymer-modified mortars during wet storage A. Jenni a, * , R. Zurbriggen b , L. Holzer c , M. Herwegh a a Institute of Geological Sciences, University of Berne, Berne, Switzerland b Elotex AG, Sempach Station, Switzerland c EMPA, Du ¨ bendorf, Switzerland Received 15 December 2004; accepted 3 June 2005 Abstract The decrease in strength of tile adhesive mortars during wet storage was investigated. In a first approach, the water resistance of the polymer phases was tested on structures isolated from the mortar and in situ. It was observed that cellulose ether and polyvinyl alcohol structures are water-soluble. Subsequent investigations on polymer mobility within the mortar showed that the migrating pore water transports cellulose ether and polyvinyl alcohol during periods of water intrusion and drying. This leads to enrichments at the mortar – substrate interface. In contrast, latices interacting with the cement are water-resistant, and therefore, immobile in the mortar. Further experiments revealed that the mortar underwent considerable volume changes depending on the storage condition. Cracking occurred mainly close to the mortar–tile interface, cement hydrates grew within these shrinkage or expansion cracks. Test results revealed that the strength decrease of wet stored tile adhesives is caused by different mechanisms related to cement hydration, volume changes of the mortar, and reversible swelling of latex films. D 2005 Elsevier Ltd. All rights reserved. Keywords: Mortar (E); Microstructure (B); Polymers (D); Wet storage; Shrinkage (C) 1. Introduction Polymer-modification is widespread in cementitious applications to improve the physical properties of building materials. As many of these materials are exposed to wet conditions during service life, numerous studies investigated the influence of water storage on their physical properties. Tile adhesives are commonly modified with cellulose ether (CE) and redispersible powder (RP), the latter containing latex and polyvinyl alcohol (PVA; for mortar formulation see Table 1). Each of these polymers fulfils different tasks during the mortar evolution [1]. CE thickens the fresh mortar, entrains air during mixing and retains water. RPs mainly provide flexibility and tensile strength in the hardened mortar. In contrast to concrete applications, such tile adhesive mortars are prepared with a high w / c (water / cement ratio) of approximately 0.8 and characterised by high air void contents of more than 20 vol.%, and low degrees of cement hydration (sometimes less than 50%, [1]). The influence of water contact on the mechanical properties of polymer-modified cementitious products, were studied extensively [2–5]. The investigations of Tubbesing [6] include a microstructural characterisation of wet stored polymer-modified mortars. Based on scanning electron microscopy (SEM) images of fracture surfaces, Schulze and Killermann [5] concluded that latex morphology undergoes no structural changes, even after 10 years of outdoor exposure. Other studies focussed on changing pore size distributions due to water contact (e.g., [7–9]). Water intrusion and shrinkage/expansion of mortars were rarely investigated [10], but more frequently in the field of concrete (e.g., [11–14]). Mortar-specific aspects like hydro- 0008-8846/$ - see front matter D 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.cemconres.2005.06.001 * Corresponding author. EPFL – STI – IMX – LMC, MXG – Ecublens, CH- 1015, Lausanne, Switzerland. Tel.: +41 21 693 28 67; fax: +41 21 693 58 00. E-mail addresses: andreas.jenni@epfl.ch (A. Jenni), roger.zurbriggen@elotex.com (R. Zurbriggen), lorenz.holzer@empa.ch (L. Holzer), marco.herwegh@geo.unibe.ch (M. Herwegh). Cement and Concrete Research 36 (2006) 79 – 90