Microstructural Evolution and Physical Properties of Polymer-Modified Mortars

Abstract

Polymer-modified cementitious materials provide the base for building materials commonly used on modern construction sites. By adding polymers, the properties of cementitious materials can be extended to suit a variety of applications. With respect to adhesion properties, for example, the first patent of latex-modified hydraulic cement systems was issued in 1924 (Lefebure 1924). In the field of tile adhesives, latex-modification allowed thinbed application, a technique that is still standard because of its economic advantages with respect to application time and resource costs. Before invention of redispersible powders, the appropriate latex was only available in the form of dispersions. Mortar mixing was an important issue, which when improperly performed, often caused cases of damage at construction sites. The development of latex in the form of redispersible powders drastically reduced this problem, because it allowed the production of one-component systems or so-called “dry mortars”, which only require the appropriate amount of water to be added before application. Mortar properties were continuously improved by optimising the formulation or enhancing the system's components. Empirical approaches dominated, in which numerous formulations were compared with each other, in terms of physical properties of the resultant mortars. To further improve these properties at the present stage, an extended understanding of the mechanisms active during mortar evolution is required. Many of these mechanisms leave characteristic marks on the mortar microstructures, which, once recognised and related to the corresponding mechanism, can be linked with the physical properties. Therefore, the microstructure represents a major key to an improved understanding of the highly complex system of polymer-modified mortars. The cementitious, mineralic microstructures can be investigated by methods commonly applied in earth and material sciences. In contrast, organic compounds like polymers can form delicate and fragile structures requiring specific techniques that originated in the field of organic chemistry and biology. Therefore, the investigation of tile adhesive requires an interdisciplinary approach, in which methods from different fields of research are adapted and combined. As is common in applied research, the investigation of polymer-modified mortars is a tightrope walk between the complex, commercial system and model systems usually based on crude simplifications. The combination of both approaches might result in large forward steps in understanding, and new insights. The present study on polymer-modified, cementitious mortars tries to incorporate the previously mentioned requirements and is organised in the following manner: (a) methods of quantitative investigations, (b) influence of polymers on microstructure and physical properties, (c) changes of microstructures and physical properties during wet storage. a) The first chapter describes the new methods developed to quantitatively investigate microstructures in polymer-modified mortars. A combination of digital light, fluorescence and electron microscopy allowed the visualisation of different mortar components such as specific polymer components, air voids, cement phases, and filler minerals. In a second step, their occurrence and spatial distribution was quantified by image analysis requiring appropriate program routines, whose use and functionality is explained. To demonstrate the power of the new quantitative approach in the field of polymer-modified tile adhesives, a selected mortar formulation was analysed as an example. The results show that the mortar fractionated during application and hardening, inducing a variety of phase enrichments or depletions. The occurrence of these microstructural heterogeneities suggests the major influence that the microstructure has on the physical properties of the mortar system. b) In the second chapter, the microstructural evolution of the mortar and the mechanisms involved were investigated by using the methodology developed above. It is shown that water flux, induced by evaporation 2 and capillary forces of the porous substrate, played the most important role in mortar fractionation. It transported cellulose ether, polyvinyl alcohol, and cement ions to the mortar interfaces, where they became accumulated. In contrast, latex components did not migrate and remained homogeneously distributed within the microstructure. Combination of quantitative with qualitative investigations allowed a reconstruction of the mechanisms forming the microstructure during the different mortar stages. By correlating microstructural observations with physical properties (e.g., adhesive strength), skinning on the mortar surface of the applied fresh paste was found to decrease adhesion strength to the tile. As a consequence, it is the mortar-tile interface that dominates the properties of the entire hardened substrate-mortar-tile system. c) In chapter three the influence of wet storage on the microstructure and its physical properties are investigated. Wet storage represents an important test criteria on the durability of polymer-modified systems exposed to wet conditions in case of outdoor or bath room applications. Tests on individual polymer structures revealed that cellulose ether and polyvinyl alcohol redissolved in the pore water, whereas latices were water-resistant. Consequently, latex distributions in the mortar measured before and after wet storage were identical because latex remained immobile, but cellulose ether and polyvinyl alcohol distributions changed. By combining these observations with microstructural investigations of the failure surfaces, pore size, shrinkage and physical test data, we were able to show that changes in the mortar volume and reinitiated cement hydration caused a decrease of the mechanical properties during wet storage. Although they remained immobile, the latex films also weakened due to water uptake and swelling, which was shown to be a reversible mechanism. The appendix A includes non-published studies in a short and descriptive form. The corresponding database is available upon request after consultation with the author and Elotex AG. Appendix B includes extended abstracts of the given talks.