Influence of polymers on microstructure and adhesive strength of cementitious tile adhesive mortars 2017-09-13

polymers

  1. Miroslav
    Introduction
    Commercially available tile adhesive mortars consist of a binder and mineral fillers, and are usually modified with cellulose ether (CE) and redispersible polymer powder (RP). These additives fulfil different tasks during the evolution from fresh to hardened mortar. The main purposes of CE are thickening, air entrainment, and water retention to establish proper workability properties. RPs further improve fresh mortar rheology, but mainly provide flexibility and tensile strength of the hardened mortar. The powder is usually manufactured by spray drying of a polyvinyl alcohol (PVA) containing latex emulsion. The most typical binder is ordinary Portland cement, used in combination with different types of mineral fillers. The simultaneous existence of binder and polymers provokes the interaction of two fundamental processes: polymer film formation and cement hydration. In comparison to common concrete technology, polymer-modified, thin-bed mortars are characterised by high water/cement ratios of about 0.8, but due to their high surface/volume ratios, they dry out more quickly. As a result, the cement has a low degree of hydration (less than 30%, instead of N90% as in concrete described in Ref. [1]). Tile adhesive mortars typically also contain a much higher air void content (25 vol.%, instead of b5% in concrete; see Fig. 1a).
    To date, the influence of polymers has generally been investigated in an empirical manner by comparison of physical properties (compressive, flexural, and adhesive strengths) from different mortar formulations (e.g., Refs. [2–7]). In general, these studies document that the increase of strength can be correlated with concentration and type of latex polymers. Furthermore, Larbi and Bijen [8] measured the pore solution chemistry of different polymer-modified mortar formulations in function of time, and concluded that latices interact with ions in pore solution. Changes in covalent latex bonds due to chemical interactions with cement ions are also documented by infrared spectroscopy [9,10]. A review of such interaction processes, mainly based on studies of ion measurements in aqueous systems, is given in Ref. [11].