Studies of moisture and alkalinity in self-levelling flooring compounds
Abstract: In recent decades there has been an increasing interest in the indoor environment and its connections with public health. One important topic discussed has been the relation between the moisture in buildings and health. Cementitious materials, when cast, contain excess water since this is necessary for workability and for flow properties. The pore solutions in cementitious materials, besides being high in water content, often have a high pH. High-moisture conditions can cause biological growth. A combination of high moisture conditions and high pH can also result in chemical degradation of other materials, affecting both indoor environment and the technical functioning and appearance of the materials involved. Self-levelling flooring compounds (SLCs) that provide smooth horizontal surfaces are used to level the substrates (mainly concrete slabs) before floor coverings are applied. Although such compounds are used extensively, only limited research on their moisture and alkali properties and their functioning in floor constructions has thus far been conducted. The aim of the present study was to investigate the moisture and alkali properties of SLCs and gain a better understanding of their interaction with other materials in floor constructions. Many traditional methods of determining the moisture properties of building materials are time-consuming and are unsuitable for SLCs. Within the project, new methods, such as a rapid method for the simultaneous determination of both the moisture sorption isotherms and of diffusion coefficients, were developed. Measurements were performed of such moisture properties as the chemical binding of water, the physical binding of it (the moisture sorption isotherm), moisture transport (diffusion), and the rate of surface evaporation and of moisture transport to a concrete substrate, with the aim of better understanding the drying behaviour involved and enabling predictions of the long-term moisture state of SLCs to be obtained. The results served as the basis for a drying model that was developed into a computer-based simulation programme for predicting drying times and long-term moisture states of SLCs. Only very low rates for the transport of hydroxide ions from the concrete substrates to the SLCs and within SLCs were found to occur. This implies that SLCs can be used as barriers for protecting floor coverings and floor adhesives from the high pH of concrete. The alkaline degradation of floor adhesives is dependent upon the pH (hydroxide ion concentration) in the zone of contact of the adhesives with the substrate. Secondary emissions from a floor construction due to alkaline degradation are thus dependent upon the pH-level and the transport of hydroxide ions in the substrate. For highly alkaline substrates, such as concrete, carbonation is essential for avoiding degradation of sensitive materials. This is not the case for SLCs that have considerably lower pH. The carbonated layer at the concrete surface may, however, due to its limited thickness, be unable to serve as a long-term protection in terms of secondary emissions. This is better provided by an SLC of lower alkalinity.
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