ALBERT

Description: The adhesive properties of smectites are widely used in many industrial applications. These pro- perties are mainly due to the reversible expansion and contraction of their interlayer spaces as a function of water activity. Being part of moulding sands, smectites are responsible for the required mechanical strength of moulds. Due to the need of industry for castings of increasing complexity but decreasing weight, it becomes essential to tailor moulding-sand mixtures beyond their current abilities. So far, however, it has not been discovered in detail why changes occurring during the casting process are partially reversible in the laboratory, but not in the circuit of the moulding sands. In general at temperatures below 300 °C and in the laboratory, de- and rehydration are reversible processes. It is therefore important to understand the complex kinetics of de- and rehydration of smectites and their influence on the mechanical behaviour of the moulding sands, before improvements can be achieved. Smectites are also widely used as adsorbents, e.g., for water. Lower water adsorption capacities and reduced adsorption rates of industrially-dried bentonites compared with bentonites dried to the same water content in the laboratory show that the kinetics of drying apparently influences interface processes. Smectites exposed to hot water vapour do not fully rehydrate in contrast to dry-heated smectites, which is another important aspect. Due to contact angle measurements, increasing hydrophobicity could be observed after vapour treatment, which, however, might depend on physical changes of aggregation (pore volume). Nevertheless, other parameters such as CEC or X-ray diffraction patterns of the 00l- reflexes did not show any conspicuous changes. Initial investigations led to the assumption that coordination of Al3+-Ions can be made responsible for these processes. However this has to be confirmed. Not only the hydration energy of the cations, but also size and charge as well as the water to smectite ratio, achievement/attainment of dispersion, speed of drying and further variables have an influence on the dynamics of dehydration (and possibly rehydration). The objective of the current project is the detailed examination of the aforementioned basic mechanisms from atomic to industrial scale in order to understand them as well as to optimize the casting processes. The following is a summary of the experiments and first interpretations of the results carried out by the project partners.