Moulding sands containing sodium silicate (water-glass) belong to the group of porous mixture with low resistance to increased humidity.
Thanks to hydrophilic properties of hardened or even overheated binder, possible is application of effective methods of hydrous
reclamation consisting in its secondary hydration. For the same reason (hydrophilia of the binder), moulds and foundry cores made of
high-silica moulding sands with sodium silicate are susceptible to the action of components of atmospheric air, including the contained
steam. This paper presents results of a research on the effect of (relative) humidity on mechanical and technological properties of
microwave-hardened moulding mixtures. Specimens of the moulding sand containing 1.5 wt% of sodium water-glass with module 2.5
were subjected, in a laboratory climatic chamber, to long-term action of steam contained in the chamber atmosphere. Concentration of
water in atmospheric air was stabilized for 28 days (672 h) according to the relative humidity parameter that was ca. 40%, 60% and 80% at
constant temperature 20 °C. In three cycles of the examinations, the specimens were taken out from the chamber every 7 days (168 h) and
their mechanical and technological parameters were determined. It was found on the grounds of laboratory measurements that moulds and
cores hardened with microwaves are susceptible to action of atmospheric air and presence of water (as steam) intensifies action of the air
components on glassy film of sodium silicate. Microwave-hardened moulding sands containing sodium silicate may be stored on a longterm
basis in strictly determined atmospheric conditions only, at reduced humidity. In spite of a negative effect of steam contained in the
air, the examined moulding mixtures maintain a part of their mechanical and technological properties, so the moulds and foundry cores
stored in specified, controlled conditions could be still used in manufacture.