Abstract
	Physical processes of dehydration in core manufacture (Hot-Box process, microwave hardening – MW) with use of Na-silicates bring the growth of strength by 60 – 100 % in comparison with chemical processes of hardening (CO2- process, Na-silicate – esters). High strength of gel of silicic acid can be explained by the presence of very small particles (micelles) with high coordination, absence of by-products of hardening reactions (e.g. Na2CO3.xH2O) and by minimum stress after dehydration (residual humidity ca 2%).
	Measurement of Na-silicates structure (M = 1.97 – 3.35) with the aid of 29Si NMR spectrometry has proved that with falling modulus the degree of polycondensation of silicate anions falls. With the aid of special additives (organosilanes and reduction products of monosaccharides) the binders were prepared that add to mixtures higher bending strength by about 25% and long-term stability of cores (96 h.) also with 81% relative humidity of air. With use of MW hardening the highest strength is achieved with Na-silicates what enables to lower considerably the binder content (about 2%) and to shorten the manufacturing cycle of cores (20 sec.). These values were achieved with the output 4 kW of the hardening unit. The exclusion of expensive teflon core boxes makes possible to use several proposed new processes with use of MW. Cores with modified Na-silicates have at the same time good shelf life and high wear resistance also under adverse climatic conditions (90% relative humidity). Re-hydration of the binding gel enables easy decoring of castings by simple dipping in water. Collapse of cores comes after 2 minutes (hot cores) and up to 60 minutes (cooled cores) without regard to the binder content.