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High residual strength of material of moulds and moulding cores after high-temperature effect by ferrous metals alloys is considered as one of the main disadvantages which restrict wide use of sand mixtures on waterglass binder in casting and foundry production. It is known that mechanical properties of sand mixtures on waterglass binder depend directly on the conditions of silicate binder film forming on the surface of refractory filling agent. Investigations aimed on examination of sodium silicate film structure forming depending on heat and chemical setting of mixtures as well as on the type of refractory filling agent were carried out at the first stage. The obtained data of this investigation showed that presence of numerous laminations, cracks in the area of seals and crystals of sodium carbonates (which are stress concentrators) in the film structure of sodium silicate aqueous solution provides the main effect on mechanical properties of sand mixtures on waterglass binder. Forming of these defects, in its turn, requires the conditions which promote running of carbonization process of sodium silicate aqueous solution.The second stage of this research included study of the effect of destruction of carbon-containing additive on integrity violation of a binder film. As a result, the suggestions about influence of crystals of sodium carbonates in sodium silicate film structure on lowering of residual strength of a waterglass mixture were confirmed.