System Na2o—CaO—SiO2
On fig. the diagramme of a condition of the investigated part of three-componental system Na2O-CaO-SiO2 on G.Moreju and N.Bouenu who in highbases areas is limited by connection Na2O-SiO2, and in highalumina - СаО-SiO2 is presented. In this part of system except already considered binary connections in private systems Na2O-SiO2 and CaO-SiO2 there are three threefold connections: Na2O-2CaO-3SiO2 (1:2:3) 2Na2O-CaO-3SiO2 (2:1:3) and Na2O-3CaO-6SiO2 (1:3:6) [in brackets the reduced designation of threefold connections accepted in this system is specified, at which on the first place the quantity of substance (moles) alkaline, on the second - shchelochno-zemelynogo oxide and on the third place - silica in the given connection is put. Except specified in system Na2O-SaO-SiO2 other threefold connections, in particular 2Na2O-8CaO-5SiO2 and 2Na2O-4CaO-3SiO2, (these structures, according to some information, represent firm solutions of alkaline silicates in orthosilicate of calcium), and also Na2O-CaO-SiO2, 4Na2O-3CaO-5SiO2, Na2O-2CaO-2SiO2 are marked also. Structures of these connections, fields of which primary crystallisation with sufficient degree of accuracy are not defined, lay outside of 69 parts of system presented on fig.
Rich of silica threefold connection d e v i t r i t Na2O-3CaO-6SiO2 melt incongruent at 1047°С, decaying on crystals β-CaO-SiO2 and alloy. Devitrit in the form of spherical crystal joints or thin needle or prism crystals often takes shape as one of phases at devitrification usual glasses. In the field of devitrit (in its top part) structures of the glasses most proof to action of water and alkaline solutions settle down.
Connection 2Na2O-CaO-3SiO2 also melt incongruent, decaying at 1140°С on crystals Na2O-2CaO-3SiO2 and a liquid. Connection Na2O-2CaO-3SiO2 melt at 1284°С without decomposition.
Structures in a considered part of system Na2O-SaO-S1O2 are characterised comparative easymelt. So, for example, all structures getting to elementary triangle Na2O-2SiO2-Na2O·3CaO·6SiO2-SiO2) begin melt at 725°С (eutectic structure About on the condition diagramme, fig.).
System Na2O-CaO-SiO2 has great value for the "know-how" alumina-sodium silicate glasses. It includes structures of some industrial glasses (window, bottle, tare and so forth) in which SiO2, Na2O and СаО are the main components.
As well as any other diagramme of a condition, system Na2O-SaO-SiO2 diagramme expresses only equilibrium conditions which, as is known, glass does not concern. Nevertheless, the knowledge of the diagramme of a condition of this system is necessary in glasses as with theoretical, and purely practical point of view. In manufacturing techniques of industrial glasses the knowledge of diagrammes of a condition of corresponding systems is necessary for struggle against one of rather widespread defects or as them sometimes name, defects of glass - stones of crystallization which represent crystal inclusions in the glass, breaking its physical and chemical uniformity. At crystallization silica-sodium silicate glasses the crystal phases existing in system Na2O-SaO-SiO2 are formed. In usual industrial glasses are formed most often trydimite, crystobalite, an vollastonit, , devitrit.
Principal cause of crystallization of glasses are incorrectly chosen structure inclined to crystallization and infringements of a temperature mode of cooking and glass development. Struggle against propensity of glasses to crystallization demands knowledge of the nature of a phase dropping out at crystallizations, temperature limits in which has flown down can crystallizations (in particular, temperatures of the beginning of crystallization), and speeds of crystallization. The condition diagramm allows not only to answer precisely at least on the two first question, but also to draw certain qualitative conclusions concerning speed of crystallization. It is known, in particular, that the glasses corresponding on structure to certain chemical compounds, have the greatest speed of crystallization. The structures forming at crystallization a firm phase, differing from structure of initial glass, will crystallize more slowly. Most difficultly with other things being equal crystallize eutectics structures.