Introduction
Physical chemistry of silicates is an individual discipline, including study about phase equilibriums and state diagrams of heterogenic systems, structure and properties of silicates in different states, also processes, stipulating formation of silicate products at high temperature synthesis.
At heating of silicate materials chemical reactions start in solid state before the beginning of melting of mixtures of components. Therefore studying of main laws, defining the reactions path between solid phases, has exceptional significance for silicate technology.
At studying reactions between solid phases, many details of those chemical processes have been revealed, which flow at producing Portland cement clinker, fireproof and ceramic masses, aluminous materials. At this optimum conditions of dosing of components of raw mixtures are defined, necessary grinding fineness of various materials, influence of mineralizing agents on the rate of reactions, influence of the rate of heating and burning temperatures and a number of other factors on the course of main technological processes.
The study about reactions in systems, formed by silicates, is based primarily on numerous researches of heterogenic equilibriums, carried out at high temperatures, advantageously in conditions of melting or crystallization of substances.
The condition of equilibrium can be reached by various ways, for example, by heating of the systems to the certain temperature or its cooling to the same temperature from the overheated state.
The use of the rule of phases at studying high temperature equilibriums in silicate systems turned out to be enough fruitful, and at present the study about heterogenic equilibriums in these systems, usually represented graphically as corresponding diagrams of the state, is the most thoroughly developed section of physical chemistry of silicates.
Practical classes № 1. Solution of tasks in diagrams of one-component systems state. Definition of number of phases, freedom degrees
One-component systems
One-component diagrams of the state are constructed in coordinates of the temperature (axis of abscissas) – pressure of gaseous phases or vapours pressure over crystalline or liquid phases (axis of coordinates).
Definition of temperatures of melting of silicates and oxides has an important significance, as most chemical reactions in silicate systems flow sufficiently intensively exactly at very high temperatures.
The melting point in exact physical and chemical meaning is the temperature of equilibrium state between crystalline and fused phases of the researched substance. From the point of view of phases rule the similar temperature point is not really invariant, not having freedom degrees, as at one component (n=1) the number of freedom degrees at the presence of two phases equals to one; but in fact they usually work at certain pressure, equal to one atmosphere, and, thus, the melting point acquires quite definite invariant character.
In one component systems, except definition of melting temperatures, it is very important to study phenomena of polymorphic transformations, observed, when the substance in crystalline state forms two or several modifications, different in reserve of internal energy, crystalline structure and physical properties.
Figure 1 - Diagram of a condition of system SiO2.