Ceramics-Silikáty 64, (1) 63 - 67 (2020)

The CaO-SiO₂ glass forming system is a typical example of a situation when the Shakhmatkin and Vedishcheva thermodynamic model cannot explain the experimentally determined Q-units distribution. As the system components are determined as stable crystalline phases found in the equilibrium phase diagram, the reason of the model failure is the missing system component representing the Q³ structural unit. We suggested a solution to the problem by adding an artificial CaO∙2SiO₂ component and we proposed a method of estimating the Gibbs energy of this component. The method is based on the linear relationship found between the reaction Gibbs energy of the formation of the system components representing the different Qⁿ units (n = 3, 2, 1, 0) divided by the number of non-bridging oxygen atoms in this particular component (i.e., 4-n) on one side and the n value on the other side. The method was qualified by the good coincidence of the model results with the MAS NMR experimentally determined Q-distribution. Moreover, the estimated value of Gibbs energy practically coincides with the optimised value obtained by minimising the sum of the squares of the deviations between the experimental and calculated Q-distribution with respect to the molar Gibbs energy of CaO∙2SiO₂.