A computer simulation of the thermodynamic stability of phases in firing ceramic silicate binders in the SiO2 - Al2O3 - B2O3-K2O - Na2O system for abrasive tools based on grained normal electrocorundum is described. The thermodynamic data for the computation are chosen from the HSC 203 data base of the Outokumpu concern and TAPP of the E. S. Microwave Inc. It is established that when the binder (67.73% SiO2, 20.0% Al2O3, 5.47% B2O3, 5.78% K2O, 2.84% Na2O with CaO, MgO and Fe2O3 impurity oxides, the remainder TiO2) is fired at 1250 degrees C, the most stable phases bear potassium (KAlSi3O3 and KAlSi2O6), the amount of the sodium-bearing phases diminishes substantially after 900-1100 degrees C with only the Na2O . Al2O3 . 2SiO(2) phase remaining stable. This seems to explain the fact that the proportion of K2O to Na2O in feldspar pegmatites and ceramic binders for abrasion tools is regulated by standards.