This article assumes that the Gibbs free energy for a mixture consisting of felsites, limestone and Fe2O3. The purpose of this paper is to study the possibility of applying felsites as clay component raw mix for the production of portland cement clinker (the system CaCO3 – CaO – felsit) and study the effect of corrective additive Fe2O3 on the system CaCO3 – CaO – felsit. By heating a mixture of 21,38 % to 78,62 % felsites limestone formed following compounds 4CaO·Al2O3·Fe2O3 = 1,59 %, 3CaO·Al2O3 = 9,15 %, 2CaO·SiO2 = 20,96 %, 3CaO·SiO2 = 64,84 %, K2O·SiO2 = 2,10 %, Na2O·SiO2 = 1,35 %. This interaction is possible according to the calculations at temperatures above 892K. Due to low content of Fe2O3 in felsite, the composition consisting of 21,38 % and 78,62 % felsites limestone has a higher alumina and silicate module. Trivalent iron oxide has been used as a corrective additive. When adding 0-3,35 % Fe2O3 formed additional 4CaO·Al2O3·Fe2O3 from 1,59 to 16,83 %. According to calculations by adding Fe2O3 temperature decreases the reaction starts from 892 to 782 K. The calculations can recommend using felsit as clay component raw mix for the production of Portland cement clinker at low firing temperatures. Proved the possibility of Fe2O3 as a corrective additive for the studied system CaCO3 – CaO – felsit. The optimal composition for the production of portland cement clinker is a low-temperature composition containing 77,28 % limestone, 20,67 % felsites and 2,05 % Fe2O3 with the start of the reaction temperature clinker above 826,59 K.