This article aims to present the possibility of reducing CO2 emission in the composite cement production, by using large amounts of industrial by-products and to present the possibility of reducing CO2 emission in the process of Portland clinker synthesis. The last one will be the result of using raw materials containing calcium compounds other than carbonates and the use of alternative fuels containing biomass for the synthesis of clinker, the combustion of which is not included in the CO2 emission balance. Replacing one mass % of CaO in the raw mix as limestone, reduces the emission by 8 kg CO2 per Mg of clinker. The reduction of CO2 emissions was evaluated and confirmed by industrial production trial. Clinker was produced using raw materials containing carbide lime or limestone fly ash. The results of the trial showed, that the use of 2%÷5% of Bełchatów calcareous fly ash in the composition of the raw mix, allows of reducing the emission by 4.0÷10.3 kg of CO2 per Mg of Portland clinker. The use of 2%÷5% of carbide lime in the composition of the raw mix leads to emission reduction by 9.5÷23.9 kg of CO2 per Mg of Portland clinker. On the other hand, the development of composite cement production with a large amount of industrial by-products, seems to be the basic solution for the coming years, allowing a significant reduction of CO2 emission in the cement industry and in the concrete production.