The cement industry is facing the challenge of a lack of supplementary cementitious materials substituting clinker. This applies primarily to granulated blast furnace slag, the most commonly used as the main component of cements, due to changes in the metallurgical industry, and to siliceous fly ash due to the abandonment of fossil fuels in the world. A potential new main component of common cements may be calcined marls, which are available in large quantities for every producer of Portland clinker. In this article, research was carried out on calcined marls from the Folwark deposit, near Opole, Poland. The conducted experiments showed that marls after calcination show pozzolanic-hydraulic properties and can replace both granulated blast furnace slag and silica fly ash in the composition of cement. The best pozzolanic-hydraulic properties are obtained by calcination of Folwark marls at 900°C