The introduction of mineral additives to binders, especially to Portland cement, is one of the promising trends for solving the resource and energy saving problems, as well as problems of environmental protection during production and application. Expanding the supplementary cementitious materials resource base can be achieved through the use of natural pozzolans and thermally activated polymineral clays(commonly known as glinites in Russia). One type of glinite is metakaolin, which is obtained by calcination of kaolin clays. Metakaolin is widely and effectively used as a pozzolanic additive due to its beneficial effect on the physical and mechanical properties of Portland cement-based materials. The obstacle to its wide production and use are the limited deposits of pure kaolin clays in many countries, including the Russian Federation. In this respect, the studies of pozzolanic activity of the most common mineral clays and their use in some countries have significantly advanced. Similar studies were widely performed in the 1940s in USSR. It seems reasonable to renew this trend to provide a scientific base for the production of local pozzolans made of clays commonly used in different regions. Comparative studies of the effect of 5 clays differing in mineral and chemical composition, calcination temperature and specific surface area, and high-quality metakaolin, on the strength of hardened Portland cement paste have been performed. It has been established that introducing 5…10 % of composite clays calcined at 400…8000 C° and milled to a specific surface area of 290…800 m2 /kg into Portland cement enhanced the strength of the hardened cement paste considerably better than the introduction of metakaolin with a specific surface area of 1200 m2 /kg. The findings of the study suggest that many kinds of commonly used polymineral clays have a specific calcination temperature and dispersity, which results in a higher pozzolanic activity compared with metakaolin.