A binder mix containing a hydraulic binding agent and an admixture mixed therewith, a process for preparing a hydraulically hardened binder mass, a calcinated kaolin product and a process for preparing the same. A paste-like composition is formed from the hydraulically hardening binding agent, a pozzolanically reacting admixture and water, which, when so desired, contains stone aggregate or similar filler, the paste-like composition is worked and the worked composition is allowed to harden to form the binder mass. The admixture comprises spherical, porous metakaolin agglomerates, the size of which is 2-200 microns and which have an open pore structure. The invention can be used to improve the manufacturing methods of cast concrete products and shorten the manufacturing times, and to provide concrete with better mechanical and chemical properties, as well as improve the fire and frost resistance of concrete.