Method for producing an aqueous suspension, which is based on a hydraulic binder, and a hardened binder product. According to the invention, a composition of blending agents is added in the aqueous phase into the binder, which composition comprises calcium carbonate particles having an average particle size of 2-200 nm, in order to generate an aqueous suspension and, if needed, mineral fillers are added into this suspension, in order to produce a hardening plaster or concrete mixture. Besides calcium carbonate, the composition of blending agents can comprise calcium hydrogen carbonate and meta-kaolin sinter, too. By using the present invention, it is possible to improve the properties of products which are bound by hydraulic binders; with the described combination of blending agents it is possible to affect for instance the early strength, the frost resistance, the phenomenon of bleading, the plasticizing, the size of the transition zone and the durability of concrete or plaster, and to reduce the quantity of the hydraulic binder.