The present invention describes a hardening accelerating admixture for hydraulic binders, the accelerator being based on transition metal silicate hydrates having the general formula: aMe.sub.xO.sub.y bMO cAl.sub.2O.sub.3 SiO.sub.2 dH.sub.2O 1) whereMe represents a transition metal whose molar coefficient a is in a range between 0.001 and 2, preferably between 0.01 and 1; M represents an alkaline earth metal whose molar coefficient b is in a range between 0 and 2, preferably between 0.3 and 1.6; The molar coefficient c for Al.sub.2O.sub.3 is in a range between 0 and 2, preferably between 0.1 and 1; H.sub.2O represents the hydration water of the silicate hydrate whose molar coefficient d can vary within a wide range between 0.5 and 20; x and y can both be equal to 1 or different, depending on the valence of the transition metal, given that the valence of the oxygen atom in the metal oxide is equal to 2.

The invention concerns a method for producing pulverulent hardening accelerators by reactive spray drying, where an aqueous phase I comprising calcium ions, and an aqueous phase II comprising sulphate ions, the molar ratio of the calcium ions to the sulphate ions being from 1/5 to 5/1, are contacted at a spray nozzle, and the phases I and II contacted with one another at the spray nozzle are sprayed in a streaming environment of drying gas. Likewise concerned are the pulverulent hardening accelerators producible by the method of the invention, and their use for accelerating the hardening of bassanite and/or anhydrite with formation of gypsum.

The invention concerns a method for producing pulverulent hardening accelerators by reactive spray drying, where an aqueous phase I comprising calcium ions, and an aqueous phase II comprising sulphate ions, the molar ratio of the calcium ions to the sulphate ions being from 1/5 to 5/1, are contacted at a spray nozzle, and the phases I and II contacted with one another at the spray nozzle are sprayed in a streaming environment of drying gas. Likewise concerned are the pulverulent hardening accelerators producible by the method of the invention, and their use for accelerating the hardening of bassanite and/or anhydrite with formation of gypsum.