Abstract: An improvement in the process for extracting alumina from gibbsitic bauxites, wherein the basic, known process comprises mixing the bauxites with a sodium hydroxide-sodium aluminate digestion liquor, digesting the bauxites at a temperature in the range of 100.degree.-180.degree. C., diluting the resulting slurry, separating the red mud therefrom, precipitating alumina hydrate by cooling and agitation of the sodium aluminate liquor, and calcining the alumina hydrate precipitate to obtain alumina, wherein the improvement is characterized by so controlling the process that, in the liquid phase of the slurry leaving the digestion reactor, the reactive hydroxide ion concentration is maintained below 6 g/l, and preferably between 0 and 2 g/l, where the reactive hydroxide ion concentration is defined by the following formula: ##EQU1## where: C.sub.Al.sbsb.2 O.sub.3,gi.sup.eq is the equilibrium solubility of Al.sub.2 O.sub.3 for gibbsite, in g/l.C.sub.Al.sbsb.2 O.sub.3 is the actual concentration of Al.sub.2 O.sub.

Abstract: A qualification method which enables a fast, exact and numerical determination of puzzoulana-properties of solid materials and thus a short evaluation of said solid materials from the point of view of industrial usability.According to this method the puzzoulana-properties and activity of solid materials can be defined by a single figure as a result of their chemical and physical properties. The material to be analyzed is brought together with fluoride ions in an acidic medium in a system having constant heat capacity and the change of the heat quantity is measured and evaluated in the initial, suitable linear, phase of the resulting reaction. The time function of the change of the heat quantity of the system is converted into a change of voltage and the rise of this curve is determined advantageously in the linear section. Advantageously the change of the heat quantity of the system is measured within less than 30 seconds, favorably at about the 20th second.