348 



G. SCHIMMEL 



Fig. 1. Monocrystalline calcium carbonate. Magnification 

 : 90,000. 



Fig. 2. Calcium liydroxyde obtained from aqueous suspen- 

 sion. Magnification 80,000. 



droxide precipitates from clear calcinated water in 

 the form of smooth, compact spherulites (hemis- 

 pheres). During the test they permitted the calcinated 

 water to dry in the air on collodion films. Since 

 then, these results have become the general scien- 

 tific property of the lime industry; they are 

 discussed, for instance, in the third volume of the 

 book Zement-Cheniie by Dr. Hans Kuehl. We re- 

 peated the investigations and obtained, at first, exactly 

 the same photographs. When heating their prepara- 

 tions by an intensive irradiation, the above-men- 

 tioned authors observed a change in the spherulites, 

 which they interpreted to be a transformation to 

 calcium oxide. It has to be borne in mind, however, 

 that it was not possible with the equipment used 

 at that time to make electron diffraction diagrams. 

 We, too, have observed corresponding changes, 

 which we have analyzed by means of diffraction. 

 Hardly any differences are detectable if the same 

 spot is examined prior to and following an intensive 

 irradiation. The proper diffraction patterns demon- 

 strate, however, that the precipitations prior to the 

 irradiation were of an amorphous nature and fol- 

 lowing the irradiation of a crystalline one. Surpris- 

 ingly, however, an evaluation of the diffraction 

 pattern revealed calcium carbonate in a calcite 

 structure to be present, rather than oxyde. Fig. 1 

 shows a monocrystalline section at which the hex- 

 agonal symmetry of the calcite is clearly noticeable 

 in the diffraction pattern. 



Radczewski, Miiller, and Eitel (1939) obtained 

 spherulitic calcium carbonate as a residuum from 

 dried calcium-bicarbonate solutions; however, they 

 did not recognize the identity with the precipitations 



of calcinated water form.ed during drying in air. A 

 light-microscopical investigation of spherulites up 

 to a diameter of 4 //, which had been skimmed off 

 from the surface of a calcium-hydroxide solution, 

 shows that the amorphous modification of the 

 carbonate is stable even if it is present in a larger 

 particle size in contrast with the above publication. 

 I have to thank Dr. Ney of the Technische Hoch- 

 schule MiJnchen for informing me of the existence of 

 such large spherulites on the surface of liquids. 



When solutions still containing undissolved hy- 

 droxide in suspension were dried, crystalline calcium 

 carbonate, exhibiting characteristic growth phenom- 

 ena, always formed during air drying. 



To avoid the hydroxide to transform into the car- 

 bonate during the drying process, the specimen grid 

 was placed into the vacuum immediately after it was 

 covered with the calcium-hydroxide solution. During 

 this process, the hydroxide precipitated while drying 

 to very small crystals of different shapes, but it 

 was always crystalline. An amorphous modification 

 of the hydroxide has never been found. Of industrial 

 importance is mainly the question in which form 

 the hydroxide is present in the commercial dry 

 hydrate and the lime paste. The amount of lime 

 contained in the mortar is determined by the plas- 

 ticity of the latter, rather than by the desired strength 

 of the solidified material. This plasticity, however, 

 will depend on the shape and size of the crystals. 

 The dry hydrate prepared from fog obtained by means 

 of ultrasonics contains numerous hexagonal scales 

 and fragments thereof. Almost without any excep- 

 tion, these crystals in their commercial form are 

 strongly agglomerated. 



A photograph of crystals of lime paste dried in 

 vacuo is shown in fig. 2. The same oblong crystals 

 are found if the water of the lime paste is substituted 

 by alcohol and the alcohol suspension is dried. The 

 following preparation technique proved to be very 

 advantageous: The lime paste spread out in a thin 

 manner is frozen rapidly by means of liquid nitrogen; 

 subsequently it is dried in vacuo in which a carbon 

 replica is prepared immediately after drying. This 

 method is particularly suited for precipitation proc- 

 esses aiming at a carbonation. 



All photographs were taken at the Battelle-Institut, 

 Frankfurt/ Main, with the Elmiskop I of the firm 

 Siemens, using a voltage of 80 kV. 



I am very grateful to the "Bundesverband der Deut- 

 schen Kalkindustrie" (Federal Association of the German 

 Lime Industry) for suggesting these studies, for supplying 

 the lime samples, and for the permission to publish the 

 photographs made in the course of these investigations 

 for the "Bundesverband". 



