Tereza Ferić

The examination of the phase transformation of aragonite into calcite by means of DSC analysis

Abstract The X-ray diffraction method was used to determine the content of aragonite and calcite in samples of precipitated calcium carbonate synthesized in the laboratory. The transformation of the aragonite modification present into the stable calcite structure was traced by means of DSC analysis. All the samples were analysed by DSC under non-isothermal conditions in the temperature range 624–772 K at heating rates of 5, 10, 15, 20 and 25 K min −1 . A mathematical analysis of experimental DSC curves defined the mechanism and the kinetics of the phase transformation of aragonite into calcite. The kinetic curves α = f(t) and α = f(T) are sigmoidal in shape, and their inflection points and the v m point of the curves v = f(t) and v = f(T) are interrelated and are defined by the concept of a stationary point. The kinetic curves indicate that v m corresponds to (d H /d t ) m of the endothermal peak of transformation of aragonite into calcite, which is characteristic of a topochemical reaction.

Effect of chrysotile asbestos on cement hydration

Abstract Hydration of cement in the presence of asbestos was traced by means of hydration heat and adsorption capabilities of asbestos. Asbestos and cement samples were prepared in laboratory in proportions similar to those found in industrial materials. Asbestos was chrysotile by its mineralogical properties, of a short-fibered sort. Hydration heat was determined calorimetrically by means of spherical thermos flasks; adsorption capability of asbestos fibers was determined relative to Ca(OH)2. The hydration process was represented by integral and differential curves, which indicated that hydration heat development was intensified in the presence of asbestos. The results obtained for adsorption have showed that asbestos surface is active, with 1018 active sites per cm2. The isotherms obtained are multistep ones and indicate the formation of two layers of adsorbate. Adsorption capabilities of asbestos intensify cement hydration.