Solid-solid reaction kinetics: Formation of tricalcium aluminate

Tricalcium aluminate is an important constituent of Portland cement, apart from having other applications. It is formed by a solid-solid reaction between CaO and Al2O3, themselves formed by solid-state decompositions of CaCO3 and Al(OH)(3), respectively. There is no unanimity in the literature about the kinetic and mechanistic aspects of its formation. In this article we report experimental studies on this system with a view to identifying the reasons for these discrepancies and to present reproducible kinetic information under a well-defined set of conditions. The experiments cover a temperature range of 1100-1300 degrees C and use CaCO3 and Al(OH)(3) gel powder as the starting materials. Reactions have been carried under a variety of conditions in an attempt to identify the experimental variables that influence the observed kinetics. The results show that mechanochemical activation can profoundly influence rates. The most reproducible and consistent results were obtained under conditions of good interparticle contact, with controlled pretreatment to define the physical structure of the reacting entity. Further, the results throw light on the sequential nature of the reaction and establish the nature of the intermediate phase. The data, when interpreted in the traditional manner, show consistent trends with the literature, but the deficiencies of such interpretation have been analyzed and the need for new models has been advanced. Because solid-solid reactions are generally less well understood than their fluid counterparts, our results argue in favor of a comprehensive modeling framework for such series reaction networks in the solid phase. (c) 2007 American Institute of Chemical Engineers.