Kinetics of the solid-phase reactions occurring during the production of clinker of a high-alumina cement by sintering

Abstract

At the present time, technological progress in the construction and maintenance (repairs) of thermal units requires mass-scale application of heat-resistant concretes whose specifications are becoming more stringent due to the developments taking place in the technology and the techniques and the intensification of the processes occurring in the thermal units. In view of the fact that high-alumina cement (HAC) is the most promising binding (bonding) agent for the refractory concretes working at a temperature exceeding 1773~ (under controlled atmospheres also), numerous studies have been carried out [1][2][3][4] for identifying the possible methods of improving its properties and for understanding the mechanisms and the kinetics of the reactions occurring during its synthesis.We carried out extensive (complex) studies aimed at identifying the optimum technological parameters for producing clinker of HAC by sintering in order to improve the service properties of the concretes based on the cement obtained from this clinker. This paper presents the results of the studies conducted on the kinetics of the solid Dhase reactions occurring during the sintering process of the clinker.In order to study the stability of the individual phases and compounds and to establish the most general regularities in the occurrence of the solid-phase reactions, thermodynamic analysis is carried out using the principle of calculating the change in the isobaric potential (AG) with respect to the compositions (constitutuents); the application of this principle involves investigation of each possible reaction under the assumption that the quantity of the initial substances is identical [5].The change AG was calculated by integrating the Gibbs-Helmholtz equation.The following equations depicting the temperature dependence of AG were obtained on the basis of the cal~ culation carried out taking into account the polymorphic transformation of 7-A1203 into ~o AI203 :A G O = --6584,470 --7,102 TIn T q-2,400 X 10-3T~--2,710 9 105T-'~-39,994T (for the aluminate having the composition ClzAT) ;A G~ = --7529,960 --6,830 T In T q-1,690,10-aT2--2,675 9 105T-'q-38,288T(for the aluminate having the composition CA) ; and 218T (for the aluminate having the composition CAz)Based on the obtained equations, the values of AG~ were calculated at different temperatures in the I073-1873"K range for the compounds CA, CAz, and C12A 7. The results of our calculations (Fig. I) showed the thermodynamic feasibility (possibility) for the formation of the aluminates CA and C12A 7 in the mixture of the experimental (system) (besides calcium dialuminate having a more negative value of AG o in the entire temperature range under consideration). The conclusion regarding the thermodynamic probability of formation of the aluminates CA, CA2, and CI2A 7 in the experimental system agree with the data of Fedorov et al. [4].It is known that the occurrence of a reaction depends not only on the magnitude of AG, but also on the kinetic factors among which the rate and the activation en...