Structure evolution of corundum concretes

Abstract

Using corundum concretes is one of the promising methods of obtaining unfired monolithic linings of the reactors used for producing technical-grade (commercial-grade) carbon that work under reducing atmospheres at elevated temperatures (up to 1800-2000~ and are subjected to the corrosive action of the drops of the atomized (sprayed) hydrocarbon raw materials, water vapors, and the particles of solid carbon and to erosion due to the fast moving gas current.Based on the studies conducted at the Ukrainian Scientific-Research Institute of Refractories, we developed corundum concretes based on a binder made from a high-alumina cement [1][2][3][4] that exhibit constancy of volume.They are intended for service in the reactors used for producing technical-grade carbon.It was shown [5] that the complex addition of calcium chromium(chrome)-ligonosulfonate (CCLS) along with technical (technological) lignosulfonate plasticizes the alumina-containing cements and that the addition of chromium oxide improves the strength of the concretes made from these cements.The optimum quantities (from the standpoint of volume constancy, strength, and thermal shock resistance) of the aforementioned additives were established [2].This paper deals with a study of the effect of these chromium-bearing additives on the structure evolution of corundum concretes based on an electrocorundum-filler having a coarseness (particle size) up to 5 mm.In order to study the effect of the additives on the hydration process of the aluminacontaining cements, we studied the kinetics of strength gain in the cement stone and the concrete baaed on it, the phase composition of the cement stone and the concrete, the changes occurring during the firing process, and the microstructure.The following alumina-containing cements were studied: alumina cement meeting the standard GOST 969~6, VTs-70 high-alumina cement meeting TU 21-20-9-73, and a Talyum type highalumina cement based on aluminium oxide and chalk that was produced at the Pilot Plant of the Ukrainian Scientific-Resrarch Institute of Refractories according to the technological route recommended by Bakalkin et el., [6]. Tables 1 and 2 show the composition and the properties of the cements.When introducing the plasticizer, decreasing the quantity of the sealing (mixing) water increases the strength of the cement stone. A study of the strength characteristics of the cement stone was carried out maintaining virtually equal spreadability of the slurry.It was shown that during the first 3-7 days, the strength gain of the cement stone obtained from the alumina cement occurs more rapidly than that of the cement stone obtained from the highalumina cement independent of the presence of the chromium-bearing additive (Fig. i). This owes to the higher initial hydraulic activity of calcium monoaluminate as compared to dialuminate (the main crystalline component of the high-alumina cement).Hydration of the alumina cement is described by the following reactions:CA + 2H~O--~2CAH lo + H~O--~C2AHs+AH3+ +gH~O, 2CA+ 11H20~C2AH~ ~...