Effect of silicon-containing additives on the properties of corundum concrete

Abstract

When developing corundum concretes suitable for lining the combustion chambers having oxidizing atmospheres and the reactors used for production of co=~ercial-grade carbon at high rates, the Ukrainian Scientific-Research Institute of Refractories carried out extensive studies for obtaining shrinkage-free concretes having the required levels of the other properties. In order to ensure volume constancy during service, the quality of the finely dispersed binder constituent is extremely important even when using fused and fired (at high temperatures) refractory oxides having a specific granulometric composition (with a proper ratio of the fractions) as fillers. It must possess high levels of properties including corrosion and erosion resistance. This can be achieved either by increasing the AI203 content of the binder and improving the structure of the products or by introducing the additives facilitating the occurance of various processes such as modification, chemical interaction, and formation of solid solutions that lead to volumetric expansion. Addition of such additives must not cause deterioration of the quality of the products. This paper deals with a study of the effect of different silicon-containing additives on the properties of corundum concretes. In order to carry out this study, we compacted a series of cylindrical specimens measuring 36 mm in diameter and 50 mm in height at a compaction pressure of I00 N/mm 2 using the corundum concretes based on an electromelted corundum filler having a grain size of 5 mm and the finely dispersed binder constituent obtained from a high-alumina cement and the GK grade alumina. Table 1 shows the chemical composition of the silicon-containing additives used in this study. The SiO 2 additive was introduced into the finely dispersed constituent part in the form of finely milled sand (its quantity amounted to 3, 5, and 8%* in the Nos. i-3 systems, respectivelz), quartzite (Nos. 4-6), and quartz sand (Nos. 7-9) and, also, into the coarse-grained filler (3 and 5%) as the minus 0.5-mm fraction of sand having natural granularity (Nos. i0 and ii) and the 3-1 mm fraction of quartzite that contains equal quantities of the 3-2 and 2-1 mm fractions (Nos. 12 and 13). A binder containing an organosilicon polymer (silicone) was introduced into the Nos. 14-16 systems; in these systems, its quantity amounted to i, 3.5, and 7%, respectively. Table 2 shows the properties of the unfired specimens of the corundum concrete. We note that mullite formation (that leads to volumetric expansion) occurs against the background of the phase transformations of the main mineralogical components of the corundum concrete and the water-setting binder and, also, the interaction between this binder and the corundum-containing component of the finely dispersed binder. The conducted studies established that when the finely milled silicon-containing additives are introduced into the finely dispersed binder constituent of the corundum concrete in the form of quartz sand, quartzite, and quartz glass, the shrinkage occuring during the sintering process in the 1400-1750~ range is not compensated. A shrinkage free corundum concrete is obtained only in the presence of 8% quartzite and in the presence of 3 and 8% quartz glass and at a temperature of 1400~ Subsequent increase of the firing temperature of the corundum concrete up to 1580 and 1750~ activates the sintering process independent of the type and the quantity of the introduced finely milled silicon-containing additive. Figure i shows the temperature dependence of the main parameters of the corundum concretes containing SiO 2.