The effect of hardening accelerators on the properties of low cement alumina-based gunning refractories with aluminum silicate plasticizer

According to ASTM C71, refractories are nonmetallic materials with chemical and physical properties that make them useful for structures or system components exposed to environments above 1000°F (~811K or 538°C). Refractory materials can be divided into shaped and unshaped (monolithic or mass) groups. Among unshaped refractories, gunning mixtures are of significant importance as they can be quickly installed and internal mass can be replaced during operation. In this case, the additives that are used to achieve different goals are very effective in gunning refractory mixtures. The additives have an effect on various properties such as spraying properties, setting and workability times, and mechanical and physical properties. In the field of additives used to improve setting properties, limited articles have investigated their effect on alumina-based gunning masses. Therefore, in this study, an attempt is made to determine the type, amount, and conditions of use of additives used in gunning materials by utilizing relevant evaluations such as X-ray diffraction analysis, physical and mechanical properties, and setting time. In this research, different hardening accelerators, sodium carbonate, sodium silicate and lithium carbonate, with different amounts were used in the samples which are of high alumina with low cement and then, the optimum amounts of these accelerators were determined. After that, the effect of these additives on the properties of monolithic alumina-based refractories has been investigated. All samples were first dried at 110°C and then, fired at 1100°C and 1550°C. In order to evaluate the desired properties, first the rheological properties of the samples were examined, and then for further investigations, bulk density (BD) and cold crushing strength (CCS) tests were performed. In addition, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and X-ray diffractometry (XRD) were used to characterize the samples. The results of this research showed that the optimum amounts for sodium carbonate, sodium silicate and lithium carbonate are 0.1%, 0.1% and 0.01%, respectively. Moreover, water consumption of the bare sample (Mix 1) and also the samples with sodium carbonate (Mix 2), sodium silicate (Mix 3) and lithium carbonate (Mix 4) is 11.25%, 11.75%, 12% and 12.5%, respectively. Based on this, CCS and BD of the samples were determined: Mix 1, Mix 2, Mix 3 and Mix 4 have experienced the highest to the lowest amounts of CCS and BD for the samples either dried at 110 °C or fired at 1100 and 1550 °C. In terms of the rheological properties of the samples, the samples containing the additives have the same rheological properties as the bare sample (without additives). Therefore, from this point of view, all the samples are identical and other properties has to be investigated to compare them. Characterization tests (XRD, SEM and EDX) have proved the formation of the phases at three temperatures. At 110°C, quartz and C3AH6, at 1100°C, gehlenite and at 1550°C, CA6 phases are formed. The main phase at all temperatures is corundum. Another result which is worth mentioning is the decrease of CCS and BD by increasing temperature from 110°C to 1100°C. The main reason for this reduction was found to be dehydration of the hydrated phases. Furthermore, sintering process and also formation of CA6 phase are the main reasons of increase in the BD and CCS of the samples fired at higher temperature of 1550°C.