| Calcium aluminate cement(CAC) has been widely used in the industries of refractories, building chemistry and so on for several advantages, including higher refractoriness, rapid strength gain, enhanced resistance to abrasion, sulfate attack and alkali-silica reaction. The rheological behavior and strength evolution of castable bonded by CAC is strongly influenced by the hydration of CAC which is dependent on its mineralogy compositions and construction conditions. About 80% production of CAC in China is CA-50 containing about 50wt% Al2O3 which is produced using bauxite and calcite as raw material and coal as fuel by rotary kilns. Some impurities are brought into CA-50 from bauxite and coal. Sulfur is the one among that. Although sulfur content in CAC is not very high, influence of it on the components, working ability of CAC and performance of castable is still unknown. Along with the enhancing awareness of environmental protection, it has more realistic significance to form a stable desulfuration product in high temperature during the combustion process of coal in order to decrease SO2 release. The distribution of sulfur in CAC clinker and gas and the influence of sulfur on the components, hydrates and hydration of CAC clinker were studied in this paper. It is meaningful for application of CAC containing sulfur and environment.Base on the above ideas, sulfur existence forms and its formation in CA-50 clinker were analyzed by XRF, XRD, SEM, EDS and Carbon – Sulfur analyzer. The influence of raw materials and coal on sulfur content and SO2 release was also analyzed. The results indicate that the sulfur in the bauxite and coal results in the formation of well – dispersed ye’elimite(C4A3?) in the clinker. The most of input sulfur is released by smoke, and the rest is fixed by the clinker. The more sulfur content in coal the more formed. During the calcination process, C4A3? is formed between 900 and 1000 centigrade degree. And its content is improving with the increase of fire temperature. The decomposition of it takes place when temperature is higher than 1300 degree. The reason of its appearance in the clinker fired at 1400 degree may relate to the occurrence of liquid phase in the system.The influence of sulfur content on the hydrates and the hydration of CAC were investigated by ICP-OES, TG-DSC and PH meter. The results show that the hydration of C4A3? retards the dissolution of anhydrous phase of CAC. The hydration of C4A3? decreases the ion concentration of Ca, Al and the PH values but increases the concentration of S at 20℃ with water cement ratio(W/C) of 10. C4A3? dissolves and releases SO42- in 5 minutes after mixing with water. More CAH10 and less C2AH8 is formed in the sample contains more sulfur at 20℃ with W/C of 0.6. The peak exothermal temperature and the time to it of cement pastes is decreased and postponed respectively by the increase of sulfur content in the CAC.The effect of sulfur content in CAC on the properties of a conventional refractory castable bonded by it was investigated. For high sulfur content CAC bonded castable, cold compressive strength(CCS) of the specimen cured for 24 h at 20℃ is higher than the castable containing a low sulfur content CAC. The influence of sulfur on the flow ability is not obvious in castable. Besides, the working performance of castable bonded by CAC containing more sulfur is more sensitive to curing temperature than the CAC containing less sulfur. The influence of silica fume, boric acid or lithium carbonate on the working performance of castable is almost not changed by the sulfur content in CAC. |
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