Cement, Energy and Environment

20 through Air Cyclone effectively. A detailed study on establishment of cost effective beneficiation technique for separation of dolomite (MgO) from limestone is essential. A study of clinker cooling reveals that under slow cooling, only 1.5% of MgO is retained in solid solution and rest crystallized as larger periclase crystals and makes the cement unsound. The rapid cooled clinker forms smaller alite and belite crystals, exhibits faster strength growth during hydration, and is able to accommodate the hydration of periclase (MgO) considerably. Various studies say that increasing the fineness of cement and addition of fly ash effectively stabilizes MgO up to 10% in Portland Cements. ii. With the lower Lime Saturation Factor (LSF) (0.94to 0.97) the high MgO in raw meal does not have any influence on the free lime generation during pyroprocessing. Whereas in case of high LSF ( 0.97-1.00) any increase of MgO in raw meal result more free lime generation and results unsound cement. The free MgO decreases in cement clinker as the iron content in the raw meal increases. It has been observed that a cement containing 5.0% MgO and 2.3 % Fe 2 O 3 had the autoclave expansion of 1.28% whereas another cement containing 4.95 %MgO and 3.6 % Fe 2 O 3 exhibit the autoclave expansion of 0.15%. The iron content in the raw meal should in the following ratio control the autoclave expansion. Fe 2 O 3 = 2 + (0.2 x MgO). Further the ratio between (Alumina +MgO) /Fe 2 O 3 should be maximum 2.7 for production of sound cement. When the MgO: Fe 2 O 3 >1.53 the autoclave expansion of the cement is in danger zone. When the ratio of MgO: Fe 2 O 3 <1.40 probability of expansion failure decrease rapidly and MgO: Fe 2 O 3 < 1.20 is the limiting value for production sound cement. iii. During pyroprocessing the C 4 AF acts as an excellent stabilizer for MgO and is capable of transforming considerable amount of MgO into non expanding compounds. The autoclave expansion due to MgO may be controlled by lowering the alumina modulus of the raw meal. This will enhance the C 4 AF content in cement and less C 3 A. The C 3 A should maintain at 6-8% in the cement clinker to control the MgO in form expansion. The higher MgO content in raw meal consumes extra thermal energy also. Incase of hard burning even the raw meal having 2.5% MgO fails in autoclave expansion. Whereas the light burning raw meal containing 5% MgO does not fail in autoclave expansion as it hydrates rapidly before the cement sets. The higher SO 3 in the Portland cement clinker also prevents the autoclave expansion in case of high MgO limestone. It has been observed that in case of 5% MgO containing clinker if the SO 3 is 2.17% the autoclave expansion is 0.44%. When the SO 3 content in the cement decreased to 1.01 % the autoclave expansion increase to 4.0%. From the above it can be concluded that in case of high MgO limestone the petcoke with high sulfur is suitable. It has been reported that the use of 0.4-0.8% fluorspar (CaF 2 ) in raw mix as mineralizer reduced the size of periclase to 1-7 micron in the cement clinker. By the fast cooling of the clinker the MgO is retained in the solution in the glass form. This reduces the expansion of the cement due to periclase. On the other hand in case of the slow cooling clinker the more periclase (MgO) crystallized out of the melt and larger crystals were formed. These large crystals of MgO are responsible for the failure of autoclave expansion of the cement. iv. The smaller size of periclase crystals in the cement does not contribute to autoclave expansion as the smaller periclase crystals are predominantly located in the interstitial phases. In an experiment it has been observed that cement clinker containing 6.5% (5.4% free periclase) MgO ground with 2.0% SO3 containing and free lime (CaO) of 0.47%. When the particle size distribution of this cement is 80% of the periclase is < 5.0 micron size and 20% periclase is size of 5.0-15.0 micron the autoclave expansion of the same cement found only 0.47%. Coarser the ground of the cement have always exhibited a greater amount of the autoclave expansion in case of clinker having high MgO content. Therefore, incase of high MgO clinker the cement should be ground finer to control expansion. It has been observed when a high MgO content clinker ground to 225 m 2 /kg was showed the autoclave expansion of 7.06%. When the fineness of the same cement enhanced to 350 m 2 /kg the autoclave expansion dropped to 1.49%. With the increase of fineness of the cement to 400 m 2 /kg the autoclave reduced to 0.24%.