Cement, Energy and Environment

19 production of more 43 and 53 grade cements. It has been observed that the demand of high grade limestone increase the mines rejects further. The limestone requirement for production of 71.28 million tonnes of clinker during 1997-98 was 106.92 million tonnes and in the year 2013-14 it has enhanced to 300.00 million tonnes. From an estimate it has been seen that for production of 500 million tonnes of Clinker (as anticipated) by the end of 2020 the limestone requirement will be around 700 million tonnes. Hence the detailed exploration and establishment of the limestone depaosits, use of high silica and high MgO limestone in cement manufacture is the need of the hour. If this trend persists the annual limestone requirement for production of 500 million tonnes (as anticipated) of clinker by the end of the year 2020 will be around 700 million tonnes. With this rate of consumption the present available limestone reserves of India can sustain only 40 years. (12 th plan report) 3.0 Effect of High MgO in Portland Cement For production of Portland cement the limiting value of MgO in case of cement grade limestone is 05.0% , however the preferable value is 3.5% to control the autoclave expansion of the cement under the prescribed value of BIS. In IS 269:2015 the limit of MgO in case OPC 33, OPC 43 & OPC 53 is 6.0% however in case of Sleeper grade 43 & 53 it is 5 %. As a result the production of Portland cement becomes more stringent from the high MgO limestone. During the pyroprossing around 02.0% MgO will dissolve in the clinker liquid predominantly in C 4 AF and contribute the liquid phase in rotary kiln. That is why some times change in MgO content of kiln feed can cause ball or coating ring formation. Above 2% MgO remains as solid phase as Periclase. Due to slow hydration of the periclase mineral present in the cement makes its unsound. When the MgO is more than 2% the rapid heating and cooling is beneficial for development of small periclase crystals in the clinker. These reactive periclase crystals are hydrated faster than the cement hardening. Therefore this do not cause any unsound by later expansion. The hard burnt MgO in cement reacts with H 2 O very slowly to form Mg(OH) 2 that causes volume expansion of 118%. This volume expansion is after the cement is hardening thus makes the cement unsound and causes cracks in concrete. The higher MgO in cement retards the initial hydration of the cement and increase the setting time. During the hydration of the cement the solubility product Mg(OH) 2 is much smaller than Ca(OH) 2 , hence the Mg(OH) 2 precipitated earlier than Ca(OH) 2 . The formation of Mg(OH) 2 reduces the Ca(OH) 2 saturation ratio, thus delaying the initiation of maximum of Ca(OH) 2 saturation ratio. When MgO hydrate in high alkali medium the Mg(OH) 2 with tiny crystals precipitates around the cement grains to form a protective layer, hence retarding further hydration of the cement grains. MgO also gives some darker colour to the clinker. 4.0 Measure to control MgO in Cement The causes of the cement expansion by the crystalline MgO (periclase) may be due to the following factors (Dreizer 1981) i. MgO Content in Raw Materials ii. Chemical & Mineralogical composition of raw materials iii. Raw meal preparation and fineness iv. Pyroprocessing of the Cement Clinker v. Size and distribution of the periclase in the cement vi. Cement Fineness vii. Cement Storage viii. Cement Additives i. Screening the limestone during crushing and blending with low MgO Limestone sometimes reduced the overall content of MgO in raw material. Grinding the raw meal finer for better burning. It has been observed that quartz and dolomites are weekly magnetic with relative attractivity of 0.37 and 0.22 respectively, whereas the calcite is nonmagnetic with relative attractivity of 0.03. Hence, with high magnetic separator the silica and dolomite can be effectively separated from limestone. In case of electrostatic separation, it is not effective to separate the calcite and silica as they are having almost same relative empirical conductivity (voltage-10,920). Whereas the relative empirical conductivity of Dolomite (voltage-8,268) is much lower than the calcite and quartz, as a result the dolomite crystals can be effectively separated from limestone by electrostatic separation methods. The magnesia content more than 5% is undesirable in cement grade limestone as it produce unsound cement. In most of the cases it has been found that the dolomitization in limestone alter the calcite in such a way that magnesia limestone nearly defies any economic separation. However, removal of dolomite crystals from limestone is possible