Calcined shale for cement manufacture

ON A specific assignment from the industry, the National Council for Cement and Building Materials (NCCB) conducted a detailed study to establish use of calcined shale in the manufacture of cement. Shale used in the present investigations is a reject generated at the captive mine of a cement plant. Detailed calcination investigations were carried out to examine the slagging behaviour of shale particularly its glass content at elevated temperatures. The shale calcined at optimised conditions was used to prepare cement blends at additions levels between 35 and 47.5 per cent alongwith clinker and gypsum.

The cement blends were prepared by intergrinding as well as by separate grinding of the ingredients. The performance evaluation of these cement blends was carried out as per BIS specifications. Results obtained indicate that 35 per cent calcined shale can be used in case of intergrinding and 40 per cent calcined shale in case of separate grinding of the materials for obtaining a cement of performance comparable to that of 43 grade OPC

Apart from utilisation of reject materials, other benefits can be expected in terms of reduction in electrical energy required for cement grinding as the Bond's grindability index of calcined shale is 6.8 kWh/short tonne compared to that of clinker which is 14.3 kWh/short tonne.

The long-term evaluation for sulphate resistance and corrosion resistance (including electrical resistivity) of concrete prepared from three blends is also being studied for establishing its possible use in special applications. - National Council for Cement and Building Material

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