Cement, Energy and Environment

4 From Fig.5 it is evident that compared to the production of Portland cement the global availability of natural and synthetic pozzolans including fly ash is much limited. Similar is the case with the ‘cementitious’ SCM like slag. The abundantly available SCMs are only fillers like limestone and clay that can be calcined into a pozzolanic substance. Hence, globally speaking, there is a renewed interest in these materials in the world. However, there is a local angle to this issue. In India, because of the high-ash coal-based thermal power generation, the current availability of fly ash has exceeded 200 million tonnes per annum and despite all the measures taken to move towards renewable energy sources, the projected generation of fly ash will be substantial. Some estimates indicate that it may 300 million tonnes per annum even by 2022. It is therefore imperative that more and more fly ash is used in the cement and concrete industry by the combined efforts of the users, generators and policy makers. As far as the granulated slag as an SCM is concerned, its use in India is small in the range of about 10 million tonnes. With the projected growth of the iron and steel industry the annual availability of granulated slag is likely to increase three folds by, say, 2030, but still it will be too small, compared to the cement production in the country. Use of slag, therefore, will not change the pattern of SCM use in India over time. The availability of limestone filler and kaolinitic clay suitable to be calcined into a pozzolan is quite large in India and a concerted focus on their use is certainly called for. Hence, an emphasis on adopting technologies for making Portland limestone Cement, Portland calcined clay limestone Cement, high-volume fly ash concrete, concrete containing high proportion of fillers, etc. is necessary in our country in place of investing more and more in clinkering plants. While the know-how for the above products is available in the country, it is important to underline certain specifics of making multi-blend Portland cements and concrete with high filler content as discussed below. Multiblend Portland Composite Cement (MPCC) The concept of manufacturing MPCC is based on the fact that two or more SCMs of widely differing particle size distribution may lead to a finished cement having the potential of high particle packing and such a cement may yield concrete with durability characteristics that are not normally encountered. This is illustrated in Fig.6, which shows the significant difference in the particle size distribution of Portland cement, slag and silica fume and the notable improvement in the mortar properties with combined addition of SCMs. The improvement occurred due to pore refinement as shown in Fig.7. Fig.6. Illustration of the effectiveness of combined addition of silica fume and Blast furnace slag on the mortar strength of the cement. Fig.7. Refinement of pore structure due to addition of two both silica fume and slag.