Dr Sujit Ghosh, Executive Director – New Product and R&D, Dalmia Cement (Bharat), discusses the alternative raw materials that can be used in the production of cement and its impact on carbon emissions while underscoring the major challenges faced in using other cementitious materials.
What are the core raw materials used in the production of cement?
The core raw materials used in the production of cement are limestone (calcium carbonate) and clay (a source of silica). First, the limestone is roasted/calcined to create activated lime (CaO) in a calciner and then the activated lime along with siliceous clay is proportioned along with some other minor ingredients into a raw mix design and charged inside a kiln to form cement clinker; which is basically made of complex compounds of calcium-silica-oxides primarily, which when mixed with water, reacts, to form a cementitious gel paste that binds all aggregates together and when dried up provides strength to the concrete/plaster, made with cement and the aggregates.
Limestone (calcium carbonate) and clay (silica), which are both available in nature, are inert materials. Only when they are heat-treated at temperatures above 900oC, they become activated lime (CaO) and activated/amorphous silica (SiO2), and fuse inside the cement kiln in liquid form to form complex calcium-silica-oxides, that is cement or cement clinker.
What are the alternative raw materials that can be used in the production of cement? How does that impact the process of production?
As explained in the previous paragraph, any activated lime (CaO) and/or activated/amorphous silica (SiO2), could be potential sources of cementitious material. These could be alternative raw materials for cement production. Thus far, the most widely found and used sources of alternative materials are primarily ‘fly ash’ and ‘blast furnace slag’. Fly ash is a waste product from the burning of coal (as in a thermal power plant etc). It primarily contains amorphous/activated silica (SiO2), but very little active lime (CaO) in the Indian context. So, it is not reactive on its own, it needs activated lime (CaO) to become cementitious – which is available from cement clinker, when the two are co-processed in a cement manufacturing plant. Blast furnace slag likewise is a waste product from the steel manufacturing process and does contain some activated silica and activated lime, but again, not in the proportion/concentration to itself become cementitious. It also has to be co-processed with a cement clinker in a cement manufacturing plant. Overall, these alternative or supplementary cementitious materials, which are other industry wastes, due to the need for co-processing with cement clinker, may add some costs to the production process, but since the use of such alternative raw materials, reduces the dependence on highly energy-intensive clinker, they are welcome by the cement manufacturing fraternity, that helps lower the carbon footprint of production. These cements are called ‘blended cements’ – either fly ash blended (popularly known as PPC) or slag blended (popularly known as PSC) or fly ash + slag blended (popularly known as PCC).
How can the cost of production be reduced by using alternative or supplementary raw materials in cement production?
Since the use of alternative / supplementary cementitious materials has been prevalent in the world and in India, for blended cement production, for the last couple of decades, the demand for such other industry wastes (primarily from thermal power plant or steel plant) has been increasing steadily. This has led to a steep increase in prices for these industry wastes (mainly slags from steel plants) which otherwise were previously dumped in landfills, by opportunistic players and profiteering groups. Also, since steel plants and thermal power plants are not co-located with cement plants geographically, transportation costs of such bulky waste materials have also been increasing. Cost of blended cement production has to reduce or at least maintain at par, as well as, at the same time assist the nation in beneficially getting rid of other-industry-wastes. Cement players can do justice to climate-change by producing less energy intensive blended cements that are in no way inferior in quality to pure-clinker cements. Transport subsidies should also be provided to cement manufacturers by the government as well as at the same time try and administer some polluter-to-pay mechanism (so that these wastes are not conveniently dumped away in nearby landfills by the relevant industries).
–Kanika Mathur
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