Manoj Kumar Rustagi, Chief Sustainability and Innovation Office (CSIO), JSW Cement, gives insights into the process of producing blended cement with supplementary cementitious materials for more strength and durability.
What are the core raw materials used in the production of cement?
Cement manufacturing is an energy and resource intensive process. Primary raw material is limestone which is mined, crushed, ground and mixed with bauxite, iron ore and other additives/correctives to make raw meal which is then heated to a temperature as high as ~1400°C in a horizontal kiln. Coal is the primary fuel which provides energy for the combustion process. The hot material is then cooled down to form clinker, an intermediate product for making cement. Clinker is further ground and blended with gypsum (mineral or chemical) to make the final product called ordinary Portland cement (OPC).
When clinker is blended with other supplementary cementitious materials like fly ash or slag or both, the product is known as blended cement.
What are the alternative raw materials that can be used in the production of cement? How does that impact the process of production?
Cement sector accounts for ~7 per cent of global CO2 emissions, and therefore it needs to be aggressive on its decarbonisation strategy wherein one of the primary lever is using alternative raw materials for the production of clinker and supplementary cementitious materials (SCMs) as cement/clinker replacements. Different fine-grained silica, silicate and alumina-silicate materials either natural or synthetic can be used in the final cement product to obtain a new eco-friendly cementitious binder with similar or better properties. The most commonly used SCMs are fly ash, granulated blast furnace slag, natural volcanic pozzolana etc.
When clinker is blended with other supplementary cementitious materials like fly ash, slag or both, products are called Portland Pozzolona Cement (PPC), Portland Slag Cement (PSC) and composite cement (CC) respectively. Blended cement products have a much lower carbon footprint than OPC. Since clinker manufacturing is the phase where most thermal energy is consumed and CO2 is emitted, reducing clinker factor in cement not only results in lowering the process CO2 but also the thermal energy and electrical energy requirements.
There are other alternative raw materials like Spent Pot Liner (SPL), red mud, lime sludge and steel slag, which are used in the clinker manufacturing to reduce consumption of limestone and consequently reducing the process CO2 that comes from limestone calcination.
Can cement maintain its quality standard with inclusion of supplementary raw materials as against limestone?
Yes, blended cement products not only maintain the most quality standards as OPC but also have superior properties in various parameters when compared to conventional OPC. Blended cements are preferred for its late strength, chemical resistance, alkali resistance and for coastal applications and dams and irrigation projects where they are technically most suitable.
The use of SCMs/mineral admixture/blended cements in concrete significantly helps in mitigating the expansion due to alkali silica reaction (ASR), due to the reduction in the availability of alkalis in the pore solution and the refinement of the pore structure. Not only does this reduce maintenance costs of infrastructure such as dams and bridges, but also allows the consumption of local aggregates that may contain deleterious materials. The reduced expansion in SCM-blended structures reduces the risk of expansion and cracking. This pozzolanic reaction also has a beneficial impact on resistance to sulphate attack.
Recently GCCA, India has published a detailed report on Benefits of Blended Cement Products, which has been prepared by NCCBM and reviewed by IIT, Madras, and that captures all the environment and technical benefits.
Explain the impact on carbon emission of the production unit when alternative raw materials are used in various proportions.
In cement manufacturing, CO2 is primarily emitted as a result of the chemical conversion process used in the production of clinker in which limestone (CaCO3) is first converted to lime (CaO) and then to hydraulic compounds. CO2 is also emitted during cement production by fossil fuel (primarily coal) combustion. Thus ~80-85 per cent of the CO2 emissions could be attributed to the production of clinker. This is partly reduced by using alternative raw materials and mineralisers in the raw mix design of clinker.
The amount of clinker in cement, known as clinker factor, determines the CO2 footprint of cement. In OPC, clinker factor is ~90 per cent thus, it has a carbon footprint of around 800 – 850 kg/MT of cement. When clinker is replaced with SCMs, the CO2 emissions are reduced as SCMs don’t have embodied carbon emissions. That is why blended cement have much lower carbon footprint than OPC. Currently in Portland Slag Cement (PSC) production almost 60 to 65 per cent of clinker is replaced with slag which results in ~60 per cent of CO2 footprint and the final carbon footprint is around 300 – 350 kg/MT. Similarly, in PPC where ~35 per cent of clinker is replaced, carbon footprint will be in the range of 500 – 550 kg/MT.
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