In the future, emitting CO2 may attract penalties. Therefore, clinker will be increasingly replaced by materials like fly ash and slag.
Incorporation of Indian Standard Specification:
In view of several benefits, BIS has brought out specification of Composite Cement covered under IS: 16415-2015 and presents a good opportunity to produce high strength concrete which is highly durable. Composite cement is a mixture of high-quality clinker (IS:16353-2015), fly ash (IS:3812 (Part 1) - 2013), granulated slag (IS:12089-1987) and gypsum.
Low Level of Production from Plants:
It is surprising to note that only a few plants have got the license to produce composite cement and producing little quantity and there are other set of plants they have license but not producing.
The reasons could be as follows:
1.Lesser availability of dry fly ash for PPC:
At present, India produces approximately 180 million-tons of fly-ash. The disposal of such huge quantity of ash is a serious issue. Around 41 % fly ash is being utilized in the PPC thanks to the customers for its acceptance. Currently, the availability of dry fly ash is less from those thermal power plants which are in the vicinity of cement plants and therefore in order to meet the production target of PPC, cement plants are utilizing pond ash since they do not have any option left. Needless to say, their chemical and physical characteristics are varying too much on account of this.
Figure 1: Fly ash production and utilization
Figure 2: Fly ash utilization
2.Slag availability in India:
In an integrated steel plant, wastes (including solid, liquid and gas) are generated for every ton of steel produced. In India, almost 100 million tons of steel is produced every year, and along with this production about 40 million tons of various slags are generated. On average, the production of one ton of steel results in 200 kg (EAF) to 400 kg (BF/BOF) of by-products generations. These include slags, dusts, sludges and other materials. Due to geographic constraints manufacture of PSC has been localized in middle and eastern part of India only.
3.Variable chemical composition of fly ash and slag:
The chemical composition of fly ash or slag is determined by the raw materials used and the conditions under which they are processed. These factors can vary, not only from one plant to another, but also within the same plant. It is natural to expect large variations in chemical composition between different batches of fly ash and slag. It is, however, generally accepted that the pozzolanic and cementitious properties of these materials are governed less by the chemistry and more by mineralogy and particle size.
Inbound movement of raw materials like fly ash and slag etc. are to be transported to the cement manufacturing plant and outbound transport of products to the markets are critical. Particularly, fly ash and slag are away from the plant location makes the production of composite cement unviable. Approximately, logistics costs constitute about 30 per cent of the total value of the finished product. Hence, cement companies focus a lot in selling closer to the plants and similarly sourcing of raw materials like fly ash and slag to bring logistics efficiencies and cost reductions. This has a direct bearing on the bottom line of the companies as the industry's ability to pass on cost increases to the markets is limited due its commodity nature.
Many major plants in India are having ball mills for finish grinding. Also, many plants are having ball mill with roller press and vertical roller mills. The physical characteristics and grindability index of clinker, slag and fly ash are different. The plants those who are having ball mill circuit should go for intergrinding of clinker + slag + fly ash + gypsum; due to change in their grindability index will not grind properly to the desired fineness which adversely affect the quality of composite cement. Many researches suggest that, separate grinding of slag, clinker + gypsum and fly ash are to be stored separately in a silo with predetermined dosages are to be mixed in a high efficiency mixer and stored in a separate silo. Many plants do not have many silos and no facility to store ground slag and clinker gypsum mixture and high efficiency mixer.
In addition to this, many cement plants are now bestowed with marginal grade limestone in their mines and hence not able to produce clinker with C3S content around 50%. This is also one of the constraints in achieving best possible initial strength as desired by the customers as It is obvious that composite cement contains lower level of clinker component.
There is also a hinderance from sales/marketing team that, they put all their efforts in converting OPC market to PPC and customers have accepted the later for structural aspects; now at this stage one more type of cement probably with lower initial strength (vis-a-vis PPC) put both marketing people and customers in confusion. The tricky issue is price fixing of composite cement versus PPC in the market.
Constraints of availability of raw materials (cement grade limestone, dry fly ash, granulated blast furnace slag), infrastructure, logistics issues, plant's infrastructure constraints are hindering the manufacture of composite cement.
Improper formulation of clinker, fly ash, and slag leads to variation in quality parameters which ultimately affect (low) durability and strength of product, which may restrain the production.
Author: Dr. S.B. Hegde, Head - Production and Quality Control, UDAIPUR CEMENT WORKS LIMITED, (A Subsidiary of JK Lakshmi Cement Ltd.) The views expressed in the article are his personal.
S.B.Hegde (2014): Secondary cementitious materials in cement and concrete, World Cement.