The worthy waste
Today fly ash and slag-based cements have become generic products that are widely available and have greater acceptance. In fact ordinary Portland cement either 43 grade or 53 grade have limited usage based on application. The overall product quality of blended cements has improved and will improve further as the quality of input materials like fly ash or slag will be more uniform and consistent. Basically fly ash based cements are seen all across the country where as slag cements are available where steel plants are operating; more particularly in the eastern part of the country.
The superiority of blended cements, the durability imparted to the structures has been discussed in length and breadth at every forum. There is nothing new to add on the durability aspects through this column. There are cement producers in the market who are producing only blended cements and have stopped producing ordinary Portland cements. We are of the opinion that companies will continue to produce ordinary Portland cement like 43 grade, 53 grade and railway sleeper grade cements, but the market share will remain restricted or shrink further. A close study of the market reveals that every company has two categories of blended cements - one is generic blended cement and the other is a premium quality of blended cement.
Let us try and appreciate why there is a need to have two categories of blended cements. A common complaint about the blended cements is slower setting and lower strength at the early age of one day and three days where as specification does not indicate strength at one day. On the other hand, the margins provided in the specifications are wide enough so that the product has very little chance of failure. Now these drawbacks of slow setting and lower strength have been addressed in the premium quality of blended cements by making use of some chemicals as additives. Therefore to some extent the higher cost of premium quality blended cements is justified. Kindly refer to the article written by Shreesh Khadilkar for more details.
There will be a section of OPC buyers, and it will be difficult for cement producers to change them to use blended cements. OPC will continue to find as a preferred product by ready mix plants, railway sleeper producers, concrete product manufacturers like spun pipes, poles, asbestos sheets etc. It is certain as expressed by Khadilkar that market share of OPC will go on reducing.
Let us now consider the challenges faced in producing quality blended cements. Making a good quality blended cement either with fly ash or slag has not been an easy job. As Khadilkar opined that in order to produce good quality PPC/PSC one has to look at the clinker quality which is the basic ingredient of cement. You have to target minimum of 63 to 64 percent of clinker lime and no less. One has to make sure that the clinker is reactive enough to absorb good percentage of slag or fly ash. There is a tendency to grind blended cement finer to improve strength, but that leads to more power consumption. It is not a correct practice. One has to balance the strength and fineness. In case of fly ash based cements it is preferred to have co-grinding of fly ash and clinker. However in case of slag a different practice has proved to be economical and efficient. Slag and clinker are ground separately and then blended together to get the final product.
Cause of variation
The next challenge is the quality of fly ash. It has been observed that there is a substantial variation in the quality of ash generated. One has to accept that fly ash is a by-product and not main product. The important reason of variation is the coal quality itself. The other reason is the location from where the fly ash is collected. The third reason is moisture content in the fly ash. As long as the variation is manageable, the quality of cement will be consistent. Such variations normally do not occur in slag therefor by and large the quality of slag from same source does not have much variation.
While we make best of cements, it is necessary to know what the end user expects. The cement user always wants a product with consistent properties. Cement being an intermediate product for making concrete or mortar or making a cement paste for water proofing or tile fixing, is converted into some other form while using and the user would like to replicate his earlier experience with cement. User's job depends on what performance cement gives him and it is absolutely necessary that the user is satisfied with the performance of cement. Fly ash and slag are materials that bring in variation in the performance of a product compared to OPC. Hence the quality of fly ash and slag are extremely important for ultimate product.
Experience of Dirk
As reported above to address the variations in the properties of fly ash, a subsidiary company of Ambuja Cement has been in the market with a product, i.e. fly ash that has definite properties and proved to be a superior product. The company has deployed a mechanical classifier, which separates the fly ash particles depending on the fineness and size. The result is a uniform quality of fly ash. This superior quality of fly ash is used by ready mix concrete plants or such users who are particular about a quality of fly ash. The cost of such processed fly ash will be slightly higher than normal.
As the processed pulverised fuel ash needs to undergo another process, which will not change the geometry of the tiny particles as the shape, which is spherical, plays very important role in the concrete. This ensures that the chemistry of the ash is not changed. Apart from this the ash needs to be processed in large volumes and at minimum cost and also in environmental-friendly manner.
Many plants today have achieved blending mode of slag up to 64 to 67 per cent and fly ash up to 30 per cent with clinker to produce ultimate product. Such products with supplementary by-products added to the main product to be cheaper which the cement industry is yet to respond to.
We draw attention of the readers to the article by Dr. Hegde on the fate of composite cement. He observes that the provision has not found takers in the commercial space for several reasons.
- VIKAS DAMLE