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The Steel Slag technology developed by CSIR-CRRI and Rejupave (see box) jointly developed by CSIR-CRRI and Verma Industries, bagged 'Product Innovation of the Year' awards at the Construction World Global Awards 2023. CW examines the two technologies... Keeping in mind the Waste to Wealth and Clean India campaigns, in collaboration with the Ministry of Steel and major steel manufacturing companies, the Central Road Research Institute (CRRI), Delhi, has developed innovative steel slag, which is waste generated during steel production. Steel slag usage is about 30 per cent cheaper than conventional methods and it prevents depletion of precious natural resources. R SRINIVASAN spoke to Satish Pandey, Principal Scientist, CSIR-Central Road Research Institute (CRRI), Ministry of Science and Technology, Government of India, to learn more. By when can we see steel slag usage on roads across India? India has embarked upon a massive road development programme under Bharatmala and the National Highway Development Programme (NHDP). For the construction of 1 km of six-lane bituminous road around 60,000 to 80,000 tonne of natural aggregates are required. Many NHAI projects face an acute shortage of this perishable natural resource. Steel slag road technology addresses the scarcity of natural aggregates as it facilitates 100 per cent substitution of natural aggregates with processed steel slag aggregates derived from steel industry waste. CSIR-CRRI has developed guidelines to process and use steel slag as processed steel slag aggregates for road construction. These guidelines were released in November 2023 for pan-India implementation and use of steel slag in road construction. India, with an impressive rate of 29 km per day up from 12 km per day in 2014, currently boasts the world's second largest National Highways network spanning 1.45 lakh km, behind only the US. In view of the total road length approved to date, what would be the construction cost per sq m of a processed steel slag road? The construction cost is largely dependent upon the bituminous and cement concrete road crust thickness and layer composition, which is determined corresponding to design traffic and subgrade strength. For an identical traffic condition, the thickness of bituminous steel slag road can be reduced to 30-35 per cent as compared to a conventional road built through natural aggregates with improved durability and load resistance capacity. The major cost component in a steel slag road is the transportation cost of processed steel slag aggregates to the project site as the source of theses aggregates are steel plants. In a 100-km radial periphery of the plant, bituminous steel slag road will cost Rs 1,100-1,200 per sq m, while for a cement concrete road it would be around Rs 1,800-2,000 per sq m. Steel slag will now be used for roads but what about all the steel slag produced over all these years – where does it otherwise go? Conventionally, steel slag generated in steel plants is largely disposed of as solid waste in and around the plants and used as landfill. Steel plants use the steel slag partially in sintering plants as raw material but this quantity is also limited to 10-15 per cent of total slag generation. Accumulation of slag inside steel plants as solid waste has created huge mounds of waste that occupy a large tract of productive land and become a source of land, air and water pollution. Legacy deposits of steel slag can be processed and then used for road construction. CSIR-CRRI has provided a customised steel slag valorisation process to different steel plants to convert different types of steel slag as processed steel slag aggregates. Is there a time ceiling for its use? There in no time ceiling. Steel slag, whether fresh or old, is subject to necessary processing to convert it as processed steel slag aggregates for road construction. It is non-perishable material so once it gets converted in to processed steel slag aggregates, it becomes chemically inert and will last like natural aggregates. Are there any disadvantages of this technology? Unprocessed steel slag can emanate heavy metal and high pH leachate to the environment. Hence, valorisation of steel slag as processed steel slag aggregates is necessary for its use in road construction. Before the utilisation of processed steel slag aggregates in road construction, upcoming CSIR-CRRI guidelines mandate the TCLP test to assess the toxicity characteristics of processed steel slag aggregates and pH of leachate to rule out the remote possibility of leaching of any heavy metal beyond permissible limits of the Central Pollution Control Board. Can slag be used in other forms of construction, apart from roads? CSIR-CRRI is working with Indian Railways to use steel slag as a substitute of railway ballast underneath the railway track in place of natural aggregates. It can also be used for reclamation of acidic land as a soil conditioner and it can be used for different marine applications.