A robust infrastructure network is a prerequisite to accelerating economic growth in any country. As India is poised to become an economic powerhouse, the Government of India has also embarked on a massive infrastructure development programme under the PM Gati Shakti Scheme, National Highway Development Programme and Bharatmala projects. India has the world’s second-largest road network in terms of length. Every year, for the maintenance and construction of this vast road network, around 1.2 billion tonne of natural aggregates are required. This huge demand is met through unsustainable quarrying and mining of stone aggregates in many areas, which is causing a serious ecological impact on natural topography, flora and fauna. Sustainable development requires reduced reliance on perishable natural resources.

India isthe world’s second-largeststeel-producing country and accounts for the world’s second-largest steel slag generation as solid waste. Most steel slag after metal recovery ends up in the waste dump or as landfill material. The utilisation of steel slag varies from 0 to 50 per cent on a plant-to-plant basis depending upon the course of action taken by steel plants for disposal and utilisation. Among all the solid and liquid wastes generated during iron and steelmaking in an integrated steel plant, the productive and safe disposal of steel slag is the most challenging task owing to its volume. Steel slag valorisation as roadmaking aggregate can provide an attractive alternative to steel industries for its safe utilisation in road construction. Under a major research projectsponsored by the Ministry of Steel along with Arcelor Mittal Nippon Steel (AM/NS) India, CSIR-CRRI has developed steel slag valorisation technology for the conversion of steel slag as roadmaking aggregate. Processed steel slag aggregates developed at the AM/NS India steel plant in Hazira have been successfully utilised in the construction of a 1-km,six-lane bituminous steel slag road connecting NH-6 to Hazira Port in Surat, Gujarat.

Steel slag and types

Steelmaking slag can be broadly classified into two categories based on steelmaking processes. The first is the basic-oxygen-furnace (BOF) process through which hot metal received from blast furnaces is converted to steel. The second is the electric arc furnace (EAF) process, which mainly recycles steel scraps. The steel from BOF and EAF can be further processed through a ladle refining unit in aladle furnace to produce high-grade steel, which is commonly referred to as the secondary steelmaking process. Each of these processes generates different types of slags.

Steel slag road

Conventionally, bituminous roads are constructed using natural aggregates and bitumen. Natural aggregates occupy 95-96 per cent of the volume of the road while bitumen is utilised as a binder along with natural aggregates in the surface layer of the road and occupies 4-5 per centof volumetric space in the surface layer. The steel slag road in Hazira, Surat, has been constructed using processed EAF steel slag aggregates by substituting natural aggregates in all layers of the bituminous road. One lakh tonne of EAF steel slag aggregates wasutilised for the construction of a 1-km, six-lane bituminous steel slag road connecting NH-6 to Hazira Port in Surat. Processed steel slag aggregates were developed at the AM/NS steel plant in Hazirausing customised steel slag valorisation technology as developed by CSIR- CRRI. All layers of the steel slag road, such as subgrade, sub-base, base andwearing course along with road shoulders and median, were built using processed steel slag aggregates.

This heavy-duty steel slag road is designed for 100 msa(million standard axle repetition) design traffic for a 20-year design life period and subjected to heavy commercial traffic,i.e. 1,000 to 1,200 commercial vehicles per day carrying a 30-40 tonne payloadfor the past two years. Owing to the novel design composition and improved stiffness of pavement layers, the road has been built with 32 per cent less thickness compared to conventional bituminous roads as recommended in the Indian Road Congress standards and guidelines.

Photos 8 and 9show the aerial and longitudinal view of the steel slag road built in Hazira. Periodic performance evaluation carried out by CSIR- CRRI shows that the structural stiffness of the road despite the reduced thickness is 40 to 50 per cent higher than conventional bituminous road.

For its technological features, this heavy-duty steel slag road built by CSIR-CRRI and AMNS India has received much national and international acclaim and was inducted in the India Book of Records as the ‘First Steel Slag Road’. After inspecting the steel slag road,RCP Singh, Former Minister of Steel, Government of India, and Dr VK Saraswat, Member,Niti Aayog, recommended its further utilisation in road construction in the country.

Summary

Steel slag roads built through processed steel slag aggregates can offer a sustainable, durable, green road network in line with Government of India's flagship programmes, Waste to Wealth and the Clean India Mission. These novel road construction concepts also fulfil the concept of circular economy to achieve the US Sustainable Development Goals for green infrastructure. The potentialutilisation of processed steel slag aggregates in roads also protects nature from unsustainable quarrying and mining by fulfilling the partial requirement of natural aggregates for road construction.

Key environmental benefits

Eco-friendly sustainable utilisation of steel slag waste

Saving of natural aggregates

Reduction of GHG emissions in road construction

Reduction in the carbon footprint of road construction

Prevention of potential land, air and water pollution from unscientific disposal of steel slag as solid waste.

Key technical benefits

Improved durability of the road with better service life

Higher load resistance capacity

Reduced road thickness

Improved skid resistance

More economical than conventional bituminous road in nearby areas of steel plants.

Challenges with steel slag utilisation for roads
The major challenges that inhibit direct utilisation of raw steel slag in roadmaking are as follows.

Volumetric expansion:Steel slag contains hydratable oxides (CaO and/or MgO) that can result in volumetric instability (expansion). Steel slag shows volumetric instability owing to the presence of free lime and free magnesia.A steelmaking slag may contain appreciable amounts of free lime (f-CaO) and small amounts of free magnesia or periclase (f- MgO), which causes expansive self-destruction by about 100 per cent volume increase because of their reactions with water. BOF slag shows substantially high volumetric expansion compared to EAF slag owing to the presence of high CaO concentration. Untreated steel slag because of high volumetric expansion can cause detrimental effects on pavement durability and may lead to longitudinal and lateral cracks with heaves in bituminous, cement concrete and granular layers. Photo 1 shows the free lime pocket in the untreated LD slag while Photo 2 shows the heave in the bituminous mix owing to volumetric expansion in LD slag aggregates.

Vesicular texture and porous structure: Compared to natural aggregates, the microscopic surface of steel slag has a pitted and vesicular texture and porous structure. The cellular or vesicular texture results from the bubbles of the gases dissolved in the liquid slag. Vesicular texture and porous structure may lead to higher water absorption, which in turn leads to higher bitumen and cement consumption compared to natural aggregates. Photo 3 shows the irregular vesicular texture at the macroscopic level while Photo 4 shows the porous structure at the microscopic level.

High pH and heavy metal leachates:Steel slags may contain heavy metals such as antimony, arsenic, barium, beryllium, cadmium, chromium, cobalt, copper, lead, mercury, nickel, selenium, thallium and vanadium. Even though these metals are available as minor constituents of steel slag, possible leaching of these heavy metals into groundwater and native soil should be prevented. Owing to the presence of high CaO, BOF steel slag can emanate high pH leachates that can make the nearby water and soil body alkaline in nature. These alkaline leachates can increase water pH and result in an increased chemical oxygen demand (COD) besides increasing salinity. Photos 5 and 6 shows the possible leachate contamination from unprocessed steel slag dumps in nearby soil and water bodies. The probability of leaching from the slag can be considerably reduced by developing stable mineralogical phases in the slag and subjecting it to appropriate processing mechanisms by adopting suitable steel slag valorisation technology based on steel slag types.

Valorisation of steel slag as aggregates: A sustainable circular economy aims to reducethe dependence on natural resources by ensuring the continued utilisation of available resources. Steel slag valorisation as roadmaking aggregate can produce good quality processed steel slag aggregates that can be utilised as a 100 per cent substitute for natural aggregates in road construction. Technological development in the production and processing phase of steel slag aggregates, as developed by CSIR-CRRI, renders the steel slag a coproduct for steel industries. Processed steel slag aggregates, owing to their improved physical and mechanical properties, can be utilised in all layers of the road as a substitutefor natural aggregates.A customised steel slag valorisation technique comprising a customised steel slag cooling procedure, followed by appropriate slag treatment and metal recovery process based on type of steel slag and chemical composition, can produce sound durable steel slag aggregates for roadmaking. Photo 7 shows the processed steel slag aggregates generated through the customised steel slag valorisation technique developed by CRRIfor the conversion of steel slag as roadmaking aggregate

About the author: Satish Pandey is Principal Scientist, CSIR-Central Road Research Institute, NewDelhi India, and Associate Professor, Academy of Scientific and Industrial Research (AcSIR).