SATISH PANDEY highlights the properties of steel slag aggregates and how they can be a 100 per cent substitute for natural aggregates in road construction, further reducing construction cost and bitumen requirement. Steel slag is one of the major solid wastes of steelmaking processes in an integrated steel plant. During steel making in steel furnace, carbon rich molten pig iron is converted into steel and all the unwanted impurities are removed through various fluxing agents in the form of steel slag for maximising the iron content in the alloy. According to the Indian Mineral Book 2018, steel slag output is approximately 20-30 per cent by mass of the crude steel production in the country, which yields around a total 18.5 million tonne of solid steel slag production annually. This quantity is slated to increase to 30 million tonne by 2030 with a likely increase in the production of steel as per the National Steel Policy 2017. A majority of steel slag after metal recovery ends up as waste dump or landfill material. Among all the solid/liquid wastes, slags generated at ironmaking and steelmaking units are in such a large quantity that management of slag has become a critical component of steel production. With increasing capacities, the mechanism for disposal of large quantities of slag that get generated has gained traction as the environmental issues it could evoke can become critical for steelmakers. In India, a huge quantity of good quality natural aggregates is required every year for the construction of new roads and maintenance of distressed road networks. Steel slag, which is rich in iron content, can become a source of good quality steel aggregates for potential utilisation as a substitute for natural aggregates in various applications in roadmaking. Based upon the steel furnace types used for steel production, steel slag can be further classified as Basic Oxygen/LD Furnace (BOF) steel slag and Electric Arc Furnace (EAF) steel slag. In both types of furnaces, steel slag is generally discharged from the furnace into the slag pot at 1,500-1,6000 C and then transported to the slag pit where molten slag is poured for air and water cooling. Picture 1 shows slag pots filled with molten BOF steel slag at RINL steel plant in Visakhapatnam, Andhra Pradesh, while Picture 2 shows the pouring of molten EAF slag into the slag pit in AMNS India steel plant in Hazira, Gujarat. After air and water cooling, the slag is further subjected to magnetic separation for entrapped metal recovery. Recovered metallic iron from steel slag is recycled in the steel furnace for steelmaking. Non-metallic steel slag component is crushed and screened into different sizes to produce steel slag aggregates for different civil engineering applications. The mineralogical and chemical composition of steel slag varies with the furnace type, steel grades and slag valorisation methodology adopted in the steel plant for treatment of the slag. To develop good quality steel slag aggregates, it is important to control the slag cooling and treatment methodology to develop stable mineralogical phases in the steel slag aggregates. CSIR-CRRI, under a sponsored R&D project by the Ministry of Steel and four major steel industries – AMNS India, Tata Steel, JSW Steel and RINL – studied the effect of slag cooling and treatment methodology on the mineralogical and chemical composition of steel slag. However, the major elemental composition in oxide form of BOF and EAF steel slag aggregates as determined though WD XRF analysis vis-a-vis composition of igneous rock group natural aggregates is shown in Table 1. BOF slag shows substantial high volumetric expansion in comparison to EAF slag owing to the presence of high CaO concentration leading to a high basicity to alkalinity ratio or basicity ratio. Under the effect of moisture, free lime (CaO) and magnesium oxide (MgO) in newly crushed steel slag are likely to hydrate and turn into calcium hydroxide (Ca(OH)2) and magnesium hydrate (Mg(OH)2) respectively, which can cause unacceptable cracks and heaves in the pavement structure. Picture 3 shows the free lime pocket in the LD slag aggregate while Picture 4 shows the possible heave in bituminous mix due to volumetric expansion in LD slag aggregates. To reduce the volumetric expansion in BOF/LD slag aggregates, it is required to carry out natural or artificial weathering or ageing of steel slag aggregate. Natural weathering/ageing processes involve prolonged exposure to slag aggregate to atmospheric agencies such as rain and temperature in an open yard for a considerably longer period of time, which is normally one to two years. Short-term weathering processes involve artificial ageing of slag aggregate using alternate wetting and drying cycles, steam aging treatment and pressurised steam aging treatment using the autoclave method. Picture 5 shows the artificial weathering of LD slag aggregate using alternate wetting and drying cycles at TATA Steel plant at Jamshedpur, Jharkhand, while Picture 6 shows the pressurised steam ageing plant at Nippon Steel facility in Japan for treatment of LD slag. After the necessary processing, processed LD/BOF steel slag and EAF steel slag aggregates exhibit better mechanical properties than natural aggregates. Picture 7 shows the different sized processed LD slag aggregates of Tata Steel Jamshedpur used in roadmaking in NH-33 from chainage 258.0 to 258.50 km on the left hand side carriageway (LHS) under the supervision of CSIR-CRRI. Table 2 shows the typical physical and mechanical properties of processed LD/BOF steel slag aggregates along with the prescribed limits in the Ministry of Road Transport and Highways (MoRTH) specifications for utilisation of aggregates in bituminous road construction in India. Processed BOF/LD slag aggregates of Tata Steel, Jamshedpur, are utilised under the technical supervision of CSIR-CRRI in the construction of NH-33 near Jamshedpur from chainage 258.0 to 258.50 km in granular layers, i.e., in base course (WMM) and sub-base course (GSB) as 100 per cent substitute of natural aggregates as shown in Picture 8 and Picture 9. The functional and mechanical properties of GSB and WMM layers constructed with processed LD/BOF steel slag aggregates were evaluated by the CRRI team who found that the base course and sub-base course layers built through processed LD slag aggregates met all the functional and serviceability requirements in the MoRTH specifications for road and bridge work. To facilitate large-scale utilisation of steel slag aggregates in road construction, as mentioned earlier, CSIR-CRRI has undertaken a major R&D project sponsored by the Ministry of Steel and four major steel industries in India for the development of design guidelines and specifications for utilisation of steel slag in road construction. The interim findings of the project study are quite encouraging and show that processed BOF and EAF steel slag aggregates can be successfully utilised as a 100 per cent substitute for natural aggregates in road construction with 30 to 40 per cent reduced pavement or road thickness. Reduction in road thickness will not only bring down the cost of construction but also save the bitumen required for construction of bituminous pavement. About the author: Satish Pandey, Principal Scientist, Flexible Pavement Division, CSIR-Central Road Research Institute, New Delhi, has 16 years of research experience in the field of transportation and highway engineering. He is the recipient of the CSIR Technology Award 2017 by the President of India for development of cold mix technology for construction maintenance and rehabilitation of bituminous pavement. He is also the recipient of the SKOCH Gold Award 2021 for utilisation of BOF steel slag of Tata Steel, Jamshedpur, in road construction.