Dr.L R Manjunatha, AVP, JSW Cement & Honorary Chairman, Indian Concrete Institute, Bangalore Centre and D Satish Kumar, R&D, JSW Steel, Bellary, present recent developments and innovations in Processing Technologies for development of eco-friendly Slag Aggregates from the steel industry.
With ever increasing demand of raw materials for roads and construction leading to rampant reclamation of natural resources, the eco-balance of the environment is under constant strain. Hence, there is an urgent need to preserve the precious natural resources and find newer materials for this purpose. Some of the feasible and economical solutions that have emerged through long research activities are to reuse the industrial process wastes through recycling or reprocessing. Promoting the use of iron and steelmaking slag as replacement of aggregates in roads and civil construction represents an excellent success story consistent with these goals.
Slags are the main wastes generated during iron and steel production in the steel industry. Steel plants generate around 400-500 kg of total slag per tonne of steel produced. Slags are generated at two different stages of steel production: Iron making and Steel making known as BF slag and Steel slag, respectively. These slags are in liquid state and are solidified in air after dumping in a pit or granulated by impinging water stream. Air cooled slags are larger in size (> 10mm) hence require crushing and sizing and are similar to coarse aggregates. Water granulated slags have a size close to river sand and or fine aggregate. Because of the high temperatures (about 1500°C) during their generation, slags do not contain any organic substances, shells, clay etc, which is a key advantage.
Slag aggregates can be used as a construction material in unbound applications (where the aggregate is not bound) as well as in bound applications (mixtures which contain binding agents like cement, bitumen or a substance that has binding properties in contact with water). Until recently, slags were not regularly utilized in civil construction due to the ease in availability of natural materials, lack of awareness of their benefits, non-availability of guidelines, and limited slag processing techniques.
With numerous studies published in recent past, on the durability and long-term impact of slag usage in roads and construction and introduction in IS-383, it is now well established that processed slag can be used as a safe replacement for natural aggregates for roads and civil construction. Use of slag aggregates in construction dates back to the Romans who used crushed slag from the crude iron production to build their roads. Slag is particularly useful as an aggregate due to its high mechanical strength which exceeds many natural aggregates. In general, the properties of iron and steel slag aggregates are comparable with the properties of natural aggregate. In some specific cases, slag aggregates are even superior to natural aggregates. Contrary to the earlier concept, iron and steel slag is not a waste any more and is a useful product capable of helping ecological balance and economic benefits.
Air Cooled Iron Slag: Coarse and Fine Aggregates
Air-cooled blast furnace slag can be defined as the material resulting from solidification of molten blast furnace slag under atmospheric conditions. The slag thus produced is in the form of large boulders. The air-cooled BF slag can be crushed, screened and directly used as coarse aggregate for roads and bases, asphalt paving, railway ballast, landfills and concrete aggregate. The solidified slag has a vesicular structure with closed pores. Air cooled BF slag exhibits similar mechanical properties as that of a natural aggregate such as good abrasion resistance, good soundness characteristics, high bearing strength etc. This slag can also be crushed to small sizes using a vertical shaft impactor and screened to convert it into fine aggregate. Fig 1 shows the Air Cooled Coarse and Fine aggregates. This light weight sand is ideal for plastering applications and matches IS-2116 (sand for masonry mortars) and IS-1542 (sand for plaster).
Figure 1: Air Cooled Coarse and Fine aggregates from iron slags
Granulated Iron Slag: Fine Aggregate
Iron making slag granulation technology has been in regular use for converting it into glassy granules for use in portland slag cement making along with clinker. Recently, ground granulated slag (GGBFS) is also developed using the granulated slag for use as a replacement of ordinary portland cement by ready mix concrete (RMC) manufacturers. However, increased steel production and lesser uptakes by cement industry force steel makers to look for alternative applications. Granulated blast furnace slag (GBS) is physically similar to sand but has low density and has strength issues when used in concrete directly and hence is not used as replacement of river sand.
Figure 2: Ironmaking slag sandHere, it is important to mention that the property requirement in granulated slags for use in cement and as aggregate is different. Slags to be used for cement making requires high glassy phase (>90%) whereas slags to be used as fine aggregate must have sufficient density (> 1400 kg/m3) or specific gravity (> 2.5). This is a standard property to meet the weight requirement in cubic meter of concrete. At JSW steel, a new processing technique  has been developed to convert this slag into fine aggregate to be used as 100 % replacement to river sand, for construction purpose. This multiple stage processing involves change in structure and shape of the slag granules. Step 1 involves the optimization of granulation parameters to achieve porosity free granules and second step changes the shape and size distribution using a vertical shaft impactor and set of screens. Fig 2 shows the slag sand developed from granulated slag.
The processed granulated blast furnace slag (PGBS) was similar to 100% true river sand. Strength, durability and workability of the concrete cast with processed granulated blast furnace slag (PGBS) were found to meet the standard requirements of the cube tests. JSW steel has become the first steel plant in the country to market and sell processed granulated slag or Slag Sand as replacement of river sand.
Air Cooled Steel Slag: Coarse and Fine Aggregate
Air cooled steel making slag from BOF (Basic oxygen furnace) or EAF (electric arc furnace) the two widely used processes of steel making, constitutes 75-80% of the total slag generated in the steel making process. These steel slags are a dense rock like material and can outperform natural aggregates in terms of resistance to crushing, skid resistance, durability and adhesion.
However, variations in compositions, size and recovery components, demand different treatments for BOF and EAF steel making slags. BOF slags due to volumetric instability in the presence of water whereas higher density and irregular shape of EAF restricts its applications. These concerns have also been addressed by the steel makers with the development of steam ageing for BOF slags and use of impactors for EAF slag. All steel slags are inevitably subjected to crushers, magnetic separator and screens to recover metallics. This metallic separation process breaks slag into different sizes. Slag sizes 10-20 mm, 20-40 mm and 40-80 mm can be used as coarse aggregates. Several experiments were conducted at JSW steel for evaluating the properties of concrete made by coarse steel slag (both aged BOF and EAF) as coarse aggregate. These experiments include water absorption, alkali-silica reaction, resistance to degradation and soundness of aggregates as well as mechanical properties of concrete specimens such as compressive strength, flexural strength, tensile strength and modulus of elasticity. Not only concrete but slags can also be used for road construction in granular sub base, dry lean concrete and asphalt mix.
Steel slags are recommended to be used in wear resistant asphalt layers as chippings. Studies have also indicated that diluting steel slag with inert materials, such as gravel and sand, can reduce the swelling potential and can be used as structural fill material in landfill embankments. pavement surfaces incorporating steel slag have shown superior skid characteristics than asphalt surfaces incorporating natural aggregates.
New processing methodology has been developed at JSW Steel to convert the crushed steel slag into fine aggregate for replacing river sand. In the developed process, the crushed slag is subjected to vertical shaft impactor to control the size and change its shape from angular to rounded. The size and shape of the processed slag particles are controlled by the feed rate and rotor speed. The product is further subjected to air classifier for separation of ultra-fine fractions (< 75 microns). The developed steel slag sand matches the specifications for fine aggregates. Cube tests conducted using the developed steel slag as fine aggregate show improved strength than the river sand owing to its higher density. Fig 3 shows the air cooled coarse and fine aggregates from iron slags.
Figure 3: Air Cooled Coarse and Fine aggregates from steel slagsGranulated Steel Slag: Fine Aggregate
Recently, new technologies of water granulation (close system) and atomization (open system) have been developed and are in practice in full scale by steel makers to granulate steel slag. These technologies help in separation of the metal and slag and also wash out free lime and MgO. Sudden quenching, of the molten slag, leads to differential contraction of metal and slag and results in good separation of both. The resulting granulated slag has good stability, size, and shape control with high glassy content.
Figure 4: Steelmaking slag sandGranulated steel slags have been successfully utilized as 100% replacement to fine natural aggregates in roads. Both steel making furnace and blast furnace slags have been successfully used in spray sealing and asphalt applications. It is now well established that steel slag can be processed into a coarse or fine aggregate material for use in dense and open-graded hot mix asphalt concrete pavements and in cold mix or surface treatment applications.Fig 4 shows the slag sand developed from granulated steel slag.
In general, steel slags are heavier than iron slags due to the presence of higher Fe content, and hence the combination of BF slags and Steel sags have been found to be suitable for many applications where restriction in weight of the concrete is not a concern. In the recent past, the major attention of the steel plants is drawn to processing and quality management of iron and steel slag products matching the broad range of civil applications. Subsequent to sustained testing and studies, the slag products have cleared environmental standards in many countries and are being used prominently in various applications with slag products occupying a major market share.
Key characteristics and applications of iron and steel slags in cement and construction are shown in Table 1. In the Indian context too, the acceptance of slag product as a replacement in roads and civil construction is on the increase.
|Table 1: Characteristics and applications of slags|
|Iron Making / BF Slag||Air/Water Cooled Slag||Size, high strength and No alkali-aggregate reaction||Coarse aggregate for concrete|
|Size, high strength and Hydraulic property||Coarse aggregate for Roads|
|Thermal insulation||Raw material for rock wool|
|Fertilizer component (CaO, SiO2)||Fertilizer|
|Granulated slag||Cementitious phases and Strong latent hydraulic property||Clinker making|
|Blending material for Slag cement and concrete|
|Latent hydraulic property, large angle of internal friction and large water permeability||Material for civil engineering works and ground improvement material|
|Size and free from impurities||Fine aggregate for concrete|
|Light weight||Fine aggregate for Mortar and Plaster|
|Fertilizer component (CaO, SiO2)||Fertilizer and Soil Improvement|
|Steelmaking slag||Air Cooled and Magnetically Separated||Hard, wear-resistant and Hydraulic property||Aggregate for Asphalt and concrete roads|
|Large angle of internal friction and high strength||Aggregates for civil foundations and engineering works, ground improvement material|
|Granulated and Magnetically Separated||Good Hydraulic property and contains (CaO, SiO2, MgO, FeO)||Raw material for Clinker|
|Blending material for Slag cement|
|Performance improver and Coloring agent in cement|
|Size, strength and free from impurities||Material for civil engineering works and ground improvement material|
|Size, strength and free from impurities||Fine aggregate for Roads and concrete and as grit|
Iron and Steel Slags are economically viable and environmentally acceptable alternative material for replacing natural aggregates in roads and civil constructions. Effective utilization of these materials has tremendous economic impact, conservation of natural resources and gainful recycling of process by-products. Collaborative efforts and investment into research to develop and validate usage of slag based products in different innovative application areas must be done. Through combined efforts of all stakeholders working together to maximize the utilization of this continuously increasing resource, slag products adding value to the sustainability of the industry and environment can be made into a real success story.