After being processed, steel slag can be used as smelter flux, cement raw material, construction aggregate, foundation backfill, railway ballast, road pavement, brick, slag fertilizer and soil amendment, etc.
Steel slag is a by-product of the steelmaking process. It is composed of various oxides oxidized in the smelting process by impurities such as silicon, manganese, phosphorus and sulfur in pig iron and salts generated by the reaction of these oxides with solvents. The mineral composition of steel slag is mainly tricalcium silicate, followed by dicalcium silicate, RO phase, dicalcium ferrite and free calcium oxide.
There are two main ways for the comprehensive utilization of steel slag as secondary resources. One is recycling as a smelting solvent in our factory, which can not only replace limestone, but also recover a large amount of metallic iron and other useful elements from it. The other is as a raw material for manufacturing road construction materials, construction materials or agricultural fertilizers.
Raw material (less than 350mm ) will be conveyed to vibrating feeder, grate of vibrating feeder is set to 100mm, material with size less than 100mm(from vibrating feeder) will be conveyed to cone crusher, material with size larger than 100mm will be conveyed to jaw crusher for primary crushing.
The material from jaw crusher will be conveyed to cone crusher for secondary crushing, one magnetic separator is used in front of cone crusher for removing iron, and another magnetic separator is used behind cone crusher for removing steel chips from slag.
The material after passing through magnetic separator will be conveyed to vibrating screen for screening; material with size larger than 10mm will be conveyed back to cone crusher for being crushed once again, material with size less than 10mm will be discharged as final product.
Steel slag is a kind of solid waste that is produced in the process of steel production, it mainly consists of blast furnace slag, steel slag, iron bearing dust (including iron oxide scale, dust, blast furnace dust, etc), coal dust, gypsum, rejected refractory, etc.
The pile of steel slag occupies a huge area of arable land, and causes environmental pollution; moreover, 7%-15% steel can be recycled from steel slag. After being processed, steel slag can be used as smelter flux, cement raw material, construction aggregate, foundation backfill, railway ballast, road pavement, brick, slag fertilizer and soil amendment, etc. Comprehensive utilization of steel slag can lead to enormous economic and social benefits.
Steel slag crushing production line adopts jaw crusher for primary crushing, and uses hydraulic cone crusher for secondary and tertiary crushing, offering high crushing efficiency, low wear, energy saving and environmental protection, it has the features of high automation, low operation cost and reasonable allocation of equipment.
4. SANME can provide technological process plans and technical support according to the actual requirements of customers, and can also design non-standard supporting components according to the actual installation conditions of customers.
BackgroundWith world steel production now well over a billion tonnes per year, the slag that arises from some of the processes involved is a major resource. Traditionally it has been used mainly as an aggregate but for some types there are other applications, such as a raw material for cement or as a fertiliser.
Slag, as the term will be used here, is any siliceous melt that arises in significant quantity from the various processes used in the production of iron and steel, and more particularly the solid materials that forms when such melts cool. Slag from the production of ferrochrome is also included here; this material is produced in substantial tonnages and the main use of ferrochrome is in the steel industry.
Slags also arise from other processes, particularly the smelting of non-ferrous metals, but these materials can be very different and each needs to be studied individually. Moreover, in colloquial English an even wider range of materials such as clinker, ash and even colliery waste is sometimes referred to as slag. Such materials are not covered here.
OriginsIn the earliest processes the slag consisted mainly of the impurities (known as the gangue) that were present in the iron ore; in more modern times there is also a large contribution from materials added to aid the underlying process or to remove some specific deleterious element. An early example of this was the lime added to the feed for blast furnaces to give a more fluid slag, to reduce loss of iron as oxide in the slag, and to remove much of the sulphur introduced by the coke.
In tonnage terms, blast furnace slag is at present the main type followed by slags from the Basic Oxygen Steelmaking (BOS) and Electric Arc Furnace (EAF) processes. Various other processes such as external desulphurisation of steel may generate significant amounts of slag at some plants but overall they are only a small proportion of the total. In a few parts of the world, older or even ancient slags may be so abundant as to constitute an important industrial resource.
DisposalDisposing of this material by dumping would not only be expensive but would also represent a waste of valuable natural resources. Thus much of this slag is utilised, and to achieve this it is normally necessary for certain properties to be kept with specified limits. For some outlets the chemical properties may be most relevant, while for others it can be the physical properties and also stability, such as susceptibility to breakdown or weathering.
Recycling slags by feeding them back into the process has the attraction of efficiency but there are usually strict limits to the amount that can be recirculated in this way. Normally one of the functions of the slag is to remove some unwanted element, in which case adding it back to the stream may simply increase the removal task at some later stage (the re-introduction of phosphorus into steel is a particular danger of this type). This can sometimes be countered by recycling to some other branch of the process stream or by separating out certain fractions and recycling those. An example of the latter is the magnetic extraction of metal globules from BOS slag and their addition to the blast furnace feed, although the amount of phosphorus in the adhering silicate slag has to be monitored closely.
CompositionIn modern ironmaking, using rich ores, the ore may contain several percent of silica and this becomes a major component in the slag. Alumina is usually present, though the levels can vary quite widely between sources. In most commercially traded ores the other elements tend to be present only at much lower levels although there are some sources which can contain significant amounts of other components. These are sometimes used for specific metallurgical purposes (for instance, to raise the manganese or titanium content of the burden) or for reasons such as local availability and cost.
The ash from the coke is another contributor to the slag, and while this is predominantly siliceous there are differences which can be linked to geography although there are many exceptions, southern hemisphere coals (including India) tend to have lower alumina contents than those from the northern hemisphere.
Other components may be included as additives of their own, to improve some part of the metallurgical process. Lime is the most important of these, and there are several points of the process before the blast furnace where it may be added; another is titanium, sometimes used to counteract wear in the furnace lining.
TreatmentAir-cooled Blast Furnace SlagThe simplest treatment of the liquid slag is to lead it into a slag pit where layers build up until the pit is full. When it has fully solidified and then cooled somewhat, the slag (commonly known as air-cooled blast furnace slag or ACBFS) is excavated and transported away for further treatment (such as magnetic removal of any iron, or crushing and screening). Slag pits beside the furnace are most convenient but where space is restricted the liquid slag can be run into slag ladles and then conveyed to pits elsewhere. While one pit is being excavated, another is being filled, and water sprays can accelerate cooling so that excavation can begin promptly.
To maintain slag quality, good housekeeping is essential at all stages. General rubbish must be kept from entering the slag pits, and the bunds or walls enclosing the pits, which may be simply a ridge of aggregate, should be made of the same material in case some is recovered with the cooling slag during excavation. As the slag is removed and processed, the different slags (and other materials) must be kept in well-separated streams.
Such air-cooled slag is usually wholly or largely crystalline. Rapid cooling gives a glassy product, and several techniques are available whereby this can be achieved with water-cooling to give a product that is in most cases fine particles or granules. In the simplest versions the molten slag stream is poured into a powerful jet of water to give granulated slag but there are other variants such as pelletising, where the slag is poured on to a spinning cylinder and flung as droplets that cool in the air. Water-cooling was also important in the foaming process, where the liquid slag was poured over a group of upward-facing sprays to give a lightweight, pumice-like product. Environmental safeguards are needed with such processes, partly for the dust or slag fibres that may be generated but also because the sulphur in the slag can react to release compounds into the air and water.
PropertiesAir-cooled blast furnace slag is a rock-like material, but often more porous than most natural rocks. This can be an advantage in some applications, such as aggregate in sewage treatment beds.
Depending on the application, there may be desirable or statutory constraints on the chemical composition. Ground granulated blast furnace slag (GGBS) can be blended with Portland cement to give a concrete that may have technical or price advantages, but in some countries there are limits on the alumina content (which may in turn restrict the use of iron ores from parts of the world where the gangue is more aluminous).
If ACBFS is used for aggregate it will be the physical properties that are most important, and the European Standards can measure various aspects of the strength. In the earlier years of the twentieth century there was concern over the presence of the form of dicalcium silicate, a mineral rare in natural rocks but which can be present to a level of several percent in ACBFS from some sources. Under certain conditions this can invert to the form while cooling, with an expansion that shatters the surrounding material. This process, known as falling, was common at some plants in the early years of the twentieth century and was found to make the slag unusable as an aggregate.
In the second half of that century a belief arose in Britain, but nowhere else, that this inversion might occur spontaneously at some substantially later time and that it might, for instance, disrupt concrete in which the slag had been used. Strict chemical limits were developed and incorporated into British Standards to guard against this postulated late falling.
When British Standards were being replaced by European Standards in the late 1990s, a review showed that the idea of late falling had been based on a misinterpretation of earlier reports and that there were no known descriptions of the phenomenon ever being observed, let alone causing any disruption to an aggregate. Thus the European Standards only specified a test to confirm whether any falling had occurred during cooling. As it is, the slag compositions used in modern ironmaking mean that falling slag in the slag pits is now a rare occurrence.
ProcessesIn the latter half of the twentieth century, steelmaking came to be dominated by two processes. These are BOS, where oxygen is blown into the molten metal, and Electric Arc, where energy is provided by large electrodes. Earlier processes included the Open Hearth, where heated air and fuel were blown across the surface of molten steel held in a vessel, and Bessemer, where air was blown through the molten steel via holes in the base of the steelmaking vessel. These processes were superseded in economies with modern industrial methods although examples may survive in areas with special considerations.
Fluxing materials may be added to accelerate the slag formation and the assimilation of additives such as lime. Among the common historical fluxes are calcium fluoride and certain borates, although a wide range of other materials has been tried at some locations such as red mud from the processing of alumina. The possible presence of such elements must be borne in mind during the environmental assessment of any such slag.
Slags from these processes are usually strong and rock-like, with the potential for use as aggregates in a variety of applications. The density is higher than that of blast furnace slag or many commonly used natural rocks, and this can influence marketing; for some applications, such as use in waterways, the density is an asset. The high total lime content, often over 40%, and the fact that much of this lime content is held by minerals that are reactive under soil conditions, allows some slags to be used as agricultural liming agents.
When molten iron (referred to as hot metal) from the blast furnace is to be converted into steel, the main tasks are the removal of carbon, silicon and phosphorus. The LD (Linz-Donawitz) process is by far the most widely used, and takes its name from the towns where it was developed. Other terms such as BOS (Basic Oxygen Steelmaking) or BOF (Basic Oxygen Furnace), which have a broader meaning, are also commonly used for this process. The word Basic is used to indicate the type of refractory lining needed, normally magnesia, to resist the lime-rich slag.
This is a batch process (with each batch known as a heat) where oxygen is blown from above into the steel, and the different elements present oxidise in a predictable order. The carbon escapes as gaseous carbon monoxide and the silicon becomes silica which enters the slag. Phosphorus is normally only present in very small amounts but its profound effect on steel properties means that it receives close scrutiny. (In modern practice there could be around 0.1-0.2% P in the hot metal, which will be reduced to about a tenth of that value, giving a content of about 1% P2O5 in the slag. The exact values will of course vary considerably depending on raw materials, plant policies, and the target steel composition at any given time.)
The key to phosphorus removal is a lime-rich slag, achieved by addition of a calculated weight of lime to each heat in the furnace. This is an area of technical trade-offs: coarse lime takes longer to dissolve into the slag and begin the removal of phosphorus but with finer lime there would be excessive dust losses. Slag with a higher lime/silica ratio is not sufficiently fluid but a lower ratio gives reduced phosphorus removal. A flux, such a fluorspar (calcium fluoride) can be added to accelerate slag formation but increases the cost of the process.
The final slag can have a lime/silica ratio of about 3 or 4, although this can vary substantially from one heat to the next. Depending on the grade of steel being produced, the details of the process for each heat may be governed largely by the target level of phosphorus. For the purposes of slag utilisation, the most important point is that several percent of free lime can remain unreacted. This can later react expansively with water and is capable of disrupting an aggregate or the structure in which it has been used.
Much research has gone into ways to counteract or modify these expansive reactions. Although there is theoretically scope for changes in various aspects of the steelmaking process, it is usually accepted that the optimisation of this process has the highest priority and so efforts to modify the slag properties begin at the next stage, when the slag is poured from the steelmaking vessel. Introducing siliceous additives could accelerate the reaction of the melt with any remaining free lime but it is necessary to ensure that such particles are not immediately surrounded by a chilled coat of solid slag as that would prevent any further reaction. Processes have been tested using injection of sand, and addition of cullet (scrap glass).
Weathering of the slag before utilisation can allow some hydration of the free lime to take place so that the potential for expansion is reduced before the slag is used as an aggregate. To assess the success of such weathering, a suitable test is needed. Many expansion tests have been devised although their suitability as indicators of behaviour under, say, conditions within a road is rarely tested. The European Standard BS EN 1744 specifies a test where the results have been correlated with a series of road trials using BOS slag. Chemical analysis for free lime can also be a useful indicator of factors such as the progress of the weathering process. The weathering can be done under atmospheric conditions, where it can typically take about a year although details may depend on local conditions. (In arid areas, for instance, little weathering will occur unless the slag piles are regularly watered.) In addition there are processes, some already in commercial operation, for accelerated weathering by use of hot water (from plant cooling circuits) or by use of steam. These can reportedly reduce the time required per batch to days or even hours.
Particles of free magnesia can also be present in steelmaking slags, usually as fragments broken from the magnesia refractories. These can hydrate expansively in the same way as free lime, although the reactions are normally slower and it is less common to find contents high enough to cause concern. In any case, any expansion from this reaction would normally be included in that measured by the test specified in BS EN 1744.
The BOS process needs to be fed with molten iron, whereas the Electric Arc Furnace (EAF) can operate with a feed consisting partly or entirely of solid metal such as scrap. This gives rise to two significant differences in the slag, which is otherwise fairly similar in chemical composition. First, because much of the phosphorus has already been removed the slag does not need to be so basic. Thus any free lime present is normally at a lower level, and less likely to cause any significant expansion of the aggregate. Secondly, depending on the nature of the scrap used, the feed may contain significant proportions of other components (such as copper from motor vehicle scrap), and some of these may enter the slag.
Historical slags In countries such as Britain, in earlier years small steel plants existed in many areas that are not now thought of as associated with that industry. Over the years large tonnages of slag may have accumulated, and these may now be encountered during operations such as the development of brownfield sites. In that case, it becomes necessary to know their nature and properties in order to establish whether they are a valuable resource or a liability. In particular, their purity needs close examination. If the slags were regarded as materials to be dumped, at the time of production, then it is quite possible that other ironworks materials may also have been dumped with them.
While each case needs individual study, some guidance is found in the nature of the processes of those times. Blast furnace slags, for instance, may have higher sulphur contents than their modern equivalents, and the longer time means that alteration reactions (such as with ground water) that are normally insignificant might be important.
Open Hearth slags were used as road aggregate in some countries when that was a major process in the industry. Although failures were not common, those that did occur were often widely reported in publications. This not only caused a loss of confidence in the material but such cases have at times been confused with BOS slag. In fact, the descriptions of the failures have usually revealed that the slag was used with no form of quality control, and in some cases the users were not even aware that of what type of slag was being used. It was such events that led to the more thorough attention that is now given to the handling, treatment and properties of modern slags.
The nature of ancient slags, in those areas where they occur, will depend on local materials and ironmaking traditions. However, slags from the earliest processes often have a high FeO content, giving a dense rocky material rich in the minerals wustite and fayalite.
Chromite is the only ore of chromium in commercial use, and as deposits of this mineral are sparsely scattered across the globe there are far fewer production sites for ferrochrome than for iron and steel. Production is usually in an electric arc furnace, and the slag arises from the other components present within the chromite. These components are mainly oxides of Fe, Mg and Al, in proportions that vary quite widely between different ore deposits. Such combinations alone will not give a good fluid slag and so silica is usually added to promote melt formation.
This gives a slag of different composition from either blast furnace or steelmaking slag, and which may be glassy or crystalline depending on the cooling routine. The result can be a strong and stable material, suitable as an aggregate for roads and similar applications. Typically the density is relatively low, closer to that of blast furnace slag than steelmaking slag, which can influence its suitability for certain markets.
Lewis Juckes graduated from the University of Natal in 1963 and worked as a geologist during the 1960s (including two years in the Antarctic). After obtaining his PhD from Birmingham University in 1969 he joined British Steel and remained with them and their successors until his retirement in 2000. His work there covered a wide range of non-metallic materials such as ores, slags and dusts. When the British Standards were being replaced by European Standards in the 1990s, he was involved in the drafting of those affecting the utilisation of slags. Since his retirement he has retained an interest in such matters, publishing two papers concerning slag utilisation.
Steel slags (SSs) are usually classified according to the type of furnace in which they are produced. The properties of the slag depend on the type of process used to produce the crude steel, the cooling conditions of the slag and the valorisation process.
In the primary process, crude steel is produced in two ways. In the first method, the iron is produced from ore in the BF, thus, generating BF slag (BFS). BBOF slag (BOFS) is produced in the steelmaking process by using the molten iron coming from the BF. In the second method, slags are generated in the scrap-based steel industry. The first stage of the scrap-based steel industry production generates EAF slag (EAFS) and a second stage is performed to refine the molten steel. The slag produced in the LF (LFS) is the result from steel refining and, therefore, it is generally a heavy metal carrier such as chrome, lead or zinc (CEDEX Ministerio de Fomento, 2018). Table 7.1 shows the characteristics of the main slag types generated by the crude steel industry.
Another aspect that determines the physical and chemical properties of the slag is the type and speed of cooling. If the cooling of the molten slag is performed slowly, the components crystallise into stable structures producing, in most cases, a dense and inert crystalline material. However, if the cooling is rapid, its components are fixed in an amorphous structure and, therefore, the slag is unstable or active in the presence of certain substances. This is the case of the granular BF slag (GBS) which presents hydraulic properties if it is rapidly cooled and is widely used as a partial substitute of clinker in Portland cements.
There are two types of BFS, see Fig. 7.1, depending on the speed of cooling. The air-cooled slags (ABS) are subjected to a slow reduction in temperature (generally to air) and, therefore, have a crystalline structure and physico-chemical stability. Once cooled, the ABS is crushed and sieved according to the geometric characteristics required by the application. The ABS has a density of around 2.5g/cm3 and is used as a concrete aggregate, for bases, subbases and road layers (EUROSLAG; Liu et al., 2013; Nippon Slag Association). The Spanish Instruction for Structural Concrete (EHE08) (Ministerio de Fomento Gobierno de Espaa, 2008; Martn-Morales et al., 2011) only allows the use of BF ABS as aggregates, provided that the requirements regarding sulphate, sulphur and volumetric stability are met.
GBS, see Fig. 7.2, present hydraulic capacity due to its amorphous or vitreous structure. Its chemical composition is the same as that of ABS, but the structure and the stability are different. To obtain rapid cooling so as to obtain a vitreous structure, the slag is granulated. After that, the ABS is ground to sizes below 100m. ABS exhibits cementitious properties and so it is highly appreciated and used in the manufacturing of Portland cements (EUROSLAG; Liu et al., 2013; Nippon Slag Association). In Spain, 100% of produced GBS are used by the cement industry.
BOFS, see Fig. 7.3, present a high content of Fe, so its specific gravity is above 3g/cm3. This type of slag is cooled slowly to cause the crystallisation and stabilisation of its components. The most widespread use is in aggregate production for concrete and road applications (Shi, 2004; Tossavainen et al., 2007; Das et al., 2007; Juckes, 2003; Mahieux et al., 2009; Poh et al., 2006; Shen et al., 2009; Waligora et al., 2010; Xuequan et al., 1999).
The two types of EAFS are presented in Table 7.1: stainless steel (EAFS-S), Fig. 7.4 (Johnson et al., 2003; Huaiwei and Xin, 2011; Rosales et al., 2017), and crude steel (EAFS-C) (Shi, 2004; Tossavainen et al., 2007; Barra et al., 2001; Luxn et al., 2000; Manso et al., 2006; Tsakiridis et al., 2008). The main differences lie in the SiO2 and Fe contents. The high amount of oxide or metallic Fe (which could not be recovered) gives the EAFS-C, Fig. 7.5, a density which is usually above 3.5g/cm3. It is estimated that for each tonne of steel produced in this furnace, between 110 and 150kg of EAFS are generated (UNESID, 2018). When they are used as aggregates, after slow cooling, they have high compressive strength and skid resistance. EAFS have been shown to be high-performance aggregates in high-strength concrete and in road layers.
LFS, see Fig. 7.6, is commonly generated in the production of low-alloy steels and after air cooling and weathering over several days, this material is completely ground into fine white particles (Manso et al., 2005, 2013). Depending on the type of process, two types of LFS can be found. Those saturated with alumina (Tossavainen et al., 2007; Adolfsson et al., 2007; Nicolae et al., 2007; Yildirim and Prezzi, 2011) and those saturated in silica (Manso et al., 2013; Qian et al., 2002; Papayianni and Anastasiou, 2006, 2010; Branca et al., 2009; Rodriguez et al., 2009; Setin et al., 2009; Montenegro et al., 2013). They differ in their composition, having either a higher content of aluminium oxides or a higher content of silica oxide. LFS can be used as raw material in the production of cement, although special care should be taken with the content of fluorine and chlorine, which could adversely affect the properties of the clinker. It accounts for between 10% and 20% of EAFS production (UNESID, 2018).
As has been known, the major concerns with the use of BOS slag in SSBC manufacture have been associated with two issues. One is whether the BOS slag has sufficient volume stability during its service period. If the stability of SSBC is not acceptable, it will lose service significance. The second issue concerns the grindability of BOS slag used in SSBC manufacture. As is known, the main energy consumed during cement manufacture is in the process of calcining and grinding. If the energy consumed in steel slag grinding/magnetic separating is more than that for calcining and grinding of raw materials and clinker, BOS slag will lose its economic significance as an additive of blended cement. Although several papers have been published dealing with steel slag use in blended cement, few have addressed the grinding aspect and little careful laboratory investigations of steel slag grinding phenomena seem to have been done to date. It is considered that the energy consumed in relation to calcining and grinding of raw materials can be saved when using steel slags as active additive materials. However, several questions exist as to the degree of grindability of BOS slag and how it compares with OPC clinker and other materials, the suitable mill feeding size and the overall assessment of grindability.
It is well known that steel slag contains similar mineral composition to that of clinker; however, because of composition fluctuations, the slag may become unstable due to excess free CaO. GBFS possesses hydraulic properties that can only be activated in the presence of an existing basic or sulfate activator such as CaO or CaSO4 (Asaga, Shibata, Hirano, Goto, & Daimon, 1981; Duda, 1987; Narang & Chopra, 1983). Steel slag contains excess CaO that could constitute this activator. These factors comprise the premise for using steel slag as a component material for SSBC manufacture. Experiments have proven that the combined use of steel slag with GBFS and/or OPC clinker can balance the composition fluctuations in the steel slag and some CaO in the steel slag can be absorbed by GBFS, thereby preventing the occurrence of instability of SSBC specimens.
Despite differences in respective quantities of the chemical and mineral constituents that exist between steel slag and OPC clinker, steel slag can be considered comparable with OPC clinker. These differences do not affect the potential use of steel slag as an active material.
Magnetic reseparation of the steel slag can improve the efficiency of intergrinding steel slag and OPC clinker by about 50% compared with intergrinding OPC clinker and nonmagnetically reseparated steel slag. The grindability of mixtures of steel slag and OPC clinker depends on the relative content and initial pregrind size of the steel slag. No decrease in grindability was measured when less than 20% of 2.364.75mm steel slag was added to the OPC clinker.
SSBC paste specimens were inspected when cured. No cracking on the surface of the samples was observed under standard curing conditions for a period of 60 days. Two specimens of each mix, cured in water for 28 days, were treated by saturated steam at 3bar (137C) for 50min. The treatment cycle consisted of 50min presoaking (temperature and pressure build-up), 50min soaking time, followed by 50min cool down period with gradual pressure reduction. Specimens from mixes of BOF slag 1 and 2 exhibited no cracking even after 100min of treatment under the same pressure. The treatment condition is more harsh than that for testing the effect of MgO on OPC. This indicates that the addition of 10% OPC clinker can effectively prevent the occurrence of instability in SSBC (Wang, 1992).
Ratio of early to late strength: the SSBC has higher strength ratio compared with OPC, strength of SSBC at 1 year increases by 152166% of that at 28 days (45.8MPa at 28 days), whereas for OPC it takes 5 years for the strength to reach 150% of 28-day strength;
Grindability is of major importance in the manufacture of slag blended cement. In terms of grindability, BFS is slightly more abrasive than clinker and cogrinding has to be performed with care (Alanyali et al., 2009).
The laboratory results are inconclusive in determining the efficiency of intergrinding steel slag and OPC clinker. Lowrison (1974) reported the Bond index for grindability of different materials: corundum 3035; silica sand 16; cement clinker 15; slag 11 (type of slag not specified).
In a SSBC pilot study conducted by the author, although the weight retained on the 75m sieve for steel slag was higher than that for OPC after grinding, about 30% of the amount of coarse OPC particles still remained as unground particles in the ball mill. This phenomenon did not occur for the steel slag. Judging from the strength of OPC and steel slag, the hardness of steel slag should be close to that of OPC. The main reasons why steel slag may be considered to be difficult to grind may be due to the incorporation of iron scrap. In the laboratory experiments reported here, about 22% by weight of steel slag was attracted by the reseparation magnet, the separated slag particles consisting mainly of fine iron particles. Nonremoval of these materials would make the slag much more difficult to grind. Magnetic reseparation is absolutely necessary, if the steel slag is to be used for manufacture of SSBC, and to ensure that the very fine particles are removed because most of them contain impurities and iron, which affects the quality of the SSBC and decreases the grindability.
Although the grindability of steel slag is rarely covered in the literature, it is of major concern in the manufacture of SSBC. Preliminary work was carried out by using a laboratory ball mill to investigate the grindability of the steel slag and OPC clinker when ground separately and interground for periods of 30 and 60min. OPC clinker having particle sizes in the range of 8.013.2mm and steel slag having particle sizes in the range of 8.013.2mm and 2.344.75mm were used in the test. Comparative tests were performed for magnetically reseparated steel slag, ordinary steel slag, and OPC clinker. The degree of grindability was assessed by particle size distribution. Results of 30min grinding have similar trends.
when 2.364.75mm particle size steel slag is interground with OPC clinker, the grindability of the composite is better than that of OPC clinker provided that the content of steel slag in the composite is not greater than 20%.
Selective use and quantification of steel slag having suitable properties are important aspects for manufacture of SSBC. The slag should be magnetically reseparated prior to grinding for SSBC. Both too fine and too coarse particles are not suitable for SSBC. If particles are very fine, impurities such as dust might be incorporated; if they are too coarse, additional crushing will be necessary and, thus, more energy will be consumed. Particle sizes within a certain range, probably 215mm, should be selected for SSBC manufacture. The incorporation of steel slag with particle size below 5mm can benefit the grinding of OPC clinker. Other particle size materials should be used for other applications (eg, road base, etc.). The addition of steel slag, at content levels of up to 20% of total solid material, is suggested as optimum with regard to stability, economy, and strength of the blended cement.
It is well known that steel slag contains similar mineral composition to that of OPC clinker; however, because of composition fluctuations, the slag may become unstable due to excess free CaO. GBFS possesses hydraulic properties that can only be activated in the presence of an existing basic or sulfate activator such as CaO or CaS (Asaga et al., 1981; Duda, 1987; Narang & Chopra, 1983). Steel slag contains excess CaO that could constitute this activator. These factors comprise the premise for using steel slag as a component material for SSBC manufacture. Experiments have proven that the combined use of steel slag with GBFS and/or OPC clinker can balance the composition fluctuations in the steel slag and some CaO in the steel slag can be absorbed by GBFS, thereby preventing the occurrence of instability of SSBC concrete.
BOS slag is produced during steelmaking by the basic oxygen process. The manufacture of steel involves the removal of excess quantities of carbon and silicon from the iron by injection of oxygen and the addition of small quantities of other constituents that are necessary for imparting special properties to the steel. A lime or dolomite flux is used that combines with the oxidized constituents to form a slag. BOS slag is decanted off from the surface of the molten steel and is normally cooled slowly, by air-cooling or water quenching, in pits or bays prior to being dug and transported to holding areas.
Despite differences in respective quantities of the chemical and mineral constituents that exist between steel slag and OPC clinker, steel slag can be considered comparable with OPC clinker. These differences do not affect the potential use of steel slag as an active material. The main differences are summarized in Table 13.6.
The addition of steel slag to the OPC clinker has to be considered in terms of f-CaO content of steel slag, total f-CaO content of SSBC, and grindability. From the results of grindability, it is known that, from grindability considerations, the optimum addition of steel slag is below 30% of total weight of SSBC (BOS slag and OPC clinker). In addition, the total f-CaO content of SSBC should be less than 2%, which is an acceptable limit for OPC and there should also be a relationship controlling the addition of BOS slag depending on its f-CaO content. From this criterion it can be shown that, provided the relative content of steel slag is controlled, steel slags with a high free calcium oxide content can be used as an ingredient of SSBC. These steel slags would not normally be suitable for other engineering applications. The addition criterion, in terms of f-CaO content, is as follows:
For a given steel slag with 1.6% f-CaO content and OPC clinker with 0.5% f-CaO, if a mixture comprising 20% BOS slag and 10% OPC clinker is interground, the resultant SSBC will be volumetrically stable if f-CaO<2%. Substituting the values into Eq.(13.2), the f-CaO content of SSBC is 0.72%. This is less than 2%, and therefore the mix is of acceptable stability.
A SSBC contains 10% OPC clinker, with 0.5% f-CaO, and 20% BOS slag. What is the maximum allowable f-CaO for the BOS slag? Substituting into Eq.(13.4) gives an answer of 11%. This means that steel slag containing<11% f-CaO can be used in SSBC.
If steel slag is used, natural resources can be preserved in steel industrial areas. Slag can be used for various purposes. There is much more to explore about steel slag as a civil engineering material, including the following:
Steel slag is an industrial byproduct obtained from the steel manufacturing industry. It is produced in large quantities during steel-making operations that use electric arc furnaces. Steel slag can also be produced by smelting iron ore in a basic oxygen furnace. According to methods for cooling molten steel slag, steel slag is classified into five types: natural air-cooling steel slag, water-spray steel slag, water-quenching steel slag, air-quenching steel slag, and shallow box chilling steel slag [47,48]. Most steel slag contains a high content of Fe1-O and other metal oxides. Fe1-O includes FeO, Fe2O3, and Fe3O4, all of which are nonstoichiometric compounds, so Fe1-O has the properties of a semiconductor. The electrical resistivity of FeO and Fe3O4 is 5102 and 4103cm, respectively, which is basically the same as that of pitch-based carbon fiber. As a result, steel slag presents good electrically conductive properties . In addition, steel slag can be used as aggregates in concrete to replace natural aggregates, because it has favorable mechanical properties, including strong bearing and shear strength, good soundness characteristics, and high resistance to abrasion and impact. Steel slag aggregates are fairly angular, roughly cubical pieces with a flat or elongated shape (as shown in Figure2.13 ). They have a rough vesicular nature with many non-interconnected cells, which gives a greater surface area than smoother aggregates of equal volume. This feature provides an excellent bond with concrete binder. Replacing some or all natural aggregates with steel slag is helpful for reducing environmental pollution and the consumption of resources [47,48]. Therefore, steel slag is a promising kind of filler because it works as both functional filler and aggregate. The incorporation of air-quenching steel slag of 0.3155mm (in which the content of Fe1-O is over 30%) into concrete to fabricate mechanically sensitive concrete was investigated by Li etal. in 2005 ; subsequently, Jia performed a systematic study of this concrete (as shown in Figure2.14) [23,50].
Property modification of steel slag was conducted after steel slag was discharged to improve the stability of steel slag. Tests in some countries have shown that adding blast furnace slag or fly ash to the steel slag improved the stability of steel slag. Some research programs resulted in patents that were reflected in Japanese Patents (JPs). These included the addition of materials containing silicate or aluminate into steel slag during discharge (JP 74-58107); putting fine steel slag into molten blast furnace slag (JP 76-61278); adding boric acid or borate into steel slag (JP 78-43690); and mixing molten BOS slag with special steel slag containing Cr2O3 to raise the content of Cr2O3 above 2% (JP 78-30997). The aim of all of these methods is to eliminate the unstable materials in steel slag.
Steel slag has become one of the major sources of aggregates for highway pavement constructions in many state Departments of Transportations (DOTs) in the United States. In the past decades, the use of steel slag in HMA pavements has proven to be extremely successful nationwide. In particular, steel slag is one of the superior aggregates for constructing smart HMA pavements for heavy truck traffic, including SMA and thin, and ultra-thin, HMA overlays, due to its unique physical and mechanical properties, such as hardness, durability, and surface texture. Another potential use for steel slag is for high friction surface treatments (HFST) that are increasingly catching the attention of pavement engineers as an effective solution to addressing high friction demand on horizontal curves. Although steel slag does not have a PSV similar to that of calcined bauxite, its local availability and low price make it a viable aggregate source for HFST, particularly for less severe geometric conditions or relatively large scale projects (Li, 2016).
In Fig. 10.7 a 19mm thick 4.75mm dense-graded HMA pavement has been placed on an interstate highway. The aggregate is compromised of 39% steel slag by weight. The pavement has performed satisfactorily with reference to surface friction and ride quality. Fig. 10.8 shows HFST test patches of steel slag lying on a multilane highway. The test patches are composed of a layer of 13mm steel slag bonded to the existing pavement surface using a specialized resin binder. These test patches functioned very well in terms of surface friction and texture after one winter maintenance season.
For steel slag used as a coarse aggregate in a bound condition, or in a rigid matrix, such as PCC, the resulting integrity and volume stability are basically controlled by the minimum allowable stress of the matrix materials, cement mortar for instance, and the maximum expansion stress, which can be deduced from the expansion force based on appropriate modeling of steel slag particles in the matrix. A usability criterion for steel slag use in confined conditions can be developed by relating the allowable stress of a known matrix material and the maximum expansion force (stress) of a steel slag particle. Because concrete is a structurally sensitive material, one localized failure (one particle failure) will be regarded as failure of the concrete. Therefore the basic disruption model for steel slag concrete should be based on a single steel slag particle. The imperative task is to determine the expansion force of bulk steel slag and an individual steel slag particle.
From the expansion force test, if the bulk steel slag sample is placed in a rigid mold and the volume expansion is completely constrained, an internal expansion force will result. The expansion force is expressed as Fex and is defined as the resultant expansion force produced by a given volume of steel slag. The expansion force is to be proportional to the volume of slag sample; that is, the greater the volume of the steel slag, the larger the expansion force will be.
where fex is the expansion force generated by a dense compacted steel slag in a unit volume, (N/m3); Fex is the measured expansion force produced by a given volume of dense compacted steel slag, (N); and Vsl is the volume of compacted steel slag, (m3). The expansion force of a unit volume of slag given by Eq.(12.17) is equal everywhere in a given volume of steel slag; that is, fex is a constant for a given slag sample. Note that this applies only to the large amount of steel slag particles in a compacted condition; it does not apply to a single steel slag particle. The three-dimensional expansion force is monopolized by expansion of steel slag in a confined condition. Both the disruption ratio, R, and expansion force, Fex, will be used in quantifying the expansion force of steel slag.
where Fec is the expansion force produced by the coarse steel slag aggregate in one cubic meter of concrete, (N); Vsc is the volume of steel slag aggregate in one cubic meter of concrete, including air voids, based on the mix proportion, (m3). From a given concrete mix proportion, with the weight of the steel slag coarse aggregate and the density of a given steel slag, Fec can be calculated.
It is reasonable to assume that only the cracked or powdered steel slag particles that have undergone the autoclave disruption test contribute to the expansion force, and the disruption ratio is equivalent to the volumetric ratio. Therefore, the actual volume of expanded steel slag, (Vse), excluding air voids, is
where Vse is the actual volume of expanded steel slag particle in concrete, (m3); is the solid volume of spheres under tightly compacted condition, which is approximately 67% (Shergold, 1953), assuming maximum volume of single-size steel slag particles occupied the volume.
where Fss is the expansion force from a single steel slag particle, (N); Vss is the volume of the single steel slag aggregate particle, ((d3)/6) (m3); and d is the nominal particle size of the steel slag aggregate, (m). The equation is illegal when R=0; that is, when the steel slag particles are volumetric stable (disruption ratio is zero) or the aggregate is natural aggregate (disruption ratio is zero).
Steel slag is a calcium-rich industrial waste. Direct aqueous carbonation is one of the routes to slag carbonation. The reaction mechanism of direct aqueous carbonation of steel slag is first discussed. Various models have been proposed to model aqueous carbonation of steel slag. The merits and shortcomings of these models are discussed. A recently proposed slag carbonation model by Gopinath and Mehra (2016) is discussed in detail. The model considers the armoring of reaction surface by two secondary phases: pore closure in one of these layers due to product precipitation and the kinetics of the reaction at the slag core. The model is analyzed, its advantages and drawbacks are discussed, and further improvements to the model are suggested.
Solid steel slag exhibits a block, honeycomb shape and high porosity. Most steel slag consists primarily of CaO, MgO, SiO2, and FeO. In low-phosphorus steelmaking practice, the total concentration of these oxides in liquid slags is in the range of 8892%. Therefore, the steel slag can be simply represented by a CaO-MgO-SiO2-FeO quaternary system. However, the proportions of these oxides and the concentration of other minor components are highly variable and change from batch to batch (even in one plant) depending on raw materials, type of steel made, furnace conditions, and so forth.
Steel slag can be air-cooled or water quenched. Most of the steel slag production for granular materials use natural air-cooling process following magnetic separation, crushing, and screening. Air-cooled steel slag may consist of big lumps and some powder. The mineral composition of cooled steel slag varies and is related to the forming process and chemical composition. Air-cooled steel slag is composed of 2CaOSiO2, 3CaOSiO2 and mixed crystals of MgO, FeO, and MnO (ie, MgOMnOFeO), which can be expressed as RO phase. CaO can also enter the RO phase. In addition, 2CaO-Fe2O3, CaOFe2O3, CaOROSiO2, 3CaORO2SiO2, 7CaOP2O32SiO2, and some other oxides exist in steel slag (Sersale, Amicarelli, Frigione et al., 1986; Shi, 2004). It was reported that the X-ray diffraction pattern of steel slag is close to that of Portland cement clinker.
Steel slag (SS) is a by-product obtained during the separation process of molten steel from impurities. Selected physical properties of steel slag are shown in Table 10.1. Depending on the used production technology, the steel slag can be divided into a basic oxygen steel slag, an electric arc furnace slag and a ladle furnace slag (De Brito and Saika, 2013). The steel slag is used as a secondary cementitious binder, or aggregates for road construction (Sheen et al., 2013; Manso et al., 2004).
Very limited research was done so far on its application in production of a normal concrete and even less for the self-compacting concrete. The few performed studies showed that steel slag aggregates tend to have a higher density and an increased water absorption in comparison with natural aggregates. At the same time the abrasion resistance tends to be enhanced (Anastasiou and Papayianni, 2006). Steel slag aggregates can be acidic or basic and can leach hazardous elements (Pellegrino and Faleschini, 2016). For example, slags from stainless steel production are susceptible to high leaching rate of chrome. The electric arc furnace slag aggregates showed tendency to expand, which is related to the presence of certain volumetrically unstable periclase and free line (Evangelista and de Brito, 2010).
In steelmaking, slagmetal mixing is a very common phenomenon and it occurs due to the shear at the slagmetal interface caused by excessive liquid steel flow.37 This mixing leads to emulsification of steel in slag, which increases the total interfacial area and consequently the rate of slagmetal reactions.
It is found by a cold model study38 that slagmetal mixing takes place only when a critical velocity (along the interface) is achieved. Below this velocity, flow is streamlined and no mixing is possible. The value of the critical velocity would depend on the properties of both liquids, namely density, viscosity and interfacial tension. It is interesting to point out that when a mixing flow is stabilized, the fraction of water in the oilwater mixing zone is very small, only a few percents. In a stabilized mixing layer, oil becomes small spherical droplets. Surrounding each oil droplet, a thin water film is formed. These oil droplets tightly packed together, so that water occupies only the voids of the drops. The amount of water in the films surrounding the oil droplets and in the voids is very small. This is in accordance with the observation found in a ladle. In a recent study,39 the slagmetal mixing zone in a ladle was sampled and analyzed. Figure9.6 presents the microphotograph of the mixing zone. It is clearly seen in this figure that metal presents in the form of very fine particles. The maximum size of the metal particles is about 140m. Chemical analysis showed that the amount of iron present in the mixing zone is about 1 per cent. The small size of the metal particles and the low content of iron in the mixing zone suggest that the metalslag system could be very similar with the wateroil system. In the mixing zone, slag takes the form of small spherical droplets, while metal forms a film around each slag sphere. It is reasonable to expect that the size of the slag droplets and the metal film thickness would be a function of the physical properties of the two liquids as well as the flow. A mathematical description of the slag mixing is highly desired for designing the processes involving slagmetal reactions.
It should be mentioned that physical properties of both liquid metal and slag vary along the process. Consequently, the behaviour of slag-metal mixing would vary as well. In Fig.9.7, the X-ray images of the iron drops surrounded by a slag in sulphurization process are compared.40 The change of the contact angle due to the change of sulphur content in the metal drop is well brought out in the figure. Hence, knowledge of the variation of the physical properties in general and interfacial tension in particular would be essential in a precision design of a process.
Ironmaking and steelmaking slags are inevitably generated as a by-product from ironmaking and steelmaking processes. Main components of the slags are CaO, SiO2, Al2O3, MgO, and iron oxides, and the compositions of slags depend on the process. In the case of Japan, three types of slags, namely BF slag, BOF slag, and EAF slag are mainly produced, which amount is shown in Figure 4.4.11  and the typical compositions of each slag are summarized in Table 4.4.2 . Main component of BF slag is SiO2 and Al2O3, coming from iron ore as gangue minerals, and CaO added as a flux during sintering process. On the other hand, BOF slag mainly contains CaO added as a refining agent, and SiO2 and iron oxides produced by oxidation refining process. The EAF slag is classified into two types, oxidation slag and reduction slag, which are produced during steel refining and reduction of iron oxide to metallic iron, respectively.
Slag compositions of constituents entrained from gangue components depend on the compositions of raw materials, while those of other constituents added as a flux for refining processes are designed to maximize its refining performance, and thus there is a wide variety of components and compositions of slags. Approximately 300kg/ton-pig iron and 100kg/ton-steel of BF slag and steelmaking slag (BOF or EAF slag) are generated. Totally 24, 11, and 2.9 million ton of BF, BOF, and EAF slags (FY2011) are produced in Japan .
Common slag treatment process in Japan is shown in Figure 4.4.12 . About 80%  of BF slag is quenched by water spray and the quenched BF slag sand produced is used mainly for cement, concrete and civil engineering resources. The rest is cooled by field air cooling and the slow cooled slag is used as a resource for road construction, concrete coarse aggregate, and so on. On the contrary, since steelmaking slag contains iron droplets at several percents in weight the slag cannot be quenched by water splashing. Therefore, slag is treated by field air cooling and then crushed and screened. Iron droplets are recovered by magnetic separation and remained slag is sold for various purposes such as civil engineering, cement or concrete resource. BF slag is completely recycled, while small fraction of steelmaking slag cannot be utilized due to the elution of hazardous elements such as heavy metals or fluorine. Development of new technologies to use such slag is an important solution to reduce the amount of slag landfilled without any utilization.
Regarding the measure to decrease the environmental load by slags generated from ironmaking and steelmaking processes, following two methods are considered. The first is the reduction of generated slag amount by development of highly efficient processes. Metallurgical slags have been mainly designed to increase the refining capability as a function of slags so far. Recent process and slag designs are based not only on the improvement of the refining capability but also on the reduction of environmental load such as the decrease of slag amount, discontinuation of the use of hazardous elements, or the development of the process which generates recyclable by-products (slags). The second measure is the development of the new utilization method of slag as a resource. Utilization method of the ironmaking and steelmaking slags as an abundant resource should be developed by creating new functions and additional values.
Among the steelmaking slags, EAF slag is worth mentioning, as its application as aggregate for concrete is gaining increasing attention. Euroslag, which is an international organization dealing with iron and steel slag matters, has estimated that about 25.9% of steel slags produced in Europe are EAF slags from carbon steel production (EAFC- EAF carbon steel), and 5.9% are EAF slag from stainless or high alloy steel production (EAFS- EAF stainless steel). In particular, the amount of the former, that is, EAFC, is expected to rise dramatically in the near future, due to the conversion of many steel processing plants into EAF technology that is more environmentally sustainable. During the melting process of the steel in this type of furnace, EAF slag (known also as black slag) is generated after the addition of certain admixtures (limestone, slag correction agents such as bauxites, and slag formers) in the molten bath of the steel, in amounts between 120 and 180kg/tonne of manufactured steel.
After cooling from 1560C, EAF slag becomes a stony, cohesive, slightly porous, heavy, hard, and tough material that appears as a black or dark-gray crushed aggregate. Generally, it has very good mechanical properties, as it is made up of particles with a hard, dense, and angular shape. It has high abrasion resistance, low aggregate crushing value, and excellent resistance to fragmentation. In addition, it is typically characterized by a heavier weight than natural aggregates, which depends on the amount of heavy metal oxides included in its composition and that varies slag by slag. Its chemical composition is principally Fe2O3, CaO, SiO2, and Al2O3, with minor amounts of MnO and MgO. Constituent materials (e.g., scraps and additions in the furnace) significantly influence slag composition and structure, as well as the cooling method, which can be rapid (through water spraying) or slow (solidification in open air). The density of the EAF slag is an important parameter that helps to identify how heterogeneous the slag might be, depending on each steelmaking process. Indeed, it has been recorded as varying within the range of 30004000kg/m3, due to differences in the content of metallic iron (8000kg/m3), iron and manganese oxides (5000kg/m3), and the internal porosity. EAF slag typically has a crystalline nature, where the principal minerals that can be detected are wustite, hematite, magnetite, merwinite, larnite, etc.
The above properties make EAF slag a good candidate for use in many areas of construction, including as a (heavyweight) aggregate for concrete. For this reason, gravity structures, hydraulic protection structures, mass concrete, and all other applications in which the heavyweight of the slag is advantageous can be identified as the best markets for EAF concrete. An interesting potential application is as radiation-shielding concrete, as an environmentally friendly substitute of natural aggregates such as barite, hematite, or limonite. One of the first attempts to assess the feasibility of using EAF slag in concrete was carried out by Al-Negheimish, Al-Sugair, and Al-Zaid (1997), who tested the main mechanical properties of EAF concrete mixtures. Subsequent research has shown that the mechanical properties and durability of concretes manufactured with this slag are well known and are similar, or even better, than those of concretes manufactured with traditional natural aggregates (Faleschini et al., 2015a; Manso, Polanco, Losanez, & Gonzalez, 2006). Almost all studies point to the positive contribution of EAF slag on concrete compressive strength, tensile strength, elastic properties, durability against carbonation, chloride penetration, and freezing/thawing cycles (Arribas, Santamara, Ruiz, Ortega-Lpez, & Manso, 2015). Poor performance is obtained only in the fresh concrete, due to the angular shape of the slag, that could be easily overcome using water-reducing admixtures in the mix. As an example, self-compacting concrete made with EAF slag has been successfully produced (Santamara et al., 2017). However, most studies show the necessity of limiting the use of the fine slag fraction, which generally cannot exceed 50% as the natural sand replacement ratio (Pellegrino, Cavagnis, Faleschini, & Brunelli, 2013).
Before being used in concrete, EAF slag must be stabilized to prevent the potential swelling that has been observed, which may exceed 2% in volume. Some of the causes of this swelling include free lime hydroxylation and subsequent carbonation, which is acknowledged as the main cause of EAF slag potential expansion, the hydroxylation and carbonation of free MgO, the long-term oxidation of metallic iron from iron +2 to iron +3, and lastly the transformation of -silicate to -silicate. To prevent slag swelling, a simple stabilization treatment that does not involve any transformation of the material can be carried out, consisting of prolonged weathering in outdoor exposure, which should last about 3 months. After this, the slag can be processed as an aggregate, and when it reaches the required grading, it should be sprayed with water for 36 days, providing alternate wetting/drying conditions. The reliability of this method has been demonstrated (Pellegrino & Gaddo, 2009). Another environmental obstacle that has discouraged the use of some slags in construction until now is the potential leaching of hazardous compounds and heavy metals. This risk seems particularly relevant for some EAFS from stainless steelmaking that contain a high quantity of chromium. Leaching from steel slags is generally characterized as a surface reaction, followed by a solidsolid diffusion process, in order to retain equilibrium in the materials. A minimization of the surface area of the slag is therefore likely to reduce leachability. The pretreatment operation discussed above is often effective not only to reduce potential swelling phenomena, but also to reduce the concentrations of harmful substances or the high leaching levels of these elements. Water used to stabilize the slag should of course be collected and treated to maximize the sustainability of the slag production plant.
Laboratory tests were also conducted to prove the suitability of using EAF slag as aggregate in reinforced concrete structures, to assess whether its heterogeneity, its high density and its poorer workability represent an obstacle for designing and constructing full-scale structures. The results obtained were positive: adjusting the mix with water reducers, it was possible to obtain both pumpable and self-compacting concrete, with appropriate flowability even in highly reinforced structural elements (Fig. 2.3). The structural response of beams subject to four-point loading was even better than for reference beams made with natural concrete, both when flexural-bending failure and shear-failure were induced (Pellegrino & Faleschini, 2013). Bond between steel and concrete is enhanced significantly when EAF slag is used as coarse aggregate, with a positive contribution for developing mechanical interlock and frictional bond mechanisms (Faleschini, Santamaria, Zanini, San Jos, & Pellegrino, 2017b). Tests were also carried out to evaluate the response of full-scale corner beam-column joints, subject to seismic-like action and gravity, to simulate one of the most stressed regions of a multistory reinforced concrete building. Results in this case also demonstrated a superior response of the elements made with EAF slag concrete, which dissipated more energy, had enhanced ultimate capacity, and remained more intact after the failure (Faleschini, Hofer, Zanini, Dalla Benetta, & Pellegrino, 2017c). A single specimen made with EAF concrete demonstrated the most important and unexpected result: it attained enhanced performances compared to the reference, but it was designed with 20% less cement, and a higher water/cement ratio, thus being significantly more environmentally sustainable than the reference. Results were also extended to other test configurations through a numerical approach, demonstrating always improved performance of the joints made with EAF slag (Faleschini, Bragolusi, Zanini, Zampieri, & Pellegrino, 2017d).
These results have encouraged some producers to use EAF slag concrete in real construction projects, even though there is still not a standard which clearly supports its use, at least in Europe. One of the most important examples of the application of the EAF slag concrete in a real structure is the basement elements of the Tecnalia experimental building KUBIK, sited in Derio, Bizkaia, on the Northern coast of Spain, which were made of premixed reinforced concrete containing up to 75% (by weight) of EAF slag aggregate. The amount of EAF concrete used was about 140m3, necessary to construct both basement walls and foundation slabs, which were manufactured in 2008. EAF concrete was poured on-site continuously with a concrete pump. Concrete strength evolution was monitored constantly for 180 days after concreting, displaying high strength development, which achieved on average 60MPa for both slabs and walls. After more than 10 years from its construction, no durability problems or swelling phenomena have been recorded. In 2015, the Port of Bilbao used EAF slag to manufacture blocks to protect the Punta Lucero dock and to build the new Punta Sollana dock, employing a large amount of this material for building these maritime structures (Santamaria, 2017). However, in the past, unsuccessful stories of EAF slag use in concrete have been also recorded, due to the incorrect management of this material, that was often mixed with the white slag (i.e., ladle furnace slag), which is very prone to swelling when in contact with water. The European standard EN 12620 (2008) considers EAF slag as an artificial aggregate, which can be used for many civil engineering applications, including as an aggregate for concrete. However, no details about the technical requirements are defined either in this standard or in other national regulations. Several research groups around the world are working on the standardization of the EAF slag in hydraulic mixes; in Europe, groups from Spain, Italy, and Greece stand out, among others. Other countries, such as Japan, have already concluded this process of standardization, and have proper standards that include this material officially as an aggregate for concrete (JIS A 5011-4, 2013).
Along with the development of steelmaking, slag processing, and treating technology, more EAF slag has been produced and used in various paving applications that are the main focuses on slag research (Hainin et al., 2014).
In the last couple of decades, other types of slag have been used in asphalt paving; for example, ferronickel slag (Wang, Thompson, & Wang, 2011), copper slag (Collins & Cielieski, 1994; Gorai, Jana, & Premchand, 2003), and boiler slag (Chesner et al., 1998).
Compared to blast-furnace slag, steelmaking slag usually contains a much higher amount of lime, which can cause formation of dicalcium silicate, 2CaO-SiO2 (sometimes formulated as 2CaOSiO2), which can cause disintegration upon cooling due to a volume increase when changing from one crystalline form to another (from the form to the form) . This transition from to form is accompanied by an increase in volume of around 12%, which results in the decomposition of slag into powder . According to Mombelli etal. , formation of only 4 wt.% of -Ca2SiO4 is enough to cause slag disintegration. The decomposition of dicalcium silicate is shown in Figure 2.8.
Based on actual experience, if there is a danger of dicalcium silicate decomposition, it will occur prior to material being placed in construction. Therefore, it does not pose a problem for the end user . However, altering the chemical composition of the slag and rapid cooling of the molten mass while preventing the crystallisation of dicalcium silicate can completely prevent this problem.
The problem of iron decomposition is considered to be rare and characteristic for slag with a high content of iron oxide . Such slag can, with a certain amount of other constituents, form compounds that will easily react with water and thus lead to the disintegration of material. However, decomposition of the form of dicalcium silicate (also called larnite) in steel slag can be avoided by the addition of melted quartz in the slag flow . Namely, quartz addition has a twofold effect: it reacts with calcium aluminates to form gehlenite, which inhibits the formation of larnite, and it simultaneously prevents its disintegration, thus avoiding the so-called dusting effect.
Solid steel slag exhibits both block shape and honeycomb shape. The former steel slag possesses luster; the latter is nonlustrous and more brittle. The specific gravity of steel slag is dependent on viscosity, surface tension of liquid steel slag and amount of dioxide contained, ferrous materials, and porosity. Moisture content of steel slag is 0.22.0%, specific gravity is 3.23.6, compressive strength is between 169 and 300MPa (43.5ksi), and the Mohs scale number is between 5 and 7. Grindability of steel slag is less than that of BF slag. Hardness and specific gravity are greater than those of BF slag. Like air-cooled BF slag, steel slag exhibits excellent skid-resistance properties. Some basic physical properties of steel slag are shown in Table 2.8.
Mineral carbonation is a promising and safe approach for permanent sequestration of CO2 via the transformation of CO2 into various carbonates. There are several elements that can be carbonated, but alkaline earth metals in terms of calcium and magnesium are the most suitable for carbonation due to their abundance and insolubility in nature (Sipil etal., 2008). Natural minerals rich in calcium or magnesium, for example, olivine (Mg2SiO4), serpentine (Mg3Si2O5(OH)4), and wollastonite (CaSiO3), are used as the feedstock for providing the Mg and Ca for the formation of carbonates. However, it could be very energy intensive for the processes of mining, mineral pretreatment (i.e., crashing, grinding, and milling, etc.), kinetic enhancement on the carbonation via temperature elevation, or acid dissolution of the natural minerals. Iron could also be employed for carbonation, but considering it is a valuable mineral resource for other industrial applications, it is less suitable for large-scale carbonation.
In addition to the natural minerals rich in Mg and Ca, there are also some industrial solid wastes containing large amounts of Mg, Ca, and even Fe. The industrial wastes include fly ash, various types of iron and steelmaking slags, carbide slag, cement dust, etc. In comparison to the natural feedstock of Mg- and Ca-containing minerals, the industrial wastes are more suitable for economical CO2 sequestration. This is because the industrial wastes are more kinetically unstable and hence are more reactive to carbonation, and therefore require less pretreatment and less energy-intensive carbonation conditions. In addition, the industrial wastes are always near the CO2 intensive point, providing a possible way for in-situ sequestration, which in turn cuts the transportation cost.
Iron and steelmaking slags are byproducts produced during the manufacturing processes of iron and steel respectively. Blast furnace slag (BFS) is a product of iron production, which has been widely investigated and utilized, particularly as a supplementary cementitious material for cement or alkali-activated material, owing to its high hydraulic property or alkali activation reactivity. Steelmaking slag is a byproduct produced in the process of refining the iron to steel in various furnaces (Shi, 2004). For a ton of steel, around 0.130.2ton of slag is produced (Yu and Wang, 2011). According to the US Geological Survey, the global production of steelmaking slag is estimated to be on the order of 170 million to 250 million tons (USGS, 2018). Unlike the iron slag, the steel slag exhibits much lower hydration reactivity and poorer hydraulic properties. Furthermore, it usually contains high content of free-CaO and periclase, which can produce excessive volume expansion as a result of hydration and therefore induce volume instability. In addition, the grinding of steel slag is energy intensive due to its poor grindability. In general, the utilization of steel slag is quite limited or of less economic value; for instance, in China only approximately 10% of steel slag has been used. Nonetheless, more approaches for the use of steel slag are still under seeking. It is found that under CO2 rich environment, the steel slag is very carbonation reactive, and hence has a big potential to be used as the feedstock for CO2 sequestration. This has attracted increasing attention from researchers and abundant work has been focused on this. As reported, up to 13 and 21.5wt% of CO2 (by weight of steel slags) could be sequestrated into stainless steel slag and basic oxygen furnace slag (BOFS) respectively (Baciocchi etal., 2009; Chen etal., 2016). To further reduce the energy consumption and cut the relevant cost for the CO2 sequestration in steel slag, more researches are conducted to integrate CO2 sequestration with value-added products development, for example, development of construction materials. This provides a novel and promising way for the economic CO2 sequestration and valorization treatment of steel slag.
Recycling of iron and steel scrap is very high; in fact, internal to steel plants virtually 100%. In general, recycling of total scrap to this industry is well above 70% and increasing. A critical raw material supply to the minimills has been scrap, and this has become true for the integrated mills with high continuous casting yields. Most minimills have scrap recycling divisions or subsidiary scrap companies in an effort to control their raw material costs, a critical factor for steel plants almost totally dependent on scrap for their raw material supply.
Recycling of oxidized iron-bearing materials and coal and coke waste products in integrated steel plants is a problem which is coming under control. These waste products amount to about 10% by weight of the total steel output. Suitable agglomeration processes can directly recycle over 80% of this material as charge material to the iron blast furnace. Other recycling opportunities exist for the remaining materials.
The increase in coated products, particularly galvanized sheet materials, has created further challenges for recycling the dust collected in the steelmaking processes which have become enriched in zinc. Several viable processes for extracting zinc from these recovered dusts have become commercial.
Producers of stainless steel face serious environmental challenges in terms of alloy waste products. One specific concern is directed toward valence five chromium ions in waste products and product solutions.
The industry has made every effort to respond to these issues. New construction costs for environmental controls for steel production installations amount to over 35% of total capital investment. This issue in the USA has been especially burdensome for coking facilities which have not been renovated or reconstructed, but have been sent offshore, exporting pollution, and at the same time employment and capital generation.
Eloneva (5) performed calcium extraction experiments with blast furnace slag, desulfurization slag, ladle slag, and steel converter slag, as mentioned earlier. From blast furnace slag and ladle slag at maximum, 20% of the calcium was extracted with various solvents, while from desulfurization and steel converter slags over 50% extraction was obtained. This comparison shows the effect of the slag mineralogy on extraction of elements from the slag material. Since steel converter slag is produced in noticeably larger quantities than desulfurization slag, the research was concentrated on steel converter slag only (76). Chemical compositions of the different studied steel converter slag samples are shown in Table 10.6. As can be observed, composition fluctuations occur especially not only regarding the major elements calcium, iron, and silicon but also for certain minor components such as manganese and vanadium.
Calcium extraction efficiency of various solvents from steel slags was also investigated by Eloneva (5). It was found that although strong acids extract calcium efficiently, the reaction is not selective, as other metals are leached from the slags as well. Moreover, since calcium carbonate precipitation requires carbonate ions, present in high concentrations only at pH values above 9 (Figure 10.5), solutions of (strong) acids were found to be too acidic after extraction. Thus, the use of an additional base such as sodium hydroxide would be required to increase the pH enough to enable efficient carbonate precipitation (5). This goes against the goal of using a recyclable solvent, and thus, it was concluded to be infeasible to use (strong) acids as leaching agents for a carbonation process.
Bao et al. (10) studied the leaching of elements from steelmaking slag with acetic acid as a solvent. They used an organic solvent, tributyl phosphate (TBP), to extract acetic acid from the aqueous phase when it is formed by calcium acetate carbonation. It is reported that besides calcium, also magnesium, iron, and aluminum were extracted from the slag. Furthermore, the authors emphasize the fact that the TBP phase can be recovered (by distillation) and reused. However, the details of the carbonation of the formed calcium and magnesium acetate and recovery of the acetic acid are not discussed by the authors.
On the contrary, aqueous solutions of a weak acid and a weak base, such as ammonium nitrate, chloride, and acetate, were found out to selectively leach calcium from slags (5). This is partly due to the pH buffering nature of these solutions, maintaining solution pH around 910 instead of 1213 when using pure water. The higher solubilities of calcium nitrate, chloride, and acetate compared to calcium hydroxide are an important benefit. Ammonium chloride was chosen to be used as a reference case in the experimental work, since it is the cheapest of the salts mentioned, it is not considered to be an explosive (such as ammonium nitrate) and does not form vaporizing compounds such as acetic acid from ammonium acetate (76). Ammonium chloride solvent has also earlier been utilized for calcium extraction from steelmaking slags (53,77,78).
After choosing an efficient solvent, different solvent molarities and solid-to-liquid ratios of slag and the aqueous solvent were studied experimentally to achieve as economical a process as possible. It was observed that solvent molarities higher than 1.0mol/L NH4Cl resulted in unwanted extraction of elements such as iron and manganese, giving strong coloring. Concentrations lower than 0.5mol/L were both kinetically and stoichiometrically infeasible, since the reaction time to obtain good extraction efficiencies was increased and the amount of ammonium ions was also theoretically too low to extract all calcium from the steel slag (5,31,76). These findings are summarized in Figure 10.6.
As a comparison, pure water has been used as a leaching agent by, for example, (42,79), but due to the lower solubility of calcium hydroxide, this resulted in low calcium extraction. If water was used as a solvent, the solid-to-liquid ratio of slag and water should be noticeably lower than for ammonium salts; otherwise, the process would not effectively utilize the calcium source.
De Windt et al. (69) stated, actually referring to the long-term leaching not only of certain elements such as chromium and vanadium but also of calcium and iron, that the controlling factor of metals leaching from slags is the limited solubility of the secondary phases such as various silicates. Also, the study concluded that the dissolution rates of brownmillerite (Ca2FeAlO5)- and wstite-type phases are slower than of lime and larnite phases. Even though water was used as solvent, these results seem to be applicable also in case of ammonium salt-enhanced leaching (31).