Q & A: A concise guide to ferrous slag
Our FAQs provide a brief, summarised overview of key questions and answers regarding ferrous slag, covering the topics of ‘Origin, Production and Composition’, ‘Technical Properties, Quality and Uses’, and ‘The Environment and Resource Conservation’.
WHAT ARE FERROUS SLAGS?
Ferous slag is a metallurgical by-product of hot metal and steel production that is essential to the manufacturing process. The products derived from it are used as construction materials in the cement and concrete industries, as industrial aggregates in various transport infrastructure applications, and as a calcareous fertiliser (converter lime) in agriculture. Depending on the metallurgical process, a distinction is made between blast furnace slag (air-cooled blast furnace slag, granulated blast furnace slag) and steel furnace slag (basic oxygen furnace slag, electric arc furnace slag from carbon steel production/from stainless steel production, secondary metallurgical slag).
WHERE DOES THE TERM ‘SLAG’ COME FROM?
Slag is a metallurgical term. Etymologically, it derives from the Low German word ‘slagge’ (meaning ‘to strike’) and originates from medieval metal extraction in a bloomery furnace, where metal and slag had to be separated from a doughy lump using wooden hammers.
HOW IS FERROUS SLAG FORMED?
As a product of a smelting process, slag consists mainly of the non-metallic components of the ores and mineral additives which are mixed with the ore to lower the melting point. Blast furnace slag is produced during the manufacture of hot metal in a blast furnace, where iron ore and other materials are melted at around 1,500 °C. Steel furnace slag forms during the production of crude steel in the molten stream at a temperature of approximately 1,600 °C, either in an LD steelworks (Linz-Donawitz process) from hot metal, steel scrap and other materials (basic oxygen furnace slag) or in an electric steelworks, predominantly from steel scrap and other materials (electric arc furnace slag; from carbon steel production abbreviated to EAF C, from stainless steel production abbreviated to EAF S). During the secondary processing of crude steel, which is necessary to produce the desired steel grades, secondary metallurgical slags (SMS) are produced. These slags perform important metallurgical functions, such as absorbing trace elements that are undesirable in the metal.
HOW IS FERROUS SLAG FORMED?
As a product of a smelting process, slag consists mainly of the non-metallic components of the ores and mineral additives which are mixed with the ore to lower the melting point. Blast furnace slag is produced during the manufacture of hot metal in a blast furnace, where iron ore and other materials are melted at around 1,500 °C. Steel furnace slag forms during the production of crude steel in the molten stream at a temperature of approximately 1,600 °C, either in an LD steelworks (Linz-Donawitz process) from hot metal, steel scrap and other materials (basic oxygen furnace slag) or in an electric steelworks, predominantly from steel scrap and other materials (electric arc furnace slag; from carbon steel production abbreviated to EAF C, from stainless steel production abbreviated to EAF S). During the secondary processing of crude steel, which is necessary to produce the desired steel grades, secondary metallurgical slags (SMS) are produced. These slags perform important metallurgical functions, such as absorbing trace elements that are undesirable in the metal.
HOW IS FERROUS SLAG PROCESSED?
Most of the blast furnace slag is converted into glassy, fine-grained sand through rapid cooling, usually by granulation with water. A smaller proportion is poured into slag beds and solidifies into crystalline air-cooled blast furnace slag, which must be crushed and screened for further processing as aggregate or as a aggregate mixture. This is also standard practice for steel furnace slag.
WHAT IS THE COMPOSITION OF FERROUS SLAG?
Yes! The decarbonisation of steel production is giving rise to ‘new slags’. Their potential applications as a construction material in cement and concrete, as well as in transport infrastructure, as fertiliser or in new fields of application are currently being researched, e.g. in the joint projects “SAVE CO2” and “DRI-EOS” coordinated by the FEhS-Institute, which focus on “granulated blast furnace slag 2.0” as an alternative, CO2-saving clinker substitute for the cement industry.
ARE SLAGS AND ASHES THE SAME THING?
No! But the term ‘slag’ is often used incorrectly. Ashes are residues from thermal combustion processes, e.g. waste incineration ashes and coal ashes. These are often mistakenly referred to as ‘slag’ as well. However, the formation and the physical, chemical and mineral characteristics of ashes are not comparable to those of ferrous slag.
WHERE IS FERROUS SLAG USED?
In Europe, around 38.0 million tonnes of ferrous slag are produced annually during the manufacture of hot metal and steel. Of this, 35.0 million tonnes, or 92%, are used as high-quality products in various applications (2024).
20.0 million tonnes of granulated blast furnace slag are mainly used as a cement component in concrete production. In addition to numerous everyday construction projects, concrete containing blast furnace slag has proven particularly effective where high technical requirements are imposed, e.g. in industrial plants, mobile phone masts, bridges, sewage treatment plants, biogas plants, coal silos, drinking water pipes and chimneys.
12.5 million tonnes of aggregate are used in transport infrastructure construction (roads, waterways, railways), for example in motorways and locks. Applications include road surfaces, e.g. asphalt, and the underlying layers.
Around 0.8 million tonnes of converter lime are used as fertiliser in agriculture, with 0.1 million tonnes used for other applications. For internal circulating flow within the plants, 1.6 million tonnes of ferrous slag are used in steelworks as a substitute for primary raw materials.
WHAT ARE THE ADVANTAGES OF FERROUS SLAG?
Ferrous slag offers numerous technical, environmental and economic advantages:
Cement and concrete
The outstanding technical properties of cements and concretes containing granulated blast furnace slag include high strength, high durability, high resistance to chemical stress, and excellent resistance to frost and heat. Slag based cements are suitable for all applications. The use of granulated blast furnace slag in cement instead of Portland cement clinker based on primary raw materials also significantly reduces the environmental footprint of cement and concrete by lowering CO2 emissions and saving natural raw materials and fuels used in clinker production.
Transport infrastructure
Transport construction materials containing aggregates made from steel furnace slag or air-cooled blast furnace slag remain stable for a long time, have a long service life, possess enormous load-bearing capacity and are resistant to heat. They can be processed all year round, as they are not affected by weather conditions, and can be used in layers with or without binders. Asphalt surfaces containing ferrous slag minimise the formation of ruts and offer excellent grip even in wet conditions. Unbound layers of ferrous slag are immediately open to traffic after laying and compaction and have an enormous load-bearing capacity.
The result: safe, durable transport routes, fewer roadworks and, consequently, savings in both time and costs.
Fertiliser
Lime fertiliser (converter lime) produced from ferrous slag is used to neutralise soil acids and stabilise an appropriate pH level in the soil, thereby maintaining long-term soil health. In addition to the alkaline components CaO and MgO in a silicate bond, converter lime contains numerous nutrients important for plants, such as calcium, magnesium and manganese, as well as a high content of soluble, plant-available silicic acid. This contributes in a special way to nutrient mobilisation, the stabilisation of soil structure and plant health. For decades, converter lime has ensured healthy, species-rich and high-yielding soils whilst protecting the environment. Converter lime can be used on all soil types and for all crops.
IS FERROUS SLAG AS RELIABLE AND SAFE AS COMPARABLE PRODUCTS?
Yes. Like all other raw materials and products, ferrous slag is subject to constant monitoring and is approved for various applications in accordance with national and international regulations. Furthermore, the sustained high global demand from industry confirms the excellent quality of the slag and slag-based products.
Example: granulated blast furnace slag
Granulated blast furnace slag replaces Portland cement clinker on a 1:1 basis in cement. Its use as a component of ordinary cement is regulated by the EN 197-1 standard. This standard defines both the minimum material requirements and the mixing ratios for the various types of cement. Furthermore, there are additional standards for specific types of cement, e.g. for sulphate-resistant cement.
Example: road construction materials
Air cooled blast furnace slag and steel furnace slag for use in road construction are subject to the same requirements as natural aggregates. The regulations are based on European standards, in particular EN 12620, EN 13043, EN 13242 and EN 13285.
Example: hydraulic engineering
Coarse stones made from steel furnace slag are ideally suited for hydraulic engineering due to their high specific gravity. They must comply with the requirements EN 13383-1 covering not only natural aggregates but also artificial hydraulic engineering stones.
SO, DOES THIS MEAN THAT PRODUCTS MADE FROM FERROUS SLAG ARE EQUIVALENT ALTERNATIVES IN TENDERS?
Yes! Nevertheless, there are still frequent tenders that disadvantage secondary building materials such as slag-based materials from the steel industry. For example, in transport infrastructure construction, there are unfortunately still tenders that explicitly permit only natural aggregates. The ambitious societal goals of promoting a circular economy and conserving resources cannot be achieved in this way. Therefore – provided they are technologically suitable, meet environmental sustainability requirements and are economically viable – by-products and other secondary building materials should be given preference in tenders. This should apply in particular to public sector tenders.
ARE FERROUS SLAGS ‘PRODUCTS’ OR ‘WASTE’?
The term ‘industrial by-product’, commonly used in professional circles, makes it clear that ferrous slags are not waste within the meaning of the European Waste Framework Directive. This is also confirmed by numerous statements from authorities and legal opinions. Furthermore, since 2010, ferrous slag has been registered as a non-hazardous substance under the European REACH chemicals regulation – an essential prerequisite for classification as a product or by-product.
CAN THE ‘NEW SLAGS’ BE USED IN THE SAME WAY AS BEFORE?
In order to ensure that the by-products of decarbonised steel production can continue to be used in a resource- and climate-friendly manner in the future, regulations must be adapted in addition to the research currently underway. To this end, the FEhS-Institute has, among other things, drawn up a proposal to amend the regulations governing their use in cement and concrete, which is being discussed with the regulatory bodies and partners in the value chain.
HOW DO FERROUS SLAGS HELP CONSERVE NATURAL RESOURCES?
Ferrous slag replaces primary raw materials, such as natural rock and quartz sand. Between 2000 and 2024, this amounted to over 1.2 billion tonnes across Europe. The use of slag-based products from the steel industry has thus helped to prevent massive disruption to the landscape.
DOES THE USE OF FERROUS SLAG REDUCE EMISSIONS?
Yes. By substituting Portland cement clinker with granulated blast furnace slag in cement production, a total of over 320 million tonnes of climate-damaging CO2 were avoided in Europe between 2000 and 2024. The use of converter lime as a lime fertiliser also prevents CO2 emissions, which are otherwise unavoidable when carbonate or oxide lime products are incorporated into the soil.
CAN STEEL FERROUS SLAG HARM WATER, SOIL AND PLANTS?
No! The concentration of environmentally relevant components in ferrous slag is low and comparable to that found in natural rock. Numerous studies confirm that neither the use of fertilisers containing slag nor the use of building materials with ferrous slag leads to any negative impact on the environment. Long-term trials confirm the safety of ferrous slag in agriculture. Other studies on the condition of trees along roads found no evidence of damage resulting from the use of ferrous slag as a road construction material. The aforementioned quality control mechanisms for building materials also guarantee the environmental compatibility of ferrous slag.
When assessing the environmental compatibility of secondary mineral building materials, such as steel furnace slag, the focus is on compliance with defined limit values for the leaching of water-soluble components. Such limits serve to protect groundwater and surface water. As part of a comprehensive scientific study, the effects of the solid content of steel furnace slag used in open road construction on the surrounding near-natural soils were investigated. It was demonstrated that, even in the worst-case scenario, only very low levels of heavy metal contamination can occur in the surrounding topsoil, limited to a maximum distance of a few metres, and that there is no cause for concern regarding the occurrence of harmful soil changes. It was also demonstrated that steel furnace slag used in unbound construction poses no risk to the soil-to-human exposure pathway. Due to the very high temperatures at which it is produced, steel furnace slag cannot inherently contain any organic pollutants
BUT DOESN’T FERROUS SLAG CONTAIN CHROMIUM?
Yes! But the latest scientific studies confirm earlier findings: steel furnace slag contains only harmless chromium(III) compounds, a trace element that is essential for humans and animals. No traces of the harmful chromium(VI) have been found in ferrous slag.
HOW IS IT PROVEN THAT FERROUS SLAGS ARE SAFE?
Ferrous slags have been subject to extensive toxicological and ecotoxicological investigations within the framework of the European chemicals legislation REACH and were registered as UVCB substances (“Unknown or Variable Composition, Complex Reaction Products or Biological Materials”). As part of the REACH registration process, different potential exposure pathways were assessed, including inhalation of respirable dust particles, oral exposure, as well as skin and eye contact. For this purpose, comprehensive in vitro and in vivo studies were conducted.
The available data indicate that ferrous slags are comparable to natural rocks. The studies performed for the REACH registration, including investigations on slags and slag eluates, did not identify hazardous properties requiring classification at the time of registration and support the conclusion that ferrous slags can be safely used when handled in accordance with applicable regulations.
Manufacturers and the REACH Ferrous Slag Consortium (RFSC) continuously work on updating the available data and ensuring the safe use of ferrous slags in accordance with applicable European legislation.
