World of Iron and Steel Slag Network

International Glossary of Terms 2020

Launched at the National Slag Association 102nd Annual Meeting

September 22, 2020



This glossary contains terms and phrases that are commonly used by those who are involved with the production, handling, management, storage, value add applications and use of iron and steel slags (ISS) across the globe.

Stakeholders within the iron and steel slags value chain have used these terms for many years, with other modern terms, e.g. co-product have been included over time which better define the essential role of slag production to the primary products of iron and steel.

The acronym, ISS, was first used in 2010 and subsequently globally adopted and encouraged through the World of Iron and Steel Slag Network (WoISS). 

This glossary of terms is an attempt to further standardise and reflect these terms in common usage across the industry.  Additionally, some allied product, environmental and regulatory terms associated with ISS are included.

Contributions, suggestions, corrections and or improvements to this ‘living document’ are welcome.  Please submit these to the Australasian (iron & steel) Slag Association which is currently the Hon. Secretary for the Network.


Aggregates – Material complying with the specified grading requirements for fine and coarse aggregates. It may be produced from rock, gravel, metallurgical slag or artificial stone Iron ore – An ore from which iron can be extracted. It is the primary raw material in the manufacture of steel. Iron ore occurs in most countries in varying forms of purity. Most iron produced in Europe is made from high purity ores from Australia, South Africa and Brazil which can contain over 70% iron.

Air- cooling – Process where the slag is allowed to cool down slowly by leaving it in contact with ambient temperature air.

Iron and Steel Slag (ISS) (also called ferrous slag) – A collective term to describe Blast Furnace Slag, Basic Oxygen Furnace Slag, Electric Arc Furnace Slag and Ladle Furnace Slag. #isslag

Air-Cooled Blast Furnace Slag (ABS) – After separating from the heavier iron, the slag is typically tapped or poured onto ground bays to air cool to form a crystalline aggregate.

Klöckner Oxygen Blowing Maximillanshuette Slag (KOBM slag) – is a co-product of the steel making process used in New Zealand. KOBM slag is typically light to medium brown in colour with a physical appearance of coarse sand, including fine size particles. Based on its chemistry, KOBM slag is pozzolanic with self-cementing properties. KOBM slag can be used as a feedstock for cement manufacture due to its pre-calcined nature, chemistry and contributions to reducing carbon emissions.

AOD Process – AOD stands for Argon Oxygen Decarburization, a refining process associated with the production of stainless steel. Most stainless steel is initially produced in an electric arc furnace before being transferred to a separate ladle furnace for refining to achieve the precise metallurgical content required – a process known as secondary metallurgy. In the AOD process, a mixture of argon and oxygen is blown through the molten steel in the ladle furnace, and the oxygen achieves the main objective of oxidizing unwanted carbon in the steel melt. But because the vital and expensive chromium contained in all stainless steels is also prone to oxidation and subsequent loss with the process slag, the argon is introduced to inhibit this reaction.

Ladle - A vessel for receiving and handling liquid steel. It is constructed of a refractory-lined steel shell.

Ash – is the solid residue from combustion processes. Industrial combustion processes mostly aim to extract energy from coal, oil or even domestic waste. The primary aim is therefore not to obtain valuable materials, as it is in metallurgical processes, where the focus is on obtaining a metal.

Ladle Furnace (LF) – A Ladle Furnace is part of the secondary metallurgy, where the steel is fine alloyed and temperature is adjusted for casting by graphite electrodes.

Hot Mix Asphalt (HMA) – A mixture of bituminous binder and aggregate with or without mineral filler produced hot in a mixing plant. It is delivered, spread and compacted while hot. The term Asphalt is an abbreviation for Asphaltic Concrete. SFS are commonly used in the manufacture of Asphalt.

Ladle Furnace Slag (LFS) (also called secondary metallurgical slag, according to REACH registration as steelmaking slag) – is a co-product of the steel-making process. The typically white slag often disintegrates due to a modification change in the mineralogy. It is used e. g. as a fertilizer.

Basic Oxygen Furnace (BOF) – also known as LD converter or Basic Oxygen System (BOS) is a steel making furnace, in which hot metal from the blast furnace – with some amounts of steel scrap- is converted into steel. High purity oxygen is blown through the molten bath to lower the carbon content. Fluxes are used to reduce other unwanted elements. Those fluxes combine with silicates and oxides to form liquid slag that floats on the surface of the hot metal.

LD Process – Linz-Donawitz process which is also known as basic oxygen process. Iron/hot metal which is still molten is transferred from the blast furnace to a large steel making vessel (LD-converter) and steel scrap and lime is added. Oxygen is blown over the metal to reduce the carbon content and specific materials are added to make various types of steel.

Basic Oxygen Furnace Slag (BOF slag) – also known as LD converter slag or Basic Oxygen System (BOS) Slag are slags from a steel making furnace which are defined as co-products. The slag is removed from the vessel after the exothermic refinement of molten iron and recycled steel in the presence of fluxes and oxygen. The slag is typically dark gray in color and characteristically harder than blast furnace slag (BFS) with a density approximately 20 - 25% greater than natural quarried aggregates or BFS. It crushes to a cubical shape.

LD Converter – A basic oxygen furnace. LD is short for Linz-Donawitz, named after the Austrian towns Linz and Donawitz (a district of Leoben) where the LD-process was developed.

Blast Furnace (BF) – A furnace used in the integrated metallurgical process in which iron ore, sinter or pellets are melted down under a hot air flow (enriched with oxygen), using Carbon in the form of coke and coke as a heating and reducing agent in the chemical process. As a result, as well a liquid hot metal ("pig iron") as a liquid slag are produced. Lime – Calcium oxide (CaO), a product derived from burnt (calcined) limestone. It is used as a fluxing agent in the iron and steel making operations. The flux, which may also include additions of dolomite or limestone combine with other non-metallic components and results in liquid slag.

Blast Furnace Slag Cement1,2  -- blast furnace slag cement can contain varying combinations ground granulated blast furnace slag and Portland cement clinker. For example in the European cement standard EN 197-1, it can contain 36–65% by weight (CEM III/A), 66–80% by weight (CEM III/B) or 81–95% by weight (CEM III/C) ground granulated blast furnace slag.

Note 1: For Australia AS3972 specifies minimum requirements for hydraulic cements including general purpose cement, general purpose limestone, and blended cement
Note 2: For the United States ASTM C595 / C595M - 20 specifies requirements blended hydraulic cements

Limestone - A natural sedimentary rock consisting primarily of calcium carbonate (CaCO3). It is used as building material, in the manufacture of lime, carbon dioxide, cement, as a fertilizer or as a fluxing agent in the iron and steel making operations.

Blast Furnace Lime (BFL) – Product from blast furnace slag obtained by milling the slag. As essential ingredients it contains calcium and magnesium silicates as well as oxides and is used as fertilizer.

Melter Slag – Melter Slag is a co-product of the steel making process that converts iron sand to iron by adding coal and limestone. Melter slag differs from other slags in that the major oxides (calcium oxide and magnesium oxide) do not occur in their free form in the slag. Unlike conventional blast furnace slag (BFS), the iron sand-based material cannot be granulated, which precludes its use as a cement replacement in concrete manufacture.

Blast Furnace Slag (BFS) – is a co-product of the iron-making process. Formed within a Blast Furnace with molten iron from iron ore in the reducing presence of heated air, coke/coal and additions like limestone. The resulting molten slag and iron, once removed from the furnace, are subject to further processing to form a crystalline aggregate (air-cooled blast furnace slag, ABS) or a glassy granulate (granulated blast furnace slag, GBFS).

Non-Ferrous Slag – Collective designation for slag that is produced in the form of molten rock during the manufacturing of lead, ferrochrome, copper, nickel, zinc and waelz oxide. Non-ferrous slag can solidify in a glassy or crystalline form

Bound Pavements – Pavements composed of granular materials incorporating sufficient amounts of binding agent(s) to produce significant flexural stiffness. ISS (see below) are commonly combined in varying proportions to create durable pavements.

Pelletisation – Molten slag is thrown through the air by a spinning drum and water is sprayed into the area from the sides and ceiling of the structure. Both air and water cool the slag. The smaller particles produced are glassy and are suitable for cementitious use. The larger lumps (greater than 10 mm) tend to be crystalline and thus used for aggregate.

CBR value – California Bearing Ratio (CBR) value is defined as the ratio of load required to cause a specified penetration of a standard plunger into the sample to the load required to produce the same penetration of same plunger into a standard aggregate sample. It is expressed as a percentage and is broadly used in design of the base and the sub-base material for pavement.

Polished Aggregate Friction Value (PAFV) – A measure, on scale of 0 to 100, of the resistance of an aggregate to polishing under the action of traffic as determined in standard laboratory tests. High values indicate good skid resistance.

Cement – Cement is a hydraulic binding agent, i.e. upon contact with water, it immediately produces strength-forming reaction products that are durable, both above water and under water. Cement is used for manufacturing concrete, mortar and other construction products. Globally, there are many different types of cement, which differ in terms of their composition, fineness and technical suitability for different applications. For example EN 197-1 for “standard cements” alone currently describes 27 different cement types and 9 different strength categories.

Portland Cement – A hydraulic material made by heating limestone, clay and other raw materials in a kiln and grinding the resulting material ("clinker") together with Sulphates to a powder.


Cementitious property A property of a material to gain strength when mixed with water. The chemical reaction is called hydration.

Portland Slag Cement – The main constituents of Portland slag cement are Portland cement clinker and ground granulated blast furnace slag. It is defined in the European cement standard EN 197-1. According to this standard, it contains 6–20% by weight (CEM II/A-S) or 21–35% by weight (CEM II/B-S) ground granulated blast furnace slag. The first German standard for Portland slag cement (known at the time as iron Portland cement) was published in 1909.  (See Blast Furnace Slag Cement)

Circular Economy (CE) – A circular economy (often referred to simply as "circularity") is an economic system or business approach aimed at eliminating waste and the continual use of resources. Circular systems employ reuse, sharing, repair, refurbishment, remanufacturing and recycling to create a close-loop system, minimising the use of finite resource inputs and the creation of waste, pollution and carbon emissions. The use of iron and steel slags contributes towards these goals. #circulareconomy

Registration, Evaluation, Authorisation and Restriction of Chemicals (REACH)  is a European Union regulation dating from 18 December 2006. REACH addresses the production and use of chemical substances, and their potential impacts on both human health and the environment.

Concrete – Concrete is a mixture of cement, aggregates (e.g. natural and slag aggregates), water, and supplementary cementitious materials (e.g. ground granulated blast furnace slag, mineral coal fly ash, silica fume or pozzolans) and chemical concrete additive agents. Initially soft and malleable, concrete hardens after curing above water or underwater, and can achieve extremely high strengths. Because of its flexible technical properties, its relatively low price, and the availability virtually anywhere on Earth of aggregates and water, concrete is the most commonly used construction material.

Secondary metallugical slag (SecMS) – see Ladle Furnace Slag

Converter Lime (CL) – Product obtained by milling of basic oxygen furnace slag (BOF slag) as well as by sieving of disintegrated basic oxygen furnace slag (BOF slag) or ladle furnace slag (LFS) from the treatment of non-alloyed steels. As essential ingredients it contains calcium and magnesium silicates as well as oxides and is used as fertilizer.

Silicate lime fertiliser – Basic slag from metallurgical processes such as blast furnaces, converters or ladle slag.

Co-Product1 (by-product2,3) – means a product that is intentionally generated during the manufacturing process or chemical reaction(s) of the primary product or process. Co-product may also be characterised as essential to the primary product or process, having a critical influence on the properties of the primary product, e.g. the manufacture of crude steel. Iron and Steel Slags are essential co-products to the production of pig iron and steel.  Common uses are slag cement and air-cooled iron blast furnace slag and steel slag aggregates.

Note 1: The term ‘co-product’ offers a more positive perspective and is in keeping with the concepts of circularity, an approach which seeks to promote the use one industry’s outputs as another industry’s raw material (e.  g. GGBFS use by cement industry). Ferrous slags are co-products during manufacturing of iron and steel.
Note 2: In USA and Australia the term by-product has occasionally been associated with waste  ,and/or recycled materials.
Note 3: In Europe, by-products are not to be considered wastes  if the following conditions are met:
(a) further use of the substance or object is certain;
(b) the substance or object can be used directly without any further processing other than normal industrial practice;
(c) the substance or object is produced as an integral part of a production process; and
(d) further use is lawful, i.e. the substance or object fulfils all relevant product, environmental and health protection requirements for the specific use and will not lead to overall adverse environmental or human health impacts.

Skid Resistance – The frictional resistance provided by the pavement surface to the vehicle tyres during braking of cornering manoeuvres. It is usually measured on wet surfaces. ISS typically have PAFV values above 70

Crude steel – The term means the first solid steel product upon solidification of liquid steel. Also known as raw steel.

Skimmer - A slag skimmer is special equipment for separation hot metal from liquid slag. Once the taphole in the blast furnace is drilled open, hot metal and slag flow down a deep trench called a "trough". The hot metal flows through a skimmer opening, down the "iron runners". Since the slag is less dense than iron, it floats on top of the iron, down the trough, hits the skimmer and is diverted into the "slag runners". The liquid slag flows into "slag pots".

Cubical Aggregate Particle – An aggregate particle which is approximately cube-shaped (length to thickness ratio ≤ 3).

Slag – Co-product of a ferrous or non-ferrous pyro-metallurgical process which is composed of those elements not required in the molten metal.

Decarburisation – Decarburisation means removal of carbon from iron or steel, usually by heating using water vapour, oxygen and carbon dioxide. This gives a low carbon composition in the outer layers of the steel, and hence a softer surface.

Slag cement – A hydraulic material including ground Portland cement clinker, ground granulated blast furnace slag (GGBS) and Sulphates.

Desulphurisation Slag – is produced due to the need to remove sulphur prior to the processing of iron to steel.

Slag Metallurgy – Slag metallurgy is concerned with all aspects of producing, curing, treating and recycling ferrous and non-ferrous slags. This includes measures in both the liquid and hardened state. The overall aim of these measures is to improve the physical and environmental properties of the slag, but also its thermal content.

Direct reduced Iron (DRI) – is a product of a direct reduction process where the removal of oxygen from iron ore occurs without melting, e. g. MIDREX® or ENERGIRON® process.

Stainless Steel Slag – Slag that is produced during the manufacture of high-alloyed steel (“stainless steel”). See Electric Arc Furnace Slag from stainless/high alloy steel process (EAF-S)

Dolomite – A natural sedimentary rock consisting principally of the mineral dolomite [CaMg(CO3)2]. It is used in the manufacture of cement, as a building material, as a fertilizer and as a fluxing agent in the iron and steel making operations.

Steel Slag, Steel Furnace Slag (SFS) – A collective term to describe co-products of steel production processes in Basic Oxygen Furnaces, Electric Arc Furnaces or Secondary metallurgical vessels.

Electric Arc Furnace (EAF) – An electrical furnace in which the thermal effect of an electric arc is used to smelt materials like steel scrap or DRI. Once the furnace is charged and covered, graphite electrode(s) are lowered through holes in the roof. The electric arc travelling between the electrodes and the metallic charge creates intense heat which melts the charge. Alloying elements can be added during the process.

Steelmaking Slag (SMS) – see Ladle Furnace Slag

Electric Arc Furnace Slag (EAF slag) – is a co-product of the steel-making process. EAF slag solidifies in a similar manner to BFS. Its cooled structure is best described as a composition of silicates and oxides. It has a density about 20 - 25% greater than basalt or BFS. It crushes to a cubical shape. EAF slag is to be distinguished in slag from carbon steel process (EAF C) and from stainless/high alloy steel process (EAF S).

Thomasphosphate – Basic slag (Thomas slag) from the "Thomas process", a process of dephosphorisation of pig iron (hot metal), with a phosphate (P2O5) content of 10 - 15 %. Nowadays, the Thomas process is displaced by other smelting processes using low phosphate iron ores. Consequently, Thomasphosphate does not arise anymore.

Ferrous Slag – see ISS

Toxicity Characteristic Leaching Procedure (TCLP) – is a soil sample extraction method for chemical analysis employed as an analytical method to simulate leaching through a landfill.

Granulation – The vitrification of molten slag by treating with large volumes of (normally) high pressure water. The water both instantaneously cools and breaks up the slag into granules up to 5 mm. Also, dry granulation technologies are existing, e.  g. air granulation or rotating cup

Vesicular Slag – The term vesicular means the particles contain voids which tend to be unconnected to each other, occurring throughout each particle and appearing as blind holes on the particle surface.

Granulated Blast Furnace Slag (GBFS) – The glassy granular material resulting from the rapid quenching of molten blast furnace slag by water. In contrary to air-cooled blast furnace slag GBFS is a latent-hydraulic material. UVCB Substance – Substances of Unknown or Variable composition, Complex reaction products or Biological materials. For identification and naming in REACH, substances are divided into “Well defined substances” and “UVCB substances”. For historical reasons slags are registered as UVCB substances.
Ground Granulated Blast Furnace Slag (GGFBS) – The glassy granular material resulting from the rapid quenching of molten blast-furnace slag which is ground to a powder with specified fineness. GGBS is a latent-hydraulic material and it is used a cement constituent or as a concrete addition.

VD process – Vacuum Degassing process, a secondary steelmaking operation in which the molten steel is exposed to vacuum. The aim is to improve the steel quality by removing gases, improving the steel cleanliness and obtaining a greater uniformity of the chemical composition.

Heavily Bound Pavement – A bound pavement layer having an Unconfined Compressive Strength (UCS) value greater than 4 MPa.

Vitrification – Making glassy substance by rapidly cooling molten slag by aid of water or air. The aim is to minimise crystalline content. Slag for cementitious purposes can only be composed of vitrified (glassy) material. Otherwise no latent hydraulic property is given.

Hot metal – Hot, liquid, metallic iron product obtained upon reduction of iron ore in a blast furnace (BF). It proceeds to the basic oxygen furnace (BOF) in molten form or is cast as pig iron.

VOD process – Vacuum Oxygen Decarburization. A refinement of stainless steel that reduces carbon content. Molten, unrefined stainless steel is heated and stirred by an electrical current while oxygen enters from the top. Many undesirable gases escape from the stainless steel and are evacuated by a vacuum pump. Alloys and other additives are then mixed in to refine the molten stainless steel further.

Hydraulic property - After mixing with water a hydraulic material (e. g. Portland cement) forms reaction products resulting in strength and being durable above and under water. A latent hydraulic material like GGBS does the same, but in addition it needs an activation in order to achieve a hydraulic reaction within an acceptable time.

Volume Stability – The ability of treated or weathered slag not to increase in volume by the interaction of water and lime.


Weathering – Subjecting stockpiles of slag to be exposed to normal weather with the principal objective of getting water to react with calcium oxide inclusions.


Weathered Slag – Steel Slag is considered to be weathered when it has been exposed in its finished form to both atmospheric and controlled conditions to allow the full hydration of any free lime present.