- Written by Howard Epstein, RVA
Stainless steel slags generated in melting and refining operations are CaO-SiO2-MgO rich with Cr2O3, Al2O3 and F- in minor quantities. A major issue is collapse of the slag structure on cooling as the high dicalcium silicate (C2S) content undergoes a phase transformation from β-C2S (monoclinic) to γ-C2S (orthorhombic). This phenomenon is accompanied by a volume increase of around 12%. Consequently, slag handling and storage are problematic. Furthermore, structural collapse prevents the realisation of commercial value of the slag. β to γ conversion of only 4% slag by weight is sufficient to cause the dusting phenomenon. CSM slags are often treated with borates to prevent β to γ transformation of C2S. However, borates are expensive and health concerns may limit their use in the future. Valoxy®, an alumina-rich material derived from the recycling of aluminium salt slags, offers an alternative route to CSM slag stabilisation in which the formation of C2S is prevented altogether.
- Written by Written by Lewis Juckes
Slags from the Iron and Steel Industry
With 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.
- Written by Nick Jones
Applications of iron and steelmaking slag products - by MPA Slag
Blast Furnace Slag (Iron Making Slag)
Granulated Blast Furnace Slag (GBS)
Granulated Blast Furnace Slag can be used in its raw/unprocessed form as a slow setting binder in hydraulically bound road construction materials. It generally requires an alkali activator such as hydrated lime or steel slag to be added to the product for this purpose.
Ground Granulated Blast Furnace Slag (GGBS)
Granulated blastfurnace slag is highly cementitious and after drying and grinding to a fine powder, it can be used as a replacement for conventional cement. The first commercially available Blastfurnace cement (a blend of Portland cement and GGBS) was produced in Germany in 1865 and currently over 200 million tonnes/annum of Blastfurnace slag cement are used worldwide.
In the UK, GGBS is normally supplied as a separate material, conforming to EN 15167-1 ‘Ground granulated blastfurnace slag for use in concrete, mortar and grout’. GGBS is blended with cement at the concrete mixer and its major use is in ready-mixed concrete. Typically the replacement level is about 50% but in some applications, GGBS can replace over 80% of the Portland cement. Specifiers of concrete are well aware of the many benefits of GGBS (e.g. it increases the durability of concrete and greatly reduces its carbon footprint) and about a third of all UK ‘ready-mix’ deliveries contain GGBS. It is also used in site-batched and precast concrete and for the in-situ stabilisation of soils.
- Written by Oktay Kutlu, Meriç Demiriz OYAK Group Adana Cement Industry, İskenderun Plant
Researchers at Adana Çimento in Turkey have undertaken a thorough investigation into the relative strengths of concretes made with normal Portland cement (CEM I) compares to those derived from GGBFS-based cements (CEM III). The results of this investigation are presented in this paper, with the central conclusion being that concretes made with cements containing up to 65% by weight of GGBFS, have improved strength, lower unit costs and a more benign environmental footprint compared with concretes made from CEM I cements.
- Written by Barry Woodbine Aumund Group
In this paper Aumund, recent winner of the Global Slag Equipment Innovation Award 2007, showcases its broad range of GBFS and GGBFS handling, conveying and storage options.
During recent years the cement industry has suffered significant increases in operating costs driven in particular by spiralling energy prices plus pressure from environmental lobbies to reduce dependence on fossil fuels and reduce CO2 emissions overall. Considering that the production of a ton of conventional Portland cement generates almost a ton of CO2, and in terms of total greenhouse gas emissions for all industries worldwide, this places cement second only to the power industry in the scale of global polluters.
These factors have generated an increased awareness of alternative fuel possibilities for kiln firing and substitute raw material options for cement production, also reflecting an increased demand for blended cements particularly including ground granulated blast furnace slag (GGBFS). This last point is particularly important; for every ton of GGBFS included in the final blended cement the total CO2 production is reduced by around 1t.