Properties and Uses of Ironmaking slag


Properties and Uses of Ironmaking slag

The majority of iron in the world is produced in the blast furnace (BF) and hence BF slag represents the largest quantity of ironmaking slag produced around the world. The BF is the primary means for reducing iron (Fe) oxides to molten, metallic iron. It is continuously charged with Fe oxide sources (ore, sinter, and pellet etc.), fluxes (limestone, and dolomite), and fuel (coke, and coal). Liquid iron collects in the bottom of the furnace and the liquid slag floats on it. Both are periodically tapped from the furnace.

BF slag is defined by the American Society for Testing and Materials (ASTM). It defines BF slag as the non-metallic product consisting essentially of silicates and alumino-silicates of calcium and other bases which is developed in a molten condition simultaneously with iron in a BF. The slag consists primarily of the impurities from the iron ore, mainly silica (SiO2) and alumina (Al2O3), combined with calcium (Ca) and magnesium (Mg) oxides from the fluxes. Sulphur (S) and ash which normally come from coke and coal are also contained in the slag. Slag comes from the furnace as a liquid at temperatures of around 1500 deg C. It is a man-made molten rock, similar in many respects to volcanic lavas.

Chemical and mineralogical composition of BF slag

Chemical analysis of BF slag normally consists of four major oxides namely (i) SiO2, (ii) Al2O3, (iii) calcium oxide (CaO), and (iv) magnesia (MgO). These oxides make up around 95 % of the total quantity. Minor elements which are present in the slag are Fe, S, manganese (Mn), alkalis, and trace amounts of several other elements. Common composition range of various components of BF slag is given in Tab 1.

Tab 1 Range of chemical analysis of BF slag
Component Unit Value
Silica (SiO2) % 30 – 40
Alumina (Al2O3) % 9 – 22
Lime (CaO) % 31 – 42
Magnesia (MgO) % 6 – 14
Manganese oxide (MnO) % 0.1 – 1.1
Iron oxide (FeO, Fe2O3) % 0.1 – 1.9
Sulphur (S) % 1 – 2

The chemical composition of the BF slag is dependent upon the composition of the available iron ore, fluxes, and fuels, and on the proportions required for the efficient operation of the BF. The BF is to be charged with uniform raw materials for consistent quality of the liquid iron produced. This also ensures uniformity in the composition of the produced BF slag, and as a result the composition of the BF slag from a given source varies within relatively narrow limits. Higher amount of variations, as shown in the overall ranges in the Tab 1, are found between sources where different raw materials are being used.

Slag which is cooled rapidly (water quenching) after emerging from the BF tends to form a glassy, non-crystalline material.  This slag is known as granulated BF slag. Slower cooling of liquid BF slag (air cooling) leads to crystallization of a number of minerals. The most common minerals are shown in Tab 2.

Tab 2 Minerals of air cooled BF slag
Mineral Chemical formula
Wollastonite CaO.SiO2
Di-calcium silicate 2CaO.SiO2
Anorthite CaO.Al2O3.2SiO2
Gehlenite 2CaO.Al2O3.SiO2
Monticellite CaO.MgO.SiO2
Akermanite 2CaO.MgO.2SiO2
Merwinite 3CaO.MgO.2SiO2

Melilite, the name applied to any of the continuous series of solid solutions formed by akermanite and gehlenite, is the most common mineral in slag. The other minerals which are present or absent in the BF slag depend upon the relative proportions of the major oxides in the slag. Most of the BF slags contain no more than four of the above minerals. The mineral dicalcium silicate can form in slags which are high in lime (CaO), and cause disintegration upon cooling by a volume increase when changing from one crystalline form to another. The S in the air cooled slag generally appears as sulphides of Ca, Fe, and Mn.

The chemical composition of BF slag is a major factor in the potential performance of the granulated slag in cementitious uses. For chemical uses, such as a raw material for manufacture of glass or mineral wool insulation and for agricultural applications, the chemical composition is also quite important. While of lesser importance in aggregate uses, the chemical composition does directly affect the slag viscosity and the rate of crystallization during cooling, and thus influences the porosity and the character and size of crystals in the solidified slag.

During the slag processing, the chemical composition cannot be controlled. It is dependent upon the available raw materials and requirements for an efficient BF operation. BF operator can only modify the slag properties to a limited extent by changing cooling conditions. Hence, it is lucky that the range of slag compositions associated with good iron production is all useful construction materials, although varying in physical properties.

However, there are factors of performance as related to the composition. These are described here. As stated earlier, slow-cooled, high-lime slags normally form di-calcium silicate which undergoes a volume increase on cooling to ambient temperatures. This results in the ‘dusting’ or ‘falling’ of the slag, plainly reducing it to a powder. Compositional changes can be readily made to avoid such problems, which are basically of concern only to the producers whose potential product is destroyed.

Experience shows that disintegration usually occurs, in susceptible slags, prior to construction use and is not a problem for the ultimate user. Rapid cooling also prevents any problems in slags destined for cementitious or chemical uses by preventing crystallization of the di-calcium silicate.

Iron unsoundness is a rarely encountered problem. Slags high in Fe oxides can, with appropriate levels of other components, form compounds which readily react with water, with resultant disintegration of the slag. In slags with normal ranges of Fe contents, it is not a problem. In other slags it is easily detected by immersing the slag in water.

Sulphur has long been looked upon as an undesirable constituent of slag, mainly because of doubts that one reaction or another is possible or can cause a problem of some kind. In actual practice no correlation of S content with performance appears to exist. Some leaching of S compounds from uncoated slag does occur. The leachates are not toxic in nature, occur only under poor drainage conditions, and are temporary or transient in existence.

In certain situations, the leachate from the BF slag may be discoloured (characteristic yellow/green color) and have a sulphurous odour. These properties appear to be associated with the presence of stagnant or slow moving water which has come in contact with the slag. The stagnant water generally shows high concentrations of Ca and sulphide, with a pH as high as 12.5. When this yellow leachate is exposed to oxygen (O2), the sulphides present react with O2 to precipitate white/yellow elemental S and produce calcium thiosulphate, which is a clear solution. Aging of the BF slag can delay the formation of yellow leachate in poor drainage conditions but does not appear to be a preventative measure, since the discoloured leachate can still form if stagnant water is left in contact with the slag for an extended period.

BF slag is mildly alkaline and shows a pH in solution in the range of 8 to 10. Although BF slag contains a small component of elemental S (1 % to 2 %), the leachate tends to be slightly alkaline and does not present a corrosion risk to steels in pilings or to the reinforcement steels embedded in concrete structures made with BF slag cement or aggregates.

Physical properties of BF slag

BF slag is tapped from the furnace as a liquid, which contains gases held in solution. The density of liquid slag at around 1450 deg C can vary in a range of 2.68 tons/cum to 2.84 tons/cum. The volume of gases (carbon di oxide, nitrogen, and oxygen) dissolved in 1 cubic metre of liquid BF gas at around 1500 deg C can reach 1 cubic metre.

The conditions of cooling control both the growth of mineral crystals and the quantity and size of gas bubbles which can escape before being trapped by solidification of the slag mass. Thus, within the limits imposed by the particular chemical composition, the cooling conditions determine the crystalline structure and the density and porosity of the solidified slag. Dependent upon the cooling methods employed, any of three distinctly different types of product can be made from the liquid BF slag (Fig 1). These are (i) air cooled slag, (ii) granulated slag, and (iii) Expanded slag also known as foamed slag.

Fig 1 Types of BF slag

Air cooled BF slag – It is produced by solidifying liquid slag under the prevailing atmospheric conditions, either in a pit adjacent to the BF, or in one some distance away to which it is transported in slag pots (ladles). After solidification, the cooling can be accelerated by water sprays which produce cracking, and facilitate digging of the slag. The solidified slag product is primarily crystalline in nature. It has textures ranging from rough, vesicular (porous) surfaces to glassy (smooth) surfaces with conchoidal fractures. The cellular or vesicular structure results from bubbles of the gases which have been dissolved in the liquid slag. After cooling, the solidified slag is dug, crushed, and screened to the required sizes. Metallic Fe in the slag is removed by powerful magnets in the crushing and screening plant.

There can be considerable variability in the physical properties of the air cooled slag depending on the production process. For example, some samples of the air cooled  slag has reported to have a compacted unit weight as high as 1940 kg /cum. Higher unit weights, if they are reported, are generally due to increased metals and iron content in the slag. The water absorption of air cooled can be as high as 6 %. Although air cooled slag can show these high absorption values, the slag can be readily dried since little water actually enters the pores of the slag and most of it is held in the shallow pits on the surface.

The air cooled slag crushes to angular, roughly cubical particles with pitted surfaces. Very good bonding of the slag is achieved either with the hydraulic cements or with the bituminous binder materials. High internal friction values and particle interlock provide excellent stability when used without cements. Bulk density and unit weight are dependent upon grading and particle size. The larger particles contain more internal cells or vesicules and have a lower bulk density. The coarse sizes can have bulk densities as much as 20 % lower than natural aggregates with the same gradation, while the fine material (passing a sieve of 4.75 mm size) is nearly equal to natural sand in density. The aggregate is highly resistant to weathering effects, and does not readily polish to produce slippery surfaces.

The air-cooled slag coarse aggregates are normally categorized with crushed stones and gravels as ‘normal weight’ aggregates. They are used for all types of construction applications similar to the natural aggregates. However, the weight saving in the case of slag becomes a significant factor in some applications. Further, because of their more porous structure air cooled slag aggregates have lower thermal conductivities than conventional aggregates. Their insulating value is of particular advantage in some applications.

Processed air cooled slag shows favourable mechanical properties for aggregate use including good abrasion resistance, good soundness characteristics, and high bearing strength. The typical mechanical properties of air cooled slag are as given below.

  • Los Angeles Abrasion (ASTM C131) – 35 % – 45 %
  • Sodium Sulphate soundness loss (ASTM C88) – 12 %
  • Angle of Internal Friction – 40-45 deg
  • Hardness – 5-6 Moh’s scale of mineral hardness
  • California Bearing ratio (CBR) – Up to 250 % for maximum size of 19 mm

Granulated BF slag – It is produced by the quick quenching (chilling) of liquid slag to produce a glassy, granular product. The most common process is quenching with water, but a combination of air and water can be used. Since the quenching being a very rapid cooling process, only a very small amount of mineral crystallization is taking place. The granulated slag can vary from a friable, popcorn-like structure to small, sand-size grains resembling a dense glass, depending upon the chemical composition, temperature at the time of quenching, and the cooling rate. The colour of granulated BF slag ranges from beige to dark to off white and it depends on the moisture content, chemistry and efficiency of granulation. When it is ground it has usually white colour.

Characteristics such as colour, moisture content, bulk density, porosity, grain shape, grading curve and grindability of BF slag are affected by different chemistry, melting temperature and the granulation process conditions. Depending on different chemistry, granulation methods and granulation parameters the morphology of granulated slag particles can vary from a dense structure without porosity to a very porous friable form. In general, the particle shape is sharp edged with occasionally elongated needle shaped forms. The physical properties of granulated slag are given in Tab 3.

Tab 3 Physical properties of granulated slag
Parameter Unit Value
Glass content volume % 70 – 100
True density grams/cum 2.8 – 3.1
Apparent density grams/cum 2.0 – 2.8
Bulk density grams/cum 0.7 – 1.4
Porosity volume % 2.5 – 31.2

Granulated BF slag is a glassy granular material that varies, depending on the chemical composition and method of production, from coarse popcorn like friable structure greater than 4.75 mm (No. 4 sieve) in diameter to dense sand size grains passing a 4.75 mm sieve. Granulated slags can be crushed, graded or ground for specific applications. When crushed or milled to very fine cement sized particles, ground granulated BF slag has cementitious properties.  This property of slag makes it a suitable partial replacement for or additive to Portland cement.

The granulated slag glass contains the same major oxides as does the Portland cement, but with considerably different proportions of CaO and SiO2. Like Portland cement, it has very good hydraulic properties and, with a suitable activator, such as calcium hydroxide [Ca(OH)2], it sets in a similar manner.

When the granulated slag is ground to the proper fineness, the chemical composition and glassy (non-crystalline) nature of vitrified slag is such that in combination with water, the vitrified slag reacts and forms cementitious hydration product. The magnitude of these cementitious reactions depends on the chemical composition, glass content, and fineness of the slag. The chemical reaction between the ground granulated slag and water is slow, but it is greatly enhanced by the presence of Ca(OH)2, alkalis and gypsum (CaSO4).

Expanded BF slag – It is also called ‘foamed slag’. It results from treatment of liquid slag with controlled quantities of water, less than what is needed for granulation. A number of pit and machine processes have been developed to combine the liquid slag with water, or with water and air, or steam. The resulting product is more cellular or vesicular in nature than the air cooled slags, and is much lighter in unit weight.

Variations in the amount of water and the process used control the cooling rate, and can result in product variations from highly crystalline material resembling very vesicular air cooled slag to glassy material closely similar to granulated slag. A pelletizing process has been recently developed which uses limited amounts of water followed by chilling of slag droplets thrown through the air by a rapidly revolving finned drum. This produces spherical pellets of highly glassy slag.

Expanded slag particles, depending upon the processing procedure, either can be angular and roughly cubical in shape or can be spherical and smooth surfaced. The cellular structure results in densities in the lightweight aggregate categories. Many of the expanded slags possess cementitious properties attributable to high glass content.

Uses of BF slag

Uses of BF slag which have been developed date back many centuries. BF slag has been used in road building as long ago as in the year 1830 and as railroad ballast since the year 1875. The use of BF slag as concrete aggregate began in the year 1880 and in bituminous surfaces in the early 1900s. Major development of slag uses is in the construction aggregate applications, with much smaller amounts going into more specialized applications such as cement manufacture and agricultural applications. Almost all the produced BF slag is being used in some application. This level of commercial application has been reached on a competitive basis with other materials. The slag is being used either because it can provide equal performance at a lower cost, or better performance for similar cost.

Air-cooled slag uses are many and varied, including all types of construction aggregate applications in addition to manufacture of mineral wool, cement and glass and as a soil conditioner. The principal uses of air cooled slag include road bases, asphalt concrete aggregate, concrete aggregate, structural fill, railroad ballast, and mineral wool. Other aggregate applications included roofing, sewage plant filter media and drainage works.

The major application is mainly in untreated in base courses, with smaller amounts used in bases stabilized with cement, asphalt, or lime-fly ash mixes. In all types of base construction, the slag is used in precisely the same manner as would any crushed, natural material. The slag has a number of desirable characteristics for this type of construction. These characteristics include (i) particle shape and texture which provides exceptionally high stability, (ii) non-plastic fines, (iii) volume stability under all weathering conditions, and (iv) lower weight per unit volume.

The above benefits are also applicable to the use in structural fills, where the lower weight is particularly important in reducing dead load on weak or unstable soils. Similar considerations are, of course, important in railroad ballast which also ranks among the five largest uses. The applications in bases fill and ballast are dependent upon the strength, stability, and durability properties of the aggregate.

The use of the air cooled slag, as aggregate in asphalt concrete, is one in which it is often the preferred material. In addition to the stability and durability characteristics mentioned above, the slag does not polish under traffic as do many natural aggregates. The slags are among the best materials to provide safe, skid resistant surface courses and are specified for this purpose in some areas. The BF slag is alkaline in reaction, coat readily with asphalt, and is not subject to stripping problems.

In concrete aggregate use, the air cooled slag offers properties of excellent bond with the cement, freedom from deleterious particles and alkali reactions, volume stability and durability, good concrete strengths and fire resistance superior to that obtained with other normal weight aggregates. The slag aggregate concretes are usually made with slag coarse aggregate and natural sand fine aggregate for workability. Structural dead loads are decreased because the unit weights of the slag concretes are normally around 160 kg/cum less than those obtained with other types of aggregates.

Granulated BF slag – It has been used in composite cements and as a cementitious component of concrete for many years. The first industrial commercial use (around 1859) was the production of bricks using unground granulated BF slag. In the second half of the 19th century the cementitious properties were discovered and by the end of 19th century the first cement containing granulated slag was produced. Since the late 1950s the use of granulated BF slag as a separately ground material added at the concrete mixer together with Portland cement has gained acceptance. It is to be noted that in some countries the term ‘slag cement” is used for pure ground granulated BF slag. Practically, there are no concrete, mortar or grout applications which preclude the use of an appropriate amount of ground granulated BF slag. The major uses of ground granulated BF slag are as follows.

  • Slag cement – Plant produced slag cement can be made in one of two ways. Either the individual components (the granulated BF slag and the Portland cement clinker) can be ground separately and subsequently blended or they can be inter-ground which mixes and grinds in a single operation.
  • Concrete – Besides as a constituent of slag cement in some places ground granulated BF slag is available as a separately ground material which can be used by the concrete producer as a cementitious component. Using slag cements or ground granulated BF slag as a concrete addition result in several advantageous concrete properties. Slag cements have a low heat of hydration. Concrete made with BF slag cement or with ground granulated BF slag as an addition has a high durability as a result of the low capillary porosity. It is resistant to chloride penetration, sulphate and thaumasite sulphate attack (attack requiring a source of sulphate and also of carbonate). Protection against alkali silica reaction, a low risk of thermal cracking, a high electrolytic resistance and a consistent light colour are further advantages. A better workability and an easier finishability are also there. These properties favour the use of slag cements or mixtures of Portland cement with ground granulated BF slag in all situations especially where high levels of durability are called for. Using ground granulated BF slag can locally cause a blue-green coloration of the fresh demoulded surface of hardened concrete. With air the typical colour vanishes within a short time.
  • Mortar – Slag used as a cementitious component in mortars enhances their workability and can allow further working time for the bricklayer.
  • Grout – Grouts containing ground granulated BF slag have been used on many occasions to control temperature rise during hydration and in areas of aggressive conditions.
  • Aggregate – Unground granulated BF slag is suitable as a normal weight aggregate in concrete.
  • Road making – Unground granulated BF slag can be used as a base layer material in road construction.

Expanded slag – It is used mainly for lightweight concrete aggregate. Much of it is used in concrete block and other precast units. Nearly 90 % of the expanded slag is used in concrete, where it provides better thermal insulation and fire resistance properties than obtainable with comparable concretes made with other aggregates. Other uses include cement manufacture, drainage facilities and use as a lightweight fill material specified under particularly adverse soil conditions.