Blast Furnace Slag

Blast Furnace Slag

Blast-furnace (BF) slag is defined by the American Society for Testing and Materials as ‘the non-metallic product consisting essentially of silicates and alumino-silicates of calcium and other bases that is developed in a molten condition simultaneously with iron in a blast furnace’.

The BF is the primary means for reducing iron oxides to liquid metallic iron. The BF is continuously charged with iron oxide (ores, pellets, and sinter), fluxes, and fuel (coke and coal). Liquid iron (hot metal) collects in the bottom of the furnace and the liquid slag floats on it. Both are periodically tapped from the furnace. The hot metal is either used for steelmaking or can be cast as pig iron to be used either in foundry or in the steelmaking. Liquid slag which comes out of BF as a liquid at temperatures of around 1500 deg C is either granulated with water or is air cooled.

BF slag is a non-metallic by-product produced in the process of the production of hot metal in the BF. The liquid slag consists primarily of the impurities from the iron ore as well as ash from coke and coal (mainly silica and alumina) combined with calcium and magnesium oxides from the flux. It consists primarily of silicates, alumino-silicates, and calcium-alumina-silicates. The major part of the sulphur (S) which comes from the BF charge material is also absorbs in the slag. The liquid BF slag comprises around 20 % to 35 % of the hot metal production. Fig 1 shows a general view of the production of BF slag and its processing.

Fig 1 Blast furnace slag and its processing

BF slag is sometimes erroneously classified, and often looked upon, as industrial waste material. In actual fact, this by-product is valuable and extremely versatile construction material. The history of slag use in road building dates back to the time of the Roman Empire, some 2000 years ago, when broken slag from the crude iron-making forges of that era were used in base construction. Applications were quite sporadic until the early twentieth century, when large quantities began to be used for a number of purposes. In relatively recent years, the need for maximum utilization and recycling of BF slag for economic and environmental reasons has led to rapid development of slag utilization. Presently the full generation of BF slag is being used in various applications.

Use of BF slag prevents it going to landfill as waste, saves energy and natural resources, and significantly reduces CO2 (carbon di-oxide) emissions in cement production. Replacing Portland cement with BF slag cement in concrete can save upto 59 % of the embodied CO2 emissions and 42 % of the embodied energy needed to produced concrete and its constituent materials. However, this does not account for the CO2 emissions associated with producing slag.

Types of BF slag

BF slag is tapped from the furnace as a liquid, which contains gases held in solution. 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 slag. Dependent upon the cooling methods used, any of the distinctly different types of product can be made from the liquid BF slag. There are two main types of BF slag categorized by how it is cooled. These are air-cooled slag and granulated slag (Fig 2).

Fig 2 Types of BF slag

Air-cooled slag – When the liquid slag is poured into beds and slowly cooled under ambient conditions, a crystalline structure is formed, and a hard, lump slag is produced, which can subsequently be crushed and screened. Liquid  slag is permitted to solidify under the prevailing atmospheric conditions, either in a pit adjacent to the furnace, or in one some distance away to which it is transported in large ladles. After solidification, the cooling can be accelerated by the water sprays which produce cracking, and facilitate digging of the slag. The product is predominantly crystalline in nature, with a cellular or vesicular structure resulting from bubbles of gases dissolved in the liquid slag. After cooling, the slag is dug, crushed, and screened to the desired sizes. Metallic iron in the slag is removed by powerful magnets in the crushing and screening plant.

Air cooled slag is hard and dense and is especially suitable for use as construction aggregate. It is also used in ready-mixed concrete, concrete products, asphaltic concrete, road bases and surfaces, fill, clinker raw material, railroad ballast, roofing, mineral wool (for use as insulation) and soil conditioner.

Granulated BF slag – Granulated BF slag is produced by quickly quenching (chilling) the liquid slag to produce a glassy, granular product. The most common process is quenching with water, but air or a combination of air and water can be used. When the liquid slag is cooled and solidified by rapid water quenching to a glassy state, little or no crystallization takes place.

This process results in the formation of sand size (or frit-like) fragments, usually with some friable clinker like material. Granulated BF slag is a glassy granular material which normally varies in size 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. The physical structure and gradation of granulated slag depend on the chemical composition of the slag, its temperature at the time of water quenching, and the method of production. Granulated slags can be crushed, graded or ground for specific applications. When crushed or milled to very fine cement-sized particles, it is known as the ground granulated BF slag. Ground granulated BF slag has cementitious properties, which make it a suitable partial replacement for or additive to Portland cement.

Concretes incorporating granulated slag generally develop strength more slowly than concretes which contain only Portland cement (the most common type of cement) but can have better long-term strength, release less heat during hydration, have reduced permeability, and normally show better resistance to chemical attack. Granulated slag can also help bring down the cost of cement. While the use of granulated slag in cement is well established, there is still potential in many places to increase the ratio of granulated slag used for this purpose. In some countries, upto 80 % of the cement contains granulated BF slag.

There are two more categories of BF slag based on the method used to cool the liquid slag. These two types of BF slag namely (i) expanded also called foamed BF slag, and (ii) pelletized BF slag are less popular.

Expanded BF slag results from treatment of liquid slag with controlled quantities of water which is less than that required 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 control in the of the cooling rate can result in product variations from highly crystalline materials resembling very vesicular air-cooled slags to glassy materials closely akin to granulated slag. Foamed slag is distinguishable from air-cooled blast furnace slag by its relatively high porosity and low bulk density.

Expanded slag particles, depending upon the processing procedure and either can be angular and roughly cubical in shape, or can be spherical and smooth surfaced. The cellular structure results in densities in the light weight aggregate categories.

The pelletizing process has been recently developed in Canada. It 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.

Properties of BF slag

Blast furnace 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 sulphur (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.

Chemical properties – In certain situations, the leachate from BF slag can be discoloured (characteristic yellow / green colour) 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 calcium and sulphide, with a pH as high as 12.5. When this yellow leachate is exposed to oxygen, the sulphides present react with oxygen to precipitate white / yellow elemental sulphur 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.

The chemical composition of the BF slag is dependent upon the composition of the BF burden materials, and on the proportions needed for efficient BF operations. The BF is to be charged with uniform raw materials if the produced hot metal is to be consistent in quality. This ensures uniformity in the composition of the BF slag, and as a result the composition of BF slag from a given source varies within relatively in narrow limits. Greater variations, as shown in the overall ranges (Tab1) can be found between sources where different raw materials are being used. The chemical composition of the BF slag normally has four major oxides namely CaO, MgO, SiO2, and Al2O3. These four oxides normally make up of around 95 % of the total. Minor elements include sulphur, iron, manganese, alkalis, and trace amounts of several others. The composition of the BF slag and its ranges is given in Tab 1.

Tab 1 Composition of BF slag
Sl. No.ComponentUnitRange
3CaO (free)%0.3-2.4
9Mn2O3%0.01- 0.28
13Na2O equivalent%0.2 – 2.6
16Insoluble residue%0.03-4.1
Note: Na2O equivalent = Na2O + 0.658 x K2O

In the BF, the chemical composition cannot be controlled, since it is dependent upon the available raw materials and requirements for an efficient ironmaking operation. During the BF operation, the slag properties can be modified to a limited extent by changing cooling conditions. However, the range of slag compositions normally associated with good iron production is all useful construction materials, although varying in physical properties. Yet, there are factors of performance as related to composition. Slow-cooled, high-lime slags can form di-calcium silicate which undergoes a volume increase on cooling to ambient temperatures. This results in the ‘dusting’ or ‘falling’ of the slag, literally reducing it to a powder. Rapid cooling also obviates any problems in slags destined for cementitious or chemical uses by preventing crystallization of the di-calcium silicate.

Liquid slag which is cooled rapidly (water quenching) after coming out from the BF tends to form a glassy, non-crystalline material. Slower cooling (air cooling) leads to crystallization of a number of minerals as shown in Tab 2. 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 are present or absent depending upon relative proportions of the major oxides in the slag. Many of the slags contain normally upto four of the minerals. The mineral di-calcium silicate can form in slags high in lime, and cause disintegration upon cooling by a volume increase when changing from one crystalline form to another. The sulphur in slow-cooled slag normally appears as sulphides of calcium, iron and manganese.

Tab 2 Minerals of air cooled BF slag
Sl. No. MineralMineral formula
7Di-calcium silicate2CaO.SiO2

The chemical composition of the BF slag is a significant factor in the potential performance of granulated material 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 very 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 thereby influences the porosity and the character and size of crystals in the solidified slag.

Iron unsoundness is a rarely encountered problem. Slags high in iron oxides can, with appropriate levels of other constituents, form compounds which readily react with water, with resultant disintegration of the slag. In slags with usual ranges of iron contents, it is not a problem.

Sulphur has long been looked upon as an undesirable component, primarily because of suspicions that one reaction or another ‘is possible’ or ‘can cause a problem’ of some kind. In actual practice no correlation of sulphur content with performance seems to exist. Some leaching of sulphur compounds from uncoated slag does occur. The leachates are not poisonous in nature, occur only under poor drainage conditions, and are temporary or transient in existence. The worst effects, unpleasant odours from stagnant water in slag fills or bases, are in the ‘nuisance’ category and are avoidable by proper design considerations.

Physical properties – The physical properties of different types of BF slags are described below.

The air-cooled slag crushes to angular, roughly cubical particles with pitted surfaces. It has textures ranging from rough, vesicular (porous) surfaces to glassy (smooth) surfaces with conchoidal fractures. Excellent bond is provided with either hydraulic cements or bituminous binder materials. High internal friction values and particle interlock provide excellent stability when used without cements. Bulk specific gravity 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 4.75 mm sieve) 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 specific gravity of the air cooled BF slag is normally in the range of 2 to 2.5 and the compacted unit weight is in the range of 1.12 tons/cum to 1.36 tons/cum. The water absorption can be in the range of 1 % to 6 %. Although air cooled slag can show these high absorption values, it 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 coarse aggregates are normally classified with crushed stones and gravels as ‘normal weight’ aggregates. They are used for all types of construction applications, just as the natural aggregates. However, the weight saving in the case of slag becomes a significant factor in some applications.

The physical properties of the granulated slag are given in Tab 3. The granulated slag has a colour which ranges from beige to dark to off white depending on the moisture content, chemistry and efficiency of granulation. When it is ground it has normally white colour. The characteristics of the granulated slag such as colour, moisture content, bulk density, porosity, grain shape, grading curve and grindability are affected by different chemistry, liquid slag temperature, and granulation process conditions. Depending on different chemistry, granulation methods and granulation parameters, the morphology of granulate 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 needled shaped forms.

Tab 3 Physical properties of granulated slag
Sl. No. PropertyUnitValue
1Glass contentVol. %60 -100
2True densityg/cc2.8-3.1
3Apparent densityg/cc2-2.84
4Bulk densityg/cc0.69-1.43
5PorosityVol. %2.5-31.2
6Sieve size (Less than 0.5 mm)Wt. %3.6-78.6
7Sieve size (Less than 3.2 mm)Wt. %81.1-100

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 slag glass contains the same major oxides as does Portland cement, but with considerably different proportions of lime and silica. Like Portland cement, it has excellent hydraulic properties and, with a suitable activator (such as calcium hydroxide) sets in a similar manner.

Crushed expanded BF slag is angular, roughly cubical in shape, and has a texture which is rougher than that of air-cooled slag. The porosity of expanded BF slag aggregates is higher than the air-cooled slag aggregates. The bulk relative density of expanded slag is difficult to determine accurately, but it is around 70 % of that of air-cooled slag. Typical compacted unit weights for expanded BF slag aggregates are in the range of 800 kg/cum to 1,040 kg/cum.

In case of the pelletized BF slag, unlike air-cooled and expanded BF slags, the slag has a smooth texture and rounded shape. As a result, the porosity and water absorption are much lower. Pellet sizes range from 0.1 mm (No. 140 sieve size) to 13 mm with the bulk of the product in the range of plus 1 mm (No. 18 sieve size) to minus 9.5 mm. Pelletized BF slag has a unit weight of about 840 kg/cum.

Mechanical properties – Of all the slag types generated, air-cooled BF slag is the type which is normally used as an aggregate material. Processed air-cooled BF slag shows favourable mechanical properties for aggregate use including good abrasion resistance, good soundness characteristics, and high bearing strength. Tab 4 gives typical mechanical properties of air-cooled BF slag aggregates.

Tab 4 Typical mechanical properties of air cooled BF slag
Sl. No.PropertyUnitValue
1Los Angeles abrasion%35-45
2Sodium sulphate soundness loss%12
3Angle of internal frictionDegrees40-45
4Hardness (measured by Moh’s scale of mineral hardness)*5-6
5California Bearing Ratio (CBR), top size 19 mm**%upto 250
*Hardness of dolomite measured on same scale is 3 to 4
**Typical CBR value for crushed limestone is 100 %

Other properties – Because of their more porous structure, BF slag aggregates have lower thermal conductivities than conventional aggregates. Their insulating value is of particular advantage in applications such as frost tapers (transition treatments in pavement sub-grades between frost susceptible and non-frost susceptible soils) or pavement base courses over frost-susceptible soils.

Uses of BF slag

BF slag is being used since many centuries. It is being used in road building as early as 1830, as railroad ballast since 1875, use as concrete aggregate began in the 1880s and in bituminous surfaces in the early 1900s. Major development of slag uses was in the construction aggregate applications, and more specialized applications such as cement manufacture and agricultural applications. The high levels of the use of BF slag have been reached on a competitive basis with other materials. The BF slag is being used either because it provides equal performance at a lower cost, or better performance for similar cost.

Air-cooled slag – The uses of air cooled slag are many and varied. It includes all types of construction aggregate applications, manufacture of mineral wool, cement and glass, and a soil conditioner. The aggregate applications include roofing, sewage plant filter media and drainage works. The major application is primarily in untreated condition 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 any crushed, natural material. The slag has a number of desirable characteristics for this type of construction, which include (i) particle shape and texture that provides exceptionally high stability, (ii) non-plastic fines, (iii) volume stability under all weathering conditions, and (iv) lower weight per unit volume.

The same benefits are 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. The applications in bases, fill, and ballast are because of 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, 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. The BF slag is alkaline in reaction, coat readily with asphalt, and is not subject to the 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 slag – The ground granulated slag has been used in composite cements and as a cementitious component of concrete for many years. The first industrial commercial use (around 1859) of granulated slag was the production of bricks. In the second half of the 19th century the cementitious properties were discovered and by the end of 19th century, the first cements containing granulated slag were produced. Since the late 1950s the use of ground granulated slag as a separately ground material added at the concrete mixer together with Portland cement has gained acceptance. In some countries the term ‘slag cement’ is used for pure ground granulated slag. Practically, there are no concrete, mortar or grout applications which preclude the use of an appropriate amount of ground granulated slag.

The main use of the granulated slag is as a constituent of slag cement. The slag cement use results in several advantageous concrete properties. Slag cements have a low heat of hydration. Concrete made with BF slag cement has a high durability as a result of the low capillary porosity. It is resistant to chloride penetration, sulphate and thaumasite sulphate attack. Protection against alkali silica reaction, a low risk of thermal cracking, a high electrolytic resistance and a consistent light colour are further advantages. Further there is a better workability and an easier finishability.  These properties favour the use of slag cements or mixtures of Portland cement with ground granulated slag in all situations especially where high levels of durability are called for.

Granulated slag is used as a cementitious component in mortars since it enhances their workability and can allow further working time for the bricklayer. Grouts containing slag have been used on many occasions to control temperature rise during hydration and in areas of aggressive conditions. Granulated slag is also suitable as a normal weight aggregate in concrete and can be used as a base layer material in road construction.

Expanded slag – Expanded slag use is largely for lightweight concrete aggregate, much of it in concrete block and other precast units. The majority 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.

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