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Compressive Strength of Concrete: What Is Blast Furnace Slag?

Granulated blast furnace slag is a mineral additive which in fact is an industrial waste, produced during the production of pig iron in iron and steel factories. Since they have a siliceous and aluminous amorphous structure, blast furnace slags show pozzolanic properties when they are ground to a very fine grained state. When the hydration product of cement combines with calcium hydroxide in an aqueous environment, it becomes a hydraulic binder. The material can be used directly in the production of Portland slag cement and as a concrete additive.

What is the purpose of adding pozzolana in cement?

Artificial or natural pozzolans react with Ca(OH)2, which occurs as a result of hydration in the cement and concrete systems in which they are incorporated, and form additional components with binding properties. Pozzolanas make the concrete more resistant to abrasive external impacts than a regular Portland cement concrete that does not contain any pozzolanas. One of the artificial pozzolanas used as a mineral additive in concrete is blast furnace slag, which we will discuss in this article.

How is granulated blast furnace slag produced?

In order to obtain iron, iron ore must be heated to very high temperatures (up to approximately 1600 ºC) in furnaces called “blast furnaces”, thus getting purified from oxygen and foreign substances. Blast furnace slags are wastes coming out of the blast furnace in a molten state, at a temperature of approximately 1500ºC. It has an amorphous structure containing high amounts of silica and alumina. The use of slag for any purpose is possible only after it is cooled. 

What is the structural feature of blast furnace slag?

The slag presents distinct structural characteristics depending on the cooling technique applied. It is usually subjected to very rapid cooling, in large amounts of water. The process of cooling the molten slag very quickly gives the slag both a granulated and amorphous (glassy) structure. The material is referred to as “granulated blast furnace slag” as, upon very rapid cooling, particles like sand grains (the largest of approximately 4 mm) are formed. 

What is the proportion of blast furnace slag used as concrete additive?

It can also be used in large quantities as a cement and concrete additive. The amount of blast furnace slag in the concrete mixture generally varies between 25% and 60% of the cement weight. 

What are the Composition Ratios of Cements Containing Blast Furnace Slag?

The composition ratios of cements containing blast furnace slag are presented in Table-1.

Cement TypesClinkerBlast Furnace SlagMinor Additional Component
CEM II/A-S80-946-200-5
CEM II/B-S65-7921-350-5
CEM III/A35-6436-650-5
CEM III/B20-3466-800-5
CEM III/C5-1981-950-5

Table-1: The Composition Ratios of Cements Containing Blast Furnace Slag in Weight (%) (TS EN 197-1:2012

Picture-1: Çimsa Izo Power 42.5 Portland Slag Cement

Using blast furnace slag in cement and concrete production, the following benefits can be obtained: 

  • Less clinker usage,
  • Reduction in GHG emissions,
  • Reduction in energy consumption,
  • Less pollution to the environment due to the use of waste.

What is the Use of Blast Furnace Slag in Concrete?

Adding ground granulated blast furnace slag into concrete improves the properties of fresh and hardened concrete. Ground blast furnace slag makes the concrete more resistant to aggressive environmental conditions than a normal concrete containing no slag. 

Research conducted to date reveal that adding slag improves concrete characteristics including corrosion resistance, durability, permeability, workability, and strength.

However, the hydration rate of slag is lower than that of Portland cement, and the incorporation of slag into the concrete delays the strength development. The degree of retardation depends on the chemical composition of the slag and Portland cement, the percentage of slag addition, and the humidity and temperature of the environment. Therefore, concrete containing slag must be cured for a longer period of time than normal Portland cement concrete. 

The research conducted reported that the early-age strength of slag-added concrete is lower than that of normal Portland cement concrete, while its late-age strength is equal to or greater than the strength of normal Portland cement concrete, provided that the concrete is well cured. 

The use of very fine-grained blast furnace slag as a mineral additive increases the workability of fresh concrete. Therefore, the need for mixing water required for a certain consistency is reduced. Using less water in the concrete mixture reduces the water/binder ratio and therefore increases its strength. 

What is the Effect of Blast Furnace Slag on Properties of Concrete?

We can list the effects of blast furnace slag used as an additive in concrete, on concrete properties as follows:

Positive effects
Increases the workability of fresh concrete
Reduces bleeding in fresh concrete.
Extends the setting time of fresh concrete.
Reduces concrete hydration heat and thermal crack formation
Increases the ultimate strength considerably.
Reduces the water permeability of hardened concrete.
Increases the resistance of concrete against negative effects
Reacts with cement hydration product calcium hydroxide, preventing efflorescence
Increases the resistance of hardened concrete to sulfates
Reduces the potential of alkali-silica reaction formation
Provides economy
Potential Harmful Effects
Delays the setting of the concrete a little, which can be a problem in cold weather.
Reduces the rate of strength of concrete in the first days
Requires the concrete to be cured for a longer period of time

What are the Chemical Compositions of Blast Furnace Slags? 

The chemical composition of blast furnace slags varies depending on the conditions of the blast furnace and the resource of the raw materials. Chemical compositions of some slags produced in various countries are given in Table 2 in comparison with those of Portland cement. The most common oxides in blast furnace slag are CaO, SiO2 and Al2O3. CaO included at a rate of 30%-40% causes the finely ground blast furnace slag to result in spontaneous binding. 

Factors affecting the binding properties of blast furnace slag are as follows;

  • Chemical composition of slag, 
  • Alkaline concentration in the reaction system, 
  • How amorphous the slag structure is, 
  • The fineness level of the ground slag, 
  • Temperature during the first phase of hydration. 

The relationship among these factors is quite complex. Therefore, the best way to investigate the availability of use of blast furnace slag in concrete is to conduct tests in compliance with the strength and durability criteria.

ComponentU.S. and CanadaSouth AfricaAustraliaTürkiyePortland Cement

Table 2. Chemical composition of BFS (Onat, 1998)

In the past years, in order to investigate whether ground blast furnace slag could show sufficient binding, its chemical composition was required to meet;

(CaO+MgO+Al2O3)/( SiO2)≥1.

In recent years, the opinion that having the appropriate chemical composition is not sufficient has gained importance, and it was decided that investigating the “slag activity index” to determine the availability of blast furnace slag as a concrete additive is a more realistic approach.

What are the Values Set by the Standard for the Availability of Blast Furnace Slag?

According to TS EN 15167-1 Granulated Blast Furnace Slag for use in concrete, mortar, and grout – Part 1: Definitions, specifications, and conformity criteria standard;

  • The chemical composition of ground granulated blast furnace slag should consist of a total mixture of calcium oxide (CaO), magnesium oxide (MgO) and silica dioxide (SiO2) in at least 2/3 of its total mass. 
  • The remaining part should consist of aluminum oxide (Al2O3) accompanied by small amounts of other components. The (CaO+MgO)/(SiO2) ratio by mass should be at least 1.0.

The chemical properties of ground granulated blast furnace slag should comply with the requirements given in Table 3.

Table 3 – Chemical requirements given as characteristic values (TS EN 15167-1)

Fineness value: The specific surface area should not be less than 275 m2/kg.

Initial setting time: The initial setting time for a mixture of 50% test cement and 50% ground granulated blast furnace slag by mass should not be longer than twice the initial setting time of the test cement determined by the same method.

Activity index: The activity index is the ratio (in percent) of the compressive strength of the mixture of 50% test cement and 50% ground granulated blast furnace slag by mass to the compressive strength of the test cement. 

Compressive strengths should be determined in accordance with EN 196-1 and the water/(Cement + ground granular blast furnace slag) and water/Cement ratios should both be 0.50.

7-day and 28-day activity indexes should not be less than 45% and 70%, respectively.

Why to Use Blast Furnace Slag?

The use of ground granulated blast furnace slag provides high durability, low heat of hydration and high ultimate strength values in concrete. Besides improving concrete performance, use of blast furnace slag also has benefits in terms of environment. It ensures less use of cement clinker and therefore less GHG emissions and less energy consumption. Blast furnace slag also causes less environmental pollution as the industrial waste of iron and steel factories are used.


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Granulated blast furnace slag is a mineral additive which in fact is an industrial waste, produced during the production of pig iron …