Ground granulated blast furnace slag (GGBFS) is a beneficial material that can improve the workability, strength, and durability of concrete. This substance is produced as a by-product during the manufacturing of iron. The process involves heating iron ore, limestone, and coke at a temperature of approximately 1500 degrees Celsius in a blast furnace. The resulting by-product is a molten slag that contains alumina, silica, and various oxides, along with molten iron.
However, GGBFS is not formed directly. The molten slag must be granulated, which involves cooling it rapidly by passing it through a high-pressure water jet. This process results in the formation of granules that are smaller than 5mm in diameter. Blast furnace slag mainly comprises CaO, SiO2, Al2O3, and MgO, which are the minerals found in most cementitious substances. After granulation, the particles are dried and ground using a rotating ball mill, which produces a fine powder known as ground granulated blast furnace slag cement.
Different methods can be used for the granulation process, resulting in different types of slag. The various types include palletized slag, foamed or expanded slag, GGBFS, or air-cooled blast furnace slag (ACBFS). Overall, GGBFS is a valuable material that can enhance the properties of concrete and is obtained through a complex process of granulation and grinding.
Composition of Ground Granulated Blast Furnace Slag (GGBFS)
The following table demonstrates the discrepancy in mineralogical composition between GGBFS and Portland cement.
To elaborate, the two materials possess different mineralogical properties that influence their physical and chemical characteristics. GGBFS, which stands for Ground Granulated Blast Furnace Slag, is a byproduct of the iron-making process and is utilized as a supplementary cementitious material. Its composition contains primarily glassy granules with a lower content of calcium silicates compared to Portland cement.
On the other hand, Portland cement is a hydraulic binder made from a mixture of clinker and gypsum, and is the most commonly used cement in the construction industry. Its mineralogical composition comprises primarily tricalcium silicate, dicalcium silicate, tricalcium aluminate, and tetracalcium aluminoferrite. The amount of each mineral present in the cement determines its properties, such as strength and setting time.
Therefore, the disparity in mineralogical composition between GGBFS and Portland cement can impact their performance and suitability for various applications. The choice of which material to use in a specific project depends on several factors, including the intended use, environmental conditions, and project specifications.
| Mineral | GGBFS | Portland Cement |
| CaO | 30-50% | 55-66% |
| SiO2 | 28-40% | 20-24% |
| Al2O3 | 8-24% | 0-8% |
| MgO | 1-18% | 5% |
GGBFS, or ground granulated blast furnace slag, differs from Portland cement in several ways. One of the most significant differences is that GGBFS contains a higher amount of glassy material, which is formed during the rapid cooling of the slag. This glassy material contributes to the pozzolanic properties of GGBFS, which means that it can react with calcium hydroxide in the presence of water to form calcium silicate hydrate (C-S-H) gel. This reaction results in an increase in the strength and durability of the resulting concrete.
Another important difference between GGBFS and Portland cement is that GGBFS contains some metals that have not undergone complete oxidation. This difference can have an impact on the structural formation of concrete, resulting in differences when compared to concrete made with Portland cement. Additionally, this difference can affect the hydration reaction and the hydration products that are formed when GGBFS is used in concrete.
Overall, these differences in composition between GGBFS and Portland cement can lead to significant changes in the properties and performance of concrete. The use of GGBFS in concrete has become increasingly popular in recent years due to its ability to improve concrete strength and durability while reducing its carbon footprint.
Physical Properties of Ground Granulated Blast Furnace Slag (GGBFS)
Ground granulated blast furnace slag (GGBFS) is a material that has a nearly white color. In contrast, ordinary Portland cement has a dark grey color. This color difference is highlighted in the figure below, which compares GGBFS and ordinary Portland cement side by side. When GGBFS is used in the manufacture of concrete, it can give the resulting concrete a lighter and brighter appearance, which is aesthetically pleasing. This is an advantage of using GGBFS over ordinary cement in terms of the visual appeal of the final product. Table-2 provides information on the typical properties of GGBFS.
Table 2. Physical properties of GGBFS (As per Tasong et.al)
| Property | Value |
| Physical Form | Off white powder |
| Bulk density (kg/m3) | 1200 |
| Specific Gravity | 2.9 |
| Specific surface (m2/kg) | 425 – 470 |
Particle Size Distribution of Ground Granulated Blast Furnace Slag
During the manufacture of GGBFS, the particle size tends to vary depending on the grinding techniques employed. The particle size distribution of the final product can be significantly influenced by the type of mill used during the grinding process. For instance, the particle size distribution of the product obtained from a ball mill tends to be wider than that obtained from an airflow mill. On the other hand, GGBFS that is ground using a vibro-mill tends to have spherical shaped particles. These observations highlight the importance of the grinding technique in determining the properties of the final product.
Using GGBFS cement
In a concrete manufacturing plant, the addition of Ground Granulated Blast Furnace Slag (GGBFS) to the mixture of Portland cement, water, and aggregates is possible without altering the normal ratio. Studies indicate that GGBFS can replace anywhere from 30 to 85% of the weight of cement used in the mixture, but typically, 40 to 50% is the norm. By incorporating GGBFS into the concrete mixture, it is possible to improve its properties, such as durability, workability, and strength. Additionally, the use of GGBFS can help to reduce the carbon footprint of concrete manufacturing.
Advantages of GGBFS in concrete
Incorporating ground granulated blast furnace slag (GGBFS) into concrete manufacturing offers numerous benefits. One of the most significant advantages is that it increases the strength and durability of concrete structures. This is due to the unique properties of GGBFS that enhance the overall quality of the concrete.
GGBFS helps to reduce the number of voids in concrete, which, in turn, reduces its permeability. This leads to a more workable mix with good pumpable and compaction characteristics. The resulting structure made of GGBFS constituents has increased resistance to sulphate attack and a decreased penetration of chloride.
Moreover, GGBFS has a lower heat of hydration compared to conventional mix hydration, and it highly resists the alkali-silica reaction. These factors make the concrete more chemically stable, resulting in a good surface finish and improved aesthetics. Additionally, the color of the concrete is more even and light, and there are lower chances of efflorescence.
The incorporation of GGBFS in concrete also has economic benefits. The maintenance and repair cost of structures are reduced, increasing the life cycle of concrete structures. Furthermore, unlike cement, GGBFS does not produce carbon dioxide, sulphur dioxide, or nitrogen oxides, which makes it an environmentally friendly choice.
GGBFS, or ground granulated blast-furnace slag, is an easy material to work with due to its high mobility characteristics. This is because the particles of GGBFS are very fine and possess a glassy texture, which increases their workability and makes them less dense. As a result, less water and Superplasticizers are needed to achieve adequate workability in common situations, and there is a lower risk of the material segregating during handling and pumping. The flowing ability of the mix is facilitated by the lower relative density of GGBFS, making it easier to pump.
Using GGBFS in concrete provides a unique feature that distinguishes it from ordinary Portland cement concrete. Concrete made from GGBFS sets slowly compared to ordinary Portland cement mix, and the more GGBFS used, the longer it takes to set. However, strength is gained over time. This slow setting can result in the formation of cold joints, but it is not suitable for situations where a faster setting time is required. GGBS composition stays plastic for a longer period, which allows for a smoother finish for contractors.
The sustainability factor of GGBFS is evident in its advantages. GGBFS is a by-product that is repurposed, reducing carbon dioxide in the structure and making it more sustainable. The glassy texture of GGBFS particles means they require less water and do not absorb water onto their surface. However, GGBFS is facing some hindrances in its implementation due to the low rate of strength gain and high sensitivity to curing conditions. Therefore, proper and high-quality curing is essential to take full advantage of GGBFS.