Blast-furnace slag cement is a combination of ordinary Portland cement and fine granulated blast furnace slag, which is a byproduct of steel manufacturing. The percentage of slag used in this mixture is less than 70% of the total cement content. Ground granulated blast furnace slag cement, also known as GGBFS, is a fine granular substance that possesses cementitious properties.
The manufacturing process of blast-furnace slag cement involves the blending of Portland cement and finely ground slag obtained from the blast furnace during steel production. The resulting mixture is then finely ground to produce a homogeneous powder with similar properties to Portland cement.
The constituents of blast-furnace slag cement include clinker, gypsum, and granulated blast furnace slag. The slag used in this type of cement is a waste product of steel production that would otherwise end up in landfills. It is finely ground to a fine powder, and when added to Portland cement, it contributes to the strength and durability of the resulting cement.
Blast-furnace slag cement has several advantageous properties, including high compressive strength, low heat of hydration, and low permeability. It is also more resistant to sulfate and chloride attacks than ordinary Portland cement. These properties make it ideal for use in various construction projects, including bridges, dams, and high-rise buildings.
However, there are some disadvantages associated with the use of blast-furnace slag cement. It has a slower setting time than ordinary Portland cement and requires longer curing periods. Additionally, it has lower early strength compared to Portland cement, which may affect the initial strength of the structure.
In summary, blast-furnace slag cement is a blend of Portland cement and granulated blast furnace slag that possesses cementitious properties. It has several advantageous properties that make it suitable for use in construction projects, but it also has some drawbacks that must be considered.
Manufacture and Constituents of Blast-Furnace Slag Cement
Fig 1: Extraction of molten Slag from Blast Furnace.
GGBFS, which stands for Ground Granulated Blast Furnace Slag, is a by-product that is obtained during the process of extracting iron from its ore. The method used for extracting iron from the ore is known as the blast furnace process. During this process, the slag, which is formed on the iron ore, is separated and allowed to cool slowly. This cooling process results in the formation of a non-reactive crystalline material, which is known as GGBFS.
The constituents of GGBFS are listed in a table, and they include silicon dioxide, aluminum oxide, calcium oxide, magnesium oxide, sulfur trioxide, and other minor components. These constituents give GGBFS its unique properties and make it a valuable material in various applications, such as cement and concrete production.
Overall, GGBFS is an important by-product that is obtained during the extraction of iron from its ore. Its unique properties and constituents make it a valuable material in various industrial applications.
Table 1: Constituents of the Ground Granulated Blast Furnace Slag
| Constituents | % by mass |
| SiO2 | 27-39% |
| Al2O3 | 8- 20% |
| CaO | 38-50% |
| MgO | <10% |
The slag is ground finely to a texture similar to that of cement and then mixed in varying proportions depending on the specific requirements of the construction project. The amount of ground granulated blast furnace slag (GGBFS) added will differ depending on the type of construction being carried out. Different percentages of GGBFS are used to achieve the desired characteristics of the construction material.
Table 2: Proportion of slag percentage for different applications.
| Application in Type of Construction | Slag Proportion in % |
| General construction | 20-40 |
| Reduction of heat hydration | 50-80 |
| Structures exposed to chloride attack | 50-81 |
| Structures exposed to sulfate attack | 50-82 |
| Marine structures | 60-80 |
Properties of Blast-Furnace Slag Cement
Table 3: Properties of Blast-Furnace Slag Cement
| Properties | Values |
| Density (g/cm3) | 3.04 |
| Specific Surface Area (cm2/g) | 4050 |
| Setting Time | |
| Initial Setting | 60min |
| Final Setting | 600min |
| Compressive Strength (N/mm2) | |
| 3days | 23.5 |
| 7days | 36.1 |
| 28days | 62.4 |
| Chemical Composition (%) | |
| Magnesium oxide | 2.88 |
| Sulphur tri oxide | 2.19 |
| Ignition loss | 1.47 |
Uses of Blast-Furnace Slag Cement
The type of concrete that is commonly utilized in ready mix concrete plants is also useful for structures that need to retain water, such as retaining walls, rivers, ports, and tunnels, because of its improved impermeability. It is also commonly used in mass concreting projects, such as dams and foundations, where a low heat of hydration is needed.
Furthermore, this type of concrete is preferred in locations that are prone to chloride and sulfate attacks, including sub-structures, bored piles, pre-cast piles, and marine structures. Its ability to resist these types of corrosive elements makes it a popular choice for construction in such areas.
Fig 2: Use of Slag cement in Sulphate and Chloride attack places.
Advantages of Blast-Furnace Slag Cement
Ground Granulated Blast Furnace Slag (GGBFS) is a type of cement that initially has lower strength compared to conventional concrete, but ultimately achieves equal or even higher strength. This is due to the fine grinding of the slag, which efficiently fills the pores, resulting in high workability and low bleeding.
Another advantage of GGBFS is its good resistance to sulphate and chloride attack, as well as a reduced risk of alkali-silica reaction with aggregates. It also has greater durability and reduced permeability due to its fineness.
The slow hydration process of GGBFS leads to longer slump retention and initial setting time. Additionally, the exothermic process of its hydration results in a slow generation of heat, making it suitable for applications where thermal cracking is a concern.
GGBFS also has a lighter color than conventional cement due to its white color, giving a lighter-colored cement upon production. Moreover, it is less expensive to produce compared to Ordinary Portland Cement (OPC).
Fig 3: Use of Slag cement in Canal Lining.
Disadvantages of Blast-Furnace Slag Cement
The cement in question has a low initial strength, which makes it unsuitable for use in reinforced concrete construction. This is because the strength of the concrete is crucial for the structural integrity of the building or structure being constructed, and a low-strength cement would compromise that integrity. As such, it is important to use a cement with a higher initial strength when working with reinforced concrete.
Additionally, this particular cement has a high initial setting time. This means that it takes longer for the cement to harden and set, which makes it unsuitable for emergency or repair work where time is of the essence. When dealing with emergencies or repairs, it is necessary to use a cement with a shorter initial setting time in order to complete the work as quickly as possible and minimize further damage or danger.