Ground Granulated Blast Furnace Slag in Concrete & its Advantages
The ground granulated blast furnace slag (GGBFS) is a by-product of iron manufacturing which when added to concrete improves its properties such as workability, strength and durability.
This material is obtained by the heating of iron ore, limestone and coke at a temperature about 1500 degree Celsius. The process is carried out in a blast furnace.
The formation of GGBFS is not direct. The by-product of iron manufacturing is a molten slag and molten iron. The molten slag consists of alumina and silica, also with the certain amount of oxides.
This slag is later granulated by cooling it. For this, it is allowed to pass through a high-pressure water get. This result in quenching of the particles which results in granules of size lesser than 5mm in diameter.
The main constituents of blast furnace slag are CaO, SiO2, Al2O3 and MgO. These are the minerals that are found in most of the cementitious substances.
The particles are further dried and ground in a rotating ball mill to form a fine powder, known as ground granulated blast furnace slag cement. Now different methods can be employed to perform the main process called as the quenching.
Based on what method is employed, it can be known as palletized slag, foamed or expanded slag, GGBFS or air-cooled blast furnace slag (ACBFS).
Composition of Ground Granulated Blast Furnace Slag (GGBFS)
The difference in mineralogical composition in GGBFS compared to Portland cement is shown in the table below.
Another significant difference in GGBFS is that some of the metals within it have not undergone complete oxidation. This change will be reflected in the structural formation in concrete when compared with Portland cement.
From the above two differences in composition compared with the Portland cement, would show changes in the hydration reaction as well as the hydration products that are given GGBFS.
Physical Properties of Ground Granulated Blast Furnace Slag (GGBFS)
Ground granulated blast furnace slag have a color nearly white in appearance. The figure below shows GGBFS and ordinary Portland cement, to clearly show the color difference between them.
Hence the use of GGBFS in concrete manufacture would give a light and brighter color to concrete unlike the dark grey color of ordinary cement concrete structures. This is an added advantage of GGBFS in terms of aesthetics.
The typical properties of GGBS are given in Table-2.
Table 2. Physical properties of GGBFS (As per Tasong et.al)
|Physical Form||Off white powder|
|Bulk density (kg/m3)||1200|
|Specific surface (m2/kg)||425 – 470|
Particle Size Distribution of Ground Granulated Blast Furnace Slag
It is observed that the particle size of GGBFS varies with the grinding techniques used during its manufacture. It is also observed that the product from ball mill has wide particles and narrow particles for the one grounded in airflow mill. Those grounded in vibro-mill have spherical shaped particles.
Using GGBFS cement
In the concrete manufacturing plant, the GGBFS can be added along with the Portland cement, water and aggregates. The normal ratio of the mixture remains the same. The studies show that the GGBFS can be replaced from 30 to 85 % of the cement weight. Most of the instances we replace 40 to 50%.
Advantages of GGBFS in concrete
The incorporation of ground granulated blast furnace slag in concrete manufacture gains many advantages which are mentioned below:
- GGBFS in concrete increases the strength and durability of the concrete structure.
- It reduces voids in concrete hence reducing permeability
- GGBFS gives a workable mix.
- It possesses good pumpable and compaction characteristics
- The structure made of GGBFS constituents help in increasing sulphate attack resistance.
- The penetration of chloride can be decreased.
- The heat of hydration is less compared to conventional mix hydration.
- The alkali-silica reaction is resisted highly.
- These make the concrete more chemically stable
- Gives good surface finish and improves aesthetics
- The color is more even and light.
- Lower chances of efflorescence
- The maintenance and repair cost of structures are reduced thus increasing the life cycle of concrete structures.
- Unlike cement, GGBFS does not produce carbon dioxide, sulphur dioxide or nitrogen oxides.
It is found that working with GGBFS is easy as it has greater mobility characteristics. This is due to its fineness and the particle shape of the GGBFS particles. These also possess a lower relative density.
The GGBFS particles have a very glassy texture that makes them increase the workability. This can help in reduction of water as well as Superplasticizers to get adequate workability in common situations.
They also have fewer chances to get segregated during handling as well as pumping of the material. Pumping is facilitated by the lower relative density and flowing ability of the mix, that is owned by GGBFS.
The usage of GGBFS in concrete provides a certain unique feature that differs from the ordinary Portland cement concrete. The concrete made from GGBFS sets slowly when compared to ordinary Portland cement mix.
More the GGBFS amount, more will be the time taken for its setting. But the strength is gained with time. This slow setting would help in the formation of cold joints. But the situations where faster setting time is required, cannot go for this replacement.
The GGBS composition stays plastic for a longer period that would help in making a smoother finish for the contractor.
The sustainability factor of GGBFS is proved from the above-mentioned advantages. The product itself is a by-product that is used for a greater recognition. The reduction of carbon dioxide in the structure make them more sustainable.
The GGBFS as a replacement has lesser water demand because of their glassy texture. The glassy surface of GGBFS particles does not absorb water onto its surface.
GGBFS is facing certain hindrances in its implementation widely due to the low rate of strength gain. It is also highly sensitive to conditions of curing. So, a better quality of curing process ensures the happening of full advantage of GGBFS.