Ground Granulated Blast Furnace Slag (GGBS)
The importance of Ground Granulated Blast Furnace Slag (GGBS) lies in its greener way to become a substitute in concrete material. GGBS being a waste material, it needs proper method for disposal. So incorporating this waste material in concrete can reduce the depletion of conventional concrete components such as cement, fine aggregate and coarse aggregate. Also the various studies show that including Ground Granulated Blast Furnace Slag in concrete does not decrease the strength of concrete.
It is a by-product from the blast furnace which is used to make iron. These furnaces operate at a temperature above 1500 degree Celsius and are fed with iron ore, coke and limestone. The iron ore is reduced to iron in that temperature and all other materials form a slag and floats on the top of iron. This slag is periodically tapped off as a molten liquid and then rapidly quenched with large volumes of water.
This process optimizes the cementitious properties and produces granules like coarse sand. This granulated slag is then dried and ground to a fine powder. It is also knows as slag cement because it has similar properties as cement clinker.
Chemical Composition of Ground Granulated Blast Furnace Slag
The composition of slag essentially depends on the raw materials used in the iron production process. Typical chemical composition contains calcium oxide 40%, silica 35%, alumina 13% and magnesia 8%. The glass content of slag’s suitable for blending with Portland cement typically varies between 90-100% and depends on the cooling methods and the temperature at which cooling is initiated. The glass structure of the quenched glass largely depends on the proportions of silica and aluminum along with calcium and magnesium.
The Physical properties are specific gravity 2.9, Bulk density 1200 Kg/m3 and fineness 350 m2/Kg.
Application of GGBS
GGBS is used along with the ordinary Portland cement or other pozzolanic materials in concrete. Its use is increasing day by day because it improves the durability of concrete which essentially increase the lifespan of concrete structures from fifty to hundred years.
GGBS are used to produce two types of cement such as Portland Blast Furnace Cement (PBFC) and High Slag Blast Furnace Cement (HSBFC) with varying amount of GGBS. The amount of GGBS in it may vary from 30 to 70%.
Ground Granulated Blast Furnace Slag concrete
GGBS hardens very slowly so it is generally used along with ordinary Portland cement in concrete. A typical combination is 50% GGBS and 50% ordinary Portland cement. But percentages of 20 to 80 are commonly used. Greater the percentage of GGBS, greater will be the effect of GGBS in concrete. The concrete made with Ground Granulated Blast Furnace Slag cement sets more slowly compared to ordinary Portland cement concrete based on the amount of GGBS in the cementations material.
It also needs longer period to gain required strength. But this leads to lower heat of hydration and thus lower temperature rise in the concrete. This may affect construction schedules which needs quick setting of concrete. Due to this property it imparts lower early age strength in concrete. Other properties of Ground Granulated Blast Furnace Slag concrete are as follows:
The setting time of concrete is mainly influenced by the temperature and water cement ratio. Using of GGBS in concrete generally increase the setting time slightly up to 30 minutes. This has an advantage that this concrete mix can be workable for a longer time and the risk of developing cold joints will be lesser. This property is helpful in construction in warm weather.
It has similar or slightly improved consistency than Portland cement concrete. This makes the concrete very easy to place and compact.
If 50% of GGBS is used in a concrete mix, it can develop almost same 28 day strength as ordinary Portland cement. At higher GGBS percentages the increased cementitious content may lead achieve equivalent 28 day strength much faster. GGBS concrete gains strength more steadily than equivalent concrete made with Portland cement.
GGBS is one of the greenest construction materials. Manufacturing of Ground Granulated Blast Furnace Slag utilizes all of the slag material and produces no significant waste. The production of GGBS requires less than a fifth the energy and produces less than a fifteenth of the carbon dioxide emissions. Further benefits of GGBS includes that the manufacturing does not requires use of any virgin materials.
Advantages of GGBS concrete
- Good workability which helps in better placing and compaction.
- Due to the less heat of hydration the temperature rise will be less avoiding the risk of thermal cracking in large volume of concrete.
- High resistance to chloride attack which reduces the risk of corrosion in concrete.
- High resistance to sulphide attack and also other chemicals.
- Good sustainability