Blast Furnace Slag Cement – Manufacture, Properties and Uses.

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Blast furnace slag cement is the mixture of ordinary Portland cement and fine granulated blast furnace slag obtained as a by product in the manufacture of steel with percent under 70% to that of cement. Ground granulated blast furnace slag cement (GGBFS) is a fine glassy granules which contain cementatious properties. In this article we discuss about the manufacture, constituents, properties, uses, advantages and disadvantages of Blast-furnace slag cement.

Manufacture and Constituents of Blast-Furnace Slag Cement

GGBFS is obtained as a by product in the extraction of iron from it ore. The process of extraction of iron is blast furnace. The slag that is obtained on the iron ore is separated and cooled down slowly, which results in the formation of nonreactive crystalline material. The constituents of the GGBFS are as shown in the table below,

Table 1: Constituents of the Ground Granulated Blast Furnace Slag

Constituents	% by mass
SiO2	27-39%
Al2O3	8- 20%
CaO	38-50%
MgO	<10%

Further this slag is grounded finely as that of cement and mixed in the proportion as per the requirement. Different percentage of GGBFS is added for different type of construction.

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 

1. Used in ready mix concrete plants.
2. Used for structures meant for water retaining such as retaining wall, rivers, ports, tunnels for improvement in impermeability.
3. Used in mass concreting works such as dams, foundations which require low heat of hydration.
4. Used in the places susceptible to chloride and sulphate attacks such as sub-structure, bored piles, pre-case piles and marine structures.

Advantages of Blast-Furnace Slag Cement 

1. The initial strength achieved is lesser than that of conventional concrete, but the higher ultimate strength gained is equal and sometimes higher that conventional concrete.
2. As the slag is grounded finely, it has the capacity to fill the pores efficiently, workability is high and bleeding is low.
3. It has good resistance towards sulphate and chloride attack and even less risk of alkali-silica reaction with aggregates.
4. Due to slow hydration process, the slump retention and initial setting time is more.
5. Greater durability and reduced permeability due to fineness.
6. the hydration process if slag is an exothermic process, thus the generation of heat of hydration is slow. Due to this property, this type of cement can be used where the problem of thermal cracking is foreseen.
7. As the colour of GGBFS is white, the cement colour obtained is lighter coloured when compared to conventional cement.
8. The production cost this cement is less when compared to OPC.

Disadvantages of Blast-Furnace Slag Cement 

1. Initial Strength is low, due to this it cannot be used in RCC works.
2. As the initial setting time is high, this cement is not used for emergency or repair works.