Introduction
1.3 Sources of Geopolymer
which leads to soil toxication and ground water contamination. Hence, the use of these by- products in concrete preparation eradicates the problem of disposal and eventually minimize the environmental impact caused by the industrial processes.
Chapter 1 Introduction
Figure 1.5: A schematic view of production of iron and BFS.
The chemical composition of BFS varies depending upon the composition of the raw materials involved in the process of production of iron. The main components of blast furnace slag are CaO (30 – 50 %), SiO2 (28 – 38 %), Al2O3 (8 – 24 %), and MgO (1 – 18 %). Increasing the CaO content of the slag results in raised slag basicity and an increase in its compressive strength.
During the production process, the slag floats on top of the iron in the blast furnace and is drawn out for separation. Slow cooling of the molten slag results in an unreactive crystalline material consisting of an assemblage of Ca-Al-Mg silicates. However, when the molten slag is rapidly cooled or quenched below 800 °C, crystallization of merwinite and melilite is prevented. Thus, a good slag reactivity is obtained. The cooling and granulation/fragmentation process is carried out by subjecting the slag to jet streams of water or air under pressure.
The process of cooling and hardening of BFS from its molten state is carried out in several ways to form several types of BFS. The various type of BFS are:-
i. Ground granulated blast furnace slag (GGBS): When the molten slag is cooled and solidified to a glassy state by rapid water quenching. In this process, crystallization does not occur and sand size fragments are formed. These are known as GGBS. The physical structure and gradation of GGBS depend on the chemical composition of the slag, its temperature at the time of water quenching and the method of production. When crushed or milled to very fine cement-sized particles, GGBS show cementitious properties.
ii. Air-cooled blast furnace slag: This type of BFS is formed when the liquid slag is poured into beds and slowly cooled under ambient conditions. A crystalline structure is formed in this process and a hard lump slag is produced, which is subsequently crushed and screened.
iii. Expanded or foamed blast furnace slag: The cellular nature of the slag can be increased by accelerated cooling and solidifying molten slag by adding controlled quantities of water, air or steam. The resultant slag is a lightweight expanded or foamed product.
This type of blast furnace slag is known as expanded or foamed blast furnace slag. It is differs from air-cooled blast furnace slag by its relatively high porosity and low bulk density.
iv. Pelletized blast furnace slag: This type of slag is produced when molten slag is cooled and solidified with water and air quenched in a spinning drum. Slag pellets are formed in this process instead of solid slag mass. The pellets can be made more crystalline by controlling the process of its formation. The crystalline pellets can be used as aggregate.
On the other hand, more vitrified (glassy) pellets can also be formed which results in slag with cementitious nature. More rapid quenching results in greater vitrification and less crystallization.
The production of iron ore was at 192.08 million tons in the year 2016-17. India is currently the 3rd largest producer of pig iron in the world. The production of pig iron was 9.39 million tons in the year 2016-17. Typically, for ore feed containing 60 to 65 % iron, BFS production ranges from about 300 to 540 kg per ton of pig iron produced [13].
1.3.2 Flyash
The residue that results from the combustion of pulverized coal and is transported from the combustion chamber by exhaust gases and collected by electrostatic precipitators known as flyash (FA). FA consists of particles which are generally spherical, typically ranging in size between 10 and 100 micron. These spherical shape contributes towards improving the fluidity and reducing the viscosity within the mix consisting of flyash and concrete preparing materials.
Fineness is one of the important property and contributes to the pozzolanic reactivity of FA.
FA is produced in the coal fired electric and steam generating plants. Usually, coal is pulverized and blown with air into the boiler's combustion chamber where it is ignited immediately. This generates heat and produces a molten mineral residue. Boiler tubes extract
Chapter 1 Introduction
heat from the boiler and cool the flue gas. This causes the molten mineral residue to harden and form ash. Coarse ash particles fall to the bottom of the combustion chamber. They are referred to as bottom ash. The lighter fine ash particles remain suspended in the flue gas. These are termed as FA. It is removed by particulate emission control devices, such as electrostatic precipitators or filter fabric baghouses before the flue gas exhausts.
FA primarily consists of silicon dioxide (SiO2), aluminium oxide (Al2O3) and calcium oxide (CaO). Very tiny amount of magnesium, potassium, sodium, titanium, and sulfur also remain in it. FA is classified as either Class C or Class F based on its chemical composition [14].
Class C FA is produced from younger lignite or sub-bituminous coals. Class C FA has the capacity of hardening and gaining strength when mixed with water. It consist primarily of calcium alumino-sulfate, quartz, tricalcium aluminate and calcium oxide (CaO). Due to presence of more than 20 % CaO, it is also known as high calcium FA. On the other hand, bituminous and anthracite coals are the sources of Class F FA. The primary constituent of Class F FA are alumino-silicate glass, quartz, mullite and magnetite. It consists of less than 10 % CaO and hence it is also known as low calcium FA. When it combines with a chemical activator such as alkali compound, it exhibits binding capacity.
In India, coal production was 662.79 million ton in 2016-17. This production amount was 3.69 % higher compared to that of the previous year. Around 82 % of coal produced was used in the Power Sector. In the year 2015, India ranked 3rd in the world in coal production [13].
1.3.3 Silica fume
Silica fume (SF) is obtained as byproduct from the process of production of silicon metal or ferrosilicon alloys. It is produced in the carbo-thermic reduction of high-purity quartz with carbonaceous materials like coal, coke, wood-chips, in electric arc furnaces. It is amorphous in nature. The silica fume is formed as an ultrafine powder consisting of spherical particles of size of less than 1 μm in diameter with an average particle size of about 0.15 μm diameter. It is also termed as micro silica. Due to the fine particles, large surface area and high SiO2 content, silica fume possess high pozzolanic characteristics.
1.3.4 Rice husk ash
Rice husk is an agricultural waste generated in the countries which produce rice. It is a hard protective covering of rice grains. About 22 % of the weight of paddy is received as husk. The husk is used as fuel for combustion in many industries, especially in rice mills. Combustion of the husk, produces rice husk ash (RHA). It consists of 85 - 90 % amorphous silica. The particles of RHA are usually finer than Portland cement particles when the rice husk is burnt completely.
Since, it is a potential source of amorphous reactive silica, it is used for preparation of geopolymer by allowing it to react with alkali compounds. In India, around 20 million tons of rice husk ash is produced per year [15].
1.3.5 Metakaolin
Metakaolin (MK) is obtained by heating china clay (mineral kaolin) to a temperature between 600 and 800 ºC. It is rather an industry manufactured product than a byproduct from industrial processes. Formation of metakaolin from kaolin is an endothermic process which involves large amount of energy to remove the chemically bonded hydroxyl ions in kaolin. Due to the pozzolanic nature of metakaolin, it is also used to prepare geopolymer.