Table 1. Fly Ash Constituents
The utilization of fly-ash in concrete as partial replacement of cement is gaining immense importance today, mainly on account of the improvement in the long term durability of concrete combined with ecological benefits. Technological improvements in thermal power plant operations and fly-ash collection systems have resulted in improving the consistency of fly-ash. The present study was carried out to determine the effect of high volume fly ash on concrete mechanical properties. Different types of concrete mixes were prepared that consist of 40%, 50%, 60% and 70% of class F fly ash by weight of cement which where compared with plain concrete. The mechanical properties of high volume fly ash concrete are evaluated for 7, 28, and 56 days.
The Indian construction industry is among top five largest construction industries in the world. It is estimated that about 1000 million tons of concrete are consumed annually by the Indian construction industry in the development of infrastructure and housing. Concrete is a unique construction material possessing superior strength and durability characteristics and thus a large number of concrete structures have come up during the past few decades. The construction industry has replaced the phase of using simple and high value fly ash in the recent times [1]. Partial replacement of Portland cement by mineral by-products such as fly ash, slag, silica fume etc can significantly reduce CO2 emission and hence Fly ash is used in concrete to achieve energy conservation and economic, ecological and technical benefits.
The proportion of fly ash used as a cementitious component in concrete depends upon several factors. The design strength and workability of the concretes, water demand, and relative cost of fly ash compared to cement are particularly important in mix proportion of concrete. From the review of literature it is generally found that the optimum proportion of fly ash in the cementitious material for high strength (100 Mpa), high workability (100 mm slump) concrete is in the range of 30-40 percent, medium strength (40-60 mpa), medium workability (50-70 mm slump) concrete 50-60 percent, and low strength (20-30 mpa), low workability (0 slump) concrete 70 to 90 percent. A higher proportion of fly ash, in the range of 70 to 80 percent, have been successfully used in roller compacted concrete mixes. Such mixes are presently being used in the construction of hydraulic structures and rigid pavements in different parts of the world.
Fly ash is an inorganic, non-combustible by-product of coal-burning power plants. As coal is burnt at high temperatures, carbon is burnt off and most of the mineral impurities are carried away by means of gas in the form of ash. The molten ash is cooled rapidly and solidifies as spherical, glassy particles. Fly ash particles range in diameter from <1 microns up to 150 microns. Fly ash is removed from the gas by means of a series of mechanical separators followed by electrostatic precipitators or bag filters. As many power companies are installing scrubber systems to remove sulfur dioxide from stack gasses, many fly ashes are being mixed with scrubber products, resulting in fly ash containing free lime and calcium sulfates or sulfites. These materials cannot be used in concrete.
Class F: The burning of harder, older anthracite and bituminous coal typically produces Class F fly ash. This fly ash is puzzolanic in nature, and contains less than 20% lime (CaO). Possessing puzzolanic properties, the glassy silica and alumina of Class F fly ash requires a cementing agent, such as Portland cement, quicklime, or hydrated lime, with the presence of water in order to react and produce cementations compounds. Alternatively, the addition of a chemical activator such as sodium (water glass) to a Class F ash can lead, to the formation of a geopolymer.
Class C: Fly ash produced from the burning of younger lignite or sub bituminous coal, in addition to having puzzolanic properties, also has some self-cementing properties. In the presence of water, Class C fly ash will harden and gain strength over time. Class C fly ash generally contains more than 20% lime (CaO). Unlike Class F, self-cementing Class C fly ash does not require an activator. Alkali and sulfate (SO4) contents are generally higher in Class C fly ashes.
Many ready-mix companies use fly ash to partially replace Portland cement in concrete. Although the addition of fly ash to concrete has economic benefits to the ready-mix producer (fly ash is typically cheaper than Portland cement), fly ash also provides enhanced fresh and hardened concrete properties. Fly ash influences the rheological properties of the fresh concrete and the strength, finish, porosity, and durability of hardened concrete.
To study the replacement of cement with fly ash at well proportions commonly encountered in current blended cement concrete mixtures that reduces the cost of expenditure without affecting the strength of concrete.
The objective of using fly ash in concrete is to achieve one or more of the following benefits:
Fly ash produces a more cementitious paste. It has a lower unit weight, which means that on a pound for pound basis, fly ash contributes roughly 30% more volume of cementitious material per pound versus cement [3-5] .
The greater the percentage of fly ash “ball bearings” in the past, the better lubricated the aggregates are and the better concrete flows. Second, fly ash reduces the amount of water needed to produce a given slump. The spherical shape of fly ash particles and its dispersive ability provide water-reducing characteristics similar to a water reducing admixture.
Typically, water demand of a concrete mix with fly ash is reduced by 2% to 10%, depending on a number of factors including the amount used and class of fly ash. Third, fly ash reduces the amount of sand needed in the mix to produce workability. Because fly ash creates more paste, and by its shape and dispersive action makes the paste more “slippery”, the amount of sand proportioned into the mix can be reduced.
The experimental program was designed to study the effect of partial replacement of cement with fly ash. For this following parameters are considered for M30 grade of concrete.
1. Plain concrete.
2. 40 % replacement of cement by fly ash.
3. 50% replacement of cement by fly ash.
4. 60% replacement of cement by fly ash.
5. 70% replacement of cement by fly ash.
Table 1 shows the chemical composition of Indian fly ash constituents.
Table 1. Fly Ash Constituents
Table 2 shows the value of specific gravity used in mixing.
Table 2. The Value of Specific Gravity used in Mixing
Table 3 shows the value of slump for replacement of fly ash.
Table 3. Value of Slump for Replacements of Fly Ash
Table 4 shows the value of compaction factors for replacement with fly ash.
Table 4. Value of Compaction Factor for Replacement with Fly Ash
Table 5 shows the value of compressive strengths for replacement with fly ash.
Table 5. Value of Compressive Strengths for Replacement with Fly Ash
Table 6 shows the properties of fine aggregate (sand, stone, dust, and fly ash), coarse aggregate and cement.
Table 6. Properties of Fine Aggregate (Sand, Stone, Dust and Fly Ash), Coarse Aggregate and Cement
The experimental exercise has helped to study the various properties of fly ash concrete and to develop the mix design curves for concrete mix proportioning with various percentages of fly ash.
Based on the studies conducted following conclusion is drawn on the fly ash concrete.
Figure 1. Graph Showing the % of Replacement of Fly Ash and its Effect on Compressive Strength of Concrete