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Properties of high lime fly ash concrete

Introduction Fly ash, is one of the residues generated during combustion of coal and comprises of the fine particles that rise with the flue gases. However, the components of fly ash vary considerably, depending upon the coal being burned.

The heavier unburnt material drops to the bottom of the furnace and is termed as bottom ash. Bottom ash is not suitable for structural concrete but is used for concrete masonry blocks. Fly ash acts as a pozzolona when used as a supplementary cementitious material in concrete. Pozzolonas are those materials which in itself do not possess any cementitious value but in its finely divided form exhibits cementitious properties when combined with Calcium Hydroxide in the presence of moisture.

The pozzolonas chemically react with Calcium Hydroxide at room temperature to form cementetious compounds.

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Similar to OPC, pozzolonas hydrate in water but do not produce the required strength as OPC and gains strength over a much longer period of time. Fly ash which is a finely divided amorphous alumino-silicate powder, reacts with the calcium hydroxide released by hydration of cement and produces various calcium-silicate hydrates C-S-H and calcium aluminum hydrates.

Thus improves the durability properties [1]. Fly ash is considered as a hazardous waste due to leaching of toxic substances into the ground water and soil when it is disposed into ponds, lagoons etc or used as a land fill. Production of Portland cement produces large amounts of carbon di oxide.

  • Utilization of fly ash as a low cost mineral admixture in concrete instead of dumping and polluting the environment seems to be the best solution;
  • Fly ash which is a finely divided amorphous alumino-silicate powder, reacts with the calcium hydroxide released by hydration of cement and produces various calcium-silicate hydrates C-S-H and calcium aluminum hydrates.

About one ton of carbon dioxide is released into the environment during the production of 1 ton of clinker besides SO2 and Properties of high lime fly ash concrete emissions. Hence, there is a need to reduce Carbon dioxide emissions, by lowering the consumption of cement. This can be achieved by replacing cement with high volumes of supplementary cementitious materials like Fly ash.

Utilization of fly ash as a low cost mineral admixture in concrete instead of dumping and polluting the environment seems to be the best solution. Fly ash is the by-product of incineration of coal. However, the strength development is slower in HVFA concrete when compared to PCC but the pozzolanic properties of fly ash result in long term strengths comparable to or better than the conventional concrete.

Thus, though it may seem that the HVFA mixture would have lower strengths at early stages due to decreased cement content, low water cement ratio and high admixture content overcome the adverse effects.

Application of fly ash in concrete will enable concrete to be more sustainable. Class F fly ash and Class C. Class F fly ash is produced by burning harder, older anthracite and bituminous coal while Class C fly ash is obtained by burning, younger lignite or subbituminous coal.

Class F fly ash is less pozzolonic in nature, while Class C fly ash has pozzolonic properties in addition to some self cementitious properties. Class C fly ash generally has higher quantities of Alkalis and Sulphates. This is because Class F fly ash possesses good pozzolonic properties and low cementitious properties. Class F fly ash is used for high volume fly ash concrete, while Class C is used for low volume fly ash concrete mixes. Class C fly ash produces more heat of hydration than Class F fly ash.

Class F Fly ash is used for structural Concrete, high performance concrete and high sulphate exposure concretes while Class C fly ash is used for residential constructions and is prohibited for high sulphate exposure environments.

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Class C fly ash mixes develop more early age strength than Class F Fly ash mixes. Low Calcium fly ash Class Fis composed of Aluminium Silicate glass crystalline quartz, hematite magnetite and mullite. These crystalline phases are generally inert in concrete and requires a source properties of high lime fly ash concrete alkali or lime Ca OH 2 to react and form cementitious hydrates. Such fly ashes are pozzolanic and display no significant hydraulic behaviour.

Whereas, high calcium fly ash class Cis composed of calcium-alumino-silicate glass in addition to those found in low calcium fly ash. Some of these crystalline phases will react with water and together with the more reactive nature of the calcium-bearing glass, makes these fly ashes react more rapidly than low-calcium fly ashes and renders the fly ash both pozzolanic and hydraulic in nature.

These fly ashes will react and harden when mixed with water due to the formation of cementitious hydration products. If the calcium content of the fly ash is high enough, it is possible to make geopolymer concrete with moderate strength. Properties of HVFA Concrete Because of lower water cement ratio, application of superplasticizer or water reducing admixture is essential to ensure workability. Air entraining admixtures can be used when frost resistance is required.

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The effectiveness of air entraining admixtures decreases with increase in fly ash content to cementitious content ratio. Higher content of air entraining admixture has to be used when the carbon content in the fly ash is higher. Workability-Slump Cone Addition of Fly ash increases workability when compared with conventional concrete with the same water content. The spherical shape and particle size distribution of fly ash improves the fluidity of concrete and thus the demand for properties of high lime fly ash concrete reduces, contributing to long term strength.

To achieve early strengths, generally low water cement ratio of about 0. The workability improves in a well-proportioned fly ash concrete.

Fly ash also improves grading of the mixture by smoothing out of fine particle distribution. The water cement ratio is generally adopted to be less than 0. HVFA concrete requires a minimum curing period of 7 days.

The most attractive property of HVFA is improvement in durability due to the reduction of Calcium Hydroxide, which is the most soluble of hydration products.

Jo Jacob Raju and Jino John [5] reported from their experimental investigations on HVFAC with polypropylene fibers that the workability of fly ash concrete decreased with addition of fibers. Hence, bleeding is usually reduced and is usually not a problem [6].

  • Production of Portland cement produces large amounts of carbon di oxide;
  • Vengata [7] has reported that the permeability of HVFA decreases considerably, even though the strength of fly ash concrete at 28 days is not encouraging;
  • To achieve early strengths, generally low water cement ratio of about 0;
  • Fly ash is considered as a hazardous waste due to leaching of toxic substances into the ground water and soil when it is disposed into ponds, lagoons etc or used as a land fill;
  • To attain early age strength, low water cement ratio is essential;
  • Class F fly ash is less pozzolonic in nature, while Class C fly ash has pozzolonic properties in addition to some self cementitious properties.

Because of sufficient air voids, freezing and thawing resistance is adequate. This increased setting time is understandable as the cement content is reduced and the reaction happen at a slower rate.

Vengata [7] has reported that the permeability of HVFA decreases considerably, even though the strength of fly ash concrete at 28 days is not encouraging. Suvimol Sujjavanich [8] concluded that HVFA concrete has lower chloride permeability and minimizes corrosion risk.

Heat of Hydration Replacement of cement with fly ash reduces the heat of hydration. Also, HVFA reduces the shrinkage cracks, due to the reduced water content and also due to the decrease in the total volume of the cement paste. The lower heat of hydration reduces the cracking due to thermal shrinkage.

The quality of fly ash also influences the strength gain and hence the creep strain.

Mechanical Properties of High Volume Fly Ash Concrete Reinforced with Hybrid Fibers

Compressive Strength The increase in compressive strength is dependent on the volume of cement replaced, the age of concrete and type of flyash. The early age strength gain is higher with class C fly ash than Class F fly ash. The long term compressive strength is higher when class F fly ash is used due to its long term pozzolanic strength contribution. The higher long term strength is also due to the smaller capillary pores and dense microstructure resulting from the pozzolanic reactions.

Hence adequate curing to a minimum of 7 days is essential to ensure that the later age strength development takes place. To attain early age strength, low water cement ratio is essential. Early compressive strength is a function of the coarseness and content of the fly ash used. He also reported that compressive strength, modulus of elasticity and abrasion resistance decrease at 28 days properties of high lime fly ash concrete age.

Flexural Strength The flexural test is one measure of the tensile strength of unreinforced concrete to resist failure in bending. Addition fibers Improves the flexural strength. If the elements such as footings, walls, columns and beams do not require early age strengths, then HVFAC can be used with a minimum of 7 days curing.

If 7 days curing cannot be provided, lower quantity of fly ash has to be used.