Global Warming Is A Silent Killer : Fly Ash

The world’s eco-system is faced with the growing problem of global warming which is associated with the emission of CO2 into the atmosphere. It is a well known fact that for every ton of Portland cement produced, approximately one ton of CO2 is released. To reduce the CO2 emissions related to cement production, the use of Portland cement needs to be reduced without compromising the performance of concrete structures. Also, the use of conventional concrete, even in common current constructions has revealed itself to be technically and economically inadequate. The reinforced concrete structures deteriorate too rapidly, causing high maintenance and repairing costs, in addition to the related decrease of the structures’ service life. Fly ash is one of the residues generated in the combustion of coal. It is generally captured from the chimneys of coal-fired power plants and causes severe pollution of air and water, and its disposal gobbles up large tracts of land. Fly ash includes substantial amounts of silicon dioxide (SiO2), which is present in two forms: amorphous, which is rounded and smooth, and crystalline, which is sharp, pointed and hazardous, aluminium oxide (Al2O3) and iron oxide (Fe2O3). Fly ash are generally spherical in shape and range in size from 0.5µm to 100µm. Owing to its pozzolanic properties, fly ash can be used as a replacement for some of the Portland cement. As pozzolan greatly improves the strength and durability of concrete, the use of a fly ash is a key factor in their preservation. Fly ash can be categorized into two types: 1.CLASS F FLY ASH: It is produced by burning of older anthracite and bituminous coal. It is pozzolanic in nature and contains less than 10% lime. Possessing pozzolanic 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 cementitious compounds. Alternatively, the addition of a chemical activator such as sodium silicate can lead to the formation of a geopolymer. 2. CLASS C FLY ASH: It is produced from the burning of younger lignite or sub bituminous coal. In addition to having pozzolanic properties, it also has some self cementing properties. In the presence of water, class C fly ash will harden and gain strength over time. It generally contains more than 20% lime. Unlike class F, self cementing class C does not requires any activator. In addition to its chemical properties fly ash’s physical properties also improve the performance of concrete. Fly ash is composed of smooth, spherical particles, which can improve the flow of fresh concrete. This more fluid mixture also requires less water, increasing the strength of hardened concrete. Clearly, the water-reducing property of fly ash can be advantageously used for achieving a considerable reduction in the drying shrinkage of concrete mixtures.

Moreover concrete is strong under compression load, when it is under pressure or supporting weight. But under tension, when it is being pulled apart, wood is stronger than concrete. Wood fibres are usually cellulosic elements that are extracted from trees, straw, bamboo, cotton seed, hemp, sugarcane and other sources. When used paper is recycled, first the ink and coating are removed. Next to be removed is the filler, made of calcium carbonate or similar materials. The left part are wood fibres some of which are too small or damaged to be recycled back into new paper. The fibre can provide certain tensile strength to the concrete. The fibres basically stitch together the micro-cracks in concrete in order to improve the tensile strength. Also, the wood chippings can be possibly used as fibres in concrete. These types of material have several potential applications such as acoustic and thermal insulation, fire resistance, cladding, etc. If wood is used as an aggregate in concrete mixture, its macro pores will reduce the capillary absorption inside the material. This mixture will prove to be light weight. The network of capillaries in wood which allow sap circulation is responsible for the hydrophilic nature of wood. When mixed directly with cement the wood aggregates /chippings could potentially affect the water cement ratio which could limit the water available for hydration due to migration of water into the wood particles. One thing that must be taken into consideration is that the wood chippings must be saturated with water before mixing with cement in order to achieve adequate hydration of the cement. Some plant fibres, like rice husk have silica content which induces pozzolanic effect. However, if a fibre does not contain silica there is a possibility of introducing silica. The wood aggregate can be bonded with cement paste by soaking the wood aggregate in sodium silicate solution prior to mixing.

The misconception that high-strength will automatically lead to high-durability has probably resulted in many cases of cracking and premature deterioration of ultra high strength concrete structures. The dosage of fly ash has a beneficial effect on the workability and cost economy of concrete but it may not be enough to sufficiently improve the durability to sulphate attack, alkali-silica expansion and thermal cracking. The reason probably lies in the mix proportions, water to cement ratio, duration of curing, etc used to achieve very high strength. Another problem associated is regarding the incorporation of wood fibres. Experiments show that concrete reinforced with wood fibres do not sustain in humid areas. This problem may solved by making the wood fibres inert to the elements present in the humid atmosphere which cause deterioration of concrete structures.
For solving these limitations of fly ash and wood fibre (or any other potential natural fibre) rich ultra high strength concrete, experiments and research is necessary to be carried out. This research work intends to investigate the possibility of producing low cost enhanced performance concrete using materials like fly ash and locally available wood aggregates, chippings or plant fibres, keeping in view certain difficulties arising with its development.