Utilization of Fly Ash In Concrete

Utilization of fly ash appears to be technically feasible in the cement industry. There are essentially three applications for fly ash in cement including,

 replacement of cement in Portland cement concrete,

 pozzolanic material in the production of pozzolanic cements,

 set retardant ingredient with cement as a replacement of gypsum.

Cement is the most cost and energy intensive component of concrete. The unit cost of concrete is reduced by partial replacement of cement with fly ash. The utilization of fly ash is partly based on economic grounds as pozzolana for partial replacement of cement, and partly because of its beneficial effects, such as, lower water demand for similar workability, reduced bleeding, and lower evolution of heat. It has been used particularly in mass concrete applications and large volume placement to control expansion due to heat of hydration and also helps in reducing cracking at early ages. This also reduces the cost of construction. Fly ash concrete provides much strong and stable protective cover to the steel against natural weathering action.

Because of the presence of cementitious compounds of calcium and a reactive glass, the high-calcium fly ash is quite suitable in Portland cement products. Several studies are being conducted to better understand the complexities of alkali aggregate reactivity and sulphate resistance with respect to fly ash in concrete. The availability

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of high-lime fly ash containing compounds found in cement has led to high strength concretes produced by the addition of fly ash and plasticizers. High-strength and high performance concrete can also be made with Class F fly ash. The utilization of fly ash in concrete produces less permeability because of the spherical particles, and therefore improved packing, i.e. more dense paste and pozzolanic reaction. In mass concrete, with high percentage replacement of cement with fly ash, there is a lower heat of hydration compared to straight Portland cement concrete, particularly when Class F fly ash is used. Class C fly ash may not lower the heat of hydration.

Mehta has discussed the factors that contribute to attack of sulphates on fly ash concrete. As noted in his review paper on this subject, the agents responsible for concrete expansion and cracking are alumina bearing hydrates, such as calcium monosulfo-aluminate and calcium aluminate hydrate, which are attacked by the sulphate ion to form ettringite and calcium trisulfoaluminate. Acidic type interactions between sulphate ions and calcium hydroxide also lead to strength and mass loss. Exhaustive research has been conducted on fly ash admixture concrete and its properties. Research showed that fly ash used as an additive to Portland cement has a number of positive effects on the resulting concrete. A decrease in water demand, decreasing the water:cement ratio. An improvement of the packing of particle size decreases air entrainment in the concrete.

Fly ash increases resistance to corrosion, and ingress of corrosive liquids by reacting with calcium hydroxide in cement into a stable, cementitious compound of calcium silicate hydrate. The original calcium hydroxide was soluble, whereas the calcium silicate hydrate is less soluble in fly ash concrete, thereby reducing the possibility of leaching of calcium hydroxide from the concrete. In addition to calcium silicate hydrate being less soluble, reaction products tend to the filling of capillary voids in the concrete mixture, thereby reducing permeability of the concrete.

Addition of fly ash as an admixture increased early age compressive strength and long term corrosion resistance characteristics of concrete. Abrasion resistance of concrete made with Class C fly ash was better than both concrete without fly ash and concretes containing Class F fly ash. High volumes of Class C and Class F fly ash can be used to produce high-quality pavements in concrete with excellent performance. Blending of Class C fly ash with Class F fly ash showed either

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comparable or better results than either of the control mixture without fly ash or the unblended Class C fly ash.

HVFAC refers to concrete where fly ash comprises more than 30% of the total cementitious materials. HVFAC has a lower cost and is more durable than conventional concrete, and affords improved resistance to alkali-sulfate reaction and sulfate attack [18].

Table 2.3 Benefits of using Fly Ash in concrete [72].

Concrete Properties Impact Of

Fly Ash Water demand of fresh concrete Reduced Component segregation of fresh concrete Reduced Workability of fresh concrete Increased Setting time of fresh concrete Elongated

Bleeding on fresh concrete Reduced

Hydration of fresh concrete Reduced

Early strength of hardened concrete Reduced Long term strength of hardened concrete Reduced Permeability of hardened concrete Reduced Freezing-Thawing of hardened concrete Reduced

Alkali-Aggregate reaction Reduced

Sulfate resistance Increased

Concrete casting in hot weather Made easy Influence On Water/Cement Ratio

The use of fly ash as an additive in Portland cement will generally reduce the water demand of the mix. Typical Portland cements require 150-250 L/m3, water, representing 15-25% of the total volume of the mix. As the water content increases, so does the drying shrinkage of the concrete and a less desirable condition is created. By adding ash to the mix, the water content can be reduced, due to the lubricating qualities of the fly ash. The ash acts as tiny 'ball bearings' and improves the workability of the cement, while decreasing the water demand of the mix. Generally, a reduction of 15-20 L/m3 water can be attained by fly ash addition to the cement mix [19].

19 Influence on Curing Temperature

In order to determine the influence of fly ash on the cement hydration and the compressive strength development, Maltaisand and Marchand made an investigation, various mortar specimens were prepared with an ordinary portland cement and two different North-American Class F fly ashes (according to ASTM C 618). The mortar mixtures were produced at a constant water/binder ratio of 0,50 and at a binder/sand ratio of 2,5. In order to evaluate the effect of fly ash on cement hydration, fly ash was used at a cement replacement level of 10%, 20% and 30% (by total mass of binder). The mortar specimens were subjected to an isothermal curing temperature of 20°C and 40o

C. Test results indicate that the use of fly ash