Fly Ash Powder

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Optimizing the Use of Fly Ash in Concrete :

• Concrete

Introduction :

• Fly ash is used as a supplementary cementitious material (SCM) in the production of portland cement concrete. A supplementary cementitious material, when used in conjunction with portland cement, contributes to the properties of the hardened concrete through hydraulic or pozzolanic activity, or both. As such, SCM's include both pozzolans and hydraulic materials. A pozzolan is defined as a siliceous or siliceous and aluminous material that in itself possesses little or nocementitious value, but that will, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide at ordinary temperatures to form compounds having cementitious properties. Pozzolans that are commonly used in concrete include fly ash, silica fume and a variety of natural pozzolans such as calcined clay and shale, and volcanic ash. SCM's that are hydraulic in behavior include ground granulated blast furnace slag and fly ashes with high calcium contents (such fly ashes display both pozzolanic and hydraulic behavior).
• The potential for using fly ash as a supplementary cementitious material in concrete has been known almost since the start of the last century (Anon 1914), although it wasn't until the mid-1900s that significant utilization of fly ash in concrete began (for example, USBR 1948) following the pioneering research conducted at the University of California, Berkeley (Davis 1937). The last 50 years has seen the use of fly ash in concrete grow dramatically with close to 15 million tons used in concrete, concrete products and grouts in the U.S. in 2005 (ACAA 2006). Historically, fly ash has been used in concrete at levels ranging from 15% to 25% by mass of the cementitious material component. The actual amount used varies widely depending on the application, the properties of the fly ash, specification limits, and the geographic location and climate. Higher levels (30% to 50%) have been used in massive structures (for example, foundations and dams) to control temperature rise. In recent decades, research has demonstrated that high dosage levels (40% to 60%) can be used in structural applications, producing concrete with good mechanical properties and durability (Marceau 2002).
• Increasing the amount of fly ash in concrete is not without shortcomings. At high levels problems may be encountered with extended set times and slow strength development, leading to low early-age strengths and delays in the rate of construction. These drawbacks become particularly pronounced in cold-weather concreting. Also, the durability of the concrete may be compromised with regards to resistance to deicer-salt scaling and carbonation.
• For any given situation there will be an optimum amount of fly ash that can be used in a concrete mixture which will maximize the technical, environmental, and economic benefits of fly ash use without significantly impacting the rate of construction or impairing the long term performance of the finished product. The optimum amount of fly ash will be a function of wide range of parameters and must be determined on a case-by-case basis.
• This report discusses issues related to using low to very high levels of fly ash in concrete and provides guidance for the use of fly ash without compromising the construction process or the quality of the finished product. For the purposes of this document the replacement levels shown in Table 1 will be used to represent low, moderate, high and very high levels of fly ash.

The advantages of fly ash concrete over the corresponding plain cement concrete are :

• Improved workability.
• Lower heat of hydration.
• The increase in creep with fly ash content upto 15% is negligible.
• Increases the modulus of elasticity of concrete when concretes of the same strength with and without fly ash are compared.
• Superior resistance to freezing and thawing.
• Improved sulphate resistance.
• Lower water and air permeability.
• Lower leaching of lime liberated during hydrated of cement.
• Reduced alkali-aggregate reactions.
• Greater resistance to attack of aggressive waters.