Nearly 60% of the electricity in the USA is produced by coal-fired plants, resulting in the production of a large quantity of coal combustion residues, among which coal fly ash is the major component. Many applications have been developed for coal fly ash. The use of coal fly ash as a cement replacement in concrete is the most attractive because of its high volume utilization and widespread construction. The replacement of cement with coal fly ash in concrete can have many advantages including better workability, higher long-term strength and improved durability of the concrete.
However, the use of fly ash is also accompanied by increased setting time and decreased early strength. Different techniques, such as grinding, elevated temperature curing and use of chemical activators, are developed to overcome those disadvantages. A comparison indicates that the use of chemical activator is the most effective and efficient technique to activate the potential pozzolanic reactivity of coal fly ashes and to improve the properties of the fly ash concrete.
Grinding of coal fly ashes
It is generally agreed that the pozzolanic reaction is a dissolution-precipitation process at early ages, which is determined by the surface area of fly ash particles. Many researchers have confirmed that the pozzolanic reactivity of fly ashes correlates well with the specific surface area of fly ashes. The size of fly ash particle ranges from 0.5 to greater than 200 µm, depending on the dust collection system. Separation technique has been used to separate those fine particles in fly ash in order to obtain highly reactive fly ashes. However, it is a complicated process and portion of small fly ash particles is very low in many cases.
Many researches have dealt with the effect of grinding on reactivity of various cementitious and pozzolanic materials. Prolonged grinding increases not only the surface area of a material, but also the number of imperfection or active centres which exist at the edges, corners, projections and places where the interatomic distances are abnormal or are embedded with foreign atoms. These centres are in a higher energy state than in the normal structure. The more the active centres are. Millers and Oulton, for example, have observed that percussive dry-grinding can cause obvious crystal distortion of kaolinite. Recently, it has been found that impaction and friction milling of high alumina cement alter its crystallinity and notably modifies its hydraulic behaviour .
In an early investigation on the reactivity of 6 siliceous rocks, Alexander observed that siliceous materials such as quartz and basic or devitrified volcanic rocks, which are not regarded as likely sources of active pozzolan, become highly reactive when ground into ultrafine powders. This is due to the presence of a disturbed layer of highly reactive material, which is formed on the surface of the mineral particles as a result of prolonged grinding. However, if grinding is sufficiently prolonged, an upper limit of activity is attained, beyond which continued increase in surface area does not produce any further increase in pozzolanic reactivity. A recent study indicated that the strengths of lime-natural pozzolan cements are linearly proportional to the Blaine fineness of the natural pozzolan.
Several studies have reported the effect of grinding on the properties of fly ashes and cement and concrete containing fly ashes . Of course, grinding decreases the particle size of the fly ash. The longer the grinding, the finer the particle size is. However, the number of spherical particles also decreases with the increase of grinding time. A short period of grinding may further decrease the water requirement of fly ash cement due to the break up of plerospheres. Prolonged grinding will increase the water requirement due to the increase of irregular-shape particles. When a fly ash is too coarse to meet the fineness requirement, grinding can be a solution.
The replacement of Portland cement with fly ash accelerates the hydration of Portland cement, improves the workability of concrete, lowers early strength, but increases the later strength of the concrete. The pozzolanic reaction between lime and fly ash is a slow process. The potential pozzolanic reactivity of fly ashes can be activated through the grinding of fly ash, elevated temperature curing of cement or concrete containing fly ashes and the addition of chemical activators to cement or concrete containing fly ashes.
A comparison based on the performance and production requirements indicate that the addition of chemical activator(s) into concrete mixtures is the most efficient and inexpensive technique for enhancing reactivity of fly ashes compared with other techniques such as grinding of fly ash and temperature curing of fly ash concrete.
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