27-12-2012, 02:22 PM
A Brief Overview of Fly Ash Brick Production
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ABSTRACT:
Fly ash is a waste material of coal firing thermal plants and its accumulation near power plants causes severe pollution problems. Therefore, its utilization as a raw material for brick making will be a very beneficial solution in terms of economical and environmental aspects. In this paper, the brick industry of Turkey was briefly presented in general terms. The methods of FA brick production and the leaching behaviours of heavy metals were discussed. The methods of producing non-fired fly ash bricks seems to be an advantageous way to confront the challenges for environment and ecologically sustainable development. Additional economical benefits will also contribute to the Turkish economy while increasing demand for greener building products will be met locally and also globally.
INTRODUCTION
Fly ash (FA) arising from the combustion of coal is being accumulated as waste material in large quantities near thermal power plants. FA which has pozzolanic properties creates serious environmental pollution problems and poses serious operational constraint and environmental hazard as a recognized environmental pollutant. In Turkey, the disposal of FA is also a significant problem and 15 million tons of FA was generated by power plants in 2000 (Tütünlü and Atalay, 2001). This output is expected to reach 50 million tons by the year of 2020 (Tütünlü and Atalay, 2001). However, only about 3 % (weight per cent) FA, mainly for cement production, is being utilized for production of building materials which is a rather low utilization ratio compared with those in the countries such as Germany, Holland, Belgium, UK, USA, and China (Tütünlü and Atalay, 2001; Aruntaş, 2006; Lingbawan, 2009). Therefore, large volume utilization of FA as a main raw material to produce building elements will be a good solution to handle such a hugely polluting material raising environmental and economical concerns. For instance, its utilization in the manufacture of FA bricks (FAB) will not only create ample opportunities for its proper and useful disposal but also help in environmental pollution control to a greater extent in the surrounding areas of power plants.
In this paper, the brick industry of Turkey is briefly presented in general terms and the technologies used to manufacture bricks are shortly described. Methods to manufacture FAB as an alternative to conventional fired brick are briefly overviewed. Leaching behaviors of heavy metals are also discussed (Cengizler, 2009; Tanrıverdi, 2006).
TURKISH BRICK INDUSTRY
Raw material: The raw material of traditional brick industry is clay which is one of the most abundant natural mineral materials on earth. Clay for the production of brick must, however, possess some specific properties and characteristics. To satisfy production requirements, clays must have plasticity, which permits them to be shaped or molded when mixed with water; and they must have sufficient wet and air-dried tensile strength to maintain their shape after forming. Also, when subjected to rising temperatures, the clay particles must fuse together.
Clays occur in three principal forms such as surface clays, shales and fire clays, all of which have similar chemical compositions but different physical characteristics. Surface clays may be the upthrusts of older deposits or of more recent, sedimentary formation. As the name implies, they are found near the surface of the earth. Shales are clays that have been subjected to high pressures until they have hardened
almost to the form of slate. Fire clays are usually mined at deeper levels than other clays and have refractory qualities. Clays are complex materials; surface clays and fire clays differ from shales more in physical structure than in chemical composition. Chemically, all three are compounds of silica and alumina with varying amounts of metallic oxides and other impurities. Although technically metallic oxides are impurities, they act as fluxes, promoting fusion at lower temperatures. Metallic oxides (particularly those of iron, magnesium and calcium) influence the color of the finished fired product.
The manufacturer minimizes variations in chemical composition and physical properties by mixing clays from different locations in the pit and from different sources. However, because clay products have a relatively low selling price, it is not economically feasible to refine clays to produce uniform raw materials. Since variations in properties of raw materials must be compensated for by varying manufacturing processes, properties of finished products from different manufacturers will also vary somewhat.
METHODS OF FAB PRODUCTION
For more than two decades, researchers have been investigating the viability of using FA for brick making. Prior to discussing them shortly, a simple classification of the methods of manufacturing FAB must be done. Generally, two fundamentally different approaches exist to make brick and other building products from FA. One is the traditional way of brick making by firing the brick material except that the FA substitutes a portion of clay or entire amount of clay in brick making. The other is based on the self cementing property of Class C FA that contain a large amount of calcium. In this case, firing or heating in kilns is not needed to obtain the final product. Instead, the bricks produced can be cured in the same way concrete is cured. Therefore, hereafter, the bricks made from FA with different clay replacing ratios fired at high temperatures such as 950-1200 oC will be referred to as fired FA bricks (FFAB) and the ones
produced by making use of other means at ambient temperatures rather than sintering will be referred to as non-fired FA bricks (NFFAB).
DISCUSSION
Of the two main methods mentioned above, the route of making fired FFAB is very similar to conventional clay brick production. Therefore, FA can be easily incorporated in conventional brick manufacturing without making any major change in the production line. These methods mentioned above (Tütünlü and Atalay, 2001; Chou et al., 2006; Chou et al., 2001; Kute and Deodhar, 2003; Lingling et al., 2005; Pimraksa et al., 2001; Kayali, 2005; Cengizler, 2008) are easily adaptable by existing clay brick factories. But, during firing process, sulfur dioxide is normally released from burning process. Besides, making fired FFAB consumes expensive heat energy which is very important from economical view point. Therefore, the second route of making NFFAB seems to be the trend to follow in terms of environmental concerns.
CONCLUSION
Several studies conducted on the viability of manufacturing fired brick partially or 100 % made of FA led to rather promising results (Chou et al., 2006; Chou et al., 2001; Kute and Deodhar, 2003; Lingling et al., 2005; Pimraksa et al., 2001; Kayali, 2005; Cengizler, 2008; Çiçek and Tanrıverdi, 2007 Tanrıverdi ve Çiçek, 2007b; Çiçek ve Tanrıverdi, 2004; Pimraksa et al., 2006; Chindaprasirt and Pimraksa, 2008). The use of such bricks is authorized in many countries such as Germany, the United Kingdom and India (Lingbawan, 2009). There are also a number of patents on the use of FA–lime mixtures for making unfired bricks (Lingbawan, 2009). Environmental concerns raised in some parts of the world, such as in India, even resulted in legislation that obliged the brick industry to incorporate at least 25 % FA in the brick making mixture if the industry was within 100 km from a coal power generation plant (Lingbawan, 2009).