Study on Fly Ash

Topics: Cement, Portland cement, Concrete Pages: 16 (4729 words) Published: April 3, 2014
COAL FLY ASH – THE MOST POWERFUL TOOL
FOR SUSTAINABILITY OF THE
CONCRETE INDUSTRY
P.K. Mehta
Professor Emeritus
Civil and Environmental Engineering
University of California, Berkley, CA 94720

Introduction
It is clear that industrialization of the world is happening at an unsustainable speed. High rates of energy consumption, short service life of manufactured products, and lack of space for safe disposal of huge volumes of solid, liquid, and gaseous wastes generated by human activities are among the important sustainability issues. However, global warming has emerged as the most serious sustainability issue of the 21st century. According to a World Watch Institute report, twenty-four of the last 27 years have been the warmest on record. Weather scientists around the world have determined that a linear relationship exists between the earth’s surface temperature and the atmospheric concentration of CO2, which makes up 85 % of the greenhouse gases. The current CO2 concentration, 387 ppm (mg/L), according to the U.S. National Oceanic and Atmospheric Administration, is highest in the recorded history. The annual global CO2 output has reached a staggering 30,000 million tonnes. Burning fossil fuels to meet the energy and transportation needs of the world is the largest source of CO2 emissions. Significant amounts of CO2 are also released during the manufacture of building materials, such as cements, clay bricks, and steel.

In a series of reports, issued in 2007 by the United Nations Intergovernmental Panel on Climate Change, leading weather scientists of the world have unequivocally stated that global warming is occurring, and that it has been triggered by human activities. They have warned about devastating consequences of global warming if immediate action is not taken by national and industry leaders to reduce the annual carbon dioxide emission rate to the 1990 level or less.

Although climate change is a global phenomenon, it has to be tackled by every country individually, and by each of the major CO2 emitting sectors of the economy. In this paper, the author has shown that both for the short term and the long term fly ash is highly effective in reducing the carbon footprints associated with the use of portland-cement clinker, which is the principle ingredient of modern cements.

Carbon Dioxide Emissions From Cement Kilns
The subject of environmental impact of the cement industry is covered by numerous publications (1-6). Typically, ordinary portland cement is composed of 95 % clinker and 5 %

gypsum. Gypsum is known as a complementary cementing material (CCM) because it enhances the cement performance by improving the setting and hardening characteristics of the product. In addition to gypsum, sometimes other mineral additives, commonly known as supplementary cementing materials (e.g., coal fly ash, granulated blast-furnace slag, natural and calcined pozzolans, pulverized limestone, and silica fume) can either be interground with clinker and gypsum or added directly during the concrete mixing operation. Pozzolanic additives like coal fly ash, when used as cement replacement materials, considerably enhance the performance of portland-cement concrete mixtures, and are emerging as an important player in sustainable development of the concrete industry. Therefore, instead of being referred to as supplementary cementing materials (SCM), it is more appropriate to refer to them as complementary cementing materials (CCM).

Global statistics for 1990 and 2005 on cement production, CCM consumption, and direct CO2 emission attributable to portland clinker manufacture, are presented in Table 1. According to the U.S. Geological Survey records, the world consumption of cement in 1990 was 1,044 million tonnes. It is estimated that, globally, the average clinker factor of cement (units of clinker per unit of cement) was 0.9, which means that 940 million tonnes of clinker and 104 million tonnes of CCM were used. The estimated...

References: 1. P.K. Mehta, and P.J.M. Monteiro, “Concrete: Microstructure, Properties, and Materials”,
McGraw-Hill, New York, 2006
3. The Concrete Center of U.K., “Sustainable Concrete”, www.concretecenter.com, 2007, 18 pages
4
www.wbcsdcement.org, Geneva, Switzerland, 2007
5
Vol. 28 No. 9, 2006, pp. 42-45
6
7, 2002, pp. 23-28
7
Coal Ash Association, Issue No. 1, pp. 7-11, 2007
8
Built with Green Concrete”, ibid., Vol. 24 No. 8, 2002, pp. 64-70
10
28 No. 7, 2006, pp. 37-42
11
Supplementary Cementing Materials for Sustainable Development, Ottawa, Canada, 2002
12
No. 7, 2007, p. 66-67
Table 1 Global Direct CO2 Emission from Cement Kilns (Million Tonnes)
CITRIS Building, Univ. of California, 2007
Fig
CITRIS Building, Univ. of California, 2007
Fig
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