Pulverized Fuel Ash (PFA) or Fly Ash (FA)

The “pulverized fuel ash” (PFA) or the so-called fly ash (FA), used as a mineral admixture in cement and concrete, is a product of the pulverized coal firing system, through conventional boilers, mostly used in the thermal power plants. While carbon burns in oxidizing surroundings, the inorganic mineral matter gets sintered and liquefied at high temperature. The melt flows down the walls of the furnace and about 25% gets collected as “bottom ash” (BA). It is crushed before disposal. The rest, PFA or FA, gets entrained in the up-flowing hot gas in the form of fine particles, which get trapped in the economizer, air-preheater, mechanical separator, and, finally, battery of electrostatic precipitators (ESP).

As a general practice in many countries, PFA and BA are mixed with water and transported to ash ponds/lagoons. The ash thus deposited in lagoons is called “lagoon ash” (LA) or “pond ash.” It causes problems besides occupying huge stretches of agricultural land. Notwithstanding the greater utilization of PFA (and BA) in recent times in cement and concrete, in bricks, and for land filling, a large quantity of ash still lies unutilized.

As per several estimates, the cement industry contributes about 5% of the global generation of carbon dioxide. The cement industry’s sustainable program developed by the World Business Council for Sustainable Development (WBCSD) prepared an “Agenda for Action” for a 5 year period from 2002 to 2007, endorsed by the leading cement manufacturers of the world. The agenda addressed the issues of (a) climate protection, (b) fuels and raw materials use, and (c) emission reduction besides other issues.

Ref: “Mineral Admixtures in Cement and Concrete”, CRC Press (http://www.crcpress.com/product/isbn/9781439817926). Author: Dr J D Bapat (http://www.drjdbapat.com)

Written for engineers, book focuses on making more workable and durable concrete using mineral admixtures. For each mineral admixture, book looks at manufacturing and processing, physical characteristics, chemical and mineralogical composition, quality control, and reported experiences. It also examines the provisions of national standards.It encourages engineers to more effectively use these and other wastes in cement and concrete to support more sustainable growth of industry. Buy this book online to obtain 20 % discount and free shipping. Download details: http://bit.ly/online_purchase

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Self-cleaning Concrete:

Self-cleaning buildings and pollution-reducing roadways: these may sound like futuristic ideas but they are realities of some of today’s concrete. Recently introduced formulations of cement are able to neutralize pollution. Harmful smog can be turned into harmless compounds and washed away. The concrete can be imparted self-cleaning properties, using a special type of cement, the so called 'Photocatalytic Cement'. It uses photocatalytic process for the in situ degradation of polluting compounds. The cement is used in the same manner as regular Portland cement, gray or white. These products provide value through unique architectural and environmental performance capabilities. The cement gains the special properties incorporating fine particles of semiconductor material, titanium dioxide, during the process of manufacture.

There are two major cement companies distributing this type of cement in the United States. Italcementi, the Italian parent company of Essroc Cement, originally brought this product to market. They have recently licensed Lehigh White Cement Company to produce and market the material in Europe and North America. In the U.S., the photocatalytic cement is marketed under the brand name 'TX Active' available as gray or white Portland cement Type I, II, or III complying with ASTM C150. In Canada, TX Active is available as gray or white Portland Cement Type GU, MS, and HE, complying with CSA A3001. The product is offered in two formulations. The first, 'TX Arca', offers the self-cleaning properties against atmospheric compounds that stain concrete over time. The second, 'TX Aria', adds depolluting qualities to the self-cleaning properties of TX Arca.

Question:

Discuss the importance of imparting self-cleaning properties to concrete in the current scenario.

Self-cleaning surfaces:

TiO₂-containing photoactive materials represent an appealing way to create self-cleaning surfaces, thus limiting maintenance costs, and to promote the degradation of polluting agents. Several mortars containing anatase TiO₂, added as suspension or as powder, were characterized: among the photocatalytic mortars, the use of a combined additive (both powder and suspension) improved the material response. The best photoefficiency was exhibited by a mortar containing TiO₂ as surface covering; nevertheless, the adhesion problems shown by the surface layer open the way for future widening of investigations focused on the optimization of layer durability. Read more .. ..

Self-cleaning Concrete: Applications

The Photocatalytic Cement is already being used for sound barriers, concrete paver blocks, façade elements. Other applications include:

* Precast and architectural concrete panels
* Pavements, road surfacing and sidewalks
* Portland cement-based plaster for finish coat applications
* Concrete masonry units, roof tiles and cement-based tiles
* Cement-based restoration products

The best applications, for cost effectiveness, include any product used in thin layers or produced in a two-stage manufacturing process (with structural and face mixtures).

In one of the more innovative applications, Italcementi has successfully partnered with lighting system makers to develop artificial lighting systems that generate enough ultraviolet light to activate the photocatalytic process. The lighting system and photocatalytic cement concrete have been used in the renovation of a heavily trafficked tunnel in Rome.

The Photocatalytic cement has already been used in North America. A white precast concrete carillon tower was constructed at Dalton State College in Georgia. The 23 m tall tower is the centerpiece of a new quadrangle project and is visible from a nearby freeway, so it is important that it remains pristine.
Hyacinth Place, an affordable “green” housing complex in Highland Park, IL, has courtyard areas with concrete pavers incorporating photocatalytic cement. The pavers help clean the air as well as store and filter storm water that would have normally been nuisance runoff. The photocatalytic cement was used to
produce two 9 m tall gateway elements at the entrances to the new I-35 W bridge in Minneapolis, MN. These gleaming white concrete sculptures represent the
international symbol for water and serve as markers to remind travelers they’re crossing the Mississippi River. With the help of advanced technology and energy from the sun, they will remain proud symbols for decades to come.

The rain washes away the pollution from the concrete surface, hence buildings stay cleaner and do not require chemical applications that are potentially harmful to the environment. Maintenance costs are reduced. This is true even for buildings in highly polluted locations. One noted application is the Air France headquarters at Roissy-Charles de Gaulle International Airport near Paris, a white concrete building that has remained white. Another is the Church of the Year 2000 in Rome.

Clean buildings are great. Perhaps even more astounding environmental benefit is the potential for cleaner air. Concrete products that are exposed to sunlight throughout their life, like precast building panels, pavers and roof tiles, are especially suited to manufacture with photocatalytic cement. For instance, city streets made with special pavers are capable of reducing the pollution at its source—where it comes out of the tailpipe.

Question:

Discuss techno-economic feasibility of using Photocatalytic Cement in a building project

Reference:

(1) Barbesta M. and Schaffer D, "Concrete that cleans itself and the air", Concrete international, February 2009, pp 49-51