Why Fly Ash Bricks Are Better Than Clay/Red Bricks

It is estimated that in India each million clay bricks consume about 200 tons of coal and emit around 270 tons of CO2; on the other hand, with fly ash bricks production in an energy-free route, there are no emissions.
Dr. N. Subramanian, Consulting Engineer,Gaithersburg, MD, USA


Dr. N. Subramanian, Consulting EngineerFly-Ash Bricks (source: Ref. 1)

The increase in greenhouse gases, of which CO2 is one of the major constituents, increases the global warming potential year after year, leading to increases in flooding, fires, hurricanes, and billions of dollars in annual damage. According to the National Oceanic and Atmospheric Administration, since the start of the Industrial Revolution in about 1750, CO2 atmospheric levels have risen steadily, causing the earth’s temperature to increase at an unusually rapid pace. Due mostly to the burning of fossil fuels, CO2 levels in our atmosphere have risen 69%, from about 280 parts per million (ppm) in 1750 to 416 ppm in April 2021(a 31% increase from the 1958 levels; 280-300 ppm may be considered as the ideal level of CO2 for human life). As a consequence, according to the National Aeronautics and Space Administration, the global average surface temperature rose from –0.07 to 0.98 degrees Celsius between 1900 and 2019—leading to 19 of the 20 warmest years on record, all of which occurred in the past couple of decades.

Although CO2 alone is singled out in many publications, there are other gases like methane (CH4), nitrous oxide (N2O), and chlorofluorocarbons (HFCs), that have a much greater effect on global warming than CO2, but their concentration in the atmosphere is less – collectively they are called greenhouse gases (For example, methane is 22 times more potent in global warming effect than CO2). It must be noted that CO2 and other gases, which exist naturally in the atmosphere, retain the Sun’s heat and create an atmosphere that sustains life on earth. The energy consumption in the USA has been broken down into five categories: electricity or power plants (consuming about 40% of the total each year), transportation (28%), industry (21%), residential (7%), and commercial (4%). Overall, CO2 emissions are roughly connected with energy consumption, meaning the largest emitter was the most obvious - electricity.

The current use of fossil fuels, which may be depleted in another 40-50 years, has resulted in the release of huge amounts of green-house gases, especially carbon dioxide (CO2), which is harmful to the environment. In 2015, the global energy-related CO2 emissions were at the level of 49 giga tons per year (Gt/yr), with over 80% coming from fossil fuel combustion. Cement and building material industry is one of the major contributors. Buildings contribute about 40% of global CO2 emission. The CO2 emission is about 90 million tons out of cement and 49 million tons out of clay bricks production in India.

It is estimated that in India each million clay bricks consume about 200 tons of coal (or any other fuel with equal quantity of thermal values) and emit around 270 tons of CO2. Fly ash bricks production in energy-free route saves the emissions totally, befitting the project to qualify under Clean Development Mechanism (CDM), as envisaged by Kyoto Protocol towards the welfare of Mother Earth. The various types of fly-ash bricks are discussed below:

Pulverized Fuel Ash Lime Bricks and Fal-G Bricks
These are made using fly ash in major quantity (conforming to IS 15648:2006), lime, and an accelerator acting as a catalyst. Pulverized fuel ash-lime bricks are generally manufactured by inter-grinding or blending various raw materials, which are then moulded into bricks and subjected to curing cycles at different temperatures and pressures. Crushed bottom fuel ash or sand may also be used in the composition as a coarser material to control water absorption in the final product. Pulverized fuel ash reacts with lime in the presence of moisture to form a calcium-silicate hydrate which acts as a binder. Thus, pulverized fuel ash-lime brick is a chemically bonded brick and hence does not require firing. The specifications for burned clay fly ash bricks are provided in IS 12894:2002. The minimum compressive strength, water absorption, and efflorescence requirements of this kind of brick are similar to that of burned clay fly ash bricks. These bricks are suitable for use in masonry construction just like common burned clay bricks. In addition to the advantages offered by burned clay fly ash bricks, these bricks have the following advantages.
  • As no burning of bricks is involved, saves energy and there is also no pollution. In addition to helping the disposal of fly ash (which is a waste product), it also conserves topsoil.
  • The bricks can be made locally, even at the job site.
  • These bricks have better mechanical properties and more durability.
  • They are about 10–20% cheaper than burned clay bricks.
The FaL-G (Fly ash-Lime-Gypsum) process was developed and patented by Dr Bhanumathidas and Mr. Kalidas and introduced in India during 1991(see www.fal-g.com). In this process, 60% fly ash, 30% sand, and 10% Portland cement are used. In the lime route, the composition is fly ash (62%), slaked lime (8%), anhydrite gypsum (5%), and stone dust/sand (25%). These ingredients are manually fed into a pan mixer, where required amount of water is added and intimately mixed. The mixture is then fed into brick making machines, and the manufactured bricks are dried and water cured for 14 days. The calcium aluminate is converted into calcium alumino-sulphate resulting in a product having high early strength. FaL-G bricks can be produced with compressive strength of 10–35 N/mm2, and water absorption of 8–15% (Bhanumathidas and Kalidas, 2003). It is notable that FaL-G bricks do not need any pressure and are cured at ambient temperature of 20–40oC. By avoiding both pressing and heating chambers, high economy is achieved and this resulted in the proliferation of 18,000 manufacturing units in India as of 2016.

Fly Ash Bricks
These are made using Class C or Class F fly ash and water. They are compressed at 28 MPa and cured for 24 h in a 66oC steam bath, then toughened with an air entrainment agent. Owing to the high concentration of calcium oxide in Class C fly ash, the brick is described as ‘self-cementing’. Use of fly ash to make the bricks results in the elimination of the whole processes of mining, transporting, mixing and grinding, and firing that are necessary in the case of the clay- and shale-based bricks. The manufacturing method saves energy, reduces pollution, and costs 20%less than traditional clay brick manufacturing.

A comparison of the properties of clay/red bricks with fly ash bricks is given in Table 1

Table 1 Comparison of Red/clay bricks with fly ash bricks
Properties

Clay/red BricksIS 1077:1992

Fal-G /fly ash bricksIS 12894:2002

Advantages of fly ash bricks

Colour Vary Uniform Good appearance
Density 15.70 -18.85 kN/m3 10 -18.85 kN/m3 Higher load bearing
Compressive strength > 5.5 MPa 10-30 MPa Higher load bearing
Water absorption 15-20% 15-20% Less dampness seen on walls
Dimensional stability Very low tolerance High tolerance Saving in mortar up to 25%
Wastage during transit Up to 10% Less than 2% savings in  cost up to 8%
Plastering Thickness may vary on both sides of the wall Thickness may be the same on both sides of the wall May result in savings in plaster  cost up to 15%
Environmental advantage Nil Absorbs CO2 from the atmosphere Eco-friendly
As the fly ash bricks may not be smooth like clay bricks, plastering on them is easy.
References
  • https://theconstructor.org/building/fly-ash-bricks/
  • Subramanian, N., Building Materials, Testing and Sustainability, Oxford University Press. New Delhi, 2019, 788 pp.
  • Subramanian, N. (2022), “Achieving Net-Zero CO2 Emissions in the Concrete Industry”, Civil Engineering & Construction Review (CE & CR), Vol. 35, No.4, Apr., pp. 32-41.
NBM ICCT, November-December 2022
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