Feature

Filling the Voids

March 2011

In the first of CW's series of features dedicated to showcasing experiences with building materials, Cyrus K Pithawala, Director, Hiranandani Group of Companies, tells us about his own pioneering use of fly ash.

Fly ash is trendy today, a material whose virtues are extolled by the movers and shakers of the construction industry. That wasn't always the case. Many believed it would reduce concrete strength; others rued the fact that there was no consistency in terms of its availability; and no literature was available about its use and applications. At such a time, one man believed. Cyrus K Pithawalla, Director, Hiranandani Group of Companies, immersed himself in R&D for half a year and used fly ash in his own proportion; the result of his experiment was put before RCC consultants to evaluate. It was a revelation. Suddenly, fly ash had arrived, and how! In conversation with Shriyal Sethumadhavan, Pithawalla elaborates on his experiences with fly ash, while highlighting its advantages, applications and future. Excerpts....

What is fly ash?

The carbon dioxide released by one cement plant is equivalent to the pollution caused by about 10,000 cars per day. One school of thought was that the use of cement provided the required strength for concrete. Slowly, this concept morphed to the need for a cement-based material that would be finer than cement but could be used in the works treated by cement. There was a myth in the construction industry that fly ash reduces the strength of concrete. This construction material is primarily derived from coal-fired electricity generating power plants. It is the mineral residue produced by burning coal. It is captured from the power plant's exhaust gases and further collected for use. The difference between fly ash and Portland cement becomes apparent under a microscope. Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures. This is one of the unique properties that make fly ash a desirable admixture for concrete.

Theory of voids

As far as aggregate and cement is concerned, there is a simple theory known as the Theory of Voids. Gaps between two aggregates have to be filled to achieve good homogeneous concrete. The components of concrete are cement, fine aggregate and coarse aggregate, and then you have fly ash. So in the first sphere, the coarse aggregates that are largest in size are filled by the smaller aggregates. As far as concrete is concerned, with the addition of fly ash, the voids in cement also get filled. In concrete making, cement is mixed with water to create the 'glue' that holds strong aggregates together. Fly ash works in tandem with cement in the production of concrete products. Concrete containing fly ash is easier to work with because the tiny, glassy beads create a lubricating effect that causes concrete to flow and pump better, to fill forms more completely, and to do it all using up to 10 per cent less water. Because the tiny fly ash particles fill microscopic spaces in the concrete, and because less water is required, concrete using fly ash is denser and more durable. And fly ash reacts chemically with lime that is given off by cement hydration, creating more of the glue that holds concrete together. That makes concrete containing fly ash stronger over time than concrete made only with cement. One has to take in to consideration that 5 per cent voids equal 30 per cent reduction in concrete strength. This makes the use of fly ash all the more essential.

Improving our environment

Thermal plants have huge piles of fly ash that lie unused. Acting against the 'greenhouse theory', this results in an increase in pollution. Also when fly ash is used, cement consumption is automatically reduced. It's a replacement method where the total volume of aggregates, cement and fly ash does not change. What changes is that if you add fly ash, cement consumption comes down proportionately. This in turn reduces carbon dioxide emissions, conserves energy and improves our environment.

Using fly ash

It is very important to use fly ash in a strategic manner. For instance, at Hiranandani, we replace 29 per cent with fly ash. So if you go beyond a percentage there could be complications and it may not give you the desired result. We produce high grade concrete up to M55 Grade with finer grade of processed fly ash using ordinary 10/7 mixer and keeping the cement content within the limit specified in IS 456 2000. It is used in high volume of almost 40 per cent in lower grades of concrete like M15. For high early strength, we use 29 per cent of fly ash in M30-M40 grades of concrete. Also, it's not just fly ash but a combination of so many materials that completes the story. These include super-plasticisers, one of the most important requirements for fly ash. Fly ash has to be properly monitored. We test at least three to four samples a month to ensure that it is in the correct grade. Being a waste product, there will be variations. Hence, you do not get an exact figure but a bandwidth - from a certain per cent to a certain per cent.

The precautions

Depending on strength, the requirement of fly ash has to be decided. For curing of concrete as per IS 456 2000, Page no:27 Clause no: 13.5.1, one must ensure 14 days of curing when fly ash is added to ensure that the secondary hydration process continues. Testing of fly ash must be done at regular intervals only in an approved laboratory as per IS 3812 2003 Part 1. For every consignment, a wet sieving test must be performed to check the percentage retained in 45 micron sieve to evaluate the fineness of fly ash; the same should be within 34 per cent limit mentioned in IS 3812 2003 Part 1. Further, the addition of fly ash on site must be done under strict supervision as per mix design and the water-binder ratio must be maintained. We use processed fly ash of different grades with varying fineness. Coarser fly ash is used for grades from M15 to M40 and finer fly ash is used for grades M45, M50 and M55.

Myriad applications

Fly ash finds application in the floor bedding of natural/artificial marble and tiles. It is used in road making and tremix concrete for better finish and in making the mortar for block masonry and plastering. It is also used in low-grade concrete to improve cohesion. We started using fly ash way back in 1998 - there isn't a single project in Mumbai we have done without fly ash. All our projects in Powai, Hiranandani Estate, Hiranandani Meadows, and our ongoing projects in Kandivali, Chandivali and Sahar are being executed using this material. We have not just used this material in bulk but have been pioneers in using this material. The material has contributed to our projects a great deal. Cement consumption has reduced and ing of concrete has reduced to a large extent. Whenever cement is mixed to create concrete, there is severe heat of hydration created that leads to s. However by adding fly ash, this heat has reduced to a great extent.

The question of availability

Initially, fly ash was not available in abundant quantity. But today, it can be sourced from two places in Mumbai: the Reliance plant in Dahanu and Dirk India's plant in Nashik. But more than availability, it's about lifting the material. Transportation of fly ash is the only thing that involves cost; else, this material is available at no cost.

The road ahead

Even today, people are sceptical about the use of fly ash. They opt instead for an imported and expensive product like micro silica fumes. On the one hand, we say India is a poor country and on the other, we invest in imported products without using resources available here. Basically, there is no substitute for fly ash. It replaces cement and has lubricant properties. As far as India is concerned, the more we use this material, the less will be the cement consumption, which in turn will reduce pollution. Also, by using this material in bulk, we will remove all the waste material from thermal plants. Engineers designing projects for public works and private developments can specify that building materials incorporate the use of fly ash. By using it in concrete and other products, architects, engineers and regulators are expressing a commitment to promote sustainable growth and exercise responsible building practices. Using fly ash is an exceptional way to 'build green', without compromising on cost or quality in concrete production.

Advantage fly ash!

Why should you use this cost-effective resource? Here are some compelling reasons:

• Reduced slump loss: More dependable concrete allows for greater working time, especially in hot weather.
• Reduced alkali silica reactivity: Fly ash combines with alkalis from cement that might otherwise combine with silica from aggregates causing destructive expansion.
• Workability: Concrete is easier to place with less effort, responding better to vibration to fill forms more completely.
• Ease of pumping: Pumping requires less energy and longer pumping distances are possible.
• Improved finishing: Sharp, clear architectural definition is easier to achieve, with less worry about in-place integrity.
• Reduced bleeding: Fewer bleed channels decreases porosity and chemical attack. Bleed streaking is reduced for architectural finishes. Improved paste to aggregate contact results in enhanced bond strengths.
• Reduced seggregation: Improved cohesiveness of fly ash concrete reduces seggregation that can lead to rock pockets and blemishes.
• Decreased permeability: Increased density and long-term pozzolanic action of fly ash, which ties up free lime, results in fewer bleed channels and decreases permeability.
• Increased durability: Dense fly ash concrete helps keep aggressive compounds on the surface, where destructive action is lessened. Fly ash concrete is also more resistant to attack by sulphate, mild acid, soft (lime hungry) water and seawater.
• Reduced efflorescence: Fly ash chemically binds free lime and salts that can create efflorescence and dense concrete holds efflorescence producing compounds on the inside.
• Reduced shrinkage: The largest contributor to drying shrinkage is water content. The lubricating action of fly ash reduces water content and drying shrinkage.
• Reduced heat of hydration: The pozzolanic reaction between fly ash and lime generates less heat, resulting in reduced thermal ing when fly ash is used to replace Portland cement.
• Spherical shape: Fly ash particles are almost totally spherical in shape, allowing them to flow and blend freely in mixtures.
• Ball-bearing effect: The 'ball-bearing' effect of fly ash particles creates a lubricating action when concrete is in its plastic state.

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