Fly ash has a high amount of silica and alumina in a reactive form. These reactive elements complement   hydration chemistry of cement. Let us take a quick tour through this exciting world of hydration chemistry. Then cement reacts with water, we say that hydration of cement has began.  On hydration, cement produces C-S-H Gel. This C-S-H Gel binds the aggregates together and strengthens our concrete!

However, one more compound is produced on hydration that is so different in behaviour.  It is non other than the Calcium Hydroxide Ca(OH)2. In our construction industry, it is generally referred to as Free Lime. Aggressive environmental agents like water, sulphates,CO₂ attack this free lime leading to deterioration of the concrete.

In the beginning of the twentieth century, fly ash was used only for the mass concrete applications?to delay the heat of hydration. However, in the early 80?s,  with the advent of the high strength cements, the undesirable side effects of free lime started surfacing.

Is free lime really bad for concrete?

No a certain amount of free lime is necessary to keep our concrete alkaline. The problem arises when our new generation - 53 grade - cements produce excessive lime which leads to the deterioration of concrete, leading to corrosion.The cement technologists observed that the reactive elements present in fly ash convert the problematic free lime into the beneficial C-H-S Gel.

It is not only the chemistry provided by fly ash that  compliments chemistry of cement, but also the physical properties of fly ash improve the rheology and  microstructure of concrete by a great extent. Fly ash, on itself, can not react with water. It needs free lime, produced on hydration of Portland cement, to trigger off its Pozzolanic effect. Once it is triggered, it can go on and on!

What Does All This Mean?

In simple words, all this means a much longer life for our concrete structure. Specific benchmarks  have been set up to evaluate the performance of concrete with respect to durability?mainly Strength and Permeability. This means to produce a durable and long lasting concrete, it must posses high strength and Low permeability

Fly ash makes concrete denser, and hence less permeable, mainly by 1. Reducing water demand in concrete and 2. Improving microstructure of concrete. At the same time, fly ash improves long term strength of concrete due to the continued Pozzolanic reaction as  discussed earlier. Following graph typically depicts the strength development with fly ash.

This is a first concrete dam of India built with roller compaction technology.  Now, for a concrete dam, it is vital that  the  heat of hydration be reduced to the  minimum. Who else could do this magic except fly ash? The Bureau of Indian Standards restricts the replacement of cement to maximum 35% with fly ash. That means, Portland Cement always remains the Big Brother.

• It delays the heat of hydration and hence reduces the thermal cracks in concrete
• It improves the workability of concrete
• It makes the mix homogeneous and hence reduces segregation and bleeding
• The concrete finish is improved due to perfectly spherical fly ash particles
• The concrete permeability is substantially reduced which enhances the life of the structure
• Fly ash contributes to the long term strength in concrete
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• Making Earth Greener?.
  

One of the greatest benefits of utilising fly ash is building a greener future.

Fly ash in itself is a waste product and it is a huge problem for disposal. From each power station, thousands of tons of fly ash are pumped into the ash lagoons every day. These fly ash lagoons are occupying millions of acres of agricultural land all over India.

When we replace every single ton of cement with fly ash, we are not only eliminating waste from our planet but also we are reducing the emission of 0.85 tons of CO2 into our atmosphere. This is a unique Win - Win solution offered by fly ash and it is no wonder that Fly Ash is at the forefront amongst the green technologies being now adopted all over the world  by leading organisations.

We have a lot of fly ash in our country -  110 million tons a year!! However, only 10% of this ash meets the requirement of an excellent cement substitute material. What do you do with the remaining 90% ash? You need investment in  plant and equipment so that more people get the benefit, structures are durable, our Earth becomes greener and for the need of strict quality assurance. No wonder, Pozzocrete - the first and only fly ash product certified with the prestigious ISI mark - is recommended by leading professionals!