The Fly Ash story begins 2000 years ago...
when the Romans built the Colosseum in the year 100 A.D. - that still stands the test of time!!
The Roman Colosseum
The ash generated from Volcanoes was used extensively in the construction of Roman structures. Colosseum is a classic example of durability achieved by using volcanic ash.
This is a building constructed 2000 years ago and still standing today!
Volcanic Ash acts just like our Fly Ash.
Only difference is, Fly Ash is generated in artificial volcanoes - non other than coal fired.
So what is so special in fly ash that makes our concrete so durable?
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: -
When 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.
From Mass Concrete To Mass Applications
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.
Ca(OH)2 + SiO2 --> C-S-H Gel
Ca(OH)2 + Al2O3 = C-Al-H gel
OR
Problem + Fly Ash --> Durable Concrete
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