Fly Ash Concrete

Chemical Composition

Fly Ash

What is Fly Ash Historical Background Chemical Nature Classification
Chemical Composition Mechanical properties Benefits High Volume
Fly Ash
Need for extending Strength Specifications to beyond 28 days – Restraints on use of Fly Ash in Highway Constructions

Chemical Composition of Fly Ash

The table below shows typical compound analyses for two fly ashes and a natural pozzolana (Class F fly ash, Class C fly ash and Class N – Natural) and ordinary portland cement.

fly ash : chemical composition

A glance at the table reveals:

1. The same compounds exist in fly ash and portland cement. Those of fly ash are amorphous (glassy) due to rapid cooling; those of cement are crystalline, formed by slower cooling.

2. The major difference between fly ash and portland cement is the relative quantity of each of the different compounds. Portland cement is rich in lime (CaO) while fly ash is low. Fly ash is high in reactive silicates while Portland cement has smaller amounts.

IS 3812 (Part 1): 2003 (Pulverized Fuel Ash — Specification for use as Pozzolana in Cement, Cement Mortar and Concrete) specifies Chemical requirements for Fly ash as tabulated below:

Chemical Requirements

S.No Characteristics Requirements for Siliceous Pulverized Fuel Ash Requirements for Calcareous Pulverized Fuel Ash
1 Silicon dioxide (SiO2 ), plus aluminium oxide (Al2O3), plus iron oxide (Fe2O3) percent by mass, min 70 50
2 Silicon dioxide (SiO2 ), percent by mass, min 35 25
3 Reactive silica in percent by mass, min 20 20
4 Magnesium oxide (MgO), percent by mass, max 5 5
5 Total Sulphur as Sulphur tri oxide (SO3), percent by mass, max 3 3
6 Available alkalis as sodium oxide (Na2O), percent by mass, Max 1.5 1.5
7 Total chlorides in percent by mass, Max 0.05 0.05
8 Loss on ignition, percent by mass, Max 5 5


Concrete-Techgroup thanks Headwaters Resources , USA for granting them permission to use their resources in developing this article.