Effect of Chemico-Mineralogical Characteristics of Indian Fly ashes on the Properties of Blended Cement Concrete

The use of fly ash as supplementary cementitious material was originally motivated by its consistent pozzolanic activity and provide for sustainable development in the cement industry. India has been a pioneer in the manufacture of blended cements. Traditionally the blended cements in the country have been produced with calcined clay, mixes of calcined clay and fly ash, fly ash and granulated blast furnace slags. The improved understanding of the beneficial properties imparted by fly ash for both the plastic and hardened stages of blended cement concrete has led to a gradual growth in acceptance for the fly ash blended cements in India in the last decade. The enhanced durability of blended cement concretes has made it the preferred cement thereby replacing the earlier skepticism regarding its performance. The choice being governed by the judiciousness of quality control of the properties of the individual components at the manufacturing stage of the blended cement or at the Ready-Mix locations.

The paper discusses the effect of compositional characteristics of Indian fly ash such as the combustible content, particle shape & size distribution and its Chemico-mineralogical characteristics i.e its oxide composition, nature & contents of the crystallites and the compositional characteristics of the amorphous glassy phase of fly ash on its enhanced pozzolanic reactivity. 

The paper also discusses in brief the available methods for assessing the pozzolanic reactivity of fly ash and further illustrates a rapid test (Alkali Reactivity test) method developed at the author’s laboratory for assessing the pozzolanic activity of Indian fly ashes. The brief description of the test along with the Alkali Reactivity (RA) values for Class-F Indian fly ashes of different sources and of varying characteristics is also illustrated.

The authors conclude that a proper understanding of the interrelations of the fly ash & OPC characteristics is of immense importance for engineering the properties of the blended cements for durable Concrete.

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