Literature Review

CHAPTER 2

LITERATURE REVIEW
2.1 INTRODUCTION

In this chapter, a brief review of the findings of earlier investigations on the important properties / parameters of fly ashes; the available literature on the mechanisms of lime-fly ash and cement-fly ash hydration processes; influence of fly ash addition on the properties of concrete, namely, workability and compressive strength , have beenpresented. A comprehensive review of the work of earlier investigators on blended cements; studies on the activation of low-calcium and high-calcium fly ashes and natural pozzolans, have also been presented. An attempt has also been made to
critically evaluate the status of activation studies on fly ash. Apart form the above,

literature relevant to the work carried out in this study, namely, on mixproportioning methods; effect of elevated temperature; influence of various aggressive environments on blended 1 fly ash concretes, have been briefly reviewed and presented.
2.2 FLY ASH - SOURCE, TERMINOLOGY AND CLASSIFICATION 2.2.1 Source

Fly ash is a by-product of the combustion of pulverized coal in thermal power plants.
B r e e different processes, namely, high-temperature combustion (1 500
-1700"

C); dry combustion (1 100 - 1400" C ) and fluidized-bed combustion (<900° C) are

employed for the combustion of pulverized coal. Ashes obtained from fluidized-bed combustion (FBC), are not genuine fly ashes, and are of little interest for building material applications (Wesche, 1991). The dust-collection system (mechanical / electrostatic precipitator - ESP) removes the fly ash , as a fineparticulate system from the combustion gases, before they are discharged into the atmosphere. The term 'fly ash' is not applied to the residue extracted from the bottom of boilers.
2.2.2 Terminology

The term 'fly ash' appears to be well accepted in English-speaking countries, except in Britain, where the term 'pulverized fuel ash' (PFA) is used. 'Cendres volantes' (in French); 'Flug ashe' ( inGerman) and 'Cenizas volantes' ( in Spanish) are the other commonly used terms.

2.2.3 <'lassi tic:ltion

Until rcccntly. 'fly ash' \\as ~.c.gardcdas a 'po~/olan'. flo\icvcr., as tligh-calolun1 fly ashcs which do not rcquire an cxtcrnal sourcc of 111ilcto
P I . O ~ L I C C' C C I ~ I C ~ I ~ ~ ~ ~ O L I S

properties' came to be niorc widcly used, than beforc, thcy are not strictly poi.zolans.Manz and others (1982) have suggested that high-calciuni fly ashes (the so-called
Class C ashcs) are best distinguished from the low-calcium (Class F) ashes by their cementing properties. Thus, a general term 'mineral admixtures' has been suggested to describe all classes of slags, ashes, pozzolans and other cement supplements, with a further distinct~onbeing drawn on the basis of theirself-cementing capab~lities.The above form of classification has been proposed as being preferable to the current division of fly ashes into two classes i.e. according to the rank of coal from which they originate, which is the current practice for Canadian (CAN3-A23.5-M86: Supplementary cementing materials and their use in concrete construction) and LIS (ASTM C618
-

92a: Standard specification for fly ashand raw or calcitied natural

pozzolan for use as a mineral admixture in Portland cement concrete) standards. The term 'mineral admixture (MA) has been criticized in the past, as it gives a misleading picture regarding the quantity actually used in concretes. Hence, in Canada, the term 'supplementary cementing materials' (SC,M) has been adopted in specifications, which describe precisely the role ofthese materials in most concretes [Malhotra and Ramezanianpour, (1994)l. Hawever, the terminology, 'high-calcium' and 'lowcalcium' have been used in this study, in general, and Class C and Class F, while referring / reporting the type of f y ashes actual used by various researchers, in their l investigations.

2.2.4 Pozzolan
Fly ashes exhibit 'pozzolanic activity'. The American Society for...