Investigations into the effect of addition of flyash and burnt clay pozzolana on certain engineering properties of cement composites

In cementitious binders when used for masonry mortars the requirements for properties of importance during early and later ages are different. Whereas during the early periods the attributes required are water retention, workability, plasticity, adhesion, etc, to allow the mortar to possess good working properties such as the ease of spreading, proper filling of joints and also to provide a water resistant crack free smooth surface, but at later ages strength becomes the main criterion to sustain the imposed load of the structure. Portland cement based mortars though harden rapidly and attain high strength but possesses a relatively poor early age properties. In composite mortars there is a common practice to incorporate lime along with portland cement, whose presence improves upon the early age rehological properties. Once the setting and hardening take place and the role of these early age properties is completed, lime has little role to play, as it harden through the lethargic process of carbonation i.e. by the chemical action of lime with atmospheric carbon dioxide forming insoluble carbonate. The process of carbonation is very slow and takes place from surface inwards. Modified composite mortars have been developed by the replacement of certain part of lime with pozzolana such as burnt clay or flyash and has been found to be of advantage. Laboratory investigations on a series of such mixtures have revealed the possession of good early age properties and at the same time better strength at later ages. Some of the results are reported in this paper. (C) 1999 Elsevier Science Ltd. All rights reserved.