Setting and Suitability of Limes
In the case of pure lime, the setting takes place partly by the absorption of carbon dioxide from the air and partly by drying which is facilitated by dry conditions; and the setting action is very slow. Slaked fat lime has a great tendency to absorb carbon dioxide from the air when it dries and hardens hut it shrinks and cracks on drying. This lime is mixed with large quantities of coarse sand, up to two to three times it volume, in the preparation of mortar which makes the mortar porous and increases the absorption of carbon dioxide for the hardening process, and also prevent shrinkage. Mortar from a mixture of fat lime and sand will set in thin wall joints and under heavy pressure. In thick-wall construction, the mortar in the interior very often never sets or hardens but crumbles into a friable powder and does not acquire any strength. As such, fat lime is suitable only for thin masonry wall joints and for interior plaster and not for works in wet foundations or under water as it dissolves in water and does not weather well in exposed positions.
Hydraulicity and setting properties of fat lime can be improved by the addition of surkhi and grinding the mixture in a mortar-mill. An addition of 10 to 15 per cent of cement to a fat lime mortar also improves its quality considerably.
The hardening of hydraulic lime does not depend on the absorption of air the setting of hydraulic limes and cement is facilitated by the presence of water. The setting action of hydraulic lime is much quicker than that of fat lime. Only eminently hydraulic lime is suitable for underwater works but it should not be immersed within 48 hours.