Tag: Fat Lime

Setting and Suitability of Limes

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.

Fat Lime (or) White Lime

Fat Lime (or) White Lime

Fat lime which is also called stone-lime or white lime is high calcium lime with about 6 per cent material insoluble in acid, chiefly obtained by burning (called calcination) in a kiln pure limestone, chalk or sea shells, etc. (calcium carbonate). By burning calcium carbonate, carbon dioxide is driven off as a gas leaving calcium oxide or quick-lime in the form of lumps. When water is poured over quick-lime it almost immediately cracks, swells and falls into powder with a hissing and creaking sound, slight explosions and considerable evolution of heat and steam. The process is called slaking or hydration, and the powder produced is called hydrated lime or slaked lime (calcium hydroxide). Quick-lime should be slaked as early as possible after it is burnt in a kiln. Over-burnt or under burnt pieces or lumps should be picked out and removed before slaking.

Fat Lime

Quick-lime if left exposed to the air will absorb moisture and carbon dioxide and become an inert powder of calcium carbonate or chalk having no cementing power. Therefore, lime should be stored in an enclosed space in large heaps and air excluded as far as possible. Unslaked lime kept in air tight vessels, and slaked lime packed in gunny bags and stored in dry place will keep sound for months. All lime that has been in any way damaged by rain, moisture or dust should be rejected.

Test for freshness of fat lime (or) white lime. Unslaked lime weights about 1050 kg/cu.m, expands on slaking and then weighs 640 kg/cu. m when fresh, increasing to about 800 kg/cu. m after 10 days.