Tag: Hydraulic 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.

Hydraulic Lime

Hydraulic Lime

Hydraulic lime is obtained by burning kankar or clayey limestones. Lime is considered to be hydraulic when it sets under water within 7 to 30 days. Lime is called feebly hydraulic, moderately hydraulic or eminently hydraulic according to its readiness to set under water and its properties which depend upon the proportion of clay in the lime, which varies from 5 to 30 per cent. The larger the proportion of clay, the more sluggish the slaking and the greater the hydraulic property. Hydraulic lime slakes very slowly taking several hours or even days depending upon its composition, and without producing much heat, noise or change in bulk. Slaking is done in the same manner as for fat limes but only just enough water is added for hydration and lime is turned over with spades. Excess of water will harden it and make it useless. Slaking action is accelerated if lime is initially pulverised in a grinding mill.

Hydraulic lime should be slaked just before use and not immediately after burning, and then passed through a 3.35 mm sieve and stored in a compact heap in an air-tight dry place.

Hydraulic lime is suitable for works under water and for all positions where strength is required as it has much less tendency to shrink or crack than fat lime, and addition of a small proportion of sand improves its qualities. It has to be a ground to a very fine powder for plaster work.