Lime

Tests for Strength of Lime Mortars

Tests for Strength of Lime Mortars

Two bricks are joined flat in a cross fashion one over the other with 12 mm mortar joint. Bricks are thoroughly soaked in water before joining and cured for 7 days after they are jointed. Load required to separate them at joint gives the adhesive strength of mortar which should not be less than 1.4 kg/sq. cm for I lime to 2 sand mortar. For testing tensile strength, briquettes are made (as for cement test), and cured for 24 days by immersion in water. A good hydraulic lime should have an ultimate tensile strength of at least 7 kg/sq. cm and a fat lime 2.8 kg/sq. cm, with 1 : 3 lime : sand mortar. The compressive strength should be 4 to 5 times its tensile strength. Mortar consisting of 1 part good quality kankar lime and 1 part sand should develop a compressive strength of over 50 kg/sq. cm after 3 months and twice that after two years. A pure surkhi mortar gives a breaking strength of about 5.6 to 6.3 kg/sq. cm if left in dry air, and 21 to 25 kg/sq. cm if left immersed under water.

Compressive strength of 1:4 cement-sand mortar after 3 months is over 210 kg/sq. cm. Simple method of testing the strength and suitability of a particular limestone or kankar is to burn the limestone, produce lime, mix it with the required proportion of sand and test the mortar as described above.

Lime Mortars

Lime Mortars

Lime mortars require grinding to slake the unslaked particles and to make an intimate mixture of the materials. Lime/surkhi mortars are ground in two operations, first only the lime and surkhi are ground together and then sand is added and again ground. For big jobs grinding is done in a bullock-driven mortar mill (or machine). In a mortar-mill the diameter of the track should not be less than 8 m. No piece larger than 6 mm should be introduced in a mortar mill. For small jobs grinding can be done by ponding the ingredients in a small pit and mixing. Small quantities of mortar can be mixed by repeatedly turning over the materials with a shovel, and afterwards with a trowel, so as to mix them very thoroughly.

Shell Lime

Shell Lime

Shell lime freshly slaked is used for polished plaster and white washing. It comes under the catagory of ‘fat limes’.

Poor Lime (or) Lean Lime

Poor Lime (or) Lean Lime

Poor lime also called Meagre or Lean lime, contains from 10 to 40 per cent impurities insoluble in acids such as sand and stones, takes longer to slake, does not increase in bulk, to such an extent (less than twice) as pure lime and has inferior plasticity colour may not be white.

Burning of Lime

Burning of Lime

Limestone is burnt in clamps or kilns. Fuel used is generally, coal-dust or fire-wood. Cowdung or litter should not be used with kankar. A clamp consists of a heap of limestone and coal stacked in alternate layers and is used for burning only small quantities of lime as it is a wasteful method.

out of 100 parts of pure limestone burnt, only 56 parts of lime are left behind.

After burning kankar lime it should be ground without delay as it deteriorates rapidly if left unground during the rains. The limes (both kankar and white lime) after being ground dry should be carried to the site of work in gunny bags, and not dumped or stacked on the ground.

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.

Methods of Slaking Lime

Methods of Slaking Lime

Platform slaking. Lime is spread over a dry nonporous platform in a 15 to 23 cm layer and water poured over it generously through a nozzle, and heaps turned over and over between each application of water until the lime disintegrates to a fine powder.

Tank slaking. The lime is placed 30 cm deep in a drum or a tub with about 90 cm of water and allowed to stand for about 24 hours or such longer period as may be necessary to slake the lime completely. It is better to add lime to the water and not water to the lime. The mixture should be well stirred.

Lime is considered to be completely slaked. when the temperature of the lime and the water ceases to rise and any further addition of water also produces no further chemical action or heat, but as a precaution, water should be allowed to stand on for 12 hours or more for white limes and until the normal temperature is restored. A vigorous slaking with heat and noise indicates a high calcium content. After slaking the lime should be screened through a 3.35 mm sieve or kept in excess of water to form lime putty according to the requirements. Limes must be thoroughly slaked especially for plaster work which is also ground very line, any unslaked particles left will produce “blisters”.

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.

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.