CONSERVATION MATERIAL: BUILDING LIME
Lime is one of the primary materials used traditionally in the construction of innumerable historic buildings. The range of buildings varies from modest vernacular dwellings to high palatial complexes. The manner in which lime structures about 2,000 years old have withstood the depredation of time bear indisputable evidence to the durability of lime mortars. It is therefore critical to understand the building limes and examine the issues which arise in the conservation of lime in historic buildings. Use of lime mortar in conservation works is essential as it is the original building material and, if well prepared, lime mortar survives for centuries as it returns to its natural state of limestone.
The method of manufacturing building limes and the manner in which they are used in construction work differ from one part of the country to another. For instance, in the south, lime mortar is generally prepared by grinding a mixture of slaked lime and sand in suitable proportions in a bullock mill while in Punjab, lime putty is mixed with sand and the mix used as mortar directly.
1.1. Production of Lime
Lime is the material produced from the heating or ‘burning’ of limestone and its subsequent ‘slaking’ with water. It can be combined with aggregate and water to produce a mortar or plaster, or diluted with water and used for lime washing.
Lime is produced by burning limestone (calcium carbonate or calcite, CaCO3) in a kiln at a high temperature which releases carbon dioxide to produce calcium oxide (CaO), a highly reactive solid known as ‘quicklime’ or ‘lump lime’. ‘Slaking’ of this calcium oxide with water results in a highly exothermic reaction to produce lime (calcium hydroxide (Ca(OH)2)), an anhydrous (free of water) material termed slaked lime, hydrated lime or Portlandite. Quick lime or hydrated lime is mixed with aggregate and water to form a mortar; alternatively the addition of excess water during slaking results in the formation of a lime putty. Calcium hydroxide in the mortar reacts with carbon dioxide (in the presence of moisture) in the atmosphere to form calcium carbonate in a reaction termed ‘carbonation’. In this series of reactions, known as the lime cycle (figure below), the material essentially returns to its original form, as the set lime is compositionally similar to its original limestone.
1.2. Advantages of Lime
Lime is naturally occurring, less vulnerable to ageing, adjusts to temperature variations and is also less prone to cracking. Lime Mortar allows the structure to breathe due to its porosity and permeability and hence, maintains the temperature and reduces dampness. Lime mortar can be made by slaking the lime and then leaving it or storing it as putty, instead of adding the aggregate immediately or turning the lime into a dry hydrate (dry hydrated lime powder). The lime putty develops a skin of carbonate, which allows the material to be stored for a long period of time before use. The benefits of storing putty have been argued, but it is considered that the process of maturation increases the plasticity, workability, and water retention of the final product.
1.2 Sources of Lime
The sources of lime are limestone, marble, seashells, chalk or kankar lime among others, while sand, mud or gravel is added to make lime mortar, depending on the application. Lime is obtained from various sources in different parts of the country.
Rajasthan is the leading producing state accounting for (21%) of the total production of limestone, followed by Madhya Pradesh & Andhra Pradesh (11% each), Chhattisgarh & Karnataka (10% each), Gujarat,Tamil Nadu & Telangana (8% each), Maharashtra & Himachal Pradesh (4% each), and the remaining 5% was contributed by Meghalaya, Odisha, Uttar Pradesh, Assam, Jharkhand, Jammu & Kashmir, Kerala and Bihar.
Organic additives used in the Indian sub-continent are rice husk, jute fibers, pulses, egg white, milk, fenugreek seeds, jaggery, yoghurt, Terminalia Chebula(Harar), CommiphoraWightii(Guggal) and extract of various plants etc.
1.3 Classification of Lime as per (IS: 712 – 1984)
Building Lime is classified as follows according to the Indian Standard of Building Lime:
Class A - Eminently hydraulic lime
(Used for structural works such as arches, domes)
Class B - Semi-hydraulic lime
(Used for constructing masonry mortars, lime concrete and plaster undercoat)
Class C – Fatlime
(Used for finishing coat in plastering, whitewashing, composite mortars, etc, and with addition of pozzolanie materials for masonry mortar)
Class D - Magnesium/dolomitic lime
(Used for finishing coat in plastering, white washing, etc.)
Class E –Kankarlime
(Used for masonry mortars. It is produced by burning Lime Nodules usually found in black cotton soils containing silica)
Class F - Siliceous dolomitic lime
(Used for undercoat and finishing coat of plaster)
1.4 Manufacture of Lime
The burning of Limestone is carried out either in clamps or kilns.
- Clamps are temporary makeshift arrangements for burning in an ordinary manner at a lower cost.
- The clamp burning proves to be uneconomical to manufacture lime on a large scale.
- The loss of heat is considerable. The mud plaster cracks by the heat from inside and allows heat to escape.
- The quality of lime produced by the clamp burning is not good.
- The quantity of fuel required is more and hence it is practised where lime stones and fuel are abundant.
- Kilns are permanent structures, built to carry out the burning operations at regular intervals or on continuous basis.
- In an Intermittent kiln, fuel is not allowed to come into contact with limestone which produces better quality of lime and the finished product is not mixed with ashes. Theyare easy to manage. There is considerable wastage of time in intermittent kilns as every operation includes loading, burning, cooling and unloading. The supply of lime is also not continuously guaranteed. Hence such kilns are used to manufacture lime on moderate scale.
- In a Continuous Kiln, mixture of lime stones and fuel is fed from the top, and a roof is provided on the top to protect the kiln. There is considerable saving of time and fuel in case of continuous kilns, but the initial cost is high. Hence these kilns are adopted to manufacture lime on a large scale.
1.5 Process for Lime Firing
1.6 Important tips in burning of limestone
- The bright red colour of stone indicates that the burning is complete. The burnt lime stones should be withdrawn from the kiln as soon as carbon dioxide is released. The dark red colour of stones indicates the presence of carbon dioxide in the stones. But when the carbon dioxide is released completely, the colour changes to brilliant white even without flame or blaze.
- The burning should be such that it does not result into over-burning or under-burning.
- The heating should be gradual. The sudden heating results in the blowing of stones to pieces due to quick release of moisture and carbon dioxide.
- The imperfectly calcined lime does not slake with water and is referred to as the dead-burnt lime.
- The lime stones should be broken into suitable sizes before they are burnt. For proper burning, fat lime stones should be broken into lumps of size 200 mm to 250 mm, and the hydraulic lime stones into lumps of size 75 mm to 100 mm.
- The quantity of fuel required in each case should be carefully decided. It is found that about 60 N of fuel is sufficient for burning 1 kN of CaCO3.
- The burning should be kept uniform for several hours. For successful burning, the temperature should be maintained at 800°C with a rapid draught of inert gases through the hot lime stones.
1.7 Disadvantages of Lime
The disadvantage of using lime is that it is slow-setting in nature and delays construction activity.
1.8 Sample Lime Mortars
|A. Lime Putty :||Aggregate|
|B. Hydrated Lime :||Aggregate|
|C. Hydrated Lime :||Aggregate||Organic Additive|
1. What are lime mortars? / Gurmeet S. Rai and ParomitaDesarkar; in association with the Lime Center; CRCI; supported by INTACH UK Trust.
2. Why Use Lime? /SangeetaBais
3. Rethinking Conservation Lime Mortar: A Traditional Building Craft / Aga Khan Trust for Culture
4. Indian Standard (IS: 712 – 1984) Specification for Building Limes, Third Edition, Bureau of Indian Standards, 1985