Lime mortar is made by mixing lime, sand and water. Lime used for mortar may be fat lime (quick or hydrated lime) or hydraulic lime. Fat lime has high calcium oxide content. Its hardening depends on loss of water and absorption of carbon dioxide from the atmosphere and possible recrystallisation in due course. Hydraulic lime contains silica, alumina and iron oxide in small quantities. When mixed with water it forms putty or mortar having the property of setting and hardening under water.
Slaked fat lime is used to prepare mortar for plastering, while hydraulic lime is used for masonry construction and are most suitable for construction of chimneys and lightly loaded superstructure of buildings. The mix proportions of lime mortar for various types of works are given in Table 12.2.
Notes:1. Sand in lime mortar is an adulterant, and reduces its shrinkage. Lime mortar becomes porous allowing air to penetrate and helps the mortar in hardening.
2. Lime mortar is not suitable for water-logged areas and damp situations.
Lime mortars have plasticity and placability, good cohesion with other surfacings and little shrinkage. They harden and develop strength very slowly continuously gaining strength over long period. Fat lime mortars do not set but stiffen only as water is lost by absorption (by masonry units) and evaporation. The gain in strength is a very slow reaction of lime with carbon dioxide absorbed from air.
Manual Mixing Lime and sand in required quantities are placed on an impervious floor or in a tank (Fig. 18). The constituents are thoroughly mixed dry by turning them up and down with spades. Water is added and mixing is done again with spades till mortar of uniform colour and consistency is obtained.
Mill Mixing Mills used for preparing lime mortars in undeveloped countries may be a chakki or ghanni run by bullocks (Fig. 18) while a pan mill (Fig. 19) is used in developed countries. In the case of ghanni the required quantity of ingredients in the form of putty is put in the trench and grinding for 100 to 200 revolutions is carried out by moving stone roller. The operation takes about 2 to 3 hours for each batch of mix; the time required in a Pan mill is much less.
Precautions Lime mortar or putty should be kept moist till use and in no case its drying is allowed. The mortar made of hydraulic lime should be consumed within one day and that with fat lime within 2-3 days.
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