Lime Mortar

Lime mortar is a mortar made from building lime (IS 712), sand and water, sometimes 'gauged' with a small proportion of cement to speed set and gain strength. Fat (non-hydraulic) lime sets slowly by carbonation (re-absorbing CO₂) and is highly workable and breathable; hydraulic lime sets partly by reaction with water and gains more strength. Mix proportions, preparation and use follow IS 2250 (code of practice for preparation and use of masonry mortars).

Though largely superseded by cement mortar for modern structural masonry, lime mortar remains important for heritage conservation and traditional construction because it is vapour-permeable ('breathable'), more flexible and self-healing of fine cracks, and chemically compatible with old soft masonry — using rigid, impermeable cement mortar on historic fabric traps moisture and accelerates decay. It is also used in lime concrete (lime-surkhi/brick-bat) terracing and as a plasticising gauge in cement-lime mortars and plasters.