Code of practice for field slaking of building lime and preparation of putty
1992 Edition

IS 1635 advises two primary slaking approaches:

1. Tank Slaking (Clauses 6.2.1 & 6.2.1.2): Use a watertight, brick-lined tank partially filled with water (around 300 mm depth). Gradually add quicklime into the water while continuously stirring to ensure lime remains submerged and to control heat generation. This method yields lime putty directly, ideal for applications requiring putty.

2. Platform Slaking (Clause 6.2.2.1): Spread quicklime in a 150 mm thick layer on a waterproof masonry platform. Water is sprinkled progressively over the lime while turning the heap repeatedly to promote uniform hydration over approximately 24 hours. The resulting hydrated lime powder is then screened through a 3.35 mm IS sieve. This method produces dry hydrated lime which can be converted to putty if necessary.

Preparation involves gradual slaking of quicklime by adding water to a 150 mm thick lime layer and turning it repeatedly over about 24 hours, followed by screening through a 3.35 mm IS sieve. To prepare putty, the dry slaked lime is mixed with water into a thick creamy consistency.

Maturation periods differ by class: Class C and D (fat limes) require at least 16 hours (or up to 3 days if matured by settling), while Class B and F (semi-hydraulic limes) should mature no more than 12 hours (or up to 2 days if produced by milk of lime settling). The putty must be stored submerged under water to prevent carbonation and drying, and used promptly for optimal performance.

Safety protocols include: storing and handling quicklime in dry conditions to prevent premature reaction; always adding lime to water (not vice versa) in a tank filled about 300 mm deep with water to avoid localized overheating and splattering; maintaining continuous stirring to keep lime submerged; monitoring the exothermic heat evolution during slaking to manage temperature; and preparing for fire hazards by implementing proper fire safety precautions around the slaking area.

Quicklime should ideally be slaked immediately after production but if storage is necessary, it must be kept in airtight metal containers or polyethylene-lined jute bags within a dry, sheltered area protected from moisture and rainfall. For dry slaked lime, short-term storage involves covered platforms shielded from weather, while longer-term storage (up to two months) requires packaging in polyethylene-lined gunny bags stored in dry, enclosed godowns to prevent moisture ingress and quality degradation.

Tank slaking entails adding quicklime to water in a brick-lined, watertight tank with constant stirring, producing uniform lime putty directly. This method offers controlled hydration and better quality.

Platform slaking involves sprinkling water onto lime spread on a waterproof platform, with manual turning over about 24 hours. It tends to yield less uniform hydration and produces dry hydrated lime powder rather than putty. Tank slaking is preferred for quality and consistency, while platform slaking is a simpler but less precise alternative.

Incomplete slaking results in unconverted quicklime remaining in the mixture, causing poor workability, uneven setting, and weak mechanical strength. Mortars and plasters may crack, powder, and exhibit poor adhesion, leading to costly repairs and reduced durability. Excess water or improper storage can further degrade quality. Proper slaking ensures complete hydration, resulting in sound, long-lasting lime-based construction materials.

IS 1635 suggests a two-tank system: an upper tank approximately 400 mm deep positioned above ground for continuous addition and stirring of quicklime into water (around 300 mm deep), and a lower tank 750–800 mm deep below ground for filtering milk of lime through a 3.35 mm IS sieve and allowing settling and maturation of lime putty. Two lower tanks can be used alternately to maintain continuous supply. Tanks should be brick-lined and watertight to resist lime’s corrosive properties.