Quick Lime Reactivity: What It Means and How It Is Tested

Ask a lime chemist what the most argued-about line on a certificate is and the answer is reactivity. Purity is a number everyone measures the same way, give or take. Reactivity is a behaviour — how fast the lime reacts with water — and there are several ways to measure it, each giving a different figure for the same lot. A buyer who specifies "high reactivity" without saying how it is measured has specified nothing; a supplier who quotes a number without a method has promised nothing.

This article explains what reactivity is, what decides it at the kiln, how the common tests work, and how to write it into a specification so both sides mean the same thing.

What reactivity is

Quick lime reacts with water to form calcium hydroxide, and the reaction releases heat:

CaO + H₂O → Ca(OH)₂ + heat

Reactivity is the rate of that reaction. A highly reactive lime dropped into water heats it quickly, breaks apart and slakes to a fine hydrate within minutes. A low-reactivity lime warms the water slowly, slakes incompletely and leaves hard grains. Both may have the same available-lime content; they are not the same material in use.

Why it matters in practice

• In a slaker at a water or chemical plant, reactivity decides how fast milk of lime is made, how fine the hydrate is and how much grit is left behind.
• In a steel furnace, it decides how quickly the lime dissolves into the slag — and, if it is too high, how much of it dusts away before it does.
• In soil stabilisation, it decides how much drying the lime does and how fast the soil becomes workable.
• In storage, a very reactive lime picks up atmospheric moisture faster and so has a shorter useful life once a bag is open.

Each process has a reactivity it wants; none wants "as high as possible" without qualification.

What decides it: the burn

Reactivity is made in the kiln. Limestone calcines when it is heated enough to drive off its carbon dioxide; the lime left behind is porous, and the pores are where water gets in. Burn at a moderate temperature for the right time and the lime is soft-burnt: porous, light, highly reactive. Burn hotter or longer and the lime sinters — the pores close, the crystals grow, the density rises — and the lime is hard-burnt: dense, slow to slake, long-lived in storage. Burn too little and the stone keeps an unburnt carbonate core that contributes nothing.

Feed size matters as much as temperature, because the heat has to reach the centre of the stone: a large stone burnt hard enough to calcine its core will be over-burnt on its surface. The how quick lime is made article follows this through the kiln. The point for a buyer is that reactivity is a property the manufacturer controls, by the stone size, the fuel and the burn, and a manufacturer that controls its own kilns can hold it from lot to lot.

How it is tested

All the common tests put a known mass of lime into a known volume of water and watch what happens. They differ in what they measure and report.

• Temperature rise over time (slaking curve). Lime is added to water at a set starting temperature in an insulated vessel with stirring, and the temperature is recorded against time. The result is reported as the temperature after a set interval, or the time to reach a set temperature, or a full curve. ASTM C110 describes a slaking-rate test of this kind, and EN 459-2 (the European test standard for building lime) reports a time to reach a set temperature rise. The method in IS 1514 is the Indian reference.
• Acid consumption (titration) tests. Lime is slaked and the hydroxide formed in a fixed time is titrated with acid; the volume of acid consumed is the result. Some steel plants in India use a test of this kind and specify a minimum volume.
• Residue on slaking. After slaking, the unreacted grit is washed over a sieve and weighed. Not a reactivity figure as such, but it tells you how much of the lime did not take part.

A figure from one test cannot be converted into a figure from another, and even within one method the starting temperature, the lime-to-water ratio, the stirring and the lime's particle size all shift the result. This is why the method, and its conditions, belong on the specification and on the certificate.

Writing it into a specification

1. Name the method — the standard, and the conditions if the standard allows a choice.
2. State the figure and whether it is a minimum or a range. A furnace or a slaker usually wants a range, not just a floor.
3. State the sample preparation: lump as received, or crushed to a stated size, because a crushed sample slakes faster.
4. Require the result on every lot certificate, by the same method.
5. Keep a retained sample from loading so a disputed result can be retested on the material that was dispatched.

Reactivity and storage

Because the reaction is with water, a reactive lime starts to slake in humid air the moment it is exposed. Loss on ignition and moisture rising across a sequence of certificates is the sign; the remedy is packing matched to the journey and dry, covered, rotated storage. The packing, storage and transport article covers it.

Testing our lime in your laboratory

The useful way to settle reactivity for a given duty is to test a sample from a tested lot, by your own method, in your own laboratory. Deshwali Minerals will send a sample with its certificate and the method used, so the comparison is like with like.

Sources

• Bureau of Indian Standards, IS 1514:1990 — Methods of sampling and test for quicklime and hydrated lime: BIS standards portal
• ASTM C110 — Standard test methods for physical testing of quicklime, hydrated lime and limestone: ASTM International
• EN 459-2 — Building lime, test methods: CEN