Quick Lime for Soil Stabilisation in Road Construction

A road is only as good as the ground under it, and across much of India that ground is clay: strong when dry, weak and heaving when wet, and impossible to compact when the monsoon has been through it. Lime stabilisation is the oldest and still one of the most economical answers. Mixed into a clay subgrade or sub-base, lime dries the soil, changes its plasticity within hours, and over weeks and months reacts with the clay to form a cemented, load-bearing layer.

This article is for the contractor or engineer buying lime for stabilisation: what the lime does, which form to use, what to specify, and how to get it onto an open site in usable condition. Mix designs, dosages and layer thicknesses are matters for the project's laboratory and the governing specification, and are not given here.

What lime does to a clay

Three things happen, on three timescales.

1. Drying. Quick lime reacts with the water in the soil and gives off heat; the soil loses moisture both to the reaction and to evaporation. A soil too wet to compact can become workable the same day. Hydrated lime does not do this.
2. Modification. Calcium ions from the lime exchange with the ions on the clay particles, and the clay flocculates: it becomes friable, its plasticity drops, it compacts more easily and swells less. This happens within hours to days with either form of lime.
3. Stabilisation. Over weeks and months, at the high pH lime creates, the silica and alumina in the clay dissolve and react with the calcium to form cementitious compounds — the pozzolanic reaction. The layer gains strength and stiffness that continue to develop for a long time. This needs enough lime, enough moisture, warmth and time, and a reactive clay.

Quick lime or hydrated lime for the job

Quick lime is the usual choice on a wet site, because of the drying effect, and because it carries more reactive calcium per tonne delivered. It must be handled with care: it reacts with the moisture on skin and in eyes, and the dust from spreading is a hazard. Hydrated lime is chosen where the soil is already at a workable moisture, where handling discipline is limited, or where a dry powder is easier to spread and mix evenly. Both work; the site decides. The quick lime vs hydrated lime article sets out the general difference.

What to specify

• Available lime — the reactive fraction, and the figure the mix design assumed. Specify a minimum and the test method.
• Fineness — for stabilisation, lime has to be distributed through the soil and react with it; a ground quick lime or a fine hydrate mixes far more evenly than lump. Deshwali Minerals supplies quick lime powder and graded fractions; the specification should state the fraction or the sieve figure the mixing method needs.
• Reactivity — for quick lime, the slaking behaviour that gives the drying effect. Specify by method.
• Impurities — silica and unburnt carbonate are dead weight; a maximum keeps the delivered tonnage useful.
• Certificate of analysis per lot, delivered with the consignment, so the site engineer can tie the lot to the mix design.

The governing document for the work — the project specification, the relevant Indian Roads Congress guidelines for soil stabilisation with lime, the engineer's instructions — sets the lime content and the acceptance tests. The lime specification above is what lets the supplier meet it.

Receiving lime on an open site

A road site has no roof. That changes the packing and the logistics:

• Packing. Lined jumbo bags for site stockpiles that will be opened over days; small bags where there is no lifting plant; bulk tanker delivery straight into a spreader or silo for large, fast-moving works. Lime left in an open stockpile hydrates and carbonates, and a week of rain can take a large part of its value.
• Timing. Deliveries matched to the spreading programme, not stockpiled for a month in advance.
• Documents. Invoice, e-way bill, weighbridge slip and certificate for each truck, so the quantity on the running bill is the quantity that arrived.
• Freight. Road sites are often far from the kiln; ask for a delivered rate to the site chainage, and a supplier who will quote it. Deshwali Minerals publishes its transport arrangements.

Safety on the spreading crew

Quick lime dust in the eyes causes serious injury, and lime on sweating skin burns. Eye protection, gloves, long sleeves and dust control are not optional, and an eye-wash supply must be within reach of the spreading operation. The handling safety article covers it.

Sources

• Indian Roads Congress — guidelines for soil and granular material stabilisation using cement, lime and fly ash (IRC:SP:89): Indian Roads Congress
• Bureau of Indian Standards, IS 1514:1990 — Methods of sampling and test for quicklime and hydrated lime: BIS standards portal
• Deshwali Minerals — lime in construction