How does lime stabilization work

Lime is used in soil stabilization applications to develop long-term permanent strength in fine-grained soils high in silt and clay content. Lime stabilization uses pozzolans, which are naturally present in clay soils, to generate cementitious bonds that permanently strengthen a soil. Pozzolans such as silica and alumina react with calcium, supplied by the lime, and water to form calcium-silicate-hydrates C-S-H and calcium-aluminate-hydrates C-A-H. The term lime can be used somewhat inconsistently.

For soil stabilization purposes, the term lime must mean either quicklime or hydrated lime. Farmers sometimes refer to agricultural lime, finely ground calcium carbonate CaCO 3 , as lime. While this form of calcium helps farmers improve their soil by amending their fields, using it would not gain any strength in the soil under the feet of a construction worker. It is worth noting that quicklime can come in two types, high calcium and dolomitic. High calcium is almost completely calcium oxide CaO , whereas dolomitic quicklime contains a portion of magnesium oxide along with calcium oxide.

While some industrial applications such as steel need the magnesium component for certain processes, for construction purposes high calcium and dolomitic are virtually indistinguishable. Hydrated lime is quicklime that has been further processed. It has been carefully hydrated with the proper amount of water and agitation to produce a very fine, high-purity product. Hydrated lime can still supply the calcium that is essential to stabilize certain soils by forming cementitious bonds.

However, since the material has already been hydrated, it does minimize much of its drying capacity that is desired on wet job sites. Also, hydrated lime is only available in certain areas and there are additional costs for the added processing. Lime kiln dust LKD , is derived during the quicklime manufacturing process as a co-product.

It is made up of the finely sized particles that are captured in the baghouse at a lime plant. These particles are high calcium or dolomitic lime plus pozzolans from the fuel used to fire the lime kiln. Because it contains both lime and pozzolans, LKD is a hybrid between lime and cement.

As previously mentioned, lime works very well with fine-grained soils. Cement, on the other hand, works very well with coarser gained soil. LKD bridges the gap between the two. Soil cement stabilization is a construction technique used to increase the strength of subgrade soil by mixing it with cement and water.

Once quicklime is mixed with the moisture bearing soil an exothermic heat-producing reaction takes place. In a homogeneous mixture, the quicklime reacts with the moisture present in the soil. Modification and stabilisation only occur with clay soils. When quicklime, hydrated lime or liquid lime is added to a clay soil, the clay platelets go through an ion exchange process, which introduces calcium into the clay surface and causes a change in the way the clay platelets align, as shown in the pictures below.

This gives an increase in soil strength and will normally occur quite rapidly usually within two hours of mixing but can take up to a day depending on site conditions. Aligned clay particles surrounded by water meaning the particles can slide easily — resulting in low strength clay soil.

Clay particles following lime modification — showing a reduced water layer and realignment — which increases the strength of the clay soil. The silica and alumina contents of clay soils will react with the calcium present in the quicklime, hydrated lime or liquid lime to form calcium silicate hydrates or calcium aluminate hydrates. These reactions are similar to those that occur in hardening cement; they are long term and strength gain can continue for many years.

The process is simple and can range from basic plough and disc harrow through to purpose built lime spreaders and powerful rotavators. Soil stabilisation using lime can be carried out on a wide range of project sites. Lime products are introduced into the soils and mixed thoroughly. The most common process is to treat the soils in horizontal layers, in situ.

Lime Content 4. Compaction 5. Curing 6. Additives Quiz. Fig 1: Lime-Soil Stabilization. Fig 2: Spreading Lime. Fig 3: Compacting and Mixing Lime-Soil.

Fig 4: Soil. Fig 5: Hydrated Lime. Fig 7: Mixing Lime-Soil. Fig 8: Fly Ash. Previous article. Next article.