Soil stabilisation for roads with lime or cement Which method is better?

In transport infrastructure projects where quality requirements are higher due to having to withstand greater traffic loads, it is advisable to use as much of the soil present on site as possible. However, soils do not always have the appropriate characteristics and mechanical strength. Fortunately, there are now various soil stabilisation techniques available to reduce soil sensitivity to water and increase its resistance to deformation under load.

The most commonly used binding agents are cement with additives and lime oxide. Both can be used in powder or slurry form. They are mixed with the soil, compacted vigorously and cured.

Next, we will look at which treatments work best depending on soil characteristics, as well as the factors that affect each type of stabilisation and their advantages and disadvantages:

Soil stabilisation with lime vs cement

In the case of the soil stabilisation for roads with lime The main purpose of using cement is to increase the bearing capacity in order to achieve maximum contact pressure so that soil failure does not occur and to reduce sensitivity to water and other unfavourable environmental conditions, as well as, in many cases, to increase its mechanical strength.

If the granular fraction of the soil is high, a moderate percentage of binder will suffice to obtain a stable material capable of withstanding long-term deformation caused by traffic.

To stabilise soil, cement is used as a binder if the soil is not very plastic, whereas if it is fine and cohesive, lime oxide should be used, although sometimes a mixed treatment may be appropriate, first with lime to reduce plasticity and then with cement to increase the bearing capacity or achieve resistance.

In this case, the structural contribution of a stabilised layer is significant, which is why it is mainly used in infrastructure esplanades that support heavy traffic. On the other hand, granular soils with limited fines and plasticity are excellent materials for the execution of soil-cement layers in semi-rigid pavements.

Soil stabilisation for roads with lime

The soil stabilisation for roads with lime It is used to develop long-term permanent resistance in fine-grained soils with high lime and clay content. Stabilisation using lime and pozzolans, the latter of which are naturally present in clay soils, generates cementitious bonds that permanently strengthen the soil. Pozzolans, such as silica and alumina, react with calcium, provided by lime, and water to form calcium silicate hydrates (C-S-H) and calcium aluminate hydrates (C-A-H). C-S-H and C-A-H are the same products responsible for the strength of materials such as concrete.

What type of lime is used in soil stabilisation for roads?

Quicklime or hydrated lime is used for soil stabilisation. Chemically, quicklime is calcium oxide (CaO) and hydrated lime is calcium hydroxide (Ca(OH)₂).

Quicklime can be of two types: high calcium content and dolomitic. High calcium lime is almost entirely calcium oxide (CaO), while dolomitic quicklime contains a portion of magnesium oxide along with calcium oxide. While some industrial applications, such as steel, require the magnesium component for certain processes, for construction purposes quicklime and dolomitic lime are virtually indistinguishable.

Hydrated lime is quicklime that has been carefully hydrated with the right amount of water and agitation to produce a very fine, high-purity product. Hydrated lime can still provide the calcium that is essential for stabilising certain soils by forming cementitious bonds. However, as the material has already been hydrated, it minimises much of its drying capacity, which is desirable in damp construction work. Furthermore, hydrated lime is only available in some areas and its processing incurs an additional cost.

Lime kiln dust is obtained as a by-product during the manufacture of quicklime. It consists of fine particles captured in the bag filter of a lime plant. These particles have a high calcium or dolomitic lime content, plus pozzolans from the fuel used to fire the lime kiln. Containing both lime and pozzolans, lime kiln dust is a hybrid between lime and cement. As mentioned above, lime works very well with fine-grained soils, while cement works very well with coarser-grained soils. Lime kiln dust bridges the gap between the two.

Factors affecting the stabilisation and mechanics of soils with lime

The following factors affect the process of soil stabilisation for roads using lime:

• Soil type: Lime stabilisation is useful for stabilising clay soils, but it is not effective for sandy soils.
• Amount of limeThe amount of lime required for stabilisation varies between 2 and 10% of the soil.
• Relationship between fly ash and limeThe ratio of fly ash to lime generally varies between 3 and 5. The fly ash used accounts for 10 to 20 per cent of the weight of the soil.
• Different types of limeQuicklime is more effective, but hydrated lime is usually used for safety and ease of handling.

Advantages and disadvantages of soil stabilisation for roads using lime

Some of the advantages of soil stabilisation with lime are as follows:

• The soil becomes more manageable.
• Overall, endurance is improved.
• Lime stabilisation increases compressive strength, sometimes up to 60 times.
• It is effective for soils.

Stabilisation with lime also has some disadvantages, which are as follows:

• • It is not effective for sandy soils.
  • The amount of lime required is limited, between 2 and 10% of the soil.

Soil stabilisation with cement

Soil stabilisation with cement is carried out by mixing pulverised soil and Portland cement with water. The mixture is then compacted to obtain a resistant material. The material obtained by mixing soil and cement is known as soil-cement. Soil-cement becomes a hard and durable structural material as the cement hydrates and develops its strength.

Stabilisation with cement is carried out while the compaction process continues. During the compaction process, a certain amount of cement is used. There may be some empty space in the soil particles. Cement is like a leg, so cement can easily fill the empty space in the soil. As a result, the proportion of voids in the soil can be reduced. After these primary tasks, when we add water during compaction, the cement reacts with the water and hardens. Therefore, the unit weight of the soil may also increase. Due to the hardening of the cement, the shear strength and load-bearing capacity are increased. Due to stabilisation, the permeability of the soil may decrease.

What is Portland cement?

Portland cement is the most common type of cement. Its basic components are calcium, silica, alumina and iron derived from limestone, sand and clay. All of these are processed, fired in a kiln and pulverised into a fine powder. The final product is what we call Portland cement. When exposed to water, it chemically hydrates, producing a gel that forms an interlocking matrix around the particles. As it cures, it hardens, giving strength to the system.

Small variations produce different types of cement. Changes in production lines, differences in raw materials and/or alterations at the end of the cement manufacturing process define the type of cement produced. Qualities such as air entrainment, or the millions of tiny air bubbles that resist stress due to freezing and thawing, are clearly distinguishable.

Factors affecting soil stabilisation with cement

The following factors affect soil stabilisation with cement:

• Soil typeCement stabilisation can be applied to fine or granular soils, however granular soils are preferable for cement stabilisation.
• Amount of cementA large amount of cement is required for cement stabilisation.
• Amount of watera: An adequate amount of water is required for stabilisation.
• Mixing, compaction and curing: Proper mixing, compaction, and curing are required for cement stabilisation.
• AdhesivesCement contains some important additives that help it form a proper bond. These additives play a vital role in the reaction between cement and water.

Advantages and disadvantages of cement stabilisation

• It is widely available.
• It is very durable.
• Floor cement is quite weather-resistant and strong.
• Granular soils with sufficient fines are ideal for cement stabilisation, as they require the least amount of cement.
• Soil-cement reduces the swelling characteristics of the soil.
• It is commonly used to stabilise sandy soils and other soils with low plasticity. The cement interacts with the silt and clay fractions and reduces their affinity for water.

The disadvantages of cement stabilisation are as follows:

• Cracks may form in the cement floor.
• It is harmful to the environment.
• Requires additional labour.
• The amount of water must be sufficient to hydrate the cement and make the mixture workable.

In summary, the type of soil and its water content, the climatic conditions of the environment, and the required performance are the main factors in selecting the most appropriate binder for the soil stabilisation for roads with lime or stabilisation with cement. In doubtful cases, a laboratory study will help to distinguish between the two.