Lime in soil stabilization can be in the form of quicklime (calcium oxide), hydrated lime (calcium hydroxide), or lime slurry. Quicklime is obtained through a chemical transformation of calcium carbonate (limestone) into calcium oxide. Hydrated lime is created from a reaction of quicklime and water, while the lime slurry is a suspension of hydrated lime in water.
Fine-grained clay soils with at least 25 per cent passing #200 sieve (74mm) and a Plasticity Index above 10 are considered to be suitable for lime treatment. Soils containing organic material in a concentration higher than around 1 per cent or sulphates in a concentration higher than 0.3 per cent are not suitable for involving lime in soil stabilization.
Lime can stabilize fine-grained subgrade or subbase. Subgrade stabilization usually involves in situ mixing of soil and stabilizer, and generally requires 3 to 6 per cent of lime by weight of the dry soil.
If quicklime is used as a form of lime in soil stabilization, it chemically reacts with water and releases heat. Soils are being dried since moisture present in the soil participates in hydration of quicklime, and as the heat generated can evaporate additional moisture.
Hydrated lime reacts with clay particles. Upon mixing, calcium ions from hydrated lime transfer to the surface of the clay particles and replace water and other ions. Consequently, the soil becomes friable and granular, which enables more efficient mixing and compaction. Also, soil Plasticity Index and shrink-swell potential largely decrease.
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When proper quantities of lime and water are added, the soil alkalinity increases to above 10.5, which causes breakdown of clay particles. Silica and alumina are consequently released and combine with calcium from the lime to form calcium-silicate-hydrates (CSH) and calcium-aluminate-hydrates (CAH). These are cementitious products which contribute to the soil strength. Therefore, the soil transforms from a granular to a hard, relatively impermeable material with significant load-bearing capacity.
Quicklime is highly preferred form of lime in soil stabilization. However, it is in form of powder and is very reactive in contact with water. Therefore, it is OHS unfriendly. The other unavoidable effect is emission of quicklime in form of dust during application and thus disturbance of nearby residents.
Two adverse chemical reactions probably occur in the lime-treated soil. The first is lime carbonation and the second is the reaction with the sulphate salt existing in the soil. Carbonation is the reaction that occurs between free lime and atmospheric carbon dioxide. Calcium carbonate is cementing material but as such has sincere drawbacks. First, it has weak bonding and thus breaks easily and represents weak spots of the treated soil. Second, calcium carbonate is soluble salt and may pulverize when exposed to air for a longer period. Furthermore, carbonation process consumes calcium ions which negatively affects the pozzolanic reaction.
Lime in soil stabilization or any calcium-based additives when in contact with soluble sulphate salt may cause soil distress and heaving, resulting in strength loss. The source of sulphate is either soil minerals, water used for mixing or groundwater. Some other noticed adverse effects are increased soil compressibility and reduced shear strength. This contribution to soil instability is due to a chemical interaction between calcium and aluminium from the soil in the presence of soluble sulphate and water, which produces ettringite and/or thaumasite.
To simplify the determination of soil suitability for treatment with certain stabilizer, Global Road Technology invented products which are effective in all soil types. Furthermore, far lower concentrations of GRT products are needed than lime in soil stabilization. In the case of soils with plasticity index higher than 20 (which is characteristic of clayey soils), a general recommendation is 3-5% of lime, while GRT7000 is needed in concentration as low as 1%. GRT products are available in liquid form and are not reactive, which make them OHS and environment-friendly. The other huge advantage of the use of GRT product instead of lime in soil stabilization is that GRT products have a very low carbon footprint, while the production of lime results in large amounts of CO2.
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