Technical Papers and Articles

»pH of Lime - Stabilized Soils

Lime stabilization of clay soils is a standard construction technique in use since Roman times. It is widely used in the United States to improve the strength of clayey soils for secure foundations and construction bases. An excellent recent publication by Professor Dallas Little covers all aspects of the technique. When good construction practices are employed, there are no adverse effects of the high pH of the lime-stabilized soils. The soil stabilization process and techniques have limited potential for surface and water pollution as will be evident from the discussion below.

Soil Stabilization Techniques

Lime, as calcium hydroxide or calcium oxide, is mixed with clays or clayey soils and water to convert the clays to non-plastic hydrous calcium alumino-silicates. The intimate mixture of calcium hydroxide, water and soil reacts chemically over an extended period with rapid strength development. The stabilized soil is compacted to form a durable construction element.

Soil Stabilization Materials

Lime, as dry calcium oxide, dry calcium hydroxide or a water slurry of calcium hydroxide, is the key ingredient for soil stabilization. All three forms show high pH, (12.4 @ 25°C) in contact with water. Ultimately, calcium ions and hydroxyl ions are the effective agents.

The class of siliceous materials which react with calcium hydroxide to form a hydrous, siliceous cement are known as pozzolans. Many clay minerals, volcanic ash, fly ash, diatomaceous earth and some glasses are pozzolans. The soil stabilization reactions are called pozzolanic reactions.

Soil Stabilization Reactions

Numerous reactions occur in the stabilization process, but a simple classification includes these:

Table 1: Chemical Reactions in the Soil Stabilization Process Using Lime  (PDF)
Soil Stabilization Practices

Good construction practices used in lime stabilization determine its effectiveness, including:

- Proper engineering lab and design work to test the soil material and determine the correct lime dosage, taking into account the soil variability.

- Use of high quality lime in sufficient quantity to achieve good pozzolanic reaction.

- Uniform application of the lime with sufficient water to complete the reactions.

- Prompt and thorough mixing of the lime and soil to bring the reactants into intimate contact.

- Weather, moisture and temperature conditions to promote the pozzolanic reactions during the curing period.

- Compaction to the specified desired density to promote strength development and limit slow, deleterious processes.

- Control of water access to the stabilized material to achieve long term protection from freeze-thaw damage, leaching and to preserve the integrity of the construction element.

Consequences for Soil and Water pH

Excess calcium hydroxide is normally added to a soil stabilization mixture to assure complete reaction and to allow for soil inhomogeneity. Normally the lime dosage amounts to 6% or less of the soil mass. This is thoroughly mixed in the treated layer thickness. Prompt mixing and compaction reduce the amount of lime exposed to the environment. After reaction and compaction, only small amounts of lime are accessible to rainfall or run-on water. As the pozzolanic reactions proceed, the lime is consumed and the pH becomes controlled by the silicate reactions. The hydrous calcium alumino-silicate minerals formed have a much lower solubility than lime and tie up the calcium ion in these cementitious phases. The compaction of the treated soils is an integral part of the lime stabilization and construction processes. This reduces the permeability to very low values which further restricts the access of water to the limed materials. The combination of (1) chemical reactions consuming lime, (2) the formation of calcium alumino-silicate mineral phases with very low solubility and (3) the low permeability of the compacted treated soils all greatly reduce the opportunity for formation of low pH waters. pH values of near-surface lime-stabilized soils are in the 10 to 12 range after a few days.

The limited solubility of lime and the accessible volume of the compacted soil limit the alkaline agent available for pH adjustment of adventitious water. Recarbonation of the excess lime near the surface forms calcium carbonate, the principal mineral in limestone, which has a pH of 7.8 – 8.4.

Good practice dictates that every effort be made to promptly mix the lime with the soil. Should excess rainfall and off-site drainage take lime solids to a body of water, the saturated calcium hydroxide pH of 12.4 can kill aquatic life. However, dilution both in the water body and by uncontaminated runoff and the natural buffering action rapidly reduce the pH to acceptable levels. These mitigating factors are:

- The natural buffer capacity of the water by which calcium hydroxide is neutralized and removed by the carbonate/bicarbonate ions in the water and the precipitation of calcium carbonate.

- Ion exchange and flocculation of untreated soils in the runoff water and over which the lime-bearing water flows.

- A ten-fold dilution drops the pH by one unit. Thus, 100 gallons of pH 12 water would result in a pH of 9 when mixed with 100,000 gallons of pure pH 7 water.

Conclusion

While direct drainage of calcium hydroxide to a body of water may temporarily raise the pH to levels dangerous to aquatic life, proper construction techniques and scheduling minimize this possibility. Calcium hydroxide is consumed in lime-stabilization of clays and by exposure to air. Waters contacting lime-stabilized soils do not show much change in pH because of the low solubility of the calcium alumino-silicate minerals and the low permeability of the compacted soils. Uncounted soil stabilization projects using hundreds of thousands of tons of lime are safely conducted every year. In the Texas Gulf Coast market, soil jobs are conducted in areas of very high rainfall without incident. The rapid reaction with the target pozzolanic soils, prompt compaction of treated material, site drainage control and recarbonation of surface lime all work to minimize undesired pH excursions.

Table 2: Estimation of Effects of Rainfall Leaching on Lime Stabilization Materials (PDF)

Article originally printed by Starr Curtis, Technical Services Manager, Chemical Lime

References:

1) Little, Dallas N., 1995, Handbook for Stabilization of Pavement Subgrades and Base Courses with Lime, NLA sponsor, 219 pages, Kendall/Hunt Publishing Co., Dubuque, IA.

2) If the lime is hydrate (calcium hydroxide) or slurry, this reaction has already occurred.
3) The low solubility of 1.5 grams Ca(OH)2 /L (= 0.15 weight percent solution) limits the pH to 12.4 @ 25°C.