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
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
Soil Stabilization Materials
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.
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
- 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
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.
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 –
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.
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
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.