A common misconception about calcium oxide (CaO) — also known as “quicklime” or simply “lime” — is that it leaches out when used for soil stabilization. While that may be possible under the wrong conditions, it’s extremely rare.
In almost all cases, when lime is suspected to have leached out, it’s because the lime wasn’t detectable using the standard test for finding the pH of the soil, which involves applying phenolphthalein and observing a color change. It should be noted that when clay soil is properly stabilized with lime, the pH should remain above a value of 10.5 throughout the life of the pavement.
Texas has many pavements with lime-stabilized subgrade that are more than 60 years old. When cores are taken from pavements with lime-stabilized subgrade, phenolphthalein can be applied to the subgrade layer and a pink or pinkish purple color will be observed, as illustrated below.
This color change indicates that the lime-stabilized layer still has a pH above 9.3. In almost all cases, when lime is used to treat a clay soil but isn’t detectable through phenolphthalein testing, it likely indicates that the soil layer was modified with lime (rather than stabilized), and one of the following likely occurred:
- The designated design procedure was not used to determine the proper amount of lime needed to stabilize the soil.
- During construction, the lime was mixed deeper than the specified lift thickness, resulting in a dilution of the amount of lime needed to stabilize the layer.
- Not enough water was used during the mixing, curing and compaction process. In that case, the chemical reaction required to stabilize the clay could not be completed.
The TxDOT procedure for lime stabilization of clay soil is Tex 121E Part 3, more commonly known as a “lime series” or “Eades & Grim” test procedure. As illustrated below, this test determines the amount of lime needed to raise the pH of the soil to a value of 12.4.
Experience has shown that once the soil reaches a pH of 12.4, it has enough lime to completely solubilize the silica and alumina in the clay soil. When that’s hydrated, it produces pozzolanic cementitious products known as calcium silicate hydrates (CSH) and calcium aluminate hydrates (CAH). The lime stabilization process permanently transforms the clay soil into CSH, which continues to gain strength throughout the life of the pavement.
The illustration to the right shows that silica and alumina (which are prevalent in all clay soils) become highly soluble at a minimum pH value of 10.
The number one reason some individuals claim lime leaches out is that at lower concentrations (less than the amount required to reach a pH of 12.4), the lime is not detectable because all the calcium gets consumed but there is not enough lime to maintain the pH above 10.5 to continue the chemical reaction. In these cases, the pH will drop back to a value less than 9.3 and not be detectable with phenolphthalein.
For example: if the lime design procedure indicates that 6% lime is needed to stabilize the soil, and only 4% got used, then several things happen:
- The plasticity index (PI) of the soil will go down
- The soil texture will change
- The soil will dry out and the gradation of the soil will become finer
These modifications occur within the soil when any amount of lime is added to clay soils. In these cases, the soil is said to be modified rather than stabilized. Lime-modified soils have a number of positive benefits, but they do not exhibit permanent strength gain over time and will eventually revert back to a more neutral pH value closer to 7, whereas a pH of 10.5 or above is needed to transform the clay soil into a pozzolanic cementitious product.
Lime modification of clay soils is sometimes done intentionally, such as when a temporary haul road or detour is built and is set to be removed later. Lime modification of clay soil can also occur by accident in some cases, when the intent was actually to stabilize the clay soil. This can happen when lime gets mixed deeper than the intended design thickness of the stabilized layer—overmixing the depth essentially results in diluting the lime concentration from what was specified. In these cases, there is not enough lime to achieve the desired pH of 12.4. Starving the lime stabilization of water can also result in modification rather than stabilization, simply because water is needed to form the hydrate portion of the chemical reaction that results in CSH and CAH formation.
In Summary
When clay soil is properly stabilized with lime, it is virtually impossible for the lime to ever leach out. In cases where clay soil is modified with lime rather than stabilized with lime, it’s common for individuals to assume the lime leached or disappeared, since phenolphthalein testing will not detect the presence of lime if the pH is less than 9.3 When the amount of lime used to treat clay soil is less than what is required by the design procedure, the lime essentially gets consumed through the chemical reaction with the clay soil and becomes undetectable. Lime leaching out of clay soils would be classified as almost entirely fiction if designed and constructed as specified.
