Technical Papers and Articles

»Dolomitic Vs. Hi-Calcium Quicklime for Use in Soil Stabilization

Is there a difference between Dolomitic and High Calcium Quicklime used for soil stabilization in California?

The current edition of the California Standard Specification manual includes dolomitic quicklime under Section 24 “Lime Treatment” as well as ASTM (C-51 & 977) definition of Quicklime.

Since Calcium lime has not been produced in California and has been in short supply for many years, Dolomitic (calcium-magnesium) lime has been used much more extensively in Northern California. Dolomitic lime has been used successfully, for the past 30 years. All credible agencies formally recognized both as equals with respect to soil stabilization.

In an effort to reacquaint all lime users with dolomite lime, the following information regarding quicklime produced in Salinas California may be of assistance in your evaluation for lime treatment projects.

ASTM C 51 describes quicklime as follows:

“ A calcined material, the major portion of which is calcium oxide or calcium oxide natural association with a lesser amount of magnesium oxide, capable of slaking with water.”

To bear out that description, we then find the following:

Chemlime, Natividad Dolomitic Quicklime
CaO (Calcium Oxide) 57.0%
MGO (Magnesium Oxide) 38.0%
-----------
Total Oxides 95.0%

Nearly all limestone in the world which is in existence today was formed by the activity of organisms present in the oceans and is believed to have formed in the Miocene and Pilocene ages which go back to between 8 and 11 Million years ago. Mollusks and other organisms assimilate calcium carbonate from sea water to form their shells or skeletons and after the death of the organism, the solid carbonate remains behind in nearly a pure state as sediment on the ocean bed.

As time passed, the formations of coral reefs, rocks, or pebbles were formed due to intense heat and pressure from below which solidified the sediment.

Dolomite is formed by the replacement of some calcium in limestone by magnesium and is believed to be caused by the action of magnesium salts in sea water shortly after the deposition of calcium carbonate and before the hardening of crystallization of the stone.

Calcium limestone is a carbonate expressed as 56% Calcium Oxide (CAO) and 44% Carbon Dioxide (CO2).

Dolomite is a double carbonate expressed as 30.4% Calcium Oxide (CAO), Magnesium Oxide 21.7% and Carbon Dioxide 47.9%.

Limestone is calcined (burned) at temperatures up to 2400°F. During the calcining process, the Carbon Dioxide in the stone is released into the atmosphere as a gas. The lime then becomes an oxide, or quicklime and, dependent upon the percent of impurities, is about 85-95% pure Calcium Oxide. However, Dolomite Lime becomes an oxide with 57% pure Calcium and 38% pure Magnesium or a total of 95-98% combined pure oxides.

When looking at soil stabilization, it is important to remember that lime does not react with clay soil until it becomes a hydroxide. Lime is hydrated by the addition of water in sufficient quantity to turn the lime oxide into a lime hydroxide.

When Quicklime is mixed with soil and water, the lime hydrates in the soil, becoming a hydroxide and then reacts with the clay portion of the soil.

Thus the type and quality of lime to be specified in soil stabilization is best defined by the amount of hydroxides produced. Both the High-Calcium and the Dolomitic lime are considered equals, even though the Dolomitic has a slightly higher amount of hydroxides.

A quick conversion factor can sometimes be used to determine the hydroxide content of quicklime when the proper amount of water is added and is as follows:

The Calcium Oxide content of the lime x a 1.32 factor equals the hydroxide content.

Thus, if the oxide content of the Hi-calcium Lime is 86%, the hydroxide content would be 86% x 1.32 = 113%. The .32 of the factor indicates water needed to overcome vapor loss plus 26% retention of chemical water, which is now part of the product.

The hydroxide content of Dolomitic lime is computed as follows:



The Dolometic lime is a considerably higher in hydroxide content, than the Hi-Calcium lime (130% vs 113%), as specified in Cal Trans Section 24

The .447 portion of the factor represents the amount of water needed to overcome vapor loss plus retention of 26% chemical water, which becomes part of the product.

STABILIZATION

As stated previously, lime does not react with clay soil until it is in a hydroxidecondition.

When lime is mixed with soil in a hydroxide state, it begins to eat into the clay particles and starts the release of silica and alumina ions. It causes a reduction in the moisture film surrounding the clay grains so that moisture is released and allowed to evaporate. This accounts for the fast drying action when lime is applied to a wet soil. During this action, silicates and aluminates are formed into a pozzalantic gel structure and act as a cement after the fine clay particles agglomerate into coarse friable particles which then can be compacted into a firm monolithic layer witch is impervious to moisture.

The last action of lime treatment is the carbonation process, which is the slow process of reabsorption of Carbon Dioxide, which was driven off during the calcining process. The CO2 absorbed from the soil and atmosphere and over a period of many years causes the lime to revert back to carbonate or rock form thus causing the treated area to become a rock-like slab.

There exists some diverse opinion that more dolomitic lime is needed to make up the difference between 57% CAO in dolomite and 85% in Calcium lime. This cannot be true since both are lime and the magnesium portion more than makes up the difference since the total Hydroxide percent is much higher than Calcium lime. Also, there is little or no difference in R-Values or compressive strength results using equal percents of either lime. You can easily confirm this in your own lab by testing both products side by side with the same soil.

The treatment of clay soils with lime of any kind is an excellent and economical method of stabilizing soils.

Section 24 of the Cal Trans Standard Specification is extremely well written and, if properly followed, will result in a very satisfactory stabilization program. The materials portion of the specification calls for either Dolometic or High-Calcium quicklime or hydrated lime.

Chemlime Dolomitic quicklime contains a minimum amount of dust and can be spread under fairly windy conditions without dusting, unlike Hi-Calcium quicklime.

Chemlime Dolomitic quicklime is granular and weighs about 63 lbs. Typical sizing of the lime indicates:

Passing:
3/8"                  98%
No. 100 Mesh   4-5%
No. 200 Mesh   2-3%

Part 2: The following information regarding Dolomitic quicklime produced in Salinas, California may be of assistance in evaluation for lime treatment projects.

Nearly all limestone in existence today was formed by the activity of organisms present in the oceans and is believed to have formed in the Miocene and Pilocene ages going back to between 8 and 11 million years ago. Mollusks and other organisms assimilate calcium carbonate from seawater to form their shells or skeletons and after the death of the organism the solid carbonate remains behind in nearly a pure state as sediment on the ocean bed.

As time passed, the formations of coral reefs, rocks, or pebbles were formed due to intense heat and pressure from below which solidified the sediment. Dolomite is formed by the replacement of some calcium in the limestone by magnesium and is believed to be caused by the action of magnesium salts in seawater shortly after the deposition of calcium carbonate and before the hardening or crystallization of the stone.

Calcium limestone is a carbonate expressed as 56% Calcium Oxide (CAO) and 39% Carbon Dioxide (CO2) and a minimum of 5% Magnesium.

Dolomite is a double carbonate expressed as 30.4% Calcium Oxide (CAO), 21.7% Magnesium Oxide (MgO) and 47.9% Carbon Dioxide (CO2).

Limestone is calcined (burned) at temperatures up to 2400°F. During the calcining process, the Carbon Dioxide in the stone is released into the atmosphere as a gas. The lime then becomes an oxide, or quicklime and, dependent upon the percent of impurities, is about 85-95% pure Calcium Oxide. However, Dolomite Lime becomes an oxide with 57% pure Calcium and 38% pure Magnesium or a total of 95-98% combined pure oxides.

When looking at soil stabilization, it is important to remember that lime does not react with clay soil until it becomes a hydroxide. Lime is hydrated by the addition of water in sufficient quantity to turn the lime oxide into a lime hydroxide.

When Quicklime is mixed with soil and water, the lime hydrates in the soil, becoming a hydroxide, and then reacts with the clay portion of the soil.

Thus the type and quality of lime to be specified in soil stabilization is best defined by the amount of hydroxides produced. Dolomitic has a slightly higher amount of hydroxides, therefore should be considered the preferred quicklime for soil stabilization use.

Hydroxide content of Dolomitic and High Calcium Quicklime

As stated previously, lime does not react with clay soil until it is in a hydroxide condition. The following is a quick conversion factor to determine the hydroxide content:

The Calcium Oxide content of the lime x a 1.32 factor equals the hydroxide content.

The .32 of the factor indicates water needed to overcome vapor loss plus 26% retention of chemical water, which is now part of the product.

Thus, if the oxide content of the High calcium quicklime is 86%, the hydroxide content would be 86% x 1.32 = 113%.

The hydroxide content of Dolomitic quicklime is computed as follows:



Dolomitic quicklime is considerably higher in hydroxide content, than the High Calcium quicklime (130% vs. 113%), as specified in Caltrans Section 24.