Lime vs. Cement: Which Soil Stabilization Method is Better?
What is Soil Stabilization?
Soil stabilization is a process by which the physical properties of a soil are transformed to provide permanent strength gains by incorporating lime or cement. Stabilized soils outperform non-stabilized soils when materials, design, and construction are properly considered. When the stabilized soil layer is incorporated into the structural design of the pavement, the subsequent layers can be thinner resulting in sizable cost savings. In addition to adding strength, stabilized soils form a solid monolith that decreases permeability, which in turn reduces the shrink/swell potential and the harmful effects of freeze/thaw cycles.
Soil stabilization can improve in situ, or natural state, soils eliminating the need for expensive remove-and-replace operations. Often the native soils where roads, building pads, parking lots, runways or other structures need to be built are naturally wet, weak soils. Those soils can be chemically treated to add strength through stabilization and improve engineering properties including moisture content and plasticity, through modification. Ex situ, or off site, soil stabilization processes are possible but are usually reserved for environmental projects rather than typical construction operations. Pre-project testing is essential to be certain that the right product is used for each project and that enough of the stabilizing agent is present to permanently stabilize the soil.
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Lime Stabilization of Soil
Why is lime used in soil stabilization?
Lime is used in soil stabilization applications to develop long-term permanent strength in fine-grained soils high in silt and clay content. Lime stabilization uses pozzolans, which are naturally present in clay soils, to generate cementitious bonds that permanently strengthen and stabilize clay soil., which are naturally present in clay soils, to generate cementitious bonds that permanently strengthen a soil. Pozzolans such as silica and alumina react with calcium, supplied by the lime, and water to form calcium-silicate-hydrates (C-S-H) and calcium-aluminate-hydrates (C-A-H). The C-S-H and C-A-H are the same products that are responsible for the strength in materials such as concrete.
What lime is used in soil stabilization?
The term lime can be used somewhat inconsistently. For soil stabilization purposes, the term lime must mean either quicklime or hydrated lime. Chemically, quicklime is calcium oxide (CaO) and hydrated lime is calcium hydroxide (Ca(OH)2). Farmers sometimes refer to agricultural lime, finely ground calcium carbonate (CaCO3), as lime. While this form of calcium helps farmers improve their soil by amending their fields, using it would not gain any strength in the soil under the feet of a construction worker.
It is worth noting that quicklime can come in two types, high calcium and dolomitic. High calcium is almost completely calcium oxide (CaO), whereas dolomitic quicklime contains a portion of magnesium oxide along with calcium oxide. While some industrial applications such as steel need the magnesium component for certain processes, for construction purposes high calcium and dolomitic are virtually indistinguishable.
Hydrated lime is quicklime that has been further processed. It has been carefully hydrated with the proper amount of water and agitation to produce a very fine, high-purity product. Hydrated lime can still supply the calcium that is essential to stabilize certain soils by forming cementitious bonds. However, since the material has already been hydrated, it does minimize much of its drying capacity that is desired on wet job sites. Also, hydrated lime is only available in certain areas and there are additional costs for the added processing.
Lime kiln dust (LKD), is derived during the quicklime manufacturing process as a co-product. It is made up of the finely sized particles that are captured in the baghouse at a lime plant. These particles are high calcium or dolomitic lime plus pozzolans from the fuel used to fire the lime kiln. Because it contains both lime and pozzolans, LKD is a hybrid between lime and cement. As previously mentioned, lime works very well with fine-grained soils. Cement, on the other hand, works very well with coarser gained soil. LKD bridges the gap between the two.
Cement Stabilization of Soil
What is soil cement stabilization?
Soil cement stabilization is a construction technique used to increase the strength of subgrade soil by mixing it with cement and water. In this soil stabilization methods, the water hydrates the cement, generating reactions that create a matrix between the soil particles and gives the soil strength. Cement stabilization is especially useful in coarse-grained soils.
What is Portland cement?
Portland cement is the most common type of cement. Its basic components are calcium, silica, alumina, and iron derived from limestone, sand and clay. All are processed, fired in a kiln and pulverized to a fine powder. The final product is what we call Portland cement. When it is exposed to water, it chemically hydrates resulting in a gel that forms an interlocking matrix around particles. As it cures, it hardens giving strength to the system.
Slight variations produce different types of cement. Changes in production lines, differences in raw materials and/or alterations at the end of the cement manufacturing process define the type of cement produced. Qualities like air entrainment, or millions of tiny air bubbles that resist stress due to freezing and thawing, are clearly distinguished. The American Society for Testing and Materials (ASTM) lists the different types of Portland cement in the following table. Type I and III are most commonly used in soil stabilization.
|Normal + Air Entrainment
|Moderate Sulfate Resistance
|Moderate Sulfate Resistance + Air Entrainment
|High Early Strength
|High Early Strength + Air Entrainment
|High Sulfate Resistance
Soil Stabilization Cost
Regardless of the specific stabilizing agent, there are a few factors that affect the cost of stabilization. They are materials, delivery, application and the availability of water. Material cost is regional, depending on local supply and demand. The costs associated with delivering material is dependent on the distance from the material production plant to the job site and trucking rates by location. While spreading with typical construction equipment, such as bulldozers and backhoes, is possible, an experienced contractor with specialized equipment is recommended to properly spread and incorporate the material with the soil. Another facet of application is depth, as more material is required whenever deeper treatment is desired. Treatment depths can range from 6 to 18 inches but are typically between 8 and 12 inches. Finally, water accessibility also affects cost. Water is necessary to hydrate both lime and cement pushing reactions to form strength generating bonds.
Regardless of all of these factors, soil stabilization is almost always the most economical option, especially when compared to remove and replace operations. Soil stabilization improves the material that is already in place without the hazards and frustrating logistics associated with hauling material to and from the site. The cost of importing engineered fill materials and exporting sub-par materials is only increasing. Soil stabilization saves time, money, materials and energy.
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