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The dominant construction-related use of lime is soil stabilization for roads, building foundations, and earthen dams. Lime is added to low-quality soils to produce a usable base and sub-base.

Hydrated lime has long been acknowledged to be a superior anti-stripping additive for asphalt pavements. It also helps resist rutting and fracture growth at low temperatures, reduces age hardening, and improves moisture resistance and durability. A recent study shows lime holds significant advantages over other additives for hot mix asphalt roads, including long-term cost savings and increased durability.

Perhaps the oldest and most traditional use of lime has been in mortar and plaster, because of its superior plasticity and workability.  This area provides additional information about the construction applications of lime.

Asphalt

Lime has been used in hot mix asphalt (HMA) to reduce moisture sensitivity and stripping since 1910 in the United States. While hydrated lime has long been an acknowledged anti-strip additive for asphalt pavements, recent studies confirm that lime imparts other important benefits:

  • It stiffens the binder and HMA to resist rutting;
  • It improves toughness and resistance to fracture growth at low temperatures;
  • Lime changes oxidation chemistry in the binder to reduce age hardening; and
  • Lime alters clay fines to improve moisture stability and durability.

Lime is also useful to upgrade marginal aggregates. In addition to the chemical effects that lime imparts to reduce stripping potential and the aging impact resulting from oxidative hardening, the “filler effect” of lime improves resistance to high-temperature rutting and adds fracture toughness at low temperatures.

Hydrated Lime

A Multi-Functional Asphalt Modifier

As numerous state highway authorities have found, modifications made to HMA with hydrated lime will add years to the pavement’s life. These modifications can reduce stripping, rutting, cracking, and aging. Hydrated lime substantially improves each of these properties when used alone, and also works well in conjunction with polymer additives, helping to create pavement systems that will perform to the highest expectations for many years.

A Superior Anti-Stripping Agent

Stripping occurs when the bond between the asphalt cement and the aggregate breaks down due to the presence of moisture, and the binder separates from the aggregate. Certain types of aggregates are particularly susceptible to stripping.  Climatic characteristics such as heat, heavy rains, freeze/thaw cycles, and traffic play a major role in stripping. Hydrated lime is the most effective anti-stripping agent available and is widely specified by states with serious stripping problems.

When lime is added to hot mix, it reacts with aggregates, strengthening the bond between the bitumen and the stone. At the same time that it treats the aggregate, lime also reacts with the asphalt itself. Lime reacts with highly polar molecules that can otherwise react in the mix to form water-soluble soaps that promote stripping. When those molecules react with lime, they form insoluble salts that no longer attract water. In addition, the dispersion of the tiny hydrated lime particles throughout the mix makes it stiffer and tougher, reducing the likelihood the bond between the asphalt cement and the aggregate will be broken mechanically, even if water is not present.

Improves Stiffness and Reduces Rutting

Rutting is permanent deformation of the asphalt, caused when elasticity is exceeded. The ability of hydrated lime to make an asphalt mix stiffer, tougher, and resistant to rutting is a reflection of its superior performance as an active mineral filler.  Unlike most mineral fillers, lime is chemically active rather than inert. It reacts with the bitumen, removing undesirable components at the same time that its tiny particles disperse throughout the mix, making the pavement more resistant to rutting and fatigue cracking.

Stiffening that results from the addition of hydrated lime can increase the performance grade (PG) rating of asphalt cement. Depending upon the amount of lime used (generally 10 to 20% by weight of asphalt), the PG rating may increase by one full grade. In other words, a PG 64-22 can be increased to a PG 70-22. Adding lime will not, however, cause the mix to become more brittle at lower temperatures. At low temperatures, hydrated lime becomes less chemically active and behaves like any other inert filler.

Reduces Oxidation and Aging

Oxidation and aging occur over time to generate a brittle pavement. Polar molecules react with the environment, breaking apart and contributing to pavement failure. Adding hydrated lime to asphalt cements reduces the rate at which the asphalt oxidizes and ages. This is a result of the chemical reactions that occur between the calcium hydroxide and the highly polar molecules in the bitumen. Hydrated lime combines with the polar molecules at the time it is added to the asphalt and thus, the polar molecules do not react with the environment. Consequently, the asphalt cement remains flexible and protected from brittle cracking for years longer than it would without the contribution of lime.

Reduces Cracking

Cracking can result from causes other than aging, such as fatigue and low temperatures. Hydrated lime reduces asphalt cracking from these causes as well. Although, in general, stiffer asphalt mixes crack more, the addition of lime improves fatigue characteristics and reduces cracking. Cracking often occurs due to the formation of micro-cracks. These micro-cracks are intercepted and deflected by tiny particles of hydrated lime. Lime reduces cracking more than inactive fillers because of the reaction between the lime and the polar molecules in the asphalt cement, which increases the effective volume of the lime particles by surrounding them with large organic chains. Consequently, the lime particles are better able to intercept and deflect micro-cracks, preventing them from growing together into large cracks that can cause pavement failure.

Synergistic Benefits

The numerous benefits resulting from the addition of hydrated lime to HMA work together to produce a superior, high-performance product. Though the benefits above have been described individually, all of them work synergistically, contributing in multiple ways to the improvement of the final product. Synergistic benefits also accrue when lime is used in conjunction with polymer modifiers. Research shows that in some situations, lime and polymers used together can produce improvements greater than when each is used alone.

Adding Hydrated Lime to Hot Mix Asphalt

Hydrated lime can be added to hot mix asphalts in a variety of ways. As a general rule, the application rate is one percent by weight of the mix. However, in cases where severe stripping is anticipated the application amount may increase. The most commonly used methods of addition are described below:

Dry Injection into Drum Mixers

This method was pioneered by the State of Georgia in the mid-1980s when the state decided to require the addition of lime to all of its HMA. One percent hydrated lime by weight of the mix is used and is added to the drum at the same time as the mineral filler. Georgia required modifications to the drum mixer to minimize the loss of lime when it is added. The hydrated lime comes into direct contact with the aggregate, thereby improving the bond between the bitumen and the stone, while the balance enters the bitumen. That portion of the lime can react with the polar molecules that contribute to both stripping and oxidation, while simultaneously stiffening and toughening the mix. The dry method is the simplest of the commonly used application methods. Since using lime, Georgia has significantly reduced its severe stripping problems as well as most of its rutting problems.

There are also other drum methods, such as ASTEC’s double barrel mixer, for example. In this system, fine materials can be added efficiently because they enter the mix in a turbulence-free zone.

Dry Lime on Damp Aggregate Method

This method is the one most commonly used throughout the country. It involves metering the lime onto a cold feed belt carrying aggregate that has been wet to approximately 2-3% over its saturated-surface-dry (SSD) condition. The lime-treated aggregate is then run through a pug mill to ensure thorough mixing before it is fed into the plant. Lime is applied to damp aggregate to ensure more complete coverage of the stone than is achieved using the dry method. Lime that does not adhere to the stone is dispersed throughout the mix where it will contribute to the other improvements that have been described. The “dry on damp” method of adding hydrated lime to a hot mix is also relatively simple, but driving off the additional water required by the process uses additional fuel and may slow down plant production to some degree. In a variation on this method, the aggregate and lime are marinated in a stockpile before use to provide additional time for the lime to react with the surface of the stone and further improve anti-stripping performance.

Slurry Method

This method utilizes a slurry mixture of lime and water that is applied at a metered rate to the aggregate, ensuring superior coverage of the stone surfaces. The aggregate can then either be fed directly into the plant or marinated in a stockpile for some time, allowing the lime to react with the aggregate. Because the lime is bound to the stone, it is also the method that results in the least dispersion of the lime throughout the rest of the mix.

Use of Lime to Recycle Asphalt Pavement

A new and growing use for lime is in cold, in-place recycling for the rehabilitation of distressed asphalt pavements. Existing asphalt pavement is pulverized using a milling machine, and a hot lime slurry is added with asphalt emulsion. The cold recycled mix is placed and compacted using conventional asphalt paving equipment and produces a smooth-based course for the new asphalt surface. The addition of lime results in superior cold recycled mixtures, with much greater early strength and resistance to moisture damage.

Building Construction

Masonry Mortars, Stuccos, and Plasters

Lime has been used as a primary ingredient in masonry mortars for centuries, and this important use continues to the present day in both historic and contemporary applications. Mortars made with lime and cement exhibit superior workability balanced with appropriate compressive strength as well as low water permeability and superior bond strength. Lime is a major constituent in exterior and interior stuccos and plasters, enhancing the strength, durability, and workability of these finishes. All of these lime applications are supported by ASTM specifications and standards. Papers and articles on a variety of building lime applications are available at www.buildinglime.org. Type S (Special) hydrated lime is a fine, white, high-purity product specially hydrated for convenient, trouble-free use in mortar applications. It is a uniquely American product, with much more stringent requirements for masonry performance than those imposed by any other country. Type SA (Special Air-Entrained) hydrated lime is similar, except that it includes an air-entraining agent that produces minute voids in the mixed mortar. Either type will provide a superior quality mortar. Both are subject to the ASTM C207 Standard Specification for Hydrated Lime for Masonry Purposes.

Modern Masonry Applications

Studies have compared the performance of cement-lime mortars to that of masonry cement mortars (which use limestone and other additives instead of hydrated lime) and mortar cements. Cement-lime mortars have shown higher bond and shear strength and lower water leakage. For more information on the use of hydrated lime for masonry purposes, click here. For a fact sheet on the use of lime-based mortars to create watertight walls, click here.

Historic Masonry Applications

Most masonry produced before the turn of the 20th century used lime-sand mortar. The elasticity of high lime-content mortars allows for the expansion and contraction of such historic masonry walls without damaging the masonry units. These units can have low compressive strengths and can be damaged by modern masonry products with higher strengths.

Plastering Uses

Type S (Special) hydrated lime shows its versatility and beauty when used for interior and exterior plaster or render. ASTM C206 Standard Specification for Finishing Hydrated Lime requires that finishing lime be free of any chemical or physical characteristics that would cause flaws in the plaster.

Other Uses of Lime in Building Construction

Limewash

Limewash is a versatile, accommodating, and robust surface covering compatible with a variety of building surfaces. It is maintainable, beautiful, stable, and long-lasting. A copy of a paper on limewash presented at the 2005 International Building Lime Symposium is available here.

Site Preparation

Lime can be used to dry wet sites. Lime can also react with clays in soil to provide a more stable base for building construction. For more information on these uses, click here.

Autoclaved Aerated Concrete (AAC)

Lime is also employed in the manufacture of innovative lightweight cellular concrete products, such as autoclaved aerated concrete (also called “aircrete”), which can be formed into block as well as large masonry units or insulation slabs. The 2005 International Building Lime Symposium included a paper on AAC.

Other Concrete Products

Hydrated lime can be added to concrete mix used to make blocks and other concrete products to produce a denser, more water-resistant product. By adding greater plasticity to the mix, lime also produces concrete products with more precise edges and corners, improves reflectivity, and reduces loss through breakage.

Calcium Silicate Brick

Calcium silicate (sand-lime) brick is employed in standard masonry construction in the same manner as common clay brick. Sand is mixed with high calcium lime (quick or hydrated) in a wet state, and then molded into bricks and autoclaved. The lime reacts with silica to form complex hydro(di)calcium silicates that bind the brick and provide high dimensional stability. Lime is also used to make hollow sand-lime building block, tile, slabs, and pipes.

Insulation Materials

Some insulating materials, molded as units, contain lime and diatomaceous earth or lime and silica. In these products, lime serves as a binding agent, reacting chemically with the available silica present in the mix to form calcium silicates. The lime-silica reaction is also employed in making microporite insulation.

Soil Stabilzation

Lime can be used to treat soils to improve their workability and load-bearing characteristics in several situations. Quicklime is frequently used to dry wet soils at construction sites and elsewhere, reducing downtime and providing an improved working surface. An even more significant use of lime is in the modification and stabilization of soil beneath roads and similar construction projects. Lime can substantially increase the stability, impermeability, and load-bearing capacity of the subgrade. Both quicklime and hydrated lime may be used for this purpose. Application of lime to subgrades can provide significantly improved engineering properties.

Soil Modification:

Lime is an excellent choice for short-term modification of soil properties. Lime can modify almost all fine-grained soils, but the most dramatic improvement occurs in clay soils of moderate to high plasticity. Modification occurs because calcium cations (KAT-eye-əns) supplied by hydrated lime replace the cations normally present on the surface of the clay mineral, promoted by the high pH environment of the lime-water system. Thus, the clay surface mineralogy is altered, producing the following benefits:

  • Plasticity reduction;
  • Reduction in moisture-holding capacity (drying);
  • Swell reduction;
  • Improved stability; and
  • Ability to construct a solid working platform.

Soil Stabilization:

Soil stabilization occurs when lime is added to a reactive soil to generate long-term strength gain through a pozzolanic reaction. This reaction produces stable calcium silicate hydrates and calcium aluminate hydrates as the calcium from the lime reacts with the aluminates and silicates solubilized from the clay. The full-term pozzolanic reaction can continue for a very long period, even decades — as long as enough lime is present and the pH remains high (above 10). As a result, lime treatment can produce high and long-lasting strength gains. The key to pozzolanic reactivity and stabilization is a reactive soil, a good mix design protocol, and reliable construction practices.

Benefits of Soil Stabilization

Lime substantially increases soil resilient modulus values (by a factor of 10 or more in many cases). In addition, when lime is added to soil, users see substantial improvements in shear strength (by a factor of 20 or more in some cases), continued strength over time, even after periods of environmental or load damage (autogenous healing), and long-term durability over decades of service even under severe environmental conditions.

Short- and Long-Term Economic Benefits

In the short term, the structural contribution of lime-stabilized layers in pavement design can create more cost-effective design alternatives. A recent interstate project in Pennsylvania, for example, began with a $29.3 million traditional design approach. An alternate design using lime stabilization, consistent with AASHTO mechanistic-empirical designs, cost only $21.6 million—more than 25 percent savings. The savings came from treating the existing subgrade material with lime, rather than removing the material and replacing it with granular material; and thinner layers of flexible pavement for the lime stabilized alternate due to the increased strength of the lime stabilized sub base. In the longer term, lime stabilization provides performance benefits that reduce maintenance costs. To illustrate, stabilizing an 8-inch native clay subgrade with lime as part of an asphalt pavement project can reduce 30-year life cycle costs from $24.49 to $22.47 per square yard. In addition to the stabilization of new materials, lime is an excellent choice for reclamation of roadbases. As more and more governmental entities are choosing to reclaim existing roadbases rather than replace them, this use of lime will become even more important.

How to Use Lime for Soil Stabilization

Lime stabilization is not difficult to carry out. After proper mix design and testing are performed, in-place mixing is usually used to add the appropriate amount of lime to soil, mixed to an appropriate depth. Pulverization and mixing are used to thoroughly combine the lime and soil. For heavy clays, preliminary mixing may be followed by 24 to 48 hours (or more) of moist curing, followed by final mixing. For maximum development of strength and durability, proper compaction is necessary.  Correct curing is also important. If sulfates are present at levels greater than 0.3 percent, special procedures are required.

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