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2 Validation of Stabilizer Selection The soil must first be classified as either a subgrade category or base category material. In order to be classified as a base material the following criteria must be met: (1) a maximum of 25 percent of the soil mass passes the No. 200 sieve (0.074 mm or 0.003 in.), (2) not more than 40 percent of the soil mass passes the No. 40 sieve (0.42 mm or 0.0165 in.), (3) a maximum plasticity index of 12 percent, and (4) a maximum liquid limit of 40 percent. Otherwise, it is classified as a subgrade material for stabilization purposes. The definition of modification and stabilization can be ambiguous. In this document modification refers soil improvement that occurs in the short term, during or shortly after mixing (within hours). This modification reduces the plasticity of the soil (improves the consistency) to the desired level and improves short-term strength to the desired level (short-term is defined as strength derived immediately within about 7-days of after compaction). Even if no significant pozzolanic or cementitious reaction occurs, the textural changes that accompany consistency improvements normally result in measurable strength improvement. Stabilization occurs when a significant, longer-term reaction takes place. This longer-term reaction can be due to hydration of calcium-silicates and/or calcium aluminates in Portland cement or class C fly ash or due to pozzolanic reactivity between free lime and soil pozzolans or added pozzolans. A strength increase of 50 psi (350 kPa) or greater (of the stabilized soil strength compared to the untreated soil strength under the same conditions of compaction and cure) is a reasonable criterion for stabilization. Construction steps in the stabilization process are not addressed in this document or in the Standard Practice associated with this document. MECHANISMS OF STABILIZATION The stabilization mechanism may vary widely from the formation of new compounds binding the finer soil particles to coating particle surfaces by the additive to limit the moisture sensitivity. Therefore, a basic understanding of the stabilization mechanisms involved with each additive is required before selecting an effective stabilizer suited for a specific application. Chemical stabilization involves mixing or injecting the soil with chemically active compounds such as Portland cement, lime, fly ash, calcium or sodium chloride or with viscoelastic materials such as bitumen. Chemical stabilizers can be broadly divided in to three groups: Traditional stabilizers such as hydrated lime, Portland cement and Fly ash; Non-traditional stabilizers comprised of sulfonated oils, ammonium chloride, enzymes, polymers, and potassium compounds; and By-product stabilizers which include cement kiln dust, lime kiln dust etc. Among these, the most widely used chemical additives are lime, Portland cement and fly ash (1). Although stabilization with fly ash may be more economical when compared to the other two, the composition of fly ash can be highly variable. The mechanisms of stabilization of the traditional stabilizers are detailed below. Traditional Stabilizers Traditional stabilizers generally rely on pozzolanic reactions and cation exchange to modify and/or stabilize. Among all traditional stabilizers, lime probably is the most routinely used. Lime is prepared by decomposing limestone at elevated temperatures. Lime-soil reactions are complex and primarily involve a two step process. The primary reaction involves cation exchange and flocculation/agglomeration that bring about rapid textural and plasticity changes (2). The altered clay structure, as a result of flocculation of clay particles due to cation exchange and short-term
3 pozzolanic reactions, results in larger particle agglomerates and more friable and workable soils. Although pozzolanic reaction processes are slow, some amount of pozzolanic strength gain may occur during the primary reactions, cation exchange and flocculation/agglomeration. Extent of this strength gain may vary with soils depending on differences in their mineralogical composition. Therefore, mellowing periods, normally about one-day in length but ranging up to about 4-days, can be prescribed to maximize the effect of short term reactions in reducing plasticity, increasing workability, and providing some initial strength improvement prior to compaction. The second step, a longer-term pozzolanic based cementing process among flocculates and agglomerates of particles, results in strength increase which can be considerable depending on the amount of pozzolanic product that develops, and this, in turn depends on the reactivity of the soil minerals with the lime or other additives used in stabilization. The pozzolanic reaction process, which can either be modest or quite substantial depending on the mineralogy of the soil, is a long term process. This is because the process can continue as long as a sufficiently high pH is maintained to solubilize silicates and aluminates from the clay matrix, and in some cases from the fine silt soil. These solubilized silicates and aluminates then react with calcium from the free lime and water to form calcium-silicate-hydrates and calcium- aluminate-hydrates, which are the same type of compounds that produce strength development in the hydration of Portland cement. However, the pozzolanic reaction process is not limited to long term effects. The pozzolanic reaction progresses relatively quickly in some soils depending on the rate of dissolution from the soil matrix. In fact, physio-chemical changes at the surface of soil particles due to pozzolanic reactions result in changes in plasticity, which are reflected in textural changes that may be observed relatively rapidly just as cation exchange reactions are. Portland cement is comprised of calcium-silicates and calcium-aluminates that hydrate to form cementitious products. Cement hydration is relatively fast and causes immediate strength gain in stabilized layers (3). Therefore, a mellowing period is not typically allowed between mixing of the components (soil, cement, and water) and compaction. In fact it is general practice to compact soil cement before or shortly after initial set, usually within about 2 hours. Unless compaction is achieved within this period traditional compaction energy may not be capable of developing target density. However, Portland cement has been successfully used in certain situations with extended mellowing periods, well beyond 2 to 4 hours. Generally, the soil is remixed after the mellowing periods to achieve a homogeneous mixture before compaction. Although the ultimate strength of a soil cement product with an extended mellowing period may be lower than one in which compaction is achieved before initial set, the strength achieved over time in the soil with the extended mellowing period may be acceptable and the extended mellowing may enhance the ultimate product by producing improved uniformity. Nevertheless, the conventional practice is to compact soil cement within 2 hours of initial mixing (4). During the hydration process, free lime, Ca(OH)2 is produced. In fact up to about 25 percent of the cement paste (cement and water mix) on a weight basis is lime. This free lime in the high pH environment has the ability to react pozzolanically with soil, just as lime does and this reaction continues as long as the pH is high enough, generally above about 10.5. Fly ash is also generally considered as a traditional stabilizer. While lime and Portland cement are manufactured materials, fly ash is a by-product from burning coal during power generation. As with other by-products, the properties of fly ash can vary significantly depending on the source of the coal and the steps followed in the coal burning process. These by-products can broadly be classified into class C (self-cementing) and class F (non-self cementing) fly ash based
4 on AASHTO M 295 (ASTM C 618). Class C fly ash contains a substantial amount of lime, CaO, but almost all of it is combined with glassy silicates and aluminates. Therefore upon mixing with water, a hydration reaction similar to that which occurs in the hydration of Portland cement occurs. As with Portland cement, this hydration reaction produces free lime. This free lime can react with other unreacted pozzolans, silicates and aluminates, available within the fly ash to produce a pozzolanic reaction, or the free lime may react pozzolanically with soil silica and/or alumina. Class F ash, on the other hand, contains very little lime and the glassy silica and/or alumina exists almost exclusively as pozzolans. Therefore, activation of these pozzolans requires additives such as Portland cement or lime, which provide a ready source of free lime. The hydration or âcementitiousâ reactions and the pozzolanic reactions that occur when fly ash is blended with water form the products that bond soil grains or agglomerates together to develop strength within the soil matrix. As discussed previously, maintenance of a high system pH is required for long term strength gain in fly ash-soil mixtures. The kinetics of the cementitious reactions and pozzolanic reactions that occur in fly ash stabilized soils vary widely depending on the type of ash and its composition. Normally, class C ashes react rapidly upon hydration. However, class F ashes activated with lime or even Portland cement produce substantially slower reactions than Portland cement â soil blends. Generally compaction practice of fly ash - soil blends varies depending on the type of ash used or whether or not an activator is used, but the standard practice is to compact within 6 hours of initial mixing (5). By-product Stabilizers Like traditional stabilizers, pozzolanic reactions and cation exchange are the primary stabilization mechanisms for many of the by-product stabilizers. Lime kiln dust (LKD) and cement kiln dust (CKD) are by-products of the production of lime and Portland cement, respectively. Lime kiln dust (LKD) normally contains between about 30 to 40 percent lime. The lime may be free lime or combined with pozzolans in the kiln. The source of these pozzolans is most likely the fuel used to provide the energy source. LKDs may be somewhat pozzolanically reactive because of the presence of pozzolans or they may be altogether non reactive due to the absence of pozzolans or the low quality of the pozzolans contained in the LKD. Cement kiln dust (CKD) is the by product of the production of Portland cement. The fines captured in the exhaust gases of the production of Portland cement are more likely (than LKD) to contain reactive pozzolans and therefore, to support some level of pozzolanic reactivity. CKD generally contains between about 30 and 40 percent CaO and about 20 to 25 percent pozzolanic material. The purpose of this document is not to establish specific guidelines regarding composition of by-product LKD or by-product CKD as the oxide composition of each can vary widely depending on the composition of the feed stock, the nature of the fuel, the burning efficiency, and the mechanism and efficiency of flue dust capture. For example if coal is used, then ash produced as a by-product of burning coal could be captured in the bag house or other mechanism used to capture exhaust fines with the by-product lime. If the source of the LKD is from the production of dolomitic lime, then magnesium oxide may form a significant part of the LKD. Magnesium oxide, MgO, takes longer and is more difficult to fully hydrate than CaO, and upon hydration it expands. If the LKD contains more than about 5 percent MgO then care should be taken to insure full hydration of the MgO if this LKD is used for modification or stabilization.
