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viii one way to restrict the formation of ettringite is by limiting one of the above components in stabilized soils. During mixing of lime, soil, and water, lime provides the required calcium needed for cation exchange and flocculation/agglomeration of clay particles. Excess lime in soil, above the amount required for cation exchange and to maintain satisfactory pozzolanic reactions with the soil, maintains the pH level above 10.5, a condition favorable for ettringite precipitation in soils. But, limiting lime concentration in soil can affect long-term strength gain in stabilized soils and hence is not generally advocated. Clay and silt fractions in soil, which provide the source for alumina and silica needed for pozzolanic reactions, also cannot be controlled during stabilization. Water is supplied to support the treatment process as part of water of construction. Water may also enter the system as ground water through processes of infiltration, through capillary rise, or through diffusion. Sulfate movement can also occur under various potentials: dissolved in water moving through gravity potential, water moving via capillary rise, water moving in vapor form due to temperature, humidity, or salt concentration differentials. Attention to drainage design can substantially limit the post-stabilization migration of water, and this may have as great an impact on reduction of sulfate-induced damage as any single design action. Perhaps the most practical construction-based approach in controlling or limiting deleterious reaction effects when treating high sulfate content soils with calcuium-based stabilizes is to create conditions in the soil system that favor the dissolution of available sulfates and force the formation of these expansive minerals before compaction. The effectiveness of this method depends primarily on the possible extent of dissolution of naturally available sulfates in the soil system. A complete dissolution of all sulfates in the soil is not practical, as the solubility of gypsum, the major sulfate source, is limited. Typically, an additional 3 to 5 percent above optimum moisture content is added to soils during mellowing time. Mellowing periods have been specified, which range from as little as 24 hours to as long as 7 days, primarily depending on soluble sulfate content in the soils. However, the amount of water typically applied during the mellowing period does not approach the amount necessary to solubilize the sulfates normally associated with the threshold level for damage (normally between 2,000 and 3,000 ppm). Even though the amount of water applied during the mixing and mellowing process is far too low to solublize sulfates, increased moisture content during mellowing does help optimize the formation of nucleation sites that trigger the development of ettringite crystal growth sites, leading to a well-dispersed or homogeneous formation of ettringite crystals, and does probably speed up the formation of ettringite and the utilization of soluble sulfates during this growth process. Since the presence of sulfate ions is the key in the formation of these deleterious minerals, sulfate quantification in soils is critical in defining the reactivity of soils. Sulfur is found in natural soils as sulfide minerals like pyrites, marcasite, and greigite and in sulfate forms like gypsum, anhydrite, barite, and jarosite. A complete sulfur characterization requires identification of different sulfur species existing in soils. Sulfur in soils is assessed under four main categories: 1. Water soluble sulfates, 2. Acid soluble sulfates, 3. Total reduced sulfur, and 4. Total sulfur. Since the sulfate availability in treated soils is dependent on dissolution and movement of sulfate ions in natural water, an extraction process using water as the solvent is acceptable.
ix Sulfate content in soils should be determined prior to construction, and techniques outlined for specific sulfate levels should be used to reduce the risk of post-compaction ettringite formation. Colorimetric techniques are fast and economical when compared to other available methods for measuring sulfate concentrations in soil. Efficacy of method AASHTO T 290 may be improved by incorporating a few changes in techniques used for sample preparation. These include: (1) reducing the size of soil particles used in testing to facilitate a faster and more complete dissolution of available sulfates in soil; (2) using a high water-to-soil dilution ratio, which will help prevent underestimation of available sulfates in soil, as the solubility is limited by saturation conditions of the solution; (3) allow the soil-water solution to set idle for at least 12 hours prior to filtration to facilitate complete dissolution of sulfates in soil.
