Soil Fly Ash Mixtures Research Articles

Leachate Concentrations from Water Leach and Column Leach Tests on Fly Ash-Stabilized Soils

Batch water leaching tests (WLTs) and column leaching tests (CLTs) were conducted on coal-combustion fly ashes, soil, and soil–fly ash mixtures to characterize leaching of Cd, Cr, Se, and Ag. The concentrations of these metals were also measured in the field at two sites where soft fine-grained soils were mechanically stabilized with fly ash. Concentrations in leachate from the WLTs on soil–fly ash mixtures are different from those on fly ash alone and cannot be accurately estimated based on linear dilution calculations using concentrations from WLTs on fly ash alone. The concentration varies nonlinearly with fly ash content due to the variation in pH with fly ash content. Leachate concentrations are low when the pH of the leachate or the cation exchange capacity (CEC) of the soil is high. Initial concentrations from CLTs are higher than concentrations from WLTs due to differences in solid–liquid ratio, pH, and solid–liquid contact. However, both exhibit similar trends with fly ash content, leachate pH, a...

Class F Fly-Ash-Amended Soils as Highway Base Materials

Class F fly ash cannot be used alone in soil stabilization applications as it is not self-cementing. An activator such as Portland cement or lime must be added to produce cementitious products often called pozzolan stabilized mixtures. The developed mixture must possess adequate strength and durability, should be easily compacted, and should be environmentally friendly. Roadways have a high potential for large volume use of the fly ash stabilized soils. The main objective of this study is to investigate the use of Class F fly ash amended soil–cement or soil–lime as base layers in highways. A battery of tests were performed on soil–fly ash mixtures prepared with cement and lime as activators. Unconfined compression, California bearing ratio, and resilient modulus tests were conducted. Finally, required base thicknesses were calculated using the laboratory-based strength parameters. Results of the study show that the strength of a mixture is highly dependent on the curing period, compactive energy, cement c...

Mechanical Stabilization of Cemented Soil–Fly Ash Mixtures with Recycled Plastic Strips

An experimental investigation was undertaken to evaluate the mechanical behavior of a soil–cement–fly ash composite, reinforced with recycled plastic strips (high-density polyethylene) that were obtained from postconsumer milk and water containers. The primary motivation for the study was to investigate the innovative reuse of several candidate waste materials in geotechnical and pavement applications. The specific objectives of the research were: (1) to evaluate the compressive, split tensile, and flexural strength characteristics of the material, and (2) to determine the effectiveness of recycled plastic strips in enhancing the toughness characteristics of the composite. Since cement-stabilized materials are weak in tension, the main focus of the experimental program was to conduct a series of specially instrumented split tensile and flexural tests on mixes containing various amounts of cement, fly ash, and plastic strips. For a meaningful comparison of test results, all specimens were prepared at a constant dry density. The standard ASTM C496 procedure for split tensile test was slightly modified by attaching two horizontal linear variable differential transformers (LVDTs) to measure the diametral deformation of the specimen due to compressive loading in an orthogonal direction. This modification enabled the evaluation of the postpeak toughness behavior of the composite. For some specimens, a strain gauge was attached to the middle of the face perpendicular to the loading plane in order to correlate the results with the one found using the LVDTs. All tests were performed with a 90 kN universal testing machine with deformation control. Experimental data show that the soil–cement matrix stabilized with 4% to 10% by weight of fly ash and reinforced with 0.25% to 0.5% (by weight) plastic strips (having lengths of 19 mm or 38 mm) can achieve a maximum compressive strength of 7000 kPa, a split tensile strength of 1000 kPa, and a flexural strength of 1200 kPa. These ranges in strength values are suitable for a high-quality stabilized base course for a highway pavement. To quantify the reinforcing effects in the postpeak region, a dimensionless toughness index is proposed. It is found that the use of fiber reinforcement significantly increases the postpeak load carrying capacity of the mix and thus the fracture energy. It is concluded that the lean cementitious mix containing recycled materials offer a lot of promise as an alternative material for civil engineering construction.

Behavior of Compacted Soil-Fly Ash-Carbide Lime Mixtures

Unconfined compression tests, Brazilian tensile tests, and saturated drained triaxial compression tests with local strain measurement were carried out to evaluate the stress-strain behavior of a sandy soil improved through the addition of carbide lime and fly ash. The effects of initial and pozzolanic reactions were investigated. The addition of carbide lime to the soil-fly ash mixture caused short-term changes due to initial reactions, inducing increases in the friction angle, in the cohesive intercept, and in the average modulus. Such improvement might be of fundamental importance to allow site workability and speeding construction purposes. In addition, under the effect of initial reactions, the maximum triaxial stiffness occurred for specimens molded on the dry side of the optimum moisture content, while the maximum strength occurred at the optimum moisture content. After 28 days, pozzolanic reactions magnified brittleness and further increased triaxial peak strength and stiffness; the maximum triaxial strength and stiffness occurred on the dry side of the optimum moisture content.

Behavior of Cement-Stabilized Fiber-Reinforced Fly Ash-Soil Mixtures

An experimental program was undertaken to study the individual and combined effects of randomly oriented fiber inclusions and cement stabilization on the geotechnical characteristics of fly ash-soil mixtures. An Indian fly ash was mixed with silt and sand in different proportions. The geotechnical characteristics of the raw fly ash-soil specimens and fly ash-soil specimens containing 1% randomly oriented polyester fiber inclusions were investigated. Unconfined compression tests were carried out on fly ash-soil specimens prepared with 3% cement content alone and also with 3% cement and 1% fiber contents, after different periods of curing. The study shows that cement stabilization increases the strength of the raw fly ash-soil specimens. The fiber inclusions increase the strength of the raw fly ash-soil specimens as well as that of the cement-stabilized specimens and change their brittle behavior to ductile behavior. Depending on the type of fly ash-soil mixture and curing period, the increase in strength caused by the combined action of cement and fibers is either more than or nearly equal to the sum of the increase caused by them individually.

Correlation Analysis of Laboratory Compaction of Fly Ashes

Fly ash, a waste from thermal power stations, has been used in many countries to build highway embankments, fills, landfill liners, and covers. The maximum dry density (MDD) and optimum moisture content (OMC) of fly ash or fly ash–soil mixtures are determined by conducting compaction test. There is a large variation in the MDD and OMC of different fly ashes due to the differences in their characteristics. In the present study, data regarding the compaction characteristics and specific gravity (G) of different fly ashes were collected. Correlation analyses of the data for 57 samples for light (standard Proctor) compaction and 21 samples for heavy (modified Proctor) compaction were carried out. Correlations have been developed for MDD and OMC for both light and heavy compaction. The correlation for MDD in terms of OMC and G and that between MDD and OMC have been expressed in the form of log-log, exponential, and linear curves. The general applicability of the correlations for light compaction has been teste...

Compressive strength of cement stabilized fly ash-soil mixtures

Rajghat fly ash from Delhi, India, and Baumineral fly ash near Bochum, Germany, were mixed with the locally available soils—silt and Yamuna sand with Rajghat fly ash and Rhine sand with Baumineral fly ash—in different proportions. Cement, varying from 3–9% was added to stabilize the fly ash-soil mixtures. Cylindrical samples were prepared at optimum moisture content and maximum dry density and were cured for different duration. Unconfined compression tests were conducted on these samples. Correlations for unconfined compressive strength and secant modulus as functions of curing time, fly ash content, and cement content have been established. The data were analyzed with other correlations recommended in literature and comparisons between the correlations have been made. Correlations for water content as functions of curing time and cement content have also been established.

Fly ash application on two acid soils and its effect on soil salinity, pH, B, P and on ryegrass growth and composition

Application of alkaline fly ash to acid soils is related to beneficial effects, such as increase of pH to a desired level and nutrient supply to plants, and to possible adverse effects, such as enrichment of soils with substances toxic to plants and animals (e.g. B, Mo, Se) and increase of salinity to undesirable levels. Therefore, use of alkaline fly ash as a beneficial amendment of acid soils needs to be evaluated with respect to phytotoxic and environmental impacts. Samples of alkaline fly ash, from two different sources, were added to two Red Mediterranean acid soils at rates equal to 5, 20 and 50 g kg −1 soil, and changes, relative to the untreated soil, of soil pH, salinity, B and P levels were measured. Ryegrass ( Lolium perenne L.) was grown in pots containing fly ash–soil mixtures for 300 days, and dry biomass yield and cumulative plant uptake of B and P were calculated. Soil application of fly ash at these rates increased the pH, up to about 8, and the electrical conductivity of the saturation extract, up to about 2.5 dS m −1, in both soils. Available soil P (0.5 M NaHCO 3 extractable) was unaffected by fly ash application. Water soluble B remained less than 1 mg litre −1 in the saturation extract, and hot water extractable B was less than 1 mg kg −1 soil. Dry biomass yield of ryegrass and cumulative plant uptake of B and P increased significantly with fly ash application. Therefore, fly ashes with low B and salt content can be used as liming agents in acid soils at rates not exceeding the 40 Mg/ha. Potential environmental impacts must also considered.

Vegetation Establishment on Soil-amended Weathered Fly Ash

A field study was conducted with the following objectives in mind: (1) to study the effect of soil addition to weathered fly ash on the establishment and survival of different grasses and legumes, (2) to identify suitable grasses and/or legume species for vegetation of fly ash, (3) to study the fertilizer N and P requirements for successful vegetation establishment on fly ash and ash-soil mixtures, (4) to examine the nutrient composition of the plant species tested, and (5) to study the plant availability of P from fly ash and ash-soil mixtures. Three rooting media were used: weathered fly ash, and 33% or 50% soil blended with the ash. Four experiments were established on each of these media to evaluate warm season grasses in pure stands, warm season grasses inter-seeded with legumes, cool season grasses, and cool season grasses inter-seeded with legumes. Soil used in this study was more acidic than the fly ash. Only the results from characterization of the rooting media, ground cover, and yield will be presented here.

National survey of elements and radioactivity in fly ashes. Absorption of elements by cabbage grown in fly ash-soil mixtures

An analytical survey of 45 elements was conducted in fly ashes from 21 states in the US, using several instrumental methods. A high degree of correlation was found between the concentrations of pairs of chemically similar elements in the fly ashes and between the magnitude of gamma emission of the fly ashes and their respective concentrations o th or U. Cabbage was grown to maturity in potted soil amended with 7% w/w of the various fly ashes. The concentrations of As, B, Mo, Se and Sr in the cabbage showed a high degree of correlation with those in the fly ashes in which the plants were cultured.