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Hydraulic conductivity of fly ash-amended mine tailings

Date

2016

Authors

Alhomair, Sultan A., author
Bareither, Christopher A., advisor
Shackelford, Charles D., committee member
Barbarick, Kenneth A., committee member

Journal Title

Journal ISSN

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Abstract

The objective of this study was to evaluate the effect of fly ash addition on hydraulic conductivity (k) of mine tailings. Fly ash-amended mine tailings have potential application as construction materials in active mines, transportation earthworks, and other geotechnical engineering projects. Addition of cementitious binder (fly ash) to mine tailings has the potential to reduce hydraulic conductivity and enhance contaminant sequestration to be feasible in earthwork projects. Mine tailings used in this study were categorized as synthetic tailings and natural tailings. Natural tailings were collected from a garnet mine located in the U.S. Two synthetic mine tailings were developed via blending commercially-available soils to create typical particle-size distributions and plasticity characteristics of actual mine tailings. The two types of fly ash used classified as off-specification, but had sufficient calcium oxide (CaO) content (17% and 18.9%) for pozzolanic activity. Hydraulic conductivity (k) was measured on pure tailings and fly ash-amended tailings in flexible-wall permeameters. All experiments were conducted following a constant head technique (Method A in ASTM D 5084). Fly ash was added to mine tailings to constitute 10% dry mass of the mixture, and specimens were cured for 7 and 28 d inside a constant humidity and temperature room (100% humidity and 21 ̊C) prior to hydraulic conductivity testing. Effluent from the experiments was measured for pH, electrical conductivity, and the presence of heavy metals to assess leaching potential of the tailings and fly ash-amended tailings mixtures. Chromium (Cr), copper (Cu), cadmium (Cd), and silver (Ag) concentrations were evaluated based on common heavy metals associated with fly ash and then compared with drinking water standards and toxicity limits. The influence of fly ash-amendment on k of mine tailings was attributed to (i) molding water content and (ii) plasticity of the mine tailings, or presence of clay particles. Average synthetic tailings that represent typical average particle-size distribution of tailings and natural tailings both classified as low-plasticity silts (ML) with clay contents less than 15%. Hydraulic conductivity of these fly ash-amended tailings were approximately equal to unamended tailings when prepared dry or near optimum water content (wopt), and two to five times lower than unamended tailings when prepared wet of wopt. Fine synthetic tailings that represent typical fine particle-size distribution of tailings classified as low-plasticity clay (CL) and contained 42% clay-sized particles, comprising primarily kaolin. The k of fine synthetic tailings increased approximately one order of magnitude with addition of fly ash for materials prepared dry or near wopt. This increase in k reduced to 3.4 times that of unamended tailings for material prepared wet of wopt. The increase in k with addition of fly ash for the clayey tailings was attributed to agglomeration of clay particles and an overall increase in average pore size to conduct flow. The decrease in k for silty tailings was attributed to formation of cementitious bonds between tailings particles that obstructed flow paths and decreased average pore size. The results also indicated that the effect of curing time on k is more pronounced during the early stages of curing (≤ 7 d), as there was negligible difference between k for 7- and 28-d cured specimens. The propensity to form cementitious bonds was evaluated via the CaO-to-SiO2 ratio, whereby fly ash with a higher CaO-to-SiO2 ratio was anticipated to yield lower k due to more cementitious bond formation. There was no distinguishable difference in the impact on k between the two fly ashes used in this study. Chemical constituents in the effluent of all hydraulic conductivity specimens were compared with literature on tailings-fly ash and soil-fly ash that have been used in geoengineering applications. Concentrations of Ag and Cd for all amended tailings were below the drinking water maximum contaminant levels (MCLs) and toxicity limits. This result was attributed to low solubility of Ag and Cd in alkaline environments (i.e., pH ≥ 7) combined with the propensity for Ag and Cd to sorb to solid particles. Concentrations of Cr and Cu for amended tailings with fly ash A (FA-A) exceeded drinking water MCLs and toxicity limits, which was attributed to low solubility and high mobility of Cr and Cu in alkaline environments. Thus, tailings amended with FA-A have potential use in transportation-related earthwork projects, but high initial concentrations of Cr and Cu must be evaluated. All tailings amended with fly ash A (FA-B) are an environmental-friendly option and can be safely used in transportation-related earthwork projects from an environmental perspective.

Description

2016 Summer.
Includes bibliographical references.

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Subject

hydraulic conductivity
synthetic tailings
transportation
mine waste
fly ash
tailings

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