Effects of highly porous components on hydraulic conductivity of simulated granular mixtures
Tyndale, Sean Matthew
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Fly ash is the lightweight byproduct that results from the combustion of coal during production of various commodities and services including electricity. In the United States, millions of tons of fly ash are generated each year in producing electricity. Approximately 40.0% of this combustion byproduct is beneficially used in applications including concrete, fills, and construction materials; however, the remaining 60.0% is geologically disposed. Historically, this ash is disposed of into surface ponds open to weathering and long-term geochemical alteration. Most significantly, this weathering can contribute to changes in the particle mineralogy, surface charge, and surface area, which in turn impact the hydraulic conductivity of the fly ash deposit. While the hydraulic conductivity of fly ash is typically similar to that of silt-sized soils, it is not uncommon to encounter pockets of fly ash that are not easily dewaterable; that is, some fly ash deposits exhibit significant water retention capacities. The work performed in this study investigated the hydraulic properties of a model particulate mixture that was composed of fine sand particles and highly porous additives including diatomaceous earth (DE) and activated carbon (AC). The highly porous additives were chosen as representative of natural materials that are commonly observed in ponded fly ash (i.e., diatoms and partially combusted carbon). Fine sand (sieved F110) was chosen as the representative matrix particulate media, and DE and AC were mixed with the sand at percentages of 2.5, 5.0, and 10.0%. The highly porous mixtures were different in nature because the DE particles were small enough to occupy the void spaces of the sand, while the AC particles were large compared to the sand grains, and existed as discrete particles in the mixture, not only occupying pore spaces between sand grains. Limiting void ratio tests demonstrated that the presence of the DE and the AC had significant impact on the fabric and structure of the mixtures, as the % high porosity components in the mixture were increased from 2.5% to 5.0% to 10.0% by volume. For mixtures with high porosity additives, emax increased in all cases (from 0.84 with 100% sand to 1.37 for the sample with 10.0% DE / 90.0% sand and to 0.98 for the sample with 10.0% AC / 90.0% sand). These data show that a higher void ratio was achieved with the inclusion of these components, which is attributable to the high porosity in these additional particles and to the less efficient packing of angular particles when mixed with sand. These same impacts on packing were observed in emin, with the minimum void ratio increasing from 0.55 for 100% sand samples to 0.59 for 10.0% DE and 0.68 for 10.0% AC samples, indicating less efficient packing for the samples when sand was replaced with the high porosity components. Measured values of hydraulic conductivity indicated that the highly porous components decreased the saturated hydraulic conductivity by one to two orders of magnitude. The DE particles were small enough to fill the void spaces of the sand matrix, while the AC particles had a larger range in grain sizes, and both packed void space, or displaced sand grains. Addition of these highly porous components act to reduce the hydraulic conductivity, and will also act to retain water in partially saturated conditions due to high capillary forces within their highly porous structure.