Impact of unburned carbon particles on the electrical conductivity of fly ash slurry

Recent stability issues in ash ponds have highlighted the importance of estimation of in situ void ratio or water content of disposed fly ash to assess the field performance and stability of these disposal units.One standard prohibits the use of fly ashwith carbon content greater than 6% in concrete, making low-cost methods of quantifying the carbon content of fly ash attractive, especially for applications that might consider mining and reuse of previously disposed fly ash. In this study, electrical conductivity/resistivity measurements were used to measure the in situ carbon content and void ratio of fly ash, with the ultimate goal of quantifying the level of unburned carbon in the material. Because unburned carbon particles are electrically conductive, the resulting media conductivity is a function of carbon content, allowing determination of the carbon content of fly ash as well as the void ratio, or water content, of the soil matrix. The authors found that the measured electrical conductivity generally increased with an increase in carbon content, and the dependency of carbon particle conduction on applied stress increased with an increase in carbon content owing to the increased number of direct contacts between electrically conductive unburned carbon particles. In addition, this study evaluated the effect of pore-water conductivity on media conductivity and carbon particle conduction.