Properties of modified Alberta fly ash in relation to utilization in waste management applications

Abstract

This paper presents the results of a laboratory investigation on hydraulic conductivity (k, also known as coefficient of permeability), compressive strength, and contaminant leachability of self-cementitious fly ash produced by two different coal-fired thermal power plants in Alberta. Hydraulic conductivity data are presented for fly ash and lime-modified fly ash samples with tap water and brine as permeants. The tap water k values of fly ash samples are almost two orders of magnitude higher than 1 × 10−7 cm/s, the usually required k value for a landfill bottom liner material. Addition of lime to fly ash increased the unconfined compressive strength and decreased the k value. The effect was directly proportional to the curing time and the percentage of lime added to the ash. The pozzolanic reactions between lime, water, and alumino-siliceous glass in the fly ash give rise to insoluble cementitious calcium alumino silicate hydrates. These pozzolanic reaction products contribute to strength gain and decrease in k value. It is also quite possible that salt precipitation during brine permeation blocked pore spaces and resulted in decreased k value. Nonetheless, the permeabilities of lime-modified fly ash samples are significantly lower than those of fly ash samples.In general, the amount of trace elements leached from test samples during tap water permeation decreased with increasing curing time and percentage of lime in samples. Exception was lead (Pb). Leachability of Pb increased with increasing lime content and curing time. Leachate pH held constant with time, which indicates that the removal of Ca(OH)2 in pozzolanic reactions did not have a significant impact on the pore solution pH. Key words: fly ash, hydraulic conductivity, contaminants, lime, leachability, liners, waste management, compressive strength, heavy metals.