Permeability and Leaching Characteristics of Fly Ash Liner Materials

Rigid-wall column tests are performed at a hydraulic gradient of 100 on 21 compacted mixture contai.ning a Class F fly ash, sand, benton.ite, and cement. Results indicate that combina1ions of the admixtures can lower the permeability of the fly ash by up to an order of magnitude. There appears to be an optimum percentage of fly ash that minimizes the permeability of fly ashsand mixtures. Specimens from four mjxtures, a well as 100 percent fly ash, are tested at four hydraulic gradient to establi h differem flow rates. Tap water is u ed as the permeanc fluid, and rhe effluent is sampled every quarter pore volume of flow and tested for 13 metal concentrations, pH , and electrical conductivity. For most of the metals most of the metal mass is leached within the first pore volume of flow. However some metals (Ba, Ca, and Sr) are not leached in significant quantities until two or three pore volumes of flow have passed through the test specimens. These two leaching behaviors suggest that there is dissolution competition between metal ions. No simple trend in leaching rate as a (unction of flow rate was observed for all mixtures and metal ·. However for specific elements between different mixtures at approximately the same flow rate, an increase in the fly ash content tended to increase the leaching rate of most metals. A 2.5 percent cement content was more effective than a 10 percent bentonite content in reducing the leaching rate of metals for mixtures containing the same amount (75 percent) of fly ash .