Root growth into sealing layers of fly ash

Fly ash can be used as sealing material on mine waste disposal sites. Our aim was to determine the penetration resistance necessary to prevent root penetration of ash sealing layers, and investigate whether plants can affect the layer strength. We analyzed the root penetration, pH, EC and penetration resistance of layers on which Phalaris arundinacea, Salix myrsinifolia, Epilobium angustifolium, and Pinus sylvestris had been growing for 4-8 months in a greenhouse. Fly ashes obtained from the incineration of biofuel or municipal solid waste were used. A penetration resistance of similar to 1.5 MPa generally prevented root penetration, but roots of P. arundinacea could grow into the surface of layers with similar to 5 MPa penetration resistance. We examined the loosening ability of agents found in root exudates and the mineralogy of rhizosphere ash. The roots increased the weathering of the ash, including dissolution of secondary calcium minerals, possibly related to saccharide exudation.