Characteristics of fly ashes, their potential uses and damaged conditions of landfill deposits

Abstract

Diverse industries produce fly ashes as thermal power plants, waste incinerators, and some recycling processes. Though some of these products can be raw materials for diverse applications, their composition can be hazardous due to their content in heavy metals. In addition, mixing fly ashes with the products of neutralization of fumes complicates the products. Then many fly ashes are landfilled in a Portland-cement matrix. However, most of the actual knowledge about their ageing derives from models the validity of which has been established only on the short term, using small sized proofs, at room conditions while matter transfers and reactions are strongly affected by temperatures reaching 80 °C for many years. Ageing leads to a mobility of chlorides, sulphates and lead, alteration of textural properties with formation of microcracks, formation of neo-formed minerals as silicatehydrates (katoite and hibschite, CSH), carbonates, tobermorite, hydrogarnets zeolites,…) that can contribute to bear Pb, Zn, Cr, Cu, in addition to inherited phases (metallic alloys, spinels, glasses, refractory oxides, titanates, phosphates,…). The alkaline hydrolysis of aluminium in saline media decreases the pH and promotes the precipitation of lead species trapped in a hydrocalumite or ettringite matrix.