Pozzolanic aggregates of fly ash in the sea

Abstract

Pulverized fuel ash (pfa) readily forms pozzolanic aggregates when wetted, and these are common at sites of ash dumping at sea, where they can be up to 2 m across. Field observations on the structure and fauna of aggregates from an ash dumping ground off the Northumberland (UK) coast were undertaken in 1975, together with experimental studies of aggregation, to determine the influence of ash aggregates on the benthic substrate and the faunistic colonisation of aggregates in situ. The aggregates are of a soft sandstone consistency, but are persistent at the sea bed. Particle analysis of aggregates showed negligible contribution by inorganic particles other than the ash, and an overrepresentation of larger diameter particles (3.5 to 4 Φ) compared to pure ash. There was an exponential relationship between aggregation and ash content of experimental sediments. It is suggested that only 15 to 17% of the ash reaching the sea bed will be involved in aggregation. The naturally occurring aggregate fauna consisted of a sparse but diverse sessile epifauna, which depended on encrusting bryozoans for initial colonisation, and a few boring species (Zirfaea crispata, Hiatella arctica, Polydora flava). The presence of 7 yr-old Z. crispata in aggregates confirms their persistence at the sea bed, and indicates that the ash is not toxic.