To meet the escalating demand of energy more coal ash and brines are inevitably produced as by-products. Large volumes of these wastes and increasing environmental awareness necessitate the development of more sustainable methods to mitigate the environmental footprint. Paste backfill is one of the potential solutions to keep the energy industry sustainable. The behaviour of pastes was investigated by strategically varying brine composition mixed with the two types of fly ash. The results showed that fly ash plays a more prominent role in the behaviour of pastes than brines. It is therefore imperative to consider both fly ash and brine characteristics i.e. constituents of paste for the development of an environmentally sound paste backfill practice. Technically there are numerous benefits in pursuing the proposed solution. (C) 2011 Elsevier Ltd. All rights reserved.