Coal wash (CW) and basic oxygen steel slag fines (BOS) are by-products of the coal mining and steel industries, respectively. Their effective reuse and recycling through large-scale geotechnical projects, such as port reclamation, is economically beneficial and environmentally sustainable. In this study, CW and BOS were blended in order to explore the possibility to obtain synthetic fills having geotechnical properties similar or superior to conventional fills, therefore suitable as a structural fill for the Port Kembla Outer Harbour reclamation near Wollongong City, Australia. A framework with four levels of acceptance is proposed in this paper to select granular waste as structural fill materials. This framework was used for optimising the CW-BOS blend. It was found that for the Port Kembla Outer Harbour reclamation, a CW-BOS blend with a BOS content between 30 and 45% can meet most geotechnical specifications (i.e. high shear strength and bearing capacity, low swelling and particle breakage levels, and adequate permeability) required for a suitable structural fill above the high tidal level.