Coastal infrastructure is under constant attack from the marine environment. Under current conditions, breakwaters and seawall armoured by rock or concrete units require regular monitoring and maintenance. But with anticipated changes to the coastal wave climate due to climate change scenarios, costal structures would be exposed to even greater wave energy, and higher rates of damage. In this project, researchers and industry partners worked together in the development and trial of a unique, sustainable high-density Geopolymer concrete mix for coastal structures. The system developed in the laboratories of UNSW was upscaled and is being tested at the northern breakwater of NSW Ports’ Port Kembla Harbour. The concrete uses steel furnace slag (SFS) aggregate in a blended fly ash-blast furnace slag binder to eliminate delayed hydration and expansion of the aggregate. The concrete properties were measured and microstructural analysis undertaken. The results show that SFS aggregate offers higher bulk density to the concrete and can reduce armour mass requirements. This important result provides a novel approach to both repair of existing structures, and construction of new structures with reductions to both cost and carbon footprint.