Abstract Life cycle assessment (LCA) provides a comprehensive framework for positioning low energy and global warming potential alternatives regarding Portland cement and concrete. Published LCA work on alkali-activated cements is, however, relatively limited. In this paper, we illustrate how LCA critically supports concrete technological studies in the search for low impact concrete mixes. Previous research on breakwater applications explored replacing a low-clinker Portland cement and natural aggregates with seven different alkali-activated blast furnace slag (bfs) binder systems and with coarse and granulated bfs aggregates. Its outcome suggested a sodium silicate-activated bfs formulation as the best match between concrete properties and environmental regulation compliance. To validate this outcome through LCA, our cradle to gate assessments followed ISO 14044 (INTERNATIONAL…, 2006b) and used Ecoinvent v.2.2 and CML baseline 2001 v.2.05. We adopted the ‘net avoided burden approach’ to handle multifunctionality intrinsic to by-product-based AAC. Whilst sodium silicate-activated mixes rivaled the reference regarding GWP, impacts in several categories were increased. LCA highlighted the implications of driving mix selection by focusing on a single environmental impact category.