From European Directive 2008/98/EC, it is clear that secondary cementitious materials such as fly ash (FA) and silica fume (SF) can no longer be seen as mere waste products. Indeed, a certain environmental impact needs to be allocated to them when performing a life cycle assessment (LCA). This paper presents the results of a probabilistic LCA of concrete with various FA contents (15%, 40%, 50%). To evaluate the influence of strength differences between concrete mixes, a centric loaded concrete column was chosen as functional unit in a cradle-to-gate LCA. To obtain a higher early strength compared to the mix with 50% FA, 10% SF was added to the concrete mix with 40% FA. Partial environmental burdens were assigned to FA and SF using mass and economic allocation. When comparing 1 m³ of each concrete mix with the reference without FA or SF, the global warming potentials (GWPs) of the mixes with 15%, 40% and 50% FA increase above the reference when applying mass (GWP + 24.5%, + 40.4%, + 95.7%) instead of economic allocation (GWP – 1.9%, – 23.9%, – 11.8% when compared to the reference). LCA scores for concrete columns show a substantial benefit of the 10% FA addition due to the strength increase: GWP + 39.4% in case of economic allocation.