Abstract The objective of this work was to assess the possibility of using biomass ashes as substitutes for cement and natural aggregates in concretes, without compromising their mechanical, chemical, and ecotoxic properties. Thirteen concrete formulations were prepared with different percentages of bottom and fly ashes produced in the power plant of a pulp and paper industry. These formulations were submitted to mechanical compressive strength assays, after 28, 60, and 90 days of cure. The reference formulation (without biomass ashes) and two formulations (with biomass ashes), were selected for further characterization. After 90 days of cure, the selected formulations were submitted to the leaching test described in the EN12457-2 (L/S = 10 L/kg, 24 h batch cycle) by using two leaching agents: a synthetic marine water (ASPM medium) and a synthetic freshwater (ISO 6341 medium). The eluates produced were submitted to chemical characterization (set of 19 metals, pH, SO 4 2 - , F−, dissolved organic carbon, chlorides, phenolic compounds, and total dissolved solids) and to an ecotoxicological characterization (for marine eluates: bioluminescence inhibition of the bacterium Vibrio fischeri; growing inhibition of the microalgae Phaedactilum tricornutum; mobility inhibition of the microcrustacean Artemia franciscana; for freshwater eluates: mobility inhibition of the microcrustacean Daphnia magna; growing inhibition of the microalgae Pseudokirchneriella subcapitata). The substitution of 10% cement by fly ashes has promoted similar to higher levels of the compressive strength to reference formulation. The new formulations presented emission levels of chemical species similar to, or even lower than, those observed for the reference formulation. The ecotoxicological levels were reduced for all of the formulations.