The synthesis and characterization of geopolymers based on metakaolin and high LOI straw ash

•The straw ash applied to the metakaolin-based geopolymers has been investigated.•The 33 wt% straw-ash addition decreases the setting time from 720 min to 120 min.•FTIR analyses have proved a formation of geopolymer bonds even after ash addition.•The compressive strengths are negatively influenced by the straw ash addition.•The straw ash incorporation reduces the polycyclic aromatic hydrocarbons extraction.