Synthesis of geopolymer composites from blends of CFBC fly and bottom ashes

► Simultaneous recycling of CFBC fly and bottom ashes by geopolymerization. ► No natural kaolinitic resource (raw or calcined kaolinite) involved. ► Modified alkali-fusion pretreatment for enhancing the reactivity of CFBC fly ash. ► An L16 (44) orthogonal array designed for optimization of the alkali fusion. ► High 7-day compressive strength of 32.7 MPa for recommended geopolymer sample.