Mechanical properties and micro mechanism of alkali-activated tannery sludge/fly ash composite cement-based recycled concrete

•A recycled concrete was synthesized by tannery sludge, fly ash and construction waste.•The alkali activator prepared by sodium silicate and NaOH was beneficial to the strength development of AA-SFRC.•The addition of alkali activator optimized the pore structure and phase composition of AA-SFRC.•The grey prediction model of strength and pore structure parameters was established.