A method for preparing a silane coupling agent/silica/plant fiber composite includes the following steps: S1: pretreating plant fiber; S2: preparing hydrolysate of a silane coupling agent; S3: preparing a silane coupling agent/plant fiber composite; S4: preparing a silica nanoparticle dispersion; and S5: preparing a silane coupling agent/silica nanoparticle/plant fiber composite. Through the covalent interaction among a silanol group (SiOH) formed by hydrolysis of the silane coupling agent, SiOH of the silica, and a hydroxyl group (OH) on the surface of the plant fiber, the present invention enables silica nanoparticles to be grafted on the surface of the plant fiber. Using a hydrophobic film formed by the silane coupling agent, harmful ions are prevented from invading, and the volume stability of the fiber is improved. Using the pozzolanic activity of the silica nanoparticles, the alkalinity and calcium hydroxide content around the fiber are reduced.

A building material includes bio-oil treated plastic and biochar adhered to an exterior surface of the bio-oil treated plastic Capturing metals from mine tailings includes combining the building material, mine tailings, and an alkaline activator to yield a mixture, and polymerizing the mixture to yield a geopolymer.