Utilization of red mud in geopolymer-based pervious concrete with function of adsorption of heavy metal ions

Red mud (RM) is a kind of industrial solid waste that is difficult to utilize，but it contains much alkali and has the ability of adsorbing some heavy metal ions. Due to these characteristics, red mud could be used as a raw material to develop a geopolymer based pervious concrete having rainwater purification function with granulated blast furnace slag (GBFS). Following this, influence of different factors including RM content, the sodium hydroxide to silicate (Na2O/SiO2) mole ratio, and the alkali concentration of the activator on the mechanical properties and ability of adsorbing heavy metal ions were investigated. XRD, SEM and EDS were then used to characterize the microstructure and morphology in attempt to reveal the mechanisms on the strength formation and on the adsorption of heavy metal ions. The results show that both the Na2O/SiO2 mole ratio and the alkali concentration of the activator had an obvious effect on the strength of the RM-GBFS-based geopolymer pervious concrete, but little effect on the adsorption of heavy metal ions. Moreover, with the increasing content of RM, the strength of the RM-GBFS-based geopolymer pervious concrete decreased, while the adsorption of heavy metal ions improved significantly. The possible reason for the excellent ability of adsorbing heavy metal ions can be attributed to the composition of the hardened RM-GBFS-based geopolymer system including geopolymeric gel, C-S-H gel with a lower Ca/Si ratio and some raw RM particles that have great ability of adsorbing heavy metal ions. As the compressive strength at 28 d of the RM-GBFS based geopolymer pervious concrete (containing 30 wt% RM in the geopolymer) can reach 18.53 MPa, it can applied in the light road pavement in China, and this pervious concrete has been demonstrated to have the function of rainwater purification due to its higher removal of heavy metal ions from solution that can reach 53%-77%. Therefore, it can be concluded that it is feasible to employ RM for manufacturing geopolymer-based pervious concrete with function of rainwater purification.