Recycling steel slag from municipal wastewater treatment plants into concrete applications - A step towards circular economy

Municipal wastewater is a major source of the release of phosphorus into the marine environment, and its excessive release could negatively impact marine flora and fauna. In this study, steel slag a by-product from steel industry was used to remove phosphorus content from the municipal wastewater. The Langmuir isotherm model revealed a maximum adsorption capacity of 3.8 mg phosphate/g of steelmaking slag. The results showed that the phosphate removal efficiency of granular slag was about 90%. The wastewater contained a significant amount of iron and manganese content that was filtered out by the slag granules. X-ray fluorescence and x-ray diffraction analysis show that the wastewater treatment process changes the chemical phase composition of slag in addition to the absorption/adsorption of various chemicals within its pore structure. To recycle this waste slag after the water treatment process, that significantly altered its chemical and mineralogical composition, it was used in concrete as a replacement of conventional coarse aggregates. The results show that the treated slag aggregates provide compressive strength improvement of 32.5% and 18% at 7 days and 8.2 and 16.8% improvement at 28 days compared to that of raw slag and conventional aggregates, respectively. In addition, due to the pozzolanic nature of slag both the raw and treated slag aggregates showed a seamless monolithic bond at the interfacial transition zone between the slag and the cement matrix. This study demonstrates that the waste slag generated from the wastewater treatment process has a great potential to be used as a replacement of conventional aggregates in concrete applications.