Basic performance, heavy metal leaching mechanism and risk assessment analysis of waste concrete.

With the acceleration of urbanization, the production of waste concrete is getting higher and higher. A large amount of outdoor accumulations of waste concrete will leach heavy metals, not only causing harm to the soil, but also posing a risk to human health. Based on this, this paper systematically studies the basic physical properties and microstructure (XRF, XRD, and SEM–EDS) of outdoor natural accumulation waste concrete, and analyzes the heavy metals in waste concrete from the aspects of existing state, leaching mechanism, human health risk analysis, and summarized the direction of resource utilization of waste concrete, calculated the carbon emission reduction during recycling. The study found that heavy metals in waste concrete mainly exist in hydration products in the form of precipitation, adsorption, and replacement, summarized the leaching mechanism from the micro- and macro-aspects. The leaching mechanism of heavy metals can be assigned to chemical (mineral dissolution and effective amount of components) and physical (advection, surface erosion, and diffusion) processes from the macro-perspective. From the micro-analysis, it can be assigned to the following five processes: acid migrates from solution to liquid–solid surface, acid migration through leaching layer, rapid dissolution reaction controlled by diffusion at leaching boundary, heavy metal through leaching layer, and heavy metals through the solid/liquid surface to the solution. In addition, the concentration and the leaching rate of heavy metals in waste concrete were analyzed. It was found that the concentration of Cr was the highest reached to 4.7 mg/kg and the leaching rate of Cd was the highest, its leaching coefficient was calculated as a result of 1.713 × 10–6. However, there was no obvious regularity in the leaching of heavy metals in different accumulate particle sizes. Through the establishment of risk assessment system was found the concentration of heavy metals in waste concrete will not cause significant harm to human health. The effective limit of heavy metals after 3 months of accumulation of waste concrete was calculated as: Cr < 0.09 mg/kg, Cd < 0.00715 mg/kg, As < 0.392 mg/kg, and Pb < 0.732 mg/kg. And the carbon emission reduction of waste concrete recycling was calculated to be 28.764kgCO2/t. All the results of this study can promote the safe and environmentally friendly utilization of waste concrete. [ABSTRACT FROM AUTHOR]