EVALUATION OF RECYCLED CONCRETE AGGREGATE AS ALTERNATIVE NEUTRALIZING MATERIAL FOR ACID MINE DRAINAGE

Acid mine drainage (AMD) effluent can be treated in either active or passive systems. Passive treatment systems have the ability to be integrated with the environment. A common type of passive treatment is a drain filled with limestone aggregate as the neutralizing material. These limestone drains are used in series or combined with other systems in oxic or anoxic environments. The ideal conditions for an oxic limestone drain, are a high dissolved oxygen content, high ferrous iron and high aluminum concentration. Meanwhile the ideal conditions for an anoxic limestone drain are opposite, a low dissolved oxygen content, low ferric iron and low aluminum concentration. Limestone in the anoxic system is limited in the effectiveness to reduce concentrations of other metals, for example, manganese. There is potential to use alternative neutralizing materials from recycled materials, such as recycled concrete aggregate (RCA). Concrete is the second most used material in the world and as a result waste concrete comprises about ¼ of the national waste stream. RCA is crushed waste concrete that has been processed to remove rebar and graded. RCA has been tested in column leach tests with AMD and has been proven to be an effective pH neutralizing agent. Previous research also suggests RCA can reduce concentrations of heavy metals in the mine drainage including aluminum, iron, manganese, and nickel. However, these studies do not compare RCA to a limestone control. In addition the data available is scattered across multiple studies with different variables, making it difficult to draw any accurate conclusions on RCA potential viability as an alternative to limestone.

This study compares the potential of RCA as an alternative to limestone to treat AMD effluent through lab testing, an environmental impact evaluation, and an economic assessment. The lab tests used a column leach test in both oxic and anoxic environments and analyzed the leachate for alkalinity, heavy metal concentrations, pH, and radium levels. The environmental impact evaluation was performed with Simapro life cycle analysis software and breaks down environmental impacts related to each aggregate when implemented into a current passive treatment system. The economic assessment was completed using modified information from a current cost estimation software, AMDTreat.