Two Waste Streams, One Solution: Reclaiming Concrete Aggregate and Treating Acid Mine Drainage through a Synergistic Recycling Process

Abstract

Concrete is the most widely used construction material in the world. Mining for and disposing natural aggregate, one of the major components of concrete, has many environmental side effects, ranging from habitat destruction to mudslides. One way to reduce the extent of aggregate mining’s negative side effects is to reuse the aggregate from demolished concrete in new components. However, there are limitations to using recycled aggregate in new concrete because it leads to low compressive strength and workability. Growing interest in acid treatments for recycling concrete to improve these characteristics led us to investigate whether acid mine drainage, a human-made, but unwanted source of sulfuric acid from coal and metal mining, could be used to treat recycled concrete. We investigated the neutralization reaction kinetics and products of cement paste and sulfuric acid by using a pH meter and Fourier Transform Infrared (FTIR) equipment. Once basic information was gathered, a 10 day sulfuric acid soak starting at a pH of 3 was conducted on recycled concrete aggregate. The aggregate was characterized before and after the treatment, and improvements in water absorbency and bulk density were noted. Five varieties of concrete with varying types of aggregate were made to compare their physical properties and mechanical properties using compressive testing. We found that concrete made with 100% recycled aggregate treated with a solution similar to acid mine drainage had over 15% higher strength than concrete made with 100% untreated recycled aggregate. We also found that within two days, acidic water was neutralized by being treated with recycled concrete aggregate, from a pH of 3 to a pH of at least 8. We concluded our study by analyzing the carbon footprint of this new process. We found that treating and reusing one tonne of recycled concrete is the saves the equivalent amount of carbon dioxide of driving a loaded truck for 40 miles. This means that our technique can simultaneously reduce negative side effects of aggregate mining and acid mine drainage.