The Influence of Biomineralized Contaminated Waste Plastics in Reinforced Cement Mortar

The demand for cement infrastructure and plastic products are continuously increasing, and the production of these materials generate greenhouse gas emissions. An additional problem is the low recycling rate of plastics — with only 9% of plastics ever produced being recycled. Instead, these waste plastics are accumulating in landfills and the environment. One contributing factor to low plastic recycling rates is that contamination from food waste and oil necessitates treatment steps that may increase the cost and reduce the quality of recycled plastic. Researchers have made headway against these challenges by using waste plastic as reinforcement in cementitious materials. However, increasing the amount of plastic reinforcement leads to a decrease in composite compressive strength. To combat the loss in compressive strength, it may be possible to use microbially induced calcium carbonate precipitation (MICP) to coat waste plastic in calcium carbonate and improve the strength of plastic-reinforced cementitious materials. The objective of our research was to optimize the amount of clean and contaminated waste plastic that can be added to plastic-reinforced mortar and whether MICP coating of waste plastic enhances the strength. The performance of plastic-reinforced mortar cylinders was investigated using compressive strength tests at a 5%, 10%, and 20% volume replacement for cement. Results indicate that even at a 20% replacement with untreated, clean post-use plastics (HDPE, PVC, LDPE1 (larger chips), and LDPE2 (smaller granules)) produced compressive strengths acceptable for several applications such as foundation walls, garages, and sidewalks. A coating of MICP on clean waste plastic did not significantly improve the compressive strength of the specimens. MICP treatment of oil-coated waste plastics (HDPE, LDPE1, and LDPE2) recovered the strength by 28.28% relative to cylinders containing untreated oil-coated plastics, on average. However, washing the same oil-coated plastics with only water resulted in compressive strengths similar to that of MICP-treated, contaminated plastics. Our results demonstrate that incorporating greater volumes of waste plastics into mortar at optimized replacement ratios could improve the sustainability of cementitious composites by the dual mechanisms of reduced cement production and repurposing plastic waste.