Sustainable Self-healing Mortar Incorporated Waste Plastic as a Microbial Carrier

Abstract This study investigates a novel approach to incorporating plastic waste into cementitious materials. While the utilization of plastic waste in concrete has been widely explored as a means to mitigate environmental pollution and promote waste reuse, its integration frequently leads to a notable decline in the mechanical and durability performance of the cementitious matrix. To address this limitation, this paper proposes the use of recycled polyethylene terephthalate (PET) and high-density polyethylene (HDPE) plastic waste, in a ratio of 70% PET to 30% HDPE, recycled three times and employed as a microbial carrier. Mortar mixes containing different percentages of plastic waste (5%, 10%, 15%, and 20%) as bacterial carriers were prepared, along with one control mortar without bacteria and plastic waste, one mortar mix containing 10% plastic waste without bacteria. Unlike previous studies that reported substantial deterioration in the mechanical and durability performance of plastic-modified mortars, the present study achieved only a modest reduction in overall performance relative to the control specimen, with compressive strength decreasing by 9.4% and 12.1% and flexural strength by 3.56% and 7.57% for 5CSPV and 10CSPV, respectively. A slight reduction in durability was also observed. This is due to the substantial microstructural enhancement achieved through the uniform distribution of microbial cells, the calcite coating of the plastic particles, which improved the interfacial transition zone between the plastic and cement matrix, and calcite precipitation that effectively filled micro-pores. These synergistic mechanisms enhanced the overall microstructural integrity of the mortar, as reflected by healing indices of 0.85 for direct bacterial incorporation and 1.01 when recycled plastic waste was used as a microbial carrier at a 10% replacement level as examined by 3D Optical Surface Profiler. The results indicate that Bacillus vallismortis-coated plastic waste at replacement levels of up to 10% can be considered a promising construction material.