Irradiated recycled plastic as a concrete additive for improved chemo-mechanical properties in hardened cement pastes

Abstract

Portland cement based concrete production contributes heavily to greenhouse gas emissions. Thus a need exists for the development of durable and sustainable concrete with a lower carbon footprint. This can be achieved when Portland cement is partially replaced with another material without compromising the concrete's strength. The use of waste plastics in concrete has been explored as a means of improving concrete's mechanical properties while also providing an efficient way to both re-purpose waste plastic and partially displace cement for the purpose of reducing carbon emissions. This replacement, however, typically comes with a sacrifice of compressive strength. This work discusses the design for and progress toward a high-strength concrete with a dense cementious matrix that contains an irradiated plastic additive. Cement samples containing various combinations of cement binder and plastic content were prepared; compressive strength tests showed that for all cement binder types, the addition of high dose irradiated plastic resulted in increased compressive strength as compared to the strengths achieved by samples with regular, non-irradiated plastic. This suggests that irradiating plastic at a high dose is a viable potential solution for gaining some of the strength back that is lost when plastic is added to concrete. To assess the internal structure of the samples and gain some insight into what aspects of their chemical compositions' contributed to the observed strength differences, a microstructural analysis -- consisting of XRD, SEM, and X-ray microtomography -- was performed. XRD analysis showed that various differences in C-S-H and C-A-S-H phase formation from the addition of both irradiated plastic and mineral additives helped to form high-density phases that contributed to higher relative strengths. BSE analysis showed that an increased alumina content among fly ash samples helped to form the high-density phases that contributed to higher relative strength among the fly ash samples, as evidenced through a ternary phase diagram. X-ray microtomography showed that the addition of high dose irradiated plastic consistently contributed to a decrease in segmented porosity, indicating that irradiated plastic may have acted as a pore-blocking agent. The results presented clearly show the benefit of using irradiated plastic as a concrete additive for improved compressive strength. By partially replacing Portland cement with a repurposed waste material, this design, when scaled to the level of mass concrete production, could contribute to reduced carbon emissions and provide a long-term solution for waste plastic storage.