On the feasibility of using Polyester (PE) waste particles from metal coating industry as a secondary raw materials in concrete

Reduction of CO₂ emissions and plastic waste are the main environmental problems that modern society is dealing with. Concrete industry is continuously investing in research and development aimed at producing sustainable cementitious materials. In the last decades, it has gained interest the possibility of reusing polymer waste (mainly PET or PP) in partial substitution of natural constituents (aggregates) or as fiber reinforcement. As a matter of fact, because of the poor mechanical characteristic of polymers compared to the one of natural aggregates, the final cementitious composite has reduced mechanical performance. In the aforesaid framework, the experimental research reported in this paper aims at verifying the feasibility of a pathway able to use fine polymer particles, in detail a Polyester resin (PE resin) which is a waste product of the coating industry, as a partial replacement of sand and, in case, of binder particles, upon a gamma irradiation process similar to the one used for the sanification of containers in food industry, also their effectiveness in performing as seeds of the cement hydration. Firstly, intrigued by a study performed by MIT researchers (in which exposure of PET waste particles to gamma irradiation has been investigated as a method to improve their mechanical performance), the influence of different gamma irradiation dosages (10 kGy or 100 kGy) on PE resin particles was investigated. However, results led to the conclusion that, even with a mere 5% by volume substitution of Portland Limestone Cement (PLC) in the mix, the process does not significantly improve the mechanical performance of cement-based composites. In a second stage, the non-irradiated particles have been employed as a replacement of the binder and/or of the sand at different volume replacement ratios (10% and 20% respectively) in mortar mix designs formulated from typical Self-Compacting Concrete (SCC) mixes. Finally, once identified the most suitable type and level of replacement as the best compromise between performance maintenance and volume of added particles, the scaling up to the concrete mix-design has been performed and the related performance thoroughly tested. The results have provided limited reduction in mechanical properties, with a 20% by volume level of substitution of cement by PE resin particles, highlighting the possibility of reusing economically viable quantities of PE resins into concrete while still being able to use the material for structural application.