Recycled Plastic Waste as Aggregates in Lightweight Concrete: A Study of Saturation Effects

The construction industry plays a significant role in the depletion of natural resources and the emission of atmospheric pollutants. As a sustainable alternative, this study investigates the reuse of polypropylene plastic waste as a partial replacement for natural coarse aggregate in lightweight concrete. The main objective was to evaluate the effects of replacing 30% of conventional crushed aggregate (coarse aggregate) with plastic waste under four pre-treatment conditions: Dry, saturated with water, saturated with cement and saturated with silica fume. Experimental tests assessed both fresh (workability and specific weight) and hardened (compressive strength, porosity, water absorption, and void ratio) properties of concrete. The results indicated that the lamellar shape and smooth surface of the plastic waste hindered its interaction with the hardened cement paste matrix, resulting in increased porosity, higher water absorption, and reduced compressive strength across all treatments. Contrary to the initial hypothesis, pre-saturation of the plastic waste did not improve the interfacial transition zone; instead, it further compromised bonding and mechanical performance. Nevertheless, the dry plastic waste mixture presented satisfactory compressive strength for lightweight concrete applications, i.e. compressive strength > 10 MPa. Overall, the use of polypropylene plastic waste contributes to reducing environmental impacts and promotes the production of low-density concrete for non-structural applications.