The use of granulated recycled plastic and sludge ash as partial replacement in concrete mixes

This study explores the sustainability potential in concrete production by integrating granulated recycled plastic and sludge ash as partial substitutes for fine aggregates and cement in concrete mixes each at a 10 % replacement rate. This percentage serves as a preliminary value aligning with the typical percentage ranges of 5–15 % reported in previous studies to explore its feasibility under the specific conditions of the waste materials at hand. A series of standard tests aiming to asses slump, fresh and hardened density as well as ultrasonic pulse velocity and time (UPV and UPT), compressive and splitting tensile strength tests were conducted as well as a microstructural analysis of the waste materials. The findings showed that replacing 10 % of fine aggregates with recycled granulated plastic waste had slump values increasing by 50 % improving workability. However, fresh and hardened density decreased by about 3.5 % and 1.5 %, respectively. In result, compressive strength decreased slightly by almost 1 % at 28 days. Using 10 % sludge ash was a waste material to replace cement increased fresh density by 3.5 % and UPV values to 7.1 %, in result workability reduced and compressive strength reduced due to decreased hydration, in comparison to full cement utilization, to 16 % lower at 28 days. The combined replacement of both materials M-RP10SA10, had the lowest strength (25.09 MPa) and lowest density (2338.75 kg/m³. In addition, the 10 % replacement of cement with sludge ash (equivalent to 40.38 kg/m 3 , and that resulted in 34.3 kg of CO 2 to be avoided per 1 m 3 of concrete — which equals on an estimate an 8.5 % reduction in total related emissions. Not to mention, the use of these waste materials supports sustainability by reducing reliance on virgin raw materials. The research highlights that, based on the achieved strength and density values, more than 25–30 MPa, the viability of modified mixes for non-structural and light structural applications such as paving blocks, kerbstones, and slabs-on-grade. • 10% GRP substitution increased slump by 50%, indicating improved rheology. • GRP-modified concrete had only a 1% drop in 28-day compressive strength. • 10% SSA raised fresh density 3.5% and UPV 7.1% but reduced compressive strength 16% due to hindered hydration. • SSA replacement cut embodied CO₂ by ∼34.3 kg/m³, an 8.5% reduction. • Compressive strengths >25 MPa, suitable for non-structural and light structural applications.