Quantifying the environmental impacts of a sustainable concrete mix for a block paving system

Abstract

A sustainable concrete mix design, incorporating industrial by-products: fly ash and recycled plastic pellets, was developed, and optimized through laboratory performance-based testing trials. The primary objective of this investigation was to offer environmentally sustainable alternatives to conventional concrete mixes that can be used for concrete block paving and aligns with circular economy principles and fosters enhanced employment opportunities and poverty reduction. Following a laboratory investigation to optimise the quantities of fly ash and plastic pellets in the concrete mix, paving blocks were produced in the laboratory using the optimised mix. The blocks were also tested to ensure compliance with performance criteria stipulated in national specifications for concrete block paving. This chapter focusses on the comprehensive life cycle assessment (LCA) conducted to investigate the environmental impacts associated with the production of the optimised concrete mix design in comparison with two references mixes. All three mixes comprised varying quantities of cement, fly ash as a partial cement replacement, and plastic pellets as a partial substitute for sand. The analysis included concrete with 100% Portland limestone cement, concrete with 50% Portland limestone cement and 50% fly ash, and concrete with 50% Portland limestone cement, 50% fly ash, and plastic pellets. The study, limited to a cradle-to-gate analysis, utilized the life cycle assessment software tool SimaPro 8.1 with the Ecoinvent Database version 3. The life cycle inventory dataset for each material was compiled, and the CML-IA Baseline World 2000 method was employed to generate and report the results. The LCA study results demonstrated that adding fly ash as a cement substitution significantly reduced the environmental impacts of concrete mixes. However, the extent of this reduction depended on the type of allocation method used. Under no allocation and economic allocation scenarios, concrete mixes with fly ash exhibited lower environmental impacts than those without fly ash. Conversely, mass allocation scenarios indicated higher environmental impacts for concrete with added fly ash more than 35%. Additionally, it was noted that environmental impacts for fly ash concrete mixes with plastic pellets as a partial substitute for sand were marginally higher than those with fly ash concrete mixes using only sand.