Engineered char from waste plastic: A review on the physicochemical properties, carbon dioxide uptake, and application in construction materials

The application of carbon-rich char-based admixtures, including biochar and plastic char, in construction products has received substantial attention from global industries due to their potential to “lock in” carbon for the long term, thus mitigating the climatic impacts of future constructions. Furthermore, a sharp rise in plastic waste generation and uncontrolled landfilling threatens natural ecosystems. Depending on type, plastic waste can be used as fuel, and the generated char (solid residue) can be reintegrated into the construction value chain by utilizing it as a carbon-sequestering admixture in construction materials. This article discusses critical factors, including the synthesis temperature, heating rate, and different activation pathways, for tuning plastic char’s porosity and surface properties, contributing to enhanced carbon fixation and CO₂ uptake. Chemical pyrolysis using alkaline agents produces microporous structure (< 2 nm) with high surface areas (> 1000 m²g⁻¹) and CO₂ uptake, ranging up to 4.6 mmolg⁻¹ while acidic agents produce a higher fraction of mesopores (> 2 nm) with lower surface areas < 1500 m²g⁻¹ and CO₂ uptake capacities (up to 1.8 mmolg⁻¹). The review finds that surface functionalization of plastic char and altering its physicochemical properties improve the engineering properties of construction binders. The locked carbon in the char, complemented by additional CO₂ uptake in the engineered pore and surface sites, can be instrumental in mitigating the embodied carbon of construction products. However, future investigations should study the microstructural interactions of engineered char within construction binders and conduct a holistic life-cycle assessment to fully realize the benefits of using engineered plastic char as a supplementary additive.