Hydrochar-amended recycled concrete aggregate as a sustainable medium for plant growth

Hydrochar, a carbon-rich material, has shown significant potential for improving soil quality and supporting plant growth. However, its use as an amendment to improve the suitability of Recycled Concrete Aggregate (RCA), a major component of construction and demolition waste, as a planting medium remains uncertain. This study investigates the effectiveness of hydrochar in transforming RCA into a viable substrate for sustainable urban landscaping and geo-environmental infrastructure development. Brassica rapa var. Chinensis seeds were grown in RCA amended with 5 % and 10 % by mass of both wood hydrochar (WHC) and eggshell hydrochar (EHC) under controlled conditions for 21 days. Plant growth parameters were measured, along with the physicochemical properties of RCA. Results reveal that WHC significantly improves organic matter content, electrical conductivity, and pH level of RCA, making it a more effective amendment than EHC. Improvements in plant performance were observed after 5 % WHC (RW5) amendment, with increases in root length (20.2 %), dry root biomass (353.3 %), leaf area (121.8 %), and shoot biomass (56.1 %) compared to the control. However, higher dosages (10 %) of WHC and EHC treatments show reduced effectiveness which can be attributed to excessive salinity or pH imbalances that create suboptimal conditions for plant growth. Additionally, WHC-amended RCA substantially increases total organic carbon content after harvest by 50.5 % and 44.8 % at 5 % and 10 % dosage, respectively, highlighting its role in carbon sequestration. These findings suggest that WHC can transform RCA into a viable planting substrate, providing a low-cost, eco-friendly solution for urban landscaping, geo-environmental infrastructure applications, and climate resilience efforts. However, these findings are limited by low and unequal sample sizes due to lower germination rate, particularly in control treatments. This indicates a need for further studies with larger and more balanced sample groups.