Application of stubble waste biochar in cementitious composites: the impact of pyrolysis temperature on its characteristics and cementitious performance

This study investigates the impact of biochar production temperature on its physicochemical properties and performance as a partial replacement of cement in mortar mixes. Rice stubble biochar was produced at 450 °C (BC₄₅₀) and 550 °C (BC₅₅₀) with consistent pyrolysis conditions of 120-minute residence time and 10 °C/min heating rate. The produced biochar’s were characterized for various physiochemical characteristics using Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), Fourier Transform Infrared Spectroscopy (FTIR), X-Ray Fluorescence (XRF), proximate analysis, pH, and water retention capacity assessments. Nine mortar mixes comprising a control and biochar-incorporated mixes at replacement levels of 2.5 %, 5 %, 7.5 %, and 10 %, were evaluated for mechanical properties (compressive and flexural strengths), durability (water absorption and void volume), and microstructural characteristics after curing periods of 7, 28, and 56 days. The findings indicated that BC₅₅₀, exhibiting superior crystallinity and alkalinity, outperformed BC₄₅₀ in improving mortar performance, attaining a compressive strength increase of up to 24.6 %, a flexural strength enhancement of 10.7 %, a reduction in water absorption by 16.5 %, and a decrease in voids by 7.3 % at a 5 % replacement. Conversely, higher replacement levels (7.5 % and 10 %) reduced performance owing to increased porosity. The study highlights the necessity of optimizing biochar production temperature and dosage to maximize its benefits in improving the mechanical and durability characteristics of sustainable cementitious composites.