Upcycling rice husk biochar into carbon-negative composites

Rice husk biochar (RHB), derived from pyrolyzed agricultural waste rice husk, is rich in carbon and silicon. RHB has a porous structure and possesses a certain degree of pozzolanic reactivity. In this study, RHB pyrolyzed at various temperatures were recycled into cement-based composites. The Frattini test results showed that RHB pyrolyzed at 700 °C exhibited the highest pozzolanic reactivity, resulting in the highest hydration degrees as well as the highest compressive strength of RHB-incorporated cement materials. These enhancements were attributed to the fact that RHB in the cement-based composites performed pozzolanic reactivity, as well as internal curing effect, which was beneficial to the hydration reactions of cement. Nanoindentation and morphologic analyses revealed a substantial accumulation of hydration products in the interfacial transition zone (ITZ), thereby improving micro-mechanical properties. Notably, the incorporation of 30 wt% RHB could produce carbon-negative cement composites with compressive strength over 42.5 MPa. This study elucidated the role of silicon-rich RHB in cement hydration and provided the theoretical foundation for the high-dose use of biochar in sustainable construction materials, advancing carbon-neutrality objectives in the construction sector.