Valorization of wasted-derived biochar in ultra-high-performance concrete (UHPC): pretreatment, characterization, and environmental benefits

This study presents the feasibility of utilizing wasted-derived biochar in ultra-high-performance concrete (UHPC) to alleviate autogenous shrinkage, mitigate environmental effects, and further improve mechanical strengths. Two types of wasted-derived biochar are investigated, including the fresh biochar from pyrolysis process of vetiver grass roots and the used biochar after purifying the contaminated water with heavy metals. The wasted-derived biochar is pre-saturated and then used to substitute the cement in UHPC. Experimental results showed that the 1 % of pre-saturated biochar in UHPC enhanced the mechanical strengths (up to 20 %) and toughness (up to 25 %) as well as reduced the autogenous shrinkage (up to 15 %). The underlying mechanisms of the property enhancement are the improved hydration degree of UHPC due to the internal curing effect. However, as non-negligible micro-defects, the utilization of over 1 % biochar reduced mechanical performance instead. Moreover, the sustainability analysis indicated the use of biochar can significantly reduce carbon emissions of UHPC. Specifically, every 1 kg biochar would reduce 1.30 kg CO₂-eq emissions. In addition, the incorporation of used biochar in UHPC eliminated the possibility of heavy metal leaching. This study successfully proposed a carbon sequestering admixture and internal curing agent in UHPC production that also provides a new avenue for wasted-derived biochar recycling.