Investigation of the macro performance and mechanism of biochar modified ultra-high performance concrete

Ultra-high performance concrete (UHPC), an extraordinary category of cementitious composites, showcases unparalleled mechanical and durability properties. Given the current disparity in carbon levels and the environmental hazards associated with cement production, various types of biochar materials have been incorporated into cement composites. This approach offers a mutually advantageous solution, effectively reducing CO2 emissions while simultaneously preserving natural resources. This study aims to evaluate the use of palm shells, a waste biomass, as a substitute for aggregate in the UHPC matrix. The investigation focused on replacing 1–40 % of sand, specifically considering apricot, date, and peach shells biochar at an 8 % replacement. A comprehensive analysis was carried out on a total of 12 combinations, encompassing a control group. For biochar-mortar admixtures, a decrease in workability and density were observed. Replacing 1 % of sand with biochar in UHPC mortar improved compressive strength, but increasing biochar dosage weakened it. Biochar-mortar was demonstrated a dual impact on drying shrinkage. Mixtures with higher biochar dosages showed a slight increase in permeability. Incorporating 1 % biochar into UHPC led to a slightly lower electrical flux value. SEM images were obtained, highlighting the compact morphological structure seen in both the control and the 1 % biochar mortar. MIP test showed nearly 1.6 times increase in the 2-dimensional porosity, from 10.49 % (control) to 16.83 % for sample which had 30 % biochar. Based on the results obtained, it could be concluded that biochar possesses the capability, which enhances the properties of UHPC mortar when used as a partial replacement for conventional fine aggregate in minor fractions.