Carbon reduction potential of cement pastes with biochar and mineralization-modified biochar

Cement production is highly carbon-intensive, and therefore developing low-carbon cementitious materials is critical for green construction. This study proposes an indirect CO 2 mineralization approach to modify biochar (BC) using carbide slag as a calcium source, producing CaCO 3 -deposited mineralized biochar (MBC). The mineralization process was optimized by varying reaction temperature, reaction time, and solid-to-liquid ratio. At the optimized conditions (30 ℃, 30 min, 500 g/L), the amount of produced CaCO 3 is about 47% of biochar mass, and it is deposited within biochar, improving the pore filling and microstructural stability. Both BC and MBC were then used as partial cement replacements (mass fractions of 10%, 20%, 30% and 40%) in cement pastes, on which the setting time and compressive strength were tested, and the carbon emission was evaluated. Results indicate MBC can weaken the retardation effect on setting time and reduce the decline of compressive strength caused by BC, due to the filling effect, packing density and promoting nucleation of deposited CaCO 3 . Using carbon-negative BC and MBC to partially replace cement can generate obvious carbon reduction potential to cement pastes, and can make cement pastes carbon-negative when their dosages (mass fraction) are larger than 20%. However, the difference in the reduction of carbon emissions due to BC and MBC is not significant when they are at the same dosage. In order to obtain a good balance among setting time, compressive strength and carbon emission for biochar-cement paste, MBC dosages of 20% and 30% are suggested.