Clarifying the effect of biochar on the hydration, setting, workability, and mechanical strength of cementitious materials

In the context of climate change, the use of biochar in construction materials is attracting increasing interest. Despite the available researches, further work is needed to understand the sole effect of biochar and in general of mix composition (biochar content, water to binder and water to cement ratios, superplasticizer (SP)) on the fresh and hardened properties and microstructure of cementitious materials. This study comprehensively investigates the effect of different contents of biochar referred to as micronized vegetable carbon (MVC) as cement substitute, water and SP, on the hydration, setting, workability, microstructure and strengths of mortars. Isothermal calorimetry revealed that, up to 10 wt% MVC, increasing both MVC and water contents improves cement hydration due to the nucleation effect, that counterbalances the dilution effect. A Vicat test-based specific method showed a slight acceleration of setting with increasing MVC content (reduction in setting time of 13 and 41 min for 5 and 10 wt% MVC) due to its water absorption. Mini-slump tests with and without SP showed that MVC has a direct impact on rheology but not on the fluidizing effect of SP. A stable microstructure is created when the water to cement ratio remains constant while the cement content decreases, resulting in higher-than-expected strengths (6 % increase at 28 days for 10 wt% MVC). The results suggest that MVC can be recycled to replace cement without compromising its properties, which is beneficial from a climate change perspective. • Up to 10 wt%, increasing biochar as cement substitute, and water content improves cement hydration. • Biochar with particle size larger than cement improves hydration via its higher specific surface area. • The water absorption by biochar has no effect on hydration but does affect workability. • The type of mixer has a significant effect on the workability due to the lighter weight of biochar. • Using fixed water content while reducing cement gives higher strength and unchanged microstructure.