Combined effects of biochar and MgO expansive additive on the autogenous shrinkage, internal relative humidity and compressive strength of cement pastes

Internal curing and expansive additives are normally used to mitigate the autogenous shrinkage of cement materials. Biochar, with a porous microstructure and water holding capacity, has a potential to be used as a novel internal curing agent, whereas few work was focused on its impact on the autogenous shrinkage of cement materials. The present work aims to investigate the effects of biochar and its combination with MgO expansive additive (MEA) on the internal relative humidity, autogenous shrinkage, compressive strength as well as microstructures of the cement pastes via using humidity sensor, laser displacement measurer, MIP, TG/DSC, and SEM. Results indicated that, compared to the plain cement paste, incorporation of biochar provided efficient internal curing, maintaining a higher internal relative humidity and hence a reduced the autogenous shrinkage by 16.3% at the age of 180 h. An obvious synergetic effect on mitigating the autogenous shrinkage of cement paste was observed when the biochar was added with the combination of MEA in the cement paste. For instance, with the addition of 2 wt% biochar and 8 wt% MEA with a high reactivity (reactivity value of 45 s) gained an expansion of 80 microstrain rather than a shrinkage at the age of 180 h. Addition of biochar increased slightly the compressive strengths of the cement pastes at the late ages of 28d and 90d. This facilitates to offset partially the compromise on compressive strength of cement paste caused by the incorporation of MEAs alone. This study provides a novel and effective approach for mitigating the autogenous shrinkage of cement materials.