Effect of Biochar Filler on the Hydration Products and Microstructure in Portland Cement–Stabilized Peat

Abstract

Laboratory tests demonstrated that biochar filler added to portland cement stabilized peat results in an increase of unconfined compressive strength comparable with that of a sand filler. The strength increase is significantly higher when biochar is ground to a size below 75 μm. This study investigated the changes in mineralogy, texture, and microstructure during the early hydration of cement mixed with peat and biochar filler to identify the mechanisms responsible for the strength increase. The results show that the biochar surface catalyzes nucleation of hydration products. Labile carbon in biochar promotes carbonation, with precipitation of calcite within its cells and on its surface, as well as formation of hemi and monocarboaluminate, two stable calcium aluminate hydrate (AFm) phases. For larger fragments of biochar, early hydration products do not reach the inner cells. Instead, the fine fragments tend to be fully covered, leading to a more homogeneous spatial distribution of cement and voids.