Relevant biochar characteristics influencing compressive strength of biochar-cement mortars

To counteract the contribution of CO2 emissions by cement production and utilization, biochar is being harnessed as a carbon-negative additive in concrete. Increasing the cement replacement and biochar dosage will increase the carbon offset, but there is large variability in methods being used and many researchers report strength decreases at cement replacements beyond 5%. This work presents a reliable method to replace 10% of the cement mass with a vast selection of biochars without decreasing ultimate compressive strength, and in many cases significantly improving it. By carefully quantifying the physical and chemical properties of each biochar used, machine learning algorithms were used to elucidate the three most influential biochar characteristics that control mortar strength: initial saturation percentage, oxygen-to-carbon ratio, and soluble silicon. These results provide additional research avenues for utilizing several potential biomass waste streams to increase the biochar dosage in cement mixes without decreasing mechanical properties. A broad selection of biochars were used to replace 10 wt.% cement in mortars, resulting in similar or improved strength.Biochar moisture saturation percentage, O/C, and soluble silicon are the most important predictors of mortar strength.The data support the hypothesis that biochar improves strength via internal curing, found to be the most important mechanism.