Performance and Durability of Biochar-Modified Concrete Incorporating Straw–Grass Biochar as a Partial Cement Substitute

Using biomass-derived biochar in cementitious materials is a way to valorise the agricultural residues and to partly replace the Portland cement. The effect of straw–grass biochar made under pyrolysing temperature of 450℃ on workability, compressive, split tensile and flexural strength, hardened density, water absorption and accelerated carbonation depth of M30 water-to-binder ratio (w/b) designated concrete is examined in this study. Cement was replaced by biochar at 0, 5, 10, 15, 20 and 25% by mass. The compressive strength at 28 days reached maximum value of 35.16 MPa at 10% replacement (14.98% increase over the control) and the amount of cement used per MPa of compressive strength decreased from 14.3 to 11.2 kg/m³. Split tensile and flexural strengths also peaked at 10%, at 3.21 MPa (+ 39.0%) and 5.42 MPa (+ 31.6%). The additional void content as determined by a change in hardened density from 2521.0 to 2348.2 kg/m3 (-6.85%) at 25% showed a reduction approximately 2.7 times greater than that predicted from the reduction in biochar's specific gravity. The lowest water absorption (4.21%) was at 10%, and was found to be higher than the control only at 25%. There was a 25% CO2 increase of 15.1 mm after 28 days of carbonation at 4% CO2 when compared with the control (3.2 mm), which was greater than the 4.7-fold increase in the cement's dilution. There was a slight reduction in slump from 98 to 90 mm. and qualitative SEM observations of 0%, 10% and 25% replacement control mixtures confirmed the trends of strength and porosity. A replacement level of ~ 10% appears to be the best compromise between strength and carbonation resistance of the levels evaluated; higher levels decrease strength and carbonation resistance. The data presented are based on three samples of each mixture and are statistically significant, using the one-way analysis of variance (ANOVA).