Strength and durability improvements of biochar-blended mortar or paste using accelerated carbonation curing

The carbonation curing method is widely used to produce low-CO2 cement-based materials. This study presents experimental studies on plain, 2 %, and 5 % biochar-blended mortar or paste with accelerated carbonation curing (ACC) and normal curing. Tests on the degree of carbonation, strength, ultrasonic pulse velocity (UPV), electrical resistivity, chloride diffusion, X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA) were performed. The results are summarized as follows: 1) Biochar-blended mortars had a higher degree of carbonation than that of plain specimens. After 28 days, the strength of the ACC specimens was approximately 1.3 times that of the sealed curing group. 2) As aging increased, the compensation effect of ACC on the quality of mortar became prominent. For ACC specimens, the strength was an exponential function of the UPV. 3) Compared with the sealed specimens, the electrical resistivity of ACC increased more obviously with an increase in the biochar replacement ratio, and the ACC specimens had much lower chloride diffusion coefficients. 4) XRD analysis revealed that the ACC specimens had no calcium hydroxide or ettringite. FTIR revealed for ACC specimens, the peak of calcium silicate hydrate shifted to approximately 1034 cm(-1) and weakened. 5) A new calculation method was proposed to exclude the weight loss of the biochar ignition in the TGA. TGA revealed that for ACC specimens, as the biochar content increased, the content of calcium carbonate increased. Carbonation curing of biochar-blended mortar or paste is a feasible method for carbon sequestration.