Sludge biochar as a green additive in cement-based composites: Mechanical properties and hydration kinetics

Biochar is a green material obtained from the thermochemical decomposition of biosolids under an anaerobic or oxygen-limited condition. Although sludge pyrolysis has been extensively studied in municipal sludge (MS) disposal, it is challenging to utilize the formed sludge biochar (SB) products. This study developed a novel and sustainable method for incorporating SB-derived biochar in cement-based composites, providing an economical and efficient way of recycling such wastes. A series of SB were prepared under different pyrolysis processes to replace 2% cement. Numerous mechanical, physical, microstructural and isothermal calorimetry analyses were used to study the performances of the SB incorporated cement composites. The results showed that the mobility of the composites declined slightly with the addition of SB. Pyrolysis regimes played a key role in influencing the strength, and the optimum performance was obtained at a pyrolysis regime of 500 °C and residual time of 60 min, which increased the 1 d and 28 d strengths by 29.3% and 5.8%, respectively. The addition of SB accelerated the formation of more hydration products, increased the hydration degree and led to a denser matrix. All these improvements were ascribed to the internal curing effect of sludge biochar and a decrease in the ratio of local water to cement. Thermal analysis by Krstulovic-Dabic Model indicated that the hydration mechanisms of biochar-cement composites belonged to nucleation and crystal growth (NG)- interactions at phase boundaries (I)-diffusion (D). The SB incorporated sample possessed more nucleation sites with a higher reaction rate constant at NG stage and shorter I stage. In addition, environmental risk assessment revealed that SB was a safe additive in cement-based composites. Therefore, pyrolysis of MS under appropriate temperature conditions has great potential to provide a green biochar in cement-based composites and thereby offers an alternative and viable way of disposal of MS.