Hydration-strength-durability-workability of biochar-cement binary blends

Cement is widely used as one of the most common materials in the construction industry. More than 2300 integrated cement plants are operated by more than 1000 cement manufacturers worldwide, producing nearly 4 billion tons of cement annually. It is worth noting that the manufacture of cement not only consumes a large amount of energy, but also produces a large amount of greenhouse gases. The CO₂ emissions of the cement industry account for 5% of global CO₂ emissions [1]. Coupled with the non-renewability of the raw materials, the cement industry faces huge challenges in terms of supply, sustainability, and carbon neutrality. The European Cement Association has set an ultimate goal of achieving zero emissions in the cement and concrete value chain by 2050. Obviously, there is an urgent need to reduce the energy consumption of the cement industry and find more environmentally friendly replacement materials to reduce the demand for cement.

Biochar is not only widely used in soil improvement because of its special properties and structure [2,3], but it is also used in construction engineering, such as for building insulation, decontamination foundations, humidity adjustment [4], resistance to electromagnetic radiation [5], and production of lightweight bricks [6]. The practicability of biochar in the field of cement substitute materials is also clear. It can reduce the utilization rate of cement while achieving carbon sequestration. Biochar has the potential to reduce net greenhouse gas emissions by approximately 870 kg CO₂ equivalent per tonne of dry feedstock [7].

Biochar is a porous carbon-rich solid product produced by the pyrolysis of raw materials under anaerobic or anoxic conditions. Its physicochemical properties are affected by the pyrolysis temperature and types of raw materials. Tan et al. [8] studied the performance of biochar blended mortars composed of waste wood biochar pyrolyzed at different temperatures (400 °C, 500 °C, 600 °C, and 700 °C) and found that the optimal pyrolysis temperature and replacement ratio to improve the mechanical properties of the blended mortar were 400–500 °C and 1%, respectively. Akhtar and Sarmah [9] and Zeidabadi et al. [10] reported the use of biochars derived from different wastes (pulp and paper mill sludge, poultry litter, rice husks, and sugarcane bagasse) to improve the performance of cement-based composites.

First, biochar improves the mechanical properties of cement-based composites. Gupta et al. [11] reported that adding 1%–2% biochar could increase the compressive strength. Praneeth et al. [12] investigated the use of biochar to replace 10%–40% of the fine aggregate (sand) and indicated that replacing sand with 20% biochar improved the flexural strength by up to 26% compared with the control mortar. Furthermore, biochar improved the autogenous shrinkage [13], self-healing [14], thermal conductivity [15,16] and compactness [12] of cement-based composites. The mechanisms of biochar in cement-based composites mainly include the internal curing and fine filler effects [17,18]. The fine filler effect is mainly affected by the particle size of the biochar and the hardness of the raw material, whereas the internal curing effect is mainly influenced by the pore structure and pore size of the biochar. Finally, there are rare studies on the durability of biochar cement-based materials. Gupta et al. reported that the mortar with 1–2% biochar reduced the strength loss compared to control when exposed to NaCl and sulfate solutions [19]. Gupta et al. further found that the addition of biochar reduced the carbonation depth of fly ash cement mortar [20].

A review of research on the application of biochar to cement-based composites shows that previous research on the properties of biochar cement-based composites has mainly focused on the mechanical properties, autogenous shrinkage, and thermal conductivity. The main limitations of previous studies are summarized as follows. First, most previous studies on the durability of biochar cement-based composites mainly focused on water absorption [17,21]. However, other durability aspects, such as chloride ingress and the electrical resistivity of biochar blended concrete, have not been widely studied [19,20]. Moreover, the monitoring of strength development using non-destructive test methods, such as ultrasonic pulse velocity, has not been carried out for biochar blended concrete. Second, previous studies measured the hydration heat and hydration products of biochar blended concrete [10,17,21]. However, the effect of biochar on the degree of hydration has not been clearly elucidated. The loss of weight of biochar during thermogravimetric analysis (TGA) tests may affect the weight loss of the biochar-cement composite, and thus affect the calculation of the degree of hydration. Third, previous studies focused on the individual properties of biochar-cement composites [9,14,15]. The relationships among different properties, such as those between mechanical properties and durability, have not been widely explored. Moreover, for plain concrete, chloride diffusivity is an inverse function of the electrical resistivity. Owing to the addition of biochar, the relationship between electrical resistivity and chloride diffusivity may be different from that of plain concrete.

To overcome the limitations of previous studies, this study presents a systematic experimental investigation of biochar-cement binary composites with elevated biochar contents. The experimental parameters consist of the workability, compressive strength, ultrasonic pulse velocity (UPV), heat of hydration, X-ray diffraction (XRD) patterns and TGA of hydration products, chloride ion diffusion coefficient, electrical resistivity, and optical micrographs and scanning electron microscopy (SEM) analyses of cement-based composites. The degree of hydration is determined based on the TGA results, considering the influence of the weight loss of biochar. Moreover, the relationships among the various experimental results are clarified.