# Enhancing Cement Paste Properties with Biochar: Mechanical and Rheological Insights
## 4 Discussion  
As shown in the Supplementary Materials, when stress is applied to the system of cement-biochar, if this stress is higher than the cohesion forces of the agglomerates, the water trapped within the agglomerates is released, thus lowering the viscosity. In this study, biochar can increase both yield strength and viscosity. The results indicate that the yield stress value is in line with that of self-compacting concrete. Dynamic oscillatory shear tests using strain sweep showed that the higher the biochar addition, the lower the LVE. Considering that biochar particles are smaller than cement ones (Figure 1), this behavior is also probably due to the limited chemical affinity between biochar and the cement gel. In addition, the sharp edges of biochar particles (Figure 3) contribute to gel structure alteration. Thus, when a reduced strain is applied to the system, the gel structure is rapidly destroyed. The initial storage modulus value increased with biochar content (Table 4), indicating that the elasticity was higher, as well as the stiffness, in agreement with steady shear measurements. The energy dissipation due to mechanical vibrations is produced by calcium silicate hydrates (C-S-H) sliding at different scales. At the nanoscale, the C-S-H sheets slide with the interlayer water acting as the lubricating layer to dissipate energy. At the microscale, gel pores are present and C-S-H globules can also slide with capillary pore water as the lubricating layer, leading to further energy dissipation [49]. Biochar addition increased the initial loss modulus (Table 4), probably because of the higher interfacial friction with C-S-H sheets. From the mechanical point of view, biochar is a good reinforcement for cement-based composites. As already investigated [16; 32; 50; 51], the addition of small amounts of B can increase the flexural strength of cementitious materials. The strength of 7-day cementitious composites depends on the densification of the matrix: the higher the packing density, the higher the mechanical properties. Furthermore, biochar can act as a starting point for hydration products' growth [52]. As a result, B samples have higher mechanical properties  
Figure 11: Linear crack path and pseudo-smooth fracture surface. Plain cement (OPC) at 28 days. Figure 12: Tortuous crack path and rough fracture surface. B 5% at 28 days. than pristine cement samples at early ages. However, the increase in mechanical strength is not as pronounced as in the case of other carbon-based reinforcements used in the literature. This is also because, as known, the dispersion of a nano-micro reinforcement within the matrix also affects the mechanical outcome. In the literature, dispersion has a fundamental role in enhancing the interaction between the reinforcement and the matrix [16; 53; 54; 55]. The literature suggests that a proper dispersion of the nano- and microparticles in the final mix is essential to achieving a homogeneous material, as this is directly proportional to the particle size: the smaller the particle size, nearer to the nanoscale, the greater the surface area per unit volume [56]. This leads to increased van der Waals forces and electrostatic force among nanoparticles, which greatly favor their re-agglomeration [55]. As seen in Figure 3, biochar particles have a wood chip structure because of the type of biomass used for the production. This structure allows a good cohesion and a good interaction with the cementitious matrix, thus guaranteeing a bond with the matrix, which explains the flexural and compressive strength enhancement [42; 44]. On the other hand, the biochar absorbs water, and it is possible that the increase in mechanical properties at 28 days, especially with large amounts of water, is due to the reduction in the w/c ratio, which consequently leads to an improvement in mechanical properties.