# Enhancing Cement Paste Properties with Biochar: Mechanical and Rheological Insights
## 1 Introduction  
The world population grew from 1 to 8.1 billion from the year 1800 onwards, while more than 50% of the population lives in urban areas [1], and this percentage will continue to grow in the future. For these reasons, today, Portland cement concrete is by far the most produced and consumed construction material in the world [2; 3; 4]. The composition of cement has evolved since the patent of [PERSON] in 1824, and nowadays, concretes should satisfy different requirements like improved workability, strength and ductility, durability, and sustainability. To meet these challenges, specific admixtures, new filler/aggregates, nanomaterials, new curing conditions, etc., have been explored [5; 6; 7; 8; 9]. Engineering materials' properties are governed by their microstructure. Currently, micro- and nano-carbon reinforcement have been shown to improve the performances of cement-based composites--for example, carbon nanotubes, graphene and carbon fibers [10; 11; 12; 13]--showing significant improvements in mechanical, thermal, and electrical properties. However, even if carbon-based reinforcement significantly influences the mechanical properties of the composites, their use has a high cost and environmental impact [14]. In addition, carbon-based reinforcements are difficult to disperse in the cementitious matrix homogeneously, due to their hydrophobic nature [15]. The use of biochar as a carbon-based reinforcement can improve the mechanical properties of cementitious materials, while at the same time being economically sustainable and environmentally friendly, mitigating CO\({}_{2}\) emissions that contribute to the greenhouse effect [16; 17; 18; 19].The use of biochar as a carbon reinforcement within cementitious matrix composites has received particular attention in recent years [20; 21; 22; 23; 24; 25; 26; 27; 28; 29]. [PERSON] et al. [30] observed that the addition of 0.08 wt. % coconut shell-derived biochar improves cement paste's mechanical properties, such as compressive strength and fracture toughness by 25% and 76%, respectively. The presence of inhomogeneous biochar particlesin the tensile plane of cement paste samples was shown to affect the rupture modulus, reducing it by 20-40%. In this article, the authors highlight the importance of particle size because the interaction between the biochar and cement particles plays an essential role in resisting crack propagation and absorbing fracture energy. Some of the authors in a previous article [31] demonstrated that hazelnut shell biochar used as a micro aggregate improves the cement paste's mechanical properties. The results suggest that adding a low percentage of biochar (0.8-1 wt. %) improves cement paste's modulus of rupture and fracture energy by 22.2 and 61%, respectively. The latter is attributed to the crack path's tortuosity, due to the introduction of biochar particles that induce the matrix's inhomogeneity and attract the crack path towards them. Furthermore, biochar derived from exhausted coffee powder is a promising low-cost carbon-based material for improving the mechanical behavior of cement-based composites [16]. [PERSON] et al. [32] discovered that integrating 1% and 2% of biochar composites enhanced the flexural strength of cement paste by up to 24% and 15%, respectively. As demonstrated by some of the authors, biochar was also used for improving the fracture energy of mortars [33]. Similarly, [PERSON] et al. [34] reported that the addition of biochar in mortar at low percentages improves the compressive strength by about 10%. Moreover, the authors investigated the use of biochar as a cement replacement in cementitious mortars with improved results, for low percentages of biochar. Nonetheless, when using 5, 10, and 15% biochar as a replacement in cement, the capability of mortar to flow was reduced by 3, 17, and 20%, respectively. [PERSON] and [PERSON] [35] also discussed the potential of using biochar as a building material to capture and \"lock\" atmospheric carbon in civil infrastructure.