## 2 Agro-Wastes as Concrete Additives
### Nutshell Waste  
[PERSON] et al. [67] studied the effects of adding different amounts of nutshells (ranging from 0.5 to 2.5% with an interval of 0.5%) obtained from walnuts, pistachios, and hazelnuts as cement supplements. They analyzed the mechanical properties and found that the highest supplement ratio correlated with the highest compressive strength. Moreover,  
Figure 6: Improvement in (**A**) flexural and (**B**) split tensile strengths after 28 days. GPS stands for geopolymer concrete; G8, G10, and G12 stand for geopolymer concrete with ground/granulated blast furnace slag at molar ratios of 8, 10, and 12, respectively; M1, M2, M3, and M4 stand for 0%, 10%, 20%, and 30% micronized rice husk silica, respectively [65]. the nutshells had a significant impact on the thermal conductivity of the mortar, wherein the 2.5% walnut supplementation decreased its thermal conductivity by 57%, compared to the pistachios and hazelnuts causing decreases of 48% and 45%, respectively. Their study found that higher nutshell ratios led to increased hardness, where pistachios showed the best results among other nutshells. Previously, [PERSON] et al. [68] declared that an up to 5% replacement of cement with hazelnut shells could meet the necessary standards (EN 197-1) [69] for compressive strength. A higher ratio of finer particles had a better influence on increasing compressive strength due to the increased interactions among these particles and the surrounding matrix. [PERSON] et al. [70] confirmed that as the additive particle size becomes finer, the slump becomes lower, which is due to the higher surface-to-area ratio. However, this is not the only reason, and a higher carbon ratio also effects the results. The addition of nutshells to fired bricks resulted in satisfactory compressive strength with respect to the several related national and international standards, whereas their addition into unfired bricks did not meet these standards. Low adhesion between the nutshells and the clay matrix was emphasized as the reason for this reduction. Nevertheless, the compressive strength of earth bricks could be enhanced by adding some other materials such as gypsum (10%), gypsum and Elazig Ferrochrome slag [71], and gypsum (3%) and lime (7%) [70]. In addition to the above research, the incorporation of plastic waste and agro-waste could improve some mechanical properties of tile. [PERSON] et al. investigated different types of polyethylene incorporated with rice husk ash and sand and obtained reasonable water absorption for floor tile composites, including 15% rice husk ash + 15% high-density polyethylene [72]. In addition, these authors obtained compressive strength and flexural strength values of 24.79 and 4.895 (N/mm\({}^{2}\)), respectively [72]. However, in another study performed by the same researchers, the best ratio for hybrid tile composed of waste plastic, rice husk ash, and sand was 1:1:2 [73]. [PERSON] et al. [74] evaluated using rice husk ash at 5% in concrete and found that this amount of ash could provide acceptable compressive strength that met the standard level. They also mentioned as an important fact that using this amount of ash could decrease the cost of preparing concrete with respect to conventional concrete. They declared that each 1 m\({}^{3}\) concrete obtained from rice husk ash could reduce the cost from USD 159 to USD 122, which shows a 22% reduction. However, it is worth noting that the cost of agro-waste materials for construction can vary widely depending on several factors, including the type of agro-waste, its availability, processing requirements, and local market conditions. In addition, agro-waste materials such as rice husk ash, bagasse ash, coconut shell ash, and others are sometimes used in construction as supplementary cementitious materials or as aggregate. Therefore, based on the above conditions, the costs of using these materials in construction vary from being almost free to costing more than conventional construction. For example, [PERSON] et al.