*2.5. Carbon Storage Properties*

Although the different studies show high insulating properties, they are not the only important characteristic of these materials. Developing a biomaterial based on hemp stalk can increase the value of the hemp crop, produce more environmentally friendly materials, and also result in materials that act as CO2 accumulators [2].

The hemp absorbs CO2; meanwhile, it is growing through the photosynthesis process. Moreover, due to very rapid growth, the biomass accumulated by hemp crops has a significant effect in absorbing atmospheric CO2; however some of the carbon stored in this biomass is turned back into the atmosphere as CO2 due to the biodegradation of leaves and roots. Assuming a concentration of 0.5 kg of carbon per kg of dry matter, it can be calculated that 1.84 kg of CO2 is sequestered per kg of dry hemp through photosynthesis during the plant's growth. This means that a ton of dry hemp can store 325 kg of CO2, taking into account the amount of emissions during the farming as the diesel consumption, transportation of the seeds, etc [119].

Hempcrete is a sustainable building material made from hemp stalks, lime, and water, and the lime in the material also takes part in the CO2 sequestration due to the carbonation process of the lime. During the carbonation process, the lime reacts with the CO2 present in the air, resulting in the conversion of calcium hydroxide (Ca(OH)2) into calcium carbonate (CaCO3) [120]. The carbonation increases the mechanical resistance of the hempcrete, and, additionally, the absorption of CO2 during this process can have significant implications for the environmental impact of this product [121]. Hempcrete can store 300 kg of CO2 per m3 [122–124]. With these two properties, we have a new material that reduces the energy consumption of the building due to the thermal insulation capacities and also stores CO2, obtaining a material that would passively help to obtain buildings with zero emissions [125].

Based on these examples, it can be concluded that the binder used in the composite material has a significant impact on its carbon footprint. The use of recyclable materials with low impact on the carbon footprint is proposed for the binder material.
