## 1 Introduction
Because the fine fraction of biochar (micrometrical size) is volatile and it could contain undesirable elements, the application of this substance in building materials or other composites is starting to gain more attention recently. Biochar has interesting properties for application in the building industry such as low thermal conductivity, high chemical stability and low flammability. Another characteristic of biochar is its capability to adsorb and desorb water, therefore materials containing biochar can be humidity control materials (HCM). The main requirement for this kind of material is good moisture absorption and desorption performance to detect and automatically adjust the relative air humidity. Various HCMs have been developed based on biomass, inorganic or organic raw materials. Specifically, in the literature, several studies have reported obtainment of HCMs using charcoal [6], diatomite [7], filled phos  
Figure 1: Scheme of combustion, gasification and pyrolysis. phates and aluminium oxide into the pores of sepiolite [8] as well as biochar-mortars [9]. Another application related to the high adsorption capacity of biochar reported in the literature is the adsorption and removal of pollutants in the aqueous phase on the surface of different kinds of geopolymers, both organics dyes and heavy metals [10]. The influence of various experimental factors such as geopolymer dosage, pH, initial dye concentration, contact time, and temperature was assessed for metakaolin geopolymers. The results demonstrated that the adsorption of the obtained material occurs spontaneously as an endothermic process, confirming that the prepared adsorbent can be used for remediation of water contaminated by methylene blue dye [11]. The activated FAs are demonstrated to be an efficient and low-cost adsorbent for decontamination of dye-containing water with the formation of new stable materials for encapsulation of aqueous solutions contaminated by high concentrations of methylene blue [12]. Biochar is also used to prepare a new biochar/geopolymer composite membrane by an in situ synchronous carbonation and self-activation process. The geopolymer membrane serves not only as a porous support, achieving good dispersion and retrieve of biochar, but also as a solid base for in situ activation of the biochar during carbonization of a lignin precursor [13]. There are also reports in the literature of the use of biochar in the construction field. In many applications, for example, cement or other building materials, an improvement of physical and thermal properties is found when biochar is added [14; 15; 16; 17]. In the last year alone, interesting works have been published in this area which we report briefly below. In a mortar with silica fumes, the introduction of biochar reduces autogenic shrinkage and drying shrinkage and improves hydration, resistance and water permeability of the mortar [18]. In ultra-high performance concrete (UHPC) biochar as a mineral additive in partial replacement of cement, improves hydration thanks to its internal polymerization and nucleation effects [19]. Combined use of waste biochar and cement-based composites with CO2 curing could be a green technology to improve cement properties (in particular mechanical strength) and carbon sequestration, together with promoting the recycling of waste [17]. The properties of biochar-cement composites depend on the lignocellulosic or non-lginocellulosic nature of the biomass from which the biochar is obtained, the former generally increasing the compressive strength, the elastic modulus and the fracture toughness [20]. Porous biochar, prepared from lignocellulosic biomass, improves the mechanical performance and durability of structural grade concrete and, in samples exposed to high temperatures, thermal damage is minimized and a good percentage of extra strength is maintained with respect to control concrete and silica fume [21]. Biochar has also been tested in lightweight aggregates (LWAs) for green roofs and bricks [22].