# Teaching Characteristics of Wood Biomass Fly Ash Cement Composites
## 1 Introduction  
Recently, in Croatia and other European Union (EU) countries, the promotion of the advantages and possibilities for electrical and thermal energy generation has resulted in a significant increase in the number of power plants that use biomass (particularly wood biomass) as a renewable energy source (RES). Worldwide, RES usage is the greatest in the electricity sector, where biomass represents the third-largest renewable source and generates 493 TWh worldwide, and in the heat generation sector where 95% of the renewable energy comes from biomass [1]. Biomass combustion generates ash, including wood biomass ash (WBA). WBA represents an environmental and economic problem for energy producers: the costs of disposal continue to increase [2,3], while the EU is attempting to limit its disposal at landfills through various directives [4,5]. According to Directive 2008/98/EC on waste [4] and the new Directive (EU) 2018/851 [5], the priority for WBA management is preventing it from being created, then recycling it, and, lastly, disposing of it. An application for WBA reuse must be identified because prevention is infeasible due to the expected increase in the use of biomass as an energy source and legislative measures decrease disposal options. During wood biomass combustion in a power plant, two primary types of WBA are generated: (1) wood biomass bottom ash, which is ash collected from the bottom of the combustion chamber, and (2) wood biomass fly ash that can consist of relatively coarse fly ash collected from the cyclone or boilers and fine fly ash collected from electrostatic filters or baghouse filters [6]. Fly WBAs andbottom WBAs have significant differences that should be considered prior to disposal. According to some authors [6; 7; 8; 9; 10; 11], heavy metals (e.g., Zn, Cd, Pb, and Hg) and organic pollutant (PCDD/F, PAH) concentrations are higher for fly WBA than bottom WBA, meaning that each type of WBA requires a different disposal strategy [8; 10]. According to [12], the greatest variations are of the volatile heavy metals concentrations such as Zn, Pb, and Cd, the concentrations of which increase as the particle size decreases. High Cd concentrations can have toxic effects on organisms in the soil, be taken up by plant cover, and, ultimately, enter the food chain [13]. A high share of Cd in some types of WBA also represents a potential health risk because it accumulates in the kidneys and affects bone density [14]. Exposure to high Zn concentrations can lead to toxic effects in brain cells [15], whereas any Pb exposure has an extremely negative impact on human health, especially cognitive function [16]. The leaching of Cu and Zn from fly WBA also damages lung tissue [17]. Furthermore, due to the leaching of heavy metals from WBA, its disposal creates a significant groundwater pollution problem [18]. Therefore, the responsible management of WBA requires an effective solution. Most studies of WBA recovery are in the field of soil enrichment [7; 9; 19; 20]. In the EU, the use of WBA in agriculture is regulated by the criteria determined by national legislation (such as in Sweden, Finland, Germany, Austria, and Croatia) that specify limits for heavy metals based on the total content of elements in the ash [21; 22; 23]. Furthermore, WBA recovery presents an array of options for the construction industry: as a raw material for alkali activation [24; 25; 26], in the production of ceramic products [27; 28], as a raw material for road construction [29; 30; 31] or as an aggregate/binder in concrete production [32; 33; 34; 35; 36; 37; 38; 39]. Cement is the main component of concrete, one of the dominant materials used in the construction industry [40; 41]. Cement production impacts the environment through its raw material processing requirements, which result in significant energy consumption accompanied by high CO\({}_{2}\) emissions. Recent research shows that the global clinker-to-cement ratio as a parameter for general estimation of CO\({}_{2}\) production, was 0.7 in 2018 [42].