# Teaching Characteristics of Wood Biomass Fly Ash Cement Composites
## 3 Results and Discussion  
### Discussion of the Application of WBA in Agriculture  
Circular WBA management is ecologically and economically viable at the local level. Based on the conducted survey made under project \"TAREC2 Transformation of Wood Biomass Ash into Resilient Construction Composites\", 61% of WBA is landfilled, while 38% is used in agriculture [63]. The heavy metals concentrations in WBA that are shown in Table 2 were compared with the limit values for heavy metals concentrations in some EU countries (Table 5) to evaluate the applicability of WBAs in agriculture.  
In Croatia, the end-of-waste criteria are specified by the \"Ordinance on by-products and end-of-waste status\" (OG 11/14) [23], through which WBAs can obtain agricultural product status in three categories, depending on the levels of heavy metals allowed. None of the tested WBAs meet the heavy metals limit criteria for use in agriculture: the Cd concentrations in samples WBA1 (3.1 mg/kg)  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Heavy Metal** & **Finland** & **Sweden** & **Denmark** & **Lithuania** & **Austria** & **Croatia** \\ \hline As & 40 & 30 & - & 30 & 20 & - \\ Cd & 25 & 30 & 15 & 30 & 5–8 & **0.7–3** \\ Cu & 700 & 400 & - & 400 & 200–250 & 70–500 \\ Pb & 150 & 300 & 120 & 300 & 100–200 & 45–200 \\ Zn & 4500 & 7000 & - & 7000 & **1200–1500** & **200–1800** \\ Cr & 300 & **100** & **100** & **100** & 150–250 & 70–250 \\ Ni & 150 & **70** & **30–60** & **70** & 150–200 & **25–100** \\ V & - & 70 & - & 70 & 100 & - \\ B & - & 500 & - & 500 & - & - \\ Hg & 1.0 & 3 & 0.8 & 3 & - & 0.4–3 \\ Co & - & - & - & - & 100 & - \\ Mo & - & - & - & - & 20 & - \\ \hline \hline \end{tabular}
\end{table}
Table 5: Heavy metals concentration limit values for the use of WBA in agriculture and forestry [mg/kg] [21; 23].  
and WBA3 (6 mg/kg), the Zn concentration in sample WBA1 (1850 mg/kg), and the Ni concentration in sample WBA2 (170.3 mg/kg) were higher than the limit values in Table 5, prohibiting their use in agriculture. Notably, the tested WBA samples met only the criteria for application to forestry in Finland. Producing WBA in one country and using it in another as part of its circular management must be economically viable.  
As shown in Table 2, the heavy metal index (HMI) was determined as an overall evaluation of the WBAs' heavy metals contents. The HMI was proposed by the authors of [6], wherein the heavy metals contents were normalised to the limit values for the standard potentially toxic elements (Cd, Hg, Cr, Cu, Pb, Ni, and Zn) to evaluate the use of waste products in agriculture [77]. Sample WBA1 has greatest HMI (18.2); it was collected from a power plant that uses a grate combustor and pulverised fuel combustor as its combustion technology (Table 1). The authors of [3] state that the temperatures in a grate combustor can reach 1000-1200 \({}^{\circ}\)C, while combustion temperatures in fluidised beds are somewhat lower (less than 900 \({}^{\circ}\)C); in pulverised fuel systems, the peak flame temperatures are commonly near 1600 \({}^{\circ}\)C, which is extremely high as compared to the other combustion systems. Because combustion temperature affects the chemical composition of the WBA, including the metals content (metals content increases with increasing temperature [33]), it could be expected that WBAs from power plants with pulverised fuel combustors cannot be used in agriculture. However, this should be investigated further.