# Teaching Characteristics of Wood Biomass Fly Ash Cement Composites
## 3 Results and Discussion
### Leading from the Monolithic Cement Samples  
The leaching characteristics of WBA used in cement composites was assessed by the cumulative leaching of heavy metals during a 64 d testing period, with the results shown Figure 2 and expressed as mg per m\({}^{2}\) of the surface area. The cumulative testing values are provided in Table 1 (Appendix A). By examining the heavy metals leaching results over time, a trend of decreasing heavy metals concentrations can be observed up to the 7 th time step of the test (at 36 d), as shown in Table 1. The exception to this trend was Ni; in the 8 th time step (at 64 d), its concentration slightly increased. Figure 2: Cumulative leaching of heavy metals from the monolithic samples over time. The initial leaching that occurs on the outer surface of the monolithic sample causes the initially larger concentration values. Over time, the heavy metals leaching occurs due to the diffusion of the heavy metals ions from the cement matrix, which was observed after the 8 th time step of the testing and agrees with [66]. In most cases, the heavy metals leaching became insignificant after long-term submersion [82]. The leaching values reducing over time indicates that the heavy metals leaching initially comes only from the surface layer of the sample. Significantly more time must elapse before the metal from the interior part of the sample is released to the external environment, due to slowed diffusion, the effect of the metals bonding to hydration products, and the reduced leaching of the majority of the heavy metals given the high pH value of the sample interior [83; 84; 85; 86; 87]. Stronger initial leaching of the heavy metals from monolithic samples containing WBA was observed by [66], and the authors suggested that the heavy metals leaching was probably caused by the diffusion of heavy metal ions from the cement matrix. Surface leaching, dissolution, and diffusion are the three dominant mechanisms for releasing heavy metals from the cement matrix [88]. The strong linear relationships observed between the cumulative leaching and time for Cr (R\({}^{2}\) = 0.99 for all mixtures) and Zn (R\({}^{2}\) = 0.92 - 0.97) indicate diffusion-controlled leaching. For Ni, delayed leaching in the initial phase [84] with subsequent diffusion-controlled leaching was observed. Cumulative leaching values for mixtures M1-15, M2-15, and M3-15 for metals Cd, Cr, Cu, and Pb are equal to or less than those of the reference mixture, as shown in Table 7. The WBA mixtures' cumulative Zn leaching values were 11.06% (M1-15) to 34.3% (M2-15) higher than reference mixture, demonstrating the sensitivity of the Zn leaching to substituting the cement with WBA. The cumulative leaching values for Ni were less than the reference mixture except for sample M2-15, for which the value was 27.42% greater than the reference. Leaching is heavily dependent on the pH value of the solution, i.e., the leaching increases as the pH value of the solution decreases [39]. The mineralogical phases of the metal oxides present in the WBA affect their sensitivity to the presence of H+ ions in leaching. At lower pH values, there is an increase in heavy metals leaching [39]. During the testing, the pH values of the eluate were between 10.8 and 11.7. The demineralised water becomes highly alkaline when it encounters concrete, especially when, as in this case, there is no water flow [82]. In the absence of national legislation regarding limit values for leaching from monolithic products, the cumulative leaching values per sample surface area obtained for certain metals were compared to the Dutch limit values given in the guideline: \"Soil Quality Degree\" [66; 84; 89] (see Table 7). Despite the increased levels of some metals (Zn and Ni) observed for the mixtures containing WBA as a partial substitute for cement, the leaching of the cement composites with 15% WBA was significantly below the limit values provided in the Dutch guidelines [66; 84; 89], i.e., the tested values of heavy metals leaching are acceptable.