**4. Ecological feasibility of using WBA as supplementary cementitious material**

In the construction sector, the use of industrial by-products as substitutes for natural raw materials is encouraged. When using alternative materials obtained as by-products from other industries, it is necessary to consider the environmental factor. One of the basic requirements for construction includes "hygiene, health, and environment" under the European regulation for construction products [61]. The assessment of the environmental impact of cement-based construction products is usually based on the determination of leaching, i.e. the potential release of ingredients such as trace elements (heavy metals) or organic compounds into the environment when the products come into direct contact with water or soil. The estimation of pollutant release can be done by standard short-term leaching tests and long-term tests [62, 63]. The Technical Committee of CEN TC 351 has developed laboratory tests to check the leaching of hazardous substances into nature using demineralized water as a leaching agent [63].

The authors [25, 41, 64] found that the concentration of heavy metals such as Zn, Cd, Pb, and Hg is higher in fly WBA samples than in bottom samples. Therefore, the leaching/stabilization behavior of cementitious composites prepared with fly WBA

**Figure 6.**

*Values of cumulative leaching in mg/m2 for different metals (M-Fi-mortar mix with 15% of fly WBA) [65].*

should be analyzed. According to the leaching results obtained by the author [65] for the observed heavy metals (Zn, Cd, CR, Cu, Ni, Pb) on monolithic specimens using 3 types of fly WBA (**Figure 6**), it was concluded that the leaching of heavy metals was acceptable, i.e., less than the limits according to the Dutch guidelines of the Soil Quality Ordinance [66] (limits for finished building materials according to the Soil Quality Ordinance for Cd: 3.8 mg/m<sup>2</sup> ; Cr: 120 mg/m<sup>2</sup> ; Cu: 98 mg/m2 ; Ni: 81 mg/m<sup>2</sup> ; Pb: 400 mg/m2 ; Zn: 800 mg/m<sup>2</sup> ). The same was confirmed by the authors [55, 67] when using ash from the combustion of pure wood biomass. This is explained by the ability of the cement matrix to physically and chemically bind contaminated elements (heavy metals) within the hydrate structure [68].
