2.1. Materials

Fluidized bed fly ash samples supplied by TAURON Wytwarzanie S.A., the leading power provider in Poland (denoted 1–4 in Table 1), carbon dioxide gas grade of 99.99% and tap water were used in the direct dry carbonation method adopted for the conversion of fluidized bed fly ash to a stable and more environmentally friendly carbonated form [33] (denoted 1C–4C, respectively), being the main solid-state component of the filling mixtures tested. The bottom ash, provided also by TAURON Wytwarzanie S.A. and sieved to a grain size below 2 mm, cement of the strength class 42.5, as well as a commercially available mineral mining binder (denoted 5–7, respectively) were applied as solid additives in the preparation of the selected filling mixtures tested. The physical and chemical characteristics of the materials are given in Table 1.

Physical and chemical properties of materials tested.

The main metal oxide components of the fluidized bed fly ash tested were silicon, aluminum and calcium (Table 1), amounting in total to approximately 71–90% w/w. The dominant oxide compound of the untreated and carbonated fluidized bed fly ash and the bottom ash was the silicon oxide (approx. 33–51% w/w), and, in the case of the remaining materials, the calcium oxide (approx. 40–64% w/w).