IMPROVING THE HARDENING OF FLY ASH FROM FLUIDIZED-BED COMBUSTION OF PEAT AND WOOD WITH THE ADDITION OF ALKALINE ACTIVATOR AND PORTLAND CEMENT

Combustion affects the physical and chemical characteristics of fly ash. Peat and biomass are typically combusted using fluidized-bed combustion (FBC) technique, which causes the fly ash to have a low chemical reactivity. Especially low-Ca FBC ashes have very weak hardening properties, which reduce their utilization potential in many applications. This study examined the effect of adding alkaline activator and Portland cement on the hardening of two peat-wood fluidized bed combustion fly ash samples one of high-calcium fly ash (24.9 % Ca) and the other low-calcium fly ash (9.7 % Ca). The reactive components of the raw materials, the mineralogical compositions, and compressive strengths of the resulting mortars were studied. Significant improvement to strength development was achieved. The addition of Portland cement proved necessary for low-Ca ash to harden at all. The ash cannot be considered solely as a filler material, since the reactive component (Ca, Si, Al, and S) contents of the ash, especially the Ca content, affect the strength development. Strength development of the mortars can be partly explained by the Ca/(Si + Al + S) ratio and the sum of the reactive components.