CaCO3-AI(OH)(3) mixtures with different Ca/(Ca+AI) molar ratios (0.33- 0.91) were calcined at different temperatures (623-1473K). The calcination product was mixed with fly ash, it contaminated by oxioanions, sand ( a-quartz) and fumed silica (amorphous) to solidify before measuring the compressive strength of the solids. The crystalline compounds that contribute to the solid strength were identified by XRD to investigate the effect of calcination conditions and the influence of oxoanions on fly ash solidifying reaction. The calcination product at 1273 K with Ca/(Ca+AI)=0.83 that contained both CaO and Ca9Al6O18, provided the highest compressive strength of solidified fly ash, forming Ca2SiO4 and subsequently Ca(3)AI(2)SiO(4)(OH)(8). The strength of solidified fly ash was lowered by oxoanion coexistence because the oxoanions consumed CaO in the calcination product, while the oxoanion leaching from fly ash was suppressed.