Distribution of iron among ferrite hydrates

The hydration products of Ca2AlFeO5 pastes at 20 degrees C were characterized by X-ray powder diffraction, secondary electron imaging and X-ray microanalysis. In pure water, two X-ray amorphous AFm phases, one Al-rich and one Fe-rich, are initially formed. AFm is then replaced by hydrogarnet (Al/Al + Fe approximate to 3/4). In the presence of gypsum, AFt needles are first formed, followed by platy sulphate-AFm (both with Al/Al + Fe approximate to 3/4). With Ca(OH)(2) and gypsum, the following was observed (cf. gypsum only): (i) higher Fe content in AFt and AFm, i.e., Al/Al + Fe approximate to 1/2; (ii) decreased amount of S in AFt; and (iii) a general decrease in the crystal size of AFt and AFm. X-ray amorphous pseudomorphs after ferrite grains were present in all runs. Despite the formation of these CaFe4O7.xH(2)O residues, much Fe enters the other products, especially when a base is added. It is proposed that the passivity of the ferrite surface is caused by a compact, X-ray amorphous layer desiccated by crystallizing AFt. (C) 1997 Elsevier Science Ltd.