Crystal structures of Boro-AFm and sBoro-AFt phases

International audienceCrystal structures of boron-containing AFm (B-AFm) and AFt (B-AFt) phases have been solved ab-initio and refined from X-ray powder diffraction. 11B NMR and Raman spectroscopies confirm the boron local environment in both compounds: three-fold coordinated in B-AFm corresponding to HBO3 2− species, and four-fold coordinated in B-AFt corresponding to B (OH)4− species. B-AFm crystallizes in the rhombohedral R 3c space group and has the 3CaO*Al2O3*CaHBO3*12H2O (4CaO*Al2O3*1/2B2O3*12.5H2O, C4AB1/2H12.5) general formulae with planar trigonal HBO32− anions weakly bonded at the centre of the interlayer region. One HBO32−anion is statistically distributed with two weakly bonded water molecules on the same crystallographic site. B-AFt crystallizes in the trigonal P3cl space group and has the 3CaO*Al2O3*Ca(OH)2*2Ca(B (OH)4)2*24H2O (6CaO*Al2O3*2B2O3*33H2O, C6AB2H33) general formulae with tetrahedral B (OH)4− anions located in the channel region of the structure. All tetrahedral anions are oriented in a unique direction, leading to a hexagonal c lattice parameter about half that of ettringite