Water- and boron speciation in hydrous soda–lime–borate glasses

Abstract

The structural investigation of hydrated borate glasses provides new insights on the influence of water on boron speciation using spectroscopic methods. In the present study three soda–lime–borate glasses (NCBx with x=5, 15 and 25 corresponding to xNa2O, 10CaO, 90−xB2O3 in mol%) were prepared with water content up to 8wt.%. The water speciation in the glasses was derived by near-infrared (NIR) spectroscopy while boron speciation was investigated by 11B MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance). For the three glasses effective molar absorption coefficients were determined experimentally for the bands at 5200cm−1 and 4600cm−1, corresponding to combination modes of H2O molecules and OH-groups, respectively. In contrast to silicate glasses, in which at most ~2wt.% H2O are dissociated to OH-groups, the amount of dissociated H2O may even exceed 5wt.% in borate glasses. The fraction of tetrahedral to total boron (N4=BIV/BIV+BIII) is predominantly controlled by the ratio of Na2O+CaO/B2O3, but only weakly affected by the water content of the glasses. When increasing the H2O content from 0 to 8wt.%, N4 increases from 25% to 26% for NCB5 and from 42% to 47% for NCB25 glasses.