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 xNa 2 O, 10CaO, 90 − xB 2 O 3 in mol%) were prepared with water content up to 8 wt.%. The water speciation in the glasses was derived by near-infrared (NIR) spectroscopy while boron speciation was investigated by 11 B MAS NMR (Magic Angle Spinning Nuclear Magnetic Resonance). For the three glasses effective molar absorption coefficients were determined experimentally for the bands at 5200 cm − 1 and 4600 cm − 1 , corresponding to combination modes of H 2 O molecules and OH-groups, respectively. In contrast to silicate glasses, in which at most ~ 2 wt.% H 2 O are dissociated to OH-groups, the amount of dissociated H 2 O may even exceed 5 wt.% in borate glasses. The fraction of tetrahedral to total boron ( N 4 = B IV / B IV + B III ) is predominantly controlled by the ratio of Na 2 O + CaO / B 2 O 3 , but only weakly affected by the water content of the glasses. When increasing the H 2 O content from 0 to 8 wt.%, N 4 increases from 25% to 26% for NCB5 and from 42% to 47% for NCB25 glasses.