Rare-earth silicate crystallization in borosilicate glasses: Effect on structural and chemical durability properties

Due to their low solubility, some elements, such as lanthanides, can lead to crystalline phases in borosilicate glass during cooling. The impact of rare earth (lanthanum and neodymium) silicate crystallization on glass structural configuration as well as on glass chemical durability was investigated in soda-lime borosilicate glasses. Partially crystallized glasses containing a single Ca2RE8(SiO4)6O2 (RE = La, Nd) phase (ranging from 1 to 36 wt.%) were synthesized by heat treating homogeneous glasses. Structural characterizations of vitreous and crystalline parts were carried out by 29Si, 11B, 23Na and 17O MAS and MQMAS NMR. The crystallization leads to a reorganization of the cation distribution around the rare earth elements. By keeping a part of the lanthanum and the calcium, crystallization causes a decrease in the number of NBOs in the vitreous part. A part of the sodium, initially near the lanthanum, becomes available to form Si-O-Na bonds and to convert trihedral boron into tetrahedral boron. Through the study of the alteration of different materials - homogeneous, crystallized and surrounding glasses, rare earth silicate ceramics, and separated mixtures of glass and crystals - a precise description could be made of the influence of the presence of crystals has on chemical durability. For the various alteration regimes, it was thus shown that the alteration kinetics increase proportionally to the crystal content, and that apatite-type crystals do not lead to a composition gradient on the glass-crystal interface. Although the crystals formed were more durable than the vitreous matrix, the chemical durability of partially crystallized glasses depends on the composition and structure of the surrounding glass.