Various characterisation techniques were used to study the composition of the glass series 55-P 2 O 5-2Cr 2 O 3-(43-x) Na 2 O-xPbO (with 8 ≤ x ≤ 38; mole %) in terms of chemical durability, IR spectroscopy and scanning electron microscopy (SEM). The change in the dissolution rate as a function of time when the studied glasses were kept submerged in distilled water at 90˚C for 20 days showed an improvement in the chemical durability when Na 2 O content was substituted to PbO content. IR spectroscopy revealed a structural change from ultraphosphate groups to pyrophosphate, orthophosphate and probably ring metaphosphate groups. SEM revealed the existence of two phases: a vitreous phase and a crystalline phase. The presence of Cr 2 O 3 , even in small amounts, seems to play an important role in the formation of crystallites in the glass network. The improved chemical durability is attributed to the replacement of the easily hydrated Na-O-P and P-O-P bonds by covalent and resistant Pb-O-P bands. Both the increase in PbO content and in the Pb + Cr/P ratio causes an increase in the number of covalent Pb-O-P and Cr-O-P bonds, making the glass structure more rigid. The increase of the covalent Pb-O-P bands leads to a clear evolution of the structure and chemical resistance, caused by grain-boundary resistance as a result of glass crystallisation. The IR spectra indicate that the increase in PbO content favours the formation of isolated 3 4 * Corresponding author. N. Beloued et al. 150 limit, the equilibrium between the glass bath and these crystallites is no longer maintained; we notice, once, a decrease in the chemical durability.

The influence of CaO on the glass forming characteristics and properties of Na2O-CaO-ZnO-P2O5 glasses has been investigated. According to the studies that we performed on phosphate based glass within system xCaO-(40-x)ZnO-15Na2O-45P2O5 (10 ≤ x ≤ 30; mol%), it was found that the increase of CaO and substitution of ZnO can give a good chemical durability. Both Cristallographies X-ray and IR spectroscopy have confirmed the structure change when the CaO content increases in the glass. This change results in the formation of metaphosphate and/or rings of metaphosphate groups at the expense of pyrophosphate. So it indicates the formation of Ca-O-P bonds in the network glass that replaces hydrated P-O-Na and P-O-P bands. The phosphate chains units can be bonded together in rings forming meta-phosphate groups. These rings likely lead to the formation of agglomerates of crystalline phases, which is the main cause of the increase in the chemical durability of the glasses when the CaO content increases. The latter may lead to wider use of these materials, especially in the biomedical field.

Various characterization techniques were used to study the composition of the glass series xAl 2 O 3 -(40 -x)CaO-10Na 2 O-50P 2 O 5 (with 0 ≤ x ≤ 10) in terms of chemical durability, X-ray diffraction, IR spectroscopy and scanning electron microscopy (SEM). The improved chemical durability was attributed to the replacement of easily hydrated P-O-P bonds by covalent and resistant Ca-O-P and Al-O-P bonds. However, the change in the dissolution rate (D R ) versus time showed a marked decrease in chemical durability with increasing the Al 2 O 3 content to the detriment of the CaO content. The X-ray diffraction analysis of glasses annealed at 550˚C and 660˚C for 48 hours indicated the presence of pyrophosphate phases and predominant metaphosphates or cyclic metaphosphate phases when the Al 2 O 3 content was ≤7.5 mol%. Nevertheless, both, X-ray diffraction and IR spectroscopy confirmed the structural tendency change from metaphosphate (Q 2 ) and pyrophosphate structural units (Q 1 ). Toward short isolated orthophosphate units (Q 0 ) when the Al 2 O 3 content above 7.5 mol%. SEM micrographs illustrated that the number of crystallites increased in the glass network when the Al 2 O 3 content increased at the expense of the CaO content. An increase in the Al 2 O 3 content to 10 mol% led to the formation of a larger number of crystallites of different sizes, dominated by small crystallite sizes assigned to short isolated orthophosphate groups. This phenomenon led to a decrease in chemical durability and seems to be a favorable factor for the formation of the apatite layers which enclose the glass, in a SBF solution test, able of regenerating bone tissue in biomedical application.

The synthesis for glasses series xFe 2 O 3 -(45-x)PbO-55P 2 O 5 , (with 0 ≤ x ≤ 20; mol%) carried out in a temperature (1050 ± 10)˚C, leads to obtaining transparent glasses, brown in color and with a non-hygroscopic appearance. The study of glasses dissolution rate, immersed in distilled water at 90˚C for 24 days, indicates a considerable chemical durability. The increase in the Fe 2 O 3 content in the vitreous network to the detriment of PbO is a favorable factor for the chemical durability improvement. Different techniques have been used such as X-ray diffraction, infrared spectroscopy, DSC, SEM and density for the study of these glasses. These techniques have led to establish correlations between chemical and structural properties. Thus the results obtained confirmed the creation of P-O-M bonds (M = Pb, Fe) with a strongly covalent nature to the detriment of the hydrated P-O-P bonds and led to the formation, mainly, of pyrophosphate groups. The low melting point of Pb-O makes it possible to play an important role, at the same time, on the viscosity, on the equilibrium between the vitreous bath and the crystallites formed. The dissolution rate obtained is 100 times smaller than that of silicate glasses used as an alternative form for the vitrification of radioactive waste.