Influence of glass composition on the properties of glass polyalkenoate cements. Part II: influence of phosphate content.

The influence of phosphate content of the glass on the formation of glass polyalkenoate cements was investigated. Glasses were synthesised based on (4.5 - 2X)SiO2-3.0 Al2O3-(3.0 - X)CaO-(1.5 + X)P2O5-2.0 CaF2 and X was varied from -1.5 to 0.8. The setting and working time of the cement pastes increased with the phosphate content of the glass (X). Increasing the phosphate content resulted in an initial increase in compressive strength followed by a sharp reduction in strength. Young's modulus and un-notched fracture strength exhibited a maximum at intermediate phosphate contents. Fracture toughness reduced at high phosphate contents, whilst toughness increased. Phosphate in the glass is thought to aid glass degradation by providing additional phosphorus-oxygen bonds for hydrolysis, but may also reduce the amount of aluminium released by reducing the susceptibility of aluminium-oxygen-silicon bonds to acid hydrolysis. The released phosphate may also compete with the carboxylate groups in the polysalt matrix cement for cations inhibiting the crosslinking reaction.