A preliminary investigation of glass polyalkenoate cements based on waste gasifier slags

Abstract

Glass polyalkenoate cements (GPC) used for dental and biomedical applications, are formed by reacting aqueous poly(acrylic acid) (PAA) with specially synthesized calcium aluminosilicate glasses, generally containing fluoride [1]. The acid degrades part of the glass network releasing metal cations, typically Ca2+ and Al3+. The cations subsequently serve to form salt bridges between the polyacid chains and result in the formation of a hard cement [2]. The setting reaction is shown schematically in Fig. 1. One of the most important properties of the glass component is its susceptibility to acid attack . This is determined by the number of hydrolyzable bonds present in the glass network and in particular the silicon to aluminum ratio. The glass can be thought of as consisting of linked [SiO4] tetrahedra and [AlO4] tetrahedra. Since an Si4+ ion is being replaced by an Al3+ ion, the network will acquire a negative charge. This is balanced by a positive charge of a network dwelling cation, for example Ca2+. It has been found that to obtain four coordinate aluminum, the calcium to aluminum ratio must be greater or equal to than 1 : 2 and the silicon to aluminum ratio must be greater than 1 : 1. The oxygen linkage between adjacent aluminum and silicon tetrahedra is vulnerable to acid attack. The difference in electronegativity between the Al3+ ion and that of the Si4+ ion results in a sufficient residual polarizability of the aluminum oxygen silicon bond to be susceptible to acid attack. The degradation mechanism of the glass network by acid is shown schematically in Fig. 2. Wilson et al. [3] and Hill et al. [4] have reviewed a wide range of simple calcium aluminosilicate glasses and related the chemical compositions to their acid degradability and the mechanical properties of the resulting polyalkenoate cements. These studies provided strong evidence that the aluminum to silicon ratio is a dominant factor determining cement properties in simple CaO-Al2O3-SiO2 glasses. Hill et al. [4] specified a Si : Al close to one, a Ca : Al ratio of 1 : 2 and a silica mole fraction of less than 0.6 for glass polyalkenaote cement formation. Rodgers et al. [5] highlighted the similarity between glass polyalkenoate cements (GPC) with “macro defect free cements” based on polyacrylamide. Hydroxyl ions present from the lime in ordinary portland cement (OPC) catalyze the partial hydrolysis of the amide functional groups in the polyacrylamide, giving rise to car-