Erosion, ion release and water absorption

Although the glass polyalkenoate cement is the most durable of all dental cements it is susceptible to attack by aqueous fluids under certain conditions. There are three related phenomena to consider erosion, ion release and water absorption. 

When fully hardened, the cement is resistant to erosion provided the solution has a pH above 4. However, the glass polyalkenoate cement is susceptible to erosion immediately after set because some of the matrixforming cations and anions are still in soluble form. In fact, the hardening process is one where these cations and anions continue to precipitate. For this reason these cements have to be protected, temporarily, by a varnish. 

When immature glass polyalkenoate cements are exposed to neutral solutions, such as normal saliva, they release ions and absorb water. The 

As the cement ages, absorption of water and loss of aluminium ions ceases (after 7 days). Other species - sodium and fluoride ions and silicic acid - continue to be eluted. The release of fluoride is important, for the glass polyalkenoate cement can be seen as a device for its sustained release. 

The three terms correspond respectively to initial washout, diffusion and erosion. Unfortunately, although the mathematical fit was good (c. 99-9 %), C and B proved to be negative, making it difficult to assign a physical meaning to the equation. 

---