The simple model of microstructure development outlined in Section 10.2.3 suggests a mechanism whereby the cement and polymer particles cure indepen- dently of each other producing a physical binding of the cement hydrates and aggregates by a distinct polymer film. In practice the situation is much more complex as discussed in the section on hydration kinetics and microstructure.
Polyvinyl acetate (PVA) offers very poor water resistance in that, when the PMC is immersed in water, it swells and undergoes partial alkaline hydrolysis (equation 10.2) to give a water soluble polyvinyl alcohol and acetic acid or calcium acetate.
Such reactions can lead to significant loss in strength after only a year in natural weathering situations. In an effort to overcome this problem, PVA has been copolymerised with ethylene to give EVA. Even then it has been found (Silva, 2002) that the acetate groups of EVA are capable of undergoing alkaline hydrolysis, interacting with calcium ions from the paste to form an organic salt, calcium acetate. SEM investigation of EVA modified, compared to unmodified, pastes showed that the calcium hydroxide content was decreased, ettringite crystals appeared to be well formed and many Hadley’s grains were observed. These observations are consistent with retardation of the hydration process. In addition a calcium-rich porous phase similar to calcium hydroxide in structure was observed, the porosity being attributed to attack by the acetic acid generated during hydrolysis.
During the 1970s chlorinated polymers such as polyvinyl di-chloride (PVdC) were popular because of their very good mechanical properties. However, it quickly became apparent that they could cause accelerated corrosion of reinforcing steel. Pore solution extraction and analysis showed, (Larbi, 1990) that, over time, an increase in the ratio of [Clÿ]:[OHÿ] ions occurred. An ion- exchange reaction occurs between hydroxyl ions in the pore solution and the chloride bound to the polymer (equation 10.3) giving a hydroxylated polymer and chloride ions in the pore solution.
More recently ter-polymers consisting of n-butyl acrylate, methyl acrylate and a `functional monomer’ have become available. The nature of this func- tional monomer is not clear but comparison with similar dental cements suggests that it may be based on an acrylic (or similar) acid which should be capable of bonding strongly (chemical anchors) to cement particles (Shaw, 1989).
