Zinc Phosphate Cement — MFDS Part 1 MCQ

Correct answer: B — Acid–base reaction

Conventional zinc phosphate cement sets principally by an exothermic acid–base reaction. Its basic, predominantly zinc oxide powder reacts with the phosphoric acid-based liquid, producing an amorphous phosphate salt matrix around residual powder particles. The detailed chemistry includes interactions involving aluminium and phosphate ions, so it should not be reduced to one simple zinc oxide neutralisation equation. Addition and condensation polymerisation are mechanisms associated with polymer-forming materials, particularly resin systems, rather than zinc phosphate cement. Chelation is classically associated with materials such as zinc oxide–eugenol cement. Although phosphate-containing reaction products may precipitate during setting, precipitation is not the accepted overall classification of the setting mechanism. A cool mixing slab dissipates reaction heat and can extend working time.

Reference: Nicholson JW, Amiri MA. The interaction of dental cements with aqueous solutions of varying pH. Journal of Materials Science: Materials in Medicine. 1998;9:549–554. https://pubmed.ncbi.nlm.nih.gov/7881266/