pH of fully set zinc phosphate:

The phosphoric acid in the liquid makes the mixture quite acidic and therefore cytotoxic when a prosthesis is luted with this cement on a prepared tooth. As setting occurs, the acidity is partially neutralized, but the cement remains acidic after 24 hours, with the pH rising from about 3 to about 6. For thin layers of dentin, a cavity liner (i.e., Ca2) is recommended to prevent an adverse pulpal response from the pressure of luting, which forces acid into the pulp tissue. Younger patients are especially susceptible because they have a more open dentin tubule area, whereas older patients with sclerotic dentin will have a more tortuous path that restricts the penetration of acid toward the pulp. Ref: Phillips Ed 12th P: 318