Which of the following is the least preferred method for increasing the working time of ZnPO4 cement?
Correct Answer: The operator can prolong the spatulation of the last increment of powder
Description: The operator can prolong the spatulation of the last increment of powder. The spatulation effectively destroys the matrix as it forms, which means that extra time is needed to rebuild the bulk of the matrix. This is not a preferred method for extending the working time.
First, the P/L ratio can be reduced to produce a thinner mixture. However, this change will result in a lower initial pH of the cement and it will adversely affect the mechanical properties of the cement.
Second, smaller portions of powder should be mixed for the first few increments. Initial small increments of powder dissolve in the liquid, which reduces the acidity of the liquid and retards the reaction rate with subsequent increments.
Third, the operator can prolong the spatulation of the last increment of powder. The spatulation effectively destroys the matrix as it forms, which means that extra time is needed to rebuild the bulk of the matrix.
Fourth, a colder mixing temperature will retard the chemical reaction between the powder and the liquid, thereby delaying formation of the matrix.
Ref: Phillips Ed-12th P: 317
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