The influence of mixing ratio on the toughening mechanisms of a hand-mixed zinc phosphate dental cement

Objective: A range of mixing ratios occur for zinc phosphate cements in clinical usage and trials. The compressive strength of these cements is dependent on mixing ratio and at a critical ratio, a sudden rise in compressive strength is known to occur. This study investigated the mechanisms behind the cement curve behavior in an attempt to explain the strength increase with mixing ratio. Methods: Crack propagation from indentations produced in cements were examined to identify if any variation in toughening mechanism existed over the range of mixing ratios investigated (1.7–3.2 g/ml). Pore distribution within the cylindrical specimens was determined using an image analysis technique. Results:Increasing the powder content from 2.3 to 2.4 g/ml increased the number of powder agglomerates formed in the cement mix over individual particles. The likelihood of introducing pores in excess of 42 μm diameter was enhanced when the ratio exceeded 2.6 g/ml. Significance: It is proposed that crack deflection by agglomerates in the cement reduces the energy of the crack fronts emanating from indentations more than would occur with individual powder particles. This decreases the cracks ability to progress and is likely to result in the marked strength increase from 2.3 to 2.4 g/ml identified previously. It is suggested that cement pores in excess of 42 μm diameter were probably generated on filling the sample molds with more viscous cements. These results emphasise that the properties of cements manipulated under optimum conditions provide little information on the cement characteristics present in clinical practice.