Studies on the Water-settable Cement

As has been indicated in a previous report [1], the authors carried out a detailed measurement of physical properties during the setting procedures of zinc phosphate cement and established the facts that 1) an increase in powder-to-liquid ratio will result in the retardation of setting speed and 2) although changes in an enviromental temperature are not directly linked to either retardation or promotion of speed in any regular manner, there is found a peculiar phenomenon in the neighborhood of 27°C. Since this is similar phenomenon which takes place in the setting mechanism of alginate impression material studied by HIGASHI [2], it provided a basis for our inference that the setting of this kind of cement may be explained as due to the gelation of colloidal particles. As for changes in pH readings, they increased in proportion to a rise in temperature but its manner of rise was somewhat checked in the neighborhood of 27°C. On the other hand, although the internal temperature of cement owing to heat generation was in parallel to that of external or environmental temperature, nothing peculiar was observed around 27°C.As the setting of this kind of cement has been hitherto ascribed to the crystallization of phosphate salt through a chemical reaction of zinc oxide with phosphoric acid, the above inference of ours may prove significant but, before it is fully substantiated, we still need many other items by way of irrefutable evidence. Along this line of thinking, our effort in this part of studies was directed to possible influence of water to be added to the reaction with specific reference to the setting speed.