Notes on the Effects of the Phosphates to the Setting of Calcined Plastar

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The setting time of the calcined plaster is changed by the phosphates in the setting water. In the present paper, these effects by ortho-, pyro-, meta-, and polyphosphate, the adsorption of these phosphates to the plaster, and the chemical change of these absorbed phosphates were examined by optical and chromatographical analysis. Some phosphate retard the setting strongly, but do not retard_the time between initial setting and final setting. From these effects, phosphates can be classified into 3 classes as follows. (1) Polyposphates retard the setting strongly and fall down the compression strength. The longer the molecule is the stronger the effect is. (2) Phosphoric acid and weak basic orthophosphates offect weakly, and have the maximam effective concentration. (3) Metaphosphates have little effects to the setting time and to the compression strength. The polyphosphates changed to the ortho-, or pyrophosphate early at the setting, but the metaphosphates remained unchanged. Acidity of these setting water shorten the setting time and fall down the compression strength.
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