石膏と石灰

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The additives which control the setting time of plaster of Paris have certain influences on the growth of dihydrate crystals, and generally lower the mechanical strength of set gypsum.
Authers examined these influences of some additives on the mechanical strength from the viewpoints of size and form of crystals at the time of initial set measured by optical and electron scanning microscope.
Accelerators such as Na₂SO₄ (1%), KNO₃ (1%), Na₂C₂O₄ (1%) and (NaPO₃) ₃ (0.02 %) crystallize out the shorter crystals than water does, and lower the strength. But dihydrate powder which has high accelerating power crystallize out very fine crystals and increase the strength of set gypsum.
Retarders such as Na₄P₂O₇ (0.05%), CaHPO₄ (0.02%), Peptone (0.1%), NH₄H₂PO₄ (0.1%), NaH₂PO₄ (1%), gelatione (0.1%) and Na-citrate (over 0.05%) crystallize out shorter crystals or very small crystals and lower the strength.
But the Na-citrate (less than 0.05%) accelerates the setting and increase the strength. This reason was discussed from the fact that addition of Na-citrate increased solubility of gypsum and the nucleation and growth of crystals were changed.

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