In production, the quality of gypsum mold is difficult to control stably because of the inherent variability of gypsum mold. One aspect of this variability is the influence of paste water ratio and slurry concentration on the performance of gypsum model. Paste water ratio is generally small, porosity is high, strength is low, service life is short. The paste has high water ratio, low porosity, high strength and long service life. The influence of the change of paste water ratio is related to the aging time of gypsum powder, especially the difference between the newly baked gypsum powder and the fully aged gypsum powder. The newly fried gypsum powder has not been metamorphosed and homogenized yet. It has high temperature, strong surface activity and large mixing water. When it is used to make the model immediately, many bubbles will form on the model surface, and the overall strength is low. Generally speaking, the newly fried gypsum powder needs to be stored for two weeks before reuse.

　　國外學者的研究結果表明，石膏粉的含水率和流變特性隨貯存時間的變化而變化，這是由于空氣中石膏的老化過程造成的。該過程可分為兩個階段:首先，將可溶性硬石膏(一種無水石膏)快速水化成半水合物;從那以后，半水石膏吸收水的速度非常慢，使晶體完整，導致混合水的量逐漸減少。當空氣相對濕度較低，水溶性硬石膏反應完全時，石膏粉可達到一定的含水量。如果空氣相對濕度較高，則水化反應繼續，會形成二水石膏，對石膏粉質量和生產穩定性控制帶來不利影響。一般來說，企業在石膏粉的儲存和使用前的性能測試，所以可以判斷石膏粉的模具要求，同時根據石膏粉的性能調整石膏的生產模具參數，達到模具的要求。

　　The research results of foreign scholars show that the moisture content and rheological properties of gypsum powder change with the change of storage time, which is caused by the aging process of gypsum in the air. The process can be divided into two stages: first, the soluble anhydrite (a kind of anhydrous gypsum) is rapidly hydrated into a hemihydrate; since then, the rate of water absorption of hemihydrate is very slow, making the crystal complete, leading to the gradual reduction of the amount of mixed water. When the relative humidity of air is low and the reaction of water-soluble anhydrite is complete, the gypsum powder can reach a certain water content. If the relative humidity of the air is high, the hydration reaction will continue, and dihydrate gypsum will be formed, which will bring adverse effects on the quality of gypsum powder and production stability control. Generally speaking, the enterprise's performance test before the storage and use of gypsum powder can judge the mold requirements of gypsum powder, and adjust the production mold parameters of gypsum according to the performance of gypsum powder to meet the mold requirements.

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