测定石膏粉的结晶水含量

Determination of crystal water content of gypsum powder

济南石膏粉是一种结晶混合物，含有生石膏、半水合物和硬石膏（超煅石膏）。在炒制过程中，在火下会出现石膏，或者火太大、炒制时间过长会出现无水石膏，影响石膏的凝固时间，影响生产的正常使用。有时由于石膏粉放置时间过长受潮，也会影响正常使用。

Jinan gypsum powder is a crystalline mixture containing raw gypsum, hemihydrate and anhydrite (super calcined gypsum). In the process of frying, gypsum will appear under the fire, or anhydrous gypsum will appear if the fire is too large and the frying time is too long, which will affect the setting time of gypsum and the normal use of production. Sometimes the gypsum powder will be affected by damp due to its long storage time, which will also affect its normal use.

据经验和资料介绍，石膏粉的好坏可以通过测定结晶水的含量来判断。一般合格石膏粉的结晶水含量在4.5-5.5%之间。如果石膏粉的结晶水含量远低于这个标准值，说明石膏粉过烧，注塑时凝固时间过长甚至不凝固；石膏粉的结晶水含量远高于这个标准值，说明石膏粉在火或潮湿下，注塑成型时凝固时间短。以上两种情况的石膏粉不合格，不能用于生产。

According to experience and information, the quality of gypsum powder can be judged by measuring the content of crystal water. Generally, the crystal water content of qualified gypsum powder is between 4.5-5.5%. If the crystal water content of gypsum powder is far lower than the standard value, it indicates that the gypsum powder is burned, and the solidification time is too long or even does not solidify during injection molding; The crystal water content of gypsum powder is much higher than this standard value, which indicates that the solidification time of gypsum powder during injection molding is short under fire or humidity. The gypsum powder in the above two cases is unqualified and cannot be used in production.

石膏粉结晶水含量的检测方法如下：精密称取试样质量G（一般为2-5g），经40℃干燥后冷却至室温，置于铂坩埚（或瓷坩埚）中)恒重。准确称量其质量为G1；在230±5℃下烧2h，取出转移到干燥器中，冷却至室温后称重质量为G2，其结晶水百分比为（Gt-G2）/GX100%。

The test method for the crystal water content of gypsum powder is as follows: accurately weigh the sample mass g (generally 2-5g), dry it at 40 ℃, cool it to room temperature, and place it in a platinum crucible (or porcelain crucible) for constant weight. Accurately weigh its mass to G1; Burn it at 230 ± 5 ℃ for 2h, take it out and transfer it to a dryer. After cooling to room temperature, the weight is G2, and the percentage of crystal water is (gt-g2) /gx100%.

测定石膏粉的凝结时间

Determination of setting time of gypsum powder

济南石膏粉的凝固时间包括初凝时间和终凝时间。标准稠度下，一般初凝时间不小于6min，终凝时间不大于25min。使用中初凝时间过短，小于6min，说明石膏粉未熟，或潮湿中含有生石，影响注塑操作，表现为生产时注桶，延误生产;如果终凝时间过长，远大于25tnin，说明石膏在油炸过程中过烧，其中含有过烧石膏，在注塑成型时容易漏浆，甚至石膏浆也会漏浆。不凝固，影响生产。

The setting time of Jinan gypsum powder includes initial setting time and final setting time. Under standard consistency, generally, the initial setting time is not less than 6min, and the final setting time is not more than 25min. The initial setting time in use is too short, less than 6min, indicating that the gypsum powder is not mature, or there is green stone in the wet, which affects the injection molding operation, which is manifested as barrel injection during production, delaying production; If the final setting time is too long, which is far greater than 25tnin, it indicates that the gypsum is burned during the frying process, which contains burned gypsum. It is easy to leak slurry during injection molding, and even the gypsum slurry will leak slurry. No solidification, affecting production.

两种情况下的石膏粉都不能用于生产。因此，凝固时间的确定可以判断石膏粉的质量，也可以决定石膏粉能否用于生产。简单的测量方法如下

In both cases, gypsum powder cannot be used for production. Therefore, the determination of setting time can determine the quality of gypsum powder and whether gypsum powder can be used in production. Simple measurement methods are as follows

(1)按制得的膏体与水的比例称取100ml水所需的石膏粉试样，将水倒入500ml容器中，1min内将试样均匀洒入水中，静置30s，快速搅拌30s，立即将石膏浆倒入放置在玻璃板上的铜管中，然后将铜管慢慢垂直向上提起20mm，使石膏浆压平成约5mm厚的试饼；

(1) Weigh the gypsum powder sample required for 100ml of water according to the ratio of the prepared paste to water, pour the water into a 500ml container, evenly sprinkle the sample into the water within 1min, stand for 30s, quickly stir for 30s, immediately pour the gypsum slurry into the copper tube placed on the glass plate, and then slowly lift the copper tube vertically upward for 20mm to flatten the gypsum slurry into a test cake about 5mm thick;

(2)用直径约5nun的玻璃棒切试饼，每30s一次，每划一次后用布擦拭玻璃棒。当划痕两侧的浆液刚刚不再合拢时，即为初凝状态。试样放入水中至初凝的时间间隔为初凝时间；

(2) Cut the test cake with a glass rod with a diameter of about 5nun, once every 30s, and wipe the glass rod with a cloth after each scratch. When the slurry on both sides of the scratch is no longer closed, it is in the initial setting state. The time interval between putting the sample into water and initial setting is the initial setting time;

(3)初凝后的试饼，用大拇指以约50N的力连续按压，间隔15s，当压印边缘无水分时，即为终凝。终凝时间是从样品开始入水到终凝的时间间隔。

(3) For the test cake after initial setting, use your thumb to press it continuously with a force of about 50N at an interval of 15s. When there is no water at the impression edge, it is the final setting. The final setting time is the time interval from the beginning of the sample entering the water to the final setting.

根据检测结果与质量标准或石膏粉在正常规模化生产中的凝固时间值的比较，可以判断石膏粉的质量是否可以用于生产。

According to the comparison between the test results and the quality standards or the setting time of gypsum powder in the normal large-scale production, we can judge whether the quality of gypsum powder can be used in production.

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