A accurate method for determining the setting time of cement
Setting time is an important index of cement, which can not only reflect whether the quality of cement meets the technical requirements of cement product standard, but also provide necessary information for the user to decide the construction progress.
The determination of setting time depends on the mixing of cement slurry, the preparation of specimen and the determination of setting time.
1.Mixing of cement slurry.
Strictly follow the operating procedures.Mixing pot and mixing wings should be wiped with a wet cloth, to prevent cement and water splashing out of the pot, stirring in strict accordance with the slow 120 seconds, stop 15 seconds, fast 120 seconds of the mixing process, can not shorten the mixing time, static slurry to stir evenly.The slurry used for setting time must be of standard consistency.In order to reduce the error of setting time, it is better to control the sinking depth of each thickener at a fixed value.
2.Preparation of two specimens.
Fill up the mold with the standard consistency static slurry, insert the knife and vibrate for several times, then smooth it out.Scraping usually should pay attention to wipe times not too much, or there will be moisture exudation;Also be careful to wipe the plane horizontally.
3.Determination of setting time.
The specimen shall be cured in the humidity curing box, and the temperature and humidity shall be strictly controlled.The first measurement shall be made after curing for 30 minutes after adding water.The metal rod of the thickening apparatus should be gently supported when being measured so as to make it drop slowly to prevent the test needle from bending, but the final result obtained by free fall shall prevail.During the test, the test needle should be at least 10mm away from the inner wall of the test mold. The test needle should not fall into the original hole after each test.
After each measurement, the round die shall be put back into the curing box and the test needle shall be wiped clean.The circular die shall not vibrate during the measurement.When the initial coagulation was near, the determination was made every 5 minutes.When the final coagulation is approaching, the determination should be made every 15 minutes, and the possible time of the two times of coagulation should be mastered. When the initial or final coagulation is approaching, the interval of the two times of determination should be shortened to avoid missing the "real" moment.
When the initial or final coagulation is reached, the measurement shall be repeated once immediately. Only when the conclusions of the two tests are the same, can the initial or final coagulation be determined to be reached.In judging whether the cement is the initial setting or the final setting, we should follow the consistent principle of the judgment standard for each experiment: the standard stipulates that the initial setting time can be reached when the measuring instrument test needle distance is 4 1mm, and it is better for us to judge by 4mm each time, so as to reduce our personal error.
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