How to accurately determine the cement setting time

The setting time is an important indicator of cement, which not only reflects whether the cement quality meets the technical requirements stipulated in the cement product standard, but also provides the necessary information for the use unit to decide on-site construction progress. How to accurately determine the setting time of cement has always been a problem we have explored.

The key to the determination of the setting time is the mixing of cement slurry, the preparation of test pieces and the determination of the setting time.

A cement slurry mixing.

We must operate strictly in accordance with the operating procedures. Stir the pan and stir wings with a damp cloth to prevent the cement and water from spilling out of the pan. Stirring should be performed strictly in accordance with 120 seconds of slow, 15 seconds of stop, and 120 seconds of rapid mixing. Do not arbitrarily shorten the mixing time. Pulp to stir evenly. This requires that we must check whether the instrument is normal before the experiment. If the mixing wings are off, the mixed slurry will be uneven, and the condensation time will not be accurate. The slurry used to set the time must be cement slurry at standard consistency. In order to reduce the condensation time error, it is best to control the depth of sinking of each coagulation instrument at a fixed value, such as this is 28mm, the next time you do it, it is still controlled at 28mm, because the amount of water added Directly affects the setting time, which reduces system errors.

The preparation of the second test piece.

The standard consistency of the prepared slurry was filled with test molds at one time, and the jigs were inserted and shaken several times with a knife to level off. Scratch should pay attention to wipe the number of times not too much, otherwise there will be water seepage, so that the consistency of the upper and lower slurry is different; but also pay attention to leveling the plane, not one side high, one low. This will affect the determination of the setting time.

Determination of the three setting times.

The test specimens are maintained in a moisture conservation tank, and the temperature and humidity must be strictly controlled. The temperature and humidity have a greater influence on the setting time. The first determination was carried out until 30 minutes after the addition of water. The metal rods of the Condensing Instrument should be lightly assisted when measuring, so that they can be lowered slowly to prevent the test needle from hitting the bend. However, the result of the free fall measurement is still the last. The position where the test needle sinks is at least 10 mm from the inner wall of the test mold, and the test needle must not be allowed to fall into the original hole. After each determination, the round mold should be placed back in the curing box and the test needle wiped. The round mold should be free from vibration during the measurement. Near the initial setting, it is measured once every 5 minutes; near the final setting, it is measured every 15 minutes, and the time when the two setting times may occur is grasped. When the initial setting or final setting is approached, it is shortened by 2 times. Determine the interval so as not to miss the "real" moment. When the initial coagulation or final coagulation is reached, the measurement should be repeated once. When the two conclusions are the same, it can be determined as the initial coagulation or final coagulation state. When judging whether the cement is initial setting or final setting, we still have to follow the principle of consistent standards for each experiment: The standard specifies that the tester needle distance is 4±1mm, the initial setting time is reached, and it is best to press it when we experiment. Judging by 4mm, it is judged every time by 4mm, which can reduce our individual error and form systematic errors. Otherwise, if this press 3mm control, next press 5mm control, the condensation time between the two is sure to be different, the error when measuring, the error when mixing the slurry, the error when the sample preparation, more Accumulated, the error is great.

Therefore, in order to accurately determine the setting time, we must strictly follow the national standards, unified operating practices, more training, more practice. Strictly control the experimental conditions to meet the requirements and strengthen the inspection and correction of experimental instruments.


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