DISCUSSION OF CEMENT
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The standard consistency of a cement paste is defined as that consistency which will permit the vicat plunger to penetrate to a point 5 to 7mm from the bottom of the vicat mould. The apparatus used in this experiment called Vicat apparatus that function to determining the normal consistency and time of setting of portland cements that consists of a rod weighing 300 grams, having a needle in each end, and supported in a frame with a graduated scale to measure the distance to which the needle penetrates the cement and this test helps to determine the water content for other tests like initial and final settings time, and soundness and compressive strength.
In this experiment, the standard consistency we get as a result is 37.33%. We also have tried this experiment by using other amount of water such as 38%, 36% and 36.5% but
unfortunately all this amount fail to satisfy the right result to complete the experiment.
The precautions step that must be alert during conducting this test are the time of gauging should not be less than 3 minutes and not more than 5 minutes. Gauging time is the time elapsing from the time of adding water to the dry cement until commencing to fill the mould. The test must also be conducted at room temperature and there must not be any vibration on the working table to get better result. The plunger should be cleaned during every repetition.
SETTING TIME OF CEMENT
Setting time is the time required for stiffening process of cement paste to a defined consistency. Setting necessary not related to hardening because hardening actually a continuation of the chemical action which began with setting time.
Based on this experiment, we get the initial setting time for the cement is 0-45 minutes and the final setting time for the cement is 45-60minutes. There are many factors that affect the setting time such as composition, powder size and distribution, amount and nature of the gravel, water composition, temperature and air humidity.
Initial setting time is the time when the paste starts losing its plasticity. Initial setting time duration is required to delay the process of hydration or hardening. Final setting time is the time when the paste loses its plasticity. It is the time taken for the cement paste or cement concrete to harden sufficiently and attain the shape of the mould in which it is cast.
Determination of final setting time period facilitates safe removal of scaffolding and form. During this period of time primary chemical reaction of cement with water is almost completed.
FINENESS OF CEMENT
Fineness of cement is property of cement that indicate particle size of cement and specific surface area and indirectly effect heat of hydration. This method only indicates the amount of cement retained on the sieve, and gives no information on the size of grains smaller than 90 μm BS sieve. The specific surface is expressed as the surface area in sq. cm/g or sq. m/kg. The finer the cement, the more is the surface.
Based on the experiment, we determine that the average fineness of cement used is 5% as an average. For the Ordinary Portland Cement (OPC), the residue by weight on BS test sieve 90µm should not exceed 10%. From the test we identify that the cement is perfectly fresh therefore it can be used for making concrete where high strength is required.
For conclusion, standard consistency test can be conducted to measure the setting times of cement. Normal consistency of standard cement can be gained by using water and cement ratio of 37.33%. The initial setting time is 0-45 minutes and the final setting time needed is 45-60 minutes. While, initial setting time is that time period between the time water is added to cement and time at section needle fails to penetrate the cement paste, placed in the vicat mould from the bottom of the mould. Final setting time is that time period between the time water is added to cement and the time at which 1 mm needle makes an impression on the paste in the mould but 5 mm attachment does not make any impression.
For the standard consistency of cement experiment, we carried out that the amount of water needed to achieve an accurate result is 112 ml. As a result we get the standard consistency of cement is 37.33%.
The given sample of cement contains 5% residue which less than 10 % by weight of material used than 90µm sieve. Therefore it satisfied the criteria as satisfied by BS 12: Part 2: 1971. The error in this experiment can occur if we sieve the cement by wrong ways. For the accurate result we need to sieve the cement with the correct technique and suitable time.
There are many types of error that can be occurred during experiment such as parallax error, this error can occur whenever there is some distance between the measuring scale and the indicator used to obtain a measurement. If the observer’s eye is not squarely aligned with the pointer and scale, the reading may be too high or low. The calibration of an instrument should be checked before taking data. If a calibration standard is not available, the accuracy of the instrument should be checked by comparing with another instrument that is at least as precise, or by consulting the technical data provided by the manufacturer. When making a measurement with a micrometre, electronic balance, or an electrical meter, always check the zero reading first. Re-zero the instrument if possible, or measure the displacement of the zero reading from the true zero and correct any measurements accordingly.