Initial and final setting time of cement

Introduction

Cement is one of the most consumed materials in the world. Because of the great importance of cement in the construction uses, we must conduct the necessary tests that will enable us to know the characteristics of the cement and the extent to which the results fit the standard specifications that must be available in the cement. One of the most important tests is to find the ideal ratio of water and know the initial and final setting time of cement. This test is of major importance in the identification of time that allows us to use the cement before it solidifies through or start losing its role.


Fresh concrete

Objectives

To determine the ideal water percentage of cement.
To determine the initial and final setting time of a sample of cement.


Theory

Cement paste setting time is affected by a number of substances including: cement fineness, w/c ratio, contents of chemicals (especially gypsum content) and admixtures in the paste. Setting tests characterizes how a specific cement paste sets. The initial set must not be too quickly, so that the workers in the site can cast the concrete before it start the be hard to work with, and the final set must not be too late, for construction purposes. Additionally, setting times can give indication of undertaking normal hydration of the concrete. Normally, two setting times are defined:
  1. Initial set: The point when the paste begins considerably to stiffen.
  2. Final set: When the cement has hardened at which it can take some load at this point.
These particular times are just subjective points used to characterize cement; they do not have any ultimate chemical significance. The most common setting time test is the Vicat needle which states initial set and final set based on the time at which a needle of specific size and weight either penetrates a cement paste sample to a given depth or doesn’t penetrate a cement paste sample.

Table A shows ASTM specified set times.

Table A: ASTM Specified Set Times by Test Method
Test Method
Set Type
Time Specification
Vicat
Initial
45 minutes
Final
375 minutes


The standard setting time tests are according to AASHTO T 131 and ASTM C 191


Apparatus

  1. Sample of cement
  2. Sample container
  3. Sensitive balance
  4. Water
  5. Trowel
  6. Vicat needle with the following apparatus:
  • Reversible stainless steel plunger (300gm) with 10mm diameter on one end and threaded 1mm diameter stainless steel needle on the other.
  • Graduated 0-50mm scale includes frame, graduated 0-50mm scale, plunger assembly, conical mold, and glass plate.


Procedure

Determination of the ideal water percentage

  1. A cement paste was prepared by gauging a 400gm sample of cement with 30% water percentage of cement for 4 minutes.
  2. The Vicat mold was completely filled with the cement paste. The surface of the paste was smoothed off making it level with the top of the mold. The cement block thus prepared in the mold was the test block.
  3. The block (test block) was positioned under the rod bearing with the 10mm diameter needle. The needle was dropped softly till it contact with the surface of the paste and it was released quickly, allowing it to penetrate the test block, and then the measured displacement was taken.
  4. This procedure was followed 4 times, each trial with different water percentage 30%, 31%, 31.5%, 32% .

Determination of the initial setting time

  1. A cement paste was prepared by gauging the cement with the ideal water percentage required to give a paste of standard consistency.
  2. A stop watch was started the moment the water was added to the cement. 
  3. The Vicat mold was completely filled with the cement paste, the mold resting in a plate which is non-porous, and the surface of the paste was leveled off by smoothing making it level with the top of the mold.
  4. The block was positioned under the rod bearing the Vicat needle. The needle was dropped softly till it contact with the surface of the paste and then released quickly, allowing it to penetrate the test block.
  5. The same procedure was repeated every hour till the needle succeeded to pierce the test block to a point 5-7mm measured from the bottom of the mold. The time period which passed between the time water was added to the paste and the time the needle succeeded to penetrate the test block by 5-7mm measured from the bottom of the mold, was the initial setting time.

Determination of the final setting time

  1. The needle which used was repositioned  by another one with an annular attachment.
  2. The test was positioned under the rod bearing the Vicat needle. The needle was dropped softly in order to make contact with the surface of the cement paste and then released quickly, allowing it to penetrate the test block.
  3. The cement is considered as finally set when, upon applying the needle softly to the surface of the test block, the needle made an impression therein, while the attachment failed to do so. The period elapsing between the time, water was added to the cement and the time, the needle made an impression on the surface of the test block, while the attachment failed to do so, was the final setting time.


Data and Calculations

Determination of the ideal water percentage

Table (1) below shows the data collected to determine the ideal percentage of water.

Table (1): Data collected to obtain the ideal percentage of water
Water percentage  (%)
Displacement from the bottom (mm)
30%
31
31%
11
31.5%
7
32%
6

Weight of Cement sample= 400g
Weight of Water added= 128g

Percentage water to cement by weight can be calculated using this equation:

 

Graph (A) below shows the plot of the displacement measured from the bottom of the mold (mm) vs. the percentage of water (%).

Graph -A: displacement measured from the bottom of the mold (mm) vs. the percentage of water (%)


Determination of the initial and final setting time

The time when the water was added to the cement was 8:39 am
The time when the initial set occurred was 12:00 pm
The time when the final set occurred was 1:50 pm
Initial time setting = 3:21 hours 
Final time setting = 5:31 hours


Results and Conclusion

Table (1) shows the results obtained for the displacements of the needle measured from the bottom of the mold when different percentages of water were used in order to obtain the ideal percentage of water.
Graph (A) is a plot of the displacement of the needle vs. the percentage of water.

As seen from the graph, the displacement of the needle measured from the bottom of the mold lies in the standard range (5-7) when the ideal percentage of water is 32%. The initial and final time settings are, 3:21 hours and 5:31 hours, respectively, and these values are allowed by the applicable specifications for Portland cement.


Engineering Use

It is important to identify the time which allows us to use the cement before it solidifies or starts losing its usability. Setting time plays an important role in controlling the impact of time of the transfer of concrete from the plant to the work site and the validity of concrete as long as the expiration of a period of time longer than the setting time during transfer process makes concrete not valid for use because of intransigence.

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