SETTING TIME OF CEMENT
When water is added to
cement, the ingredients of cement react chemically with the water and form a
complicated chemical compound. The mixing of cement with water results in a
sticky cement paste and it gradually goes on thickening in course of time. It
is found that ordinary cement achieves 70 per cent of its final strength in 20
days and 90 per cent in 1 year or so.
The
time of setting is greatly influenced by the following factors:
1. The temperature at which the cement paste is allowed
to set.
2. The percentage of water mixed with cement in making the
paste.
3. The humidity at which the setting is allowed.
Setting time is distinguished into initial setting time
and final setting time on the basis of the time taken by the test specimen to set
to a specified minimum depth.
A Vicat needle apparatus is used for the determination of setting time (Figure).
 |
| Vicat needle apparatus |
Apparatus
1. It consists of a frame with a movable rod fitted with
a cap.
2. A needle of a 1 mm square cross-section is attached to
the lower end of the rod for the determination of the initial setting time. The
total weight of the rod along with the needle is 300 g.
3. Another needle like the above mentioned but with a
hollow metallic attachment with a circular cutting edge of 5 mm diameter and has
a 0.5 mm projection at the end is used to determine the final setting time.
4. A standard Vicat mould in which the specimen is
allowed to set.
Initial setting time – a procedure
a. Take
300 g by weight of cement and mix with 0.85 times the water required to give a
paste of standard consistency.
b. Start
the stopwatch at the instant water is added to the cement.
c. Fill
the Vicat mould with the cement paste and smooth the surface.
d. Place
the square needle of cross-section 1 mm to the moving rod of the Vicat
apparatus.
e. Lower
the needle gently bringing it in contact with the surface and quickly release
allowing it to penetrate the paste.
f. In
the beginning the needle will completely pierce the test block. Repeat the
procedure in a fresh place until the needle, when brought in contact with the
test block and released fails to pierce the block for 5 mm measured from the
bottom.
The initial setting time is the interval between the
addition of water to the cement and the stage when the needle fails to pierce
the test block for 5 mm measured from the bottom.
Final Setting Time – Procedure
a. Replace
the needle for the initial setting time by the needle with an annular attachment
for the final setting time.
b. The
cement shall be considered as finally set, when upon applying the needle gently
to the surface of the test block, the needle makes an impression thereon while
the attachment fails to do so.
The final setting time is the interval between the
addition of water to the cement and the time at which the needle makes an
impression while the attachment fails to make an impression on the surface of
the test block. The following table shows the initial and final setting time of
various grades of cements.
Table:
The Initial and Final Setting Time of Various Grades of Cement
|
Type
of cement |
Initial
setting time |
Final
setting time |
|
1. Ordinary |
It shall not be less than 30 minutes. |
It shall not be more than 10 hours. |
|
2. Rapid hardening |
It shall not be less than 30 minutes. |
It shall not be more than 10 hours. |
|
3. Low heat |
It shall not be less than 60 minutes. |
It shall not be more than 10 hours. |
If you find
This information helpful, please share it.
Thanks! For reading the article.
No comments:
Post a Comment