This is the term used to describe the stiffening of the cement paste. Broadly speaking, setting refers to a change from a fluid to a rigid state. Setting is mainly caused by a selective hydration of C3A and C3S and is accompanied by temperature rises in the cement paste; initial set corresponds to a rapid rise and final set corresponds to the peak temperature. Initial and final sets should be distinguished from false set which sometimes occurs within a few minutes of mixing with water (ASTM C 451-05). No heat is evolved in a false set and the concrete can be re-mixed without adding water. Flash set has previously been mentioned and is characterized by the liberation of heat.
For the determination of initial set, the Vicat apparatus is again used, this time with a 1 mm (0.04 in.) diameter needle, acting under a prescribed weight on a paste of standard consistence. When the needle penetrates to a point 5 mm (0.2 in.) from the bottom of a special mould, initial set is said to occur (time being measured from adding the mixing water to the cement). A minimum time of 45 min is prescribed by BS EN 197-1 for cements of strength classes 52.5 N and 62.5 N whereas 60 minutes applies to strength classes of 32.5 N and R and 42.5 N and R.
A similar procedure is specified by ASTM C 191 04b except that a smaller depth of penetration is required; a minimum setting time of 60 min is prescribed for Portland cements (ASTM C 150-05).
Final set is determined by a needle with a metal attachment hollowed out so as to leave a circular cutting edge 5 mm (0.2 in.) in diameter and set 0.5 mm (0.02 in.) behind the tip of the needle. Final set is said to have occurred when the needle makes an impression on the paste surface but the cutting edge fails to do so. British Standards prescribe the final setting time as a maximum of 10 hours for Portland cements, which is the same as that of the American Standards. An alternative method is that of the Gillmore test, as prescribed by ASTM C 266-04.
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SOUNDNESS
The initial and final setting times are approximately related:
final time (min.) = 90 + 1.2 [initial time (min.)]
(except for high alumina cement). Since temperature affects the setting times, BS EN 196-3: 1995 specifies that the mixing has to be undertaken at a temperature of 20 + 2 °C (68 ± 4 °F) and minimum relative humidity of 65 per cent, and the cement paste stored at 20 + 1 °C (68 + 2 °F) and maximum relative humidity of 90 per cent.
Soundness
It is essential that the cement paste, once it has set, does not undergo a large change in volume. One restriction is that there must be no appreciable expansion, which under conditions of restraint could result in disruption of the hardened cement paste. Such expansion may occur due to reactions of free lime, magnesia and calcium sulfate, and cements exhibiting this type of expansion are classified as unsound.
Free lime is present in the clinker and is intercrystallized with other compounds; consequently, it hydrates very slowly occupying a larger volume than the original free calcium oxide. Free lime cannot be determined by chemical analysis of cement because it is not possible to distinguish between unreacted CaO and Ca(OH)2 produced by a partial hydration of the silicates when the cement is exposed to the atmosphere.
Magnesia reacts with water in a manner similar to CaO, but only the crystalline form is deleteriously reactive so that unsoundness occurs.
Calcium sulfate is the third compound liable to cause expansion through the formation of calcium sulfoaluminate (ettringite) from excess gypsum (not used up by C,A during setting).
Le Chatelier's accelerated test is prescribed by BS EN 196-3: 1995 for detecting unsoundness due to free lime only. Essentially, the test is as fol-lows. Cement paste of standard consistence is stored in water for 24 hours.
The expansion is determined after increasing the temperature and boiling for 1 hour, followed by cooling to the original temperature. If the expan-sion exceeds a specified value, a further test is made after the cement has been spread and aerated for 7 days. At the end of this period, lime may have hydrated or carbonated, so that a second expansion test should fall within 50 per cent of the original specified value. A cement which fails to satisfy at least one of these tests should not be used. In practice, unsound-ness due to free lime is very rare.
Magnesia is rarely present in large quantities in the raw materials used for making cement in the UK, but in the US this is not the case. For this reason, ASTM C 151-05 specifies the autoclave test which is sensitive to both free magnesia and free lime. Here, a neat cement paste specimen of known length is cured in humid air for 24 hours and then heated by high-pressure steam (2 MPa (295 psi)) for about 1 hour so that a temperature of 216 °C (420 °F) is attained. After maintaining that temperature and
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pressure for a further 3 hours, the autoclave is cooled so that the pressure falls within 1.5 hours and the specimen is cooled in water to 23 °C (73 °F) in 15 min. After a further 15 min, the length of the specimen is measured:
the expansion due to autoclaving must not exceed 0.8 per cent of the ori-ginal length. This accelerated test gives no more than a broad indication of the risk of long-term expansion in practice.
No test is available for the detection of unsoundness due to an excess of calcium sulfate, but its content can be easily determined by chemical analysis.
Strength
Strength tests are not made on neat cement paste because of difficulties in obtaining good specimens and in testing with a consequent large variabil-ity of test results. Cement-sand mortar and, in some cases, concrete of prescribed proportions, made with specified materials under strictly Table 2.6: BS EN 197-1: 2000 and ASTM C 150-05 requirements for minimum
strength of cement (MPa (psi))
Age BS EN 197-1 : 2000 (mortar prism), , strength class (days)
(days)
32.5 N 32.5 R 42.5 N 42.5 R 52.5 N 62.5 R
2 _ 10 10 20 20 20
- (1450) (1450) (2900) (2900) (2900)
7 16 - - -
-(2300) - - - -
-28 32.5* 32.5* 42.5 42.5** 52.5 62.5
(4700) (4700) (6200) (6200) (7600) (9100) Age ASTM C 150-05 (mortar cube), cement type (Table 2.7)
(days) I IA II# IIA# III IIIA IV V
1 _ _ _ 12.0 10.0 _
- - - - (1740) (1450)
-3 12.0 10.0 10.0 8.0 24.0 19.0 - 8.0 (1740) (1450) (450) (1160) (3480) (2760) (1160)
7 19.0 16.0 17.0 14.0 - - 7.0 15.0
(2760) (2320) (2470) (2030) - - (1020) (2180) 28 28.0a 22.0a 28.0a 22.0" - 17.0 21.0
(4060) (3190) (4080) (3190) - - (2470) (3050)
* and not more than 52.5 (7600); ** and not more than 62.5 (9100)
* Strength values depend on specified heat of hydration or chemical limits of tricalcium silicate and tricalcium aluminate
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