Civil Engineering Reference
In-Depth Information
alumina modulus (MA= A/F) of the raw mix. The increase of short-term strength was
explained by a lowering of the C/S molar ratio in the alite formed (from 3.07 to 2.84),
which resulted in an increase of the overall alite content in clinker at the expense of
belite. Furthermore, under such conditions a fraction of the C
3
A present decomposes to
C
12
A
7
+C, and the released free lime reacts with belite to yield alite. In parallel, C
12
A
7
reacts with the aluminoferrite phase, increasing its A/F ratio and thus its reactivity:
(2.3)
Garbacik (1997) reported the possibility of producing a clinker with an elevated belite
content, which exhibits favorable strength properties, by burning at maximum
temperatures as low as 1250-1300 °C a raw mix containing chalky and marly starting
constituents and very fine crystalline reactive forms of silicates. The alite and belite
phases formed under these conditions exhibit very small dimensions of their crystals, and
are exceptionally reactive.
C
linkers with increased C
2
S and reduced C
3
S contents are employed in the production
of cements with reduced hydration heat evolution (see section 19), such as moderate heat
of hardening cement (corresponding to ASTM Type II cement) and low-heat cement
(corresponding to ASTM Type IV cement). To slow down the rate of heat evolution,
cements employed for this purpose are usually ground to a relatively low specific surface
area. Cements of this type are used in applications in which a reduced release of
hydration heat is required, as in the construction of dams and other bulk concrete
structures (Kelham and Moir, 1992; Sone
et al.,
1992). High-C
2
S clinkers, especially
those with a reduced C
3
A content, are also constituents of some oil well cements (see
section 27). Low-C
3
S clinkers are less suitable for the production of blended cements,
owing to the reduced amount of free calcium hydroxide produced in the hydration of such
clinkers.
B
elite-rich clinkers (with C
2
S contents above 55%) have also been developed in which,
in addition to C
3
S, the amounts of C
3
A and C
4
AF have also been reduced significantly, to
less than 3% and 8% respectively (Okamura
et al.,
1998). Cements made from such
clinkers usually exhibit excellent fluidity; however, this depends greatly on the amount
and form of calcium sulfate present.
2.5
HIGH-C
3
A PORTLAND CEMENT
High-C
3
A Portland cement contains elevated amounts of tricalcium aluminate in its
clinker, compared with ordinary Portland cement. Such clinkers may be produced by
increasing the Al
2
O
3
content in the raw meal and thus lowering its silica modulus
, and by increasing the alumina modulus . High-
C
3
A clinkers are used for producing type S expansive cements. To produce such cement
the clinker must be ground with relatively high amounts of calcium sulfate, to permit the
formation of high amounts of expansive ettringite in the course of hydration. (For more
