Civil Engineering Reference
In-Depth Information
8.0
REGULATED-SET CEMENT
8.1
Description
The current state of disrepair of the North American concrete
infrastructure has had a significant impact on strategic planning related to
maintenance, repair, and service-life prediction of the concrete structures.
The relatively slow setting properties of portland cement and portland
cement concretes are inadequate for many repair applications. The devel-
opment of a modified portland cement composition capable of developing
a high early set strength upon hydration was an attempt to address this need.
The high early strength development is due to the rapid formation of
ettringite. This cement is known as regulated-set cement.
Regulated-set cement (often referred to as Reg Set or Jet Set) is a
modified portland cement which contains as one ingredient a substantial
amount of a ternary compound, essentially a calcium halo-aluminate,
having the chemical formula 11CaO•7Al
2
O
3
•CaX
2
wherein X is a halogen,
i.e., fluorine, chlorine, bromine, or iodine.
[72]
The preferred ternary calcium
halo-aluminate to be incorporated into the cement is calcium fluoroaluminate
having the formula 11CaO•7Al
2
O
3
•CaF
2
. The modified cement contains
between 1 and 30 % by weight of the calcium halo-aluminate.
Reg Set can be made by mixing finely divided 11CaO•7Al
2
O
3
•CaF
2
with portland cement or directly by the production of clinker containing the
fluoroaluminate compound, by using fluoride as one of the raw materials in
the manufacture of portland cement clinker. Chemical analysis of Reg Set
cement used by various investigators is given in Table 6. Reg Set cements
contain more SO
3
than portland cements because the fluoride complex
requires additional amounts of SO
3
for the formation of ettringite.
8.2
Paste and Mortar Hydration
Uchikawa and Tsukiyama have followed the hydration of Jet Set
cement paste (w/c = 0.40) at 20°C.
[73]
They utilized two Jet Set cements —
one containing 2% calcium carbonate and citric acid (0.2%), the other 2.5%
calcium sulfate hemihydrate. In the early stages of hydration, the cement
using calcium sulfate hemihydrate as a set-regulator displayed a higher rate
of hydration than that regulated by citric acid; at a later period this behavior
was reversed. The cement regulated with citric acid then had a higher degree
of hydration, lower porosity, a larger percentage of small pores, and a larger
