Civil Engineering Reference
In-Depth Information
content have a higher pH of their pore solution and a better capacity to protect steel from
corrosion than sulfoaluminate cements low in Fe
2
O
3
. Janotka and Krajci (1999) found an
insufficient degree of steel passivation in a sulfobelite cement mortar after 90 days.
However, a suitable increase of pH and adequate corrosion protection of the steel
reinforcement may be achieved by adding Portland cement to the mix in amounts of at
least 15%. In separate work Andac and Glasser (1999) found in the pore solution of a
commercial china-made calcium sulfoaluminate cement pH values of 12.8-13.8 between
1 and 60 days of hydration. The main components in the solutions were K and Al, with a
maximum for potassium at about three days.
Sulfobelite cements do not meet the requirements of most specifications for Portland
cement, because their SO
3
content is higher than permitted. However, special standards
for sulfobelite cements were issued in China, and about a million tons of this type of
cement are produced annually in that country (Zhang
et al.,
1999).
4.3
SULFOALITE CEMENT
Sulfoalite cement is characterized by the simultaneous presence of the phases C
3
S and
C
4
A
3
in its clinker. Other phases may include C
2
S, C
2
(A,F) and C
3
A. The possibility of
producing such clinkers initially appeared uncertain, as the optimum temperatures for the
synthesis of the two phases differ significantly: whereas C
4
A
3
is formed at around
1250-1300°C and decomposes above 1300°C, a temperature of at least 1350°C is
required to synthesize C
3
S at an acceptable rate. Moreover, it has been reported that the
presence of greater amounts of SO
3
in the raw meal tends to reduce or prevent
completely the formation of C
3
S (Strunge
et al.,
1985; Cheong
et al.,
1992; Moranville-
Regour and Boikova, 1992). However, it has been found that clinkers containing both
phases may be produced at burning temperatures of 1230-1300°C if small amounts of
fluorspar (CaF
2
) are added to the raw mix (Abdul-Maula and Odler, 1992; Odler and
Zhang, 1996). At the same time the phase composition of the clinker that is produced
may be varied over a wide range by varying the oxide composition of the raw meal. The
energy consumption in the production of sulfoalitic clinkers is lower than in the
production of ordinary Portland clinker, but higher than in the production of sulfobelitic
clinkers, owing to the presence of tricalcium silicate. Just like sulfobelitic clinkers,
sulfoalitic clinkers are easy to grind.
The strength development of sulfoalite cements may be controlled by varying the
phase composition of the clinker and the amount of interground calcium sulfate. This
amount is generally higher than is common in ordinary Portland cement, but it must not
exceed an optimum amount, to prevent unwanted expansion of the hardened paste. An
advantage of sulfoalite cements compared with sulfobelite cements is their more
favorable strength development, especially between 1 and 28 days. Table 4.3 shows the
strength development of a typical sulfoalite and sulfobelite cement and that of a Portland
and a belite cement, for comparison.
Unlike sulfobelite cements, the hardened paste of sulfoalite cements will contain
distinct amounts of free calcium hydroxide, in addition to C-S-H and AFt. As with
