Special Portland cements - Special Inorganic Cements - page 15

in combination with appropriate amounts of calcium sulfate, must be ground to a specific
surface area higher than that common in ordinary Portland cement: that is, to 400-500
m 2 /kg (Blaine), rather than the 300-350 m 2 /kg (Blaine) typical for ordinary Portland
cement.
2.4
PORTLAND CEMENT WITH ELEVATED C 2 S CONTENT
Some Portland cements may contain elevated amounts of belite (C 2 S), at the expense of
alite (C 3 S). To produce such clinkers, the lime saturation factor of the raw meal must be
reduced, typically to values between about LSF=80 and LSF=90, instead of the LSF>90
that exists in ordinary Portland cement. (Cements made from clinkers with even lower
lime saturation factors, which contain no or only very limited amounts of alite, are called
belite cements, and are discussed in section 3.2.)
Raw meals for high-belite clinkers are relatively easy to burn, and the consumption of
energy is reduced, mainly because of a reduced CaCO 3 content. Compared with ordinary
Portland cement clinkers, it is possible to reduce the burning temperature by up to about
100 °C and along with it the NO x emission. At the same time the grindability of the
clinker is improved (Ludwig and Pohlman, 1986).
I n the formed clinker the M 1 modification of C 3 S prevails. Belite is present mostly in
its modification, but some - and ′ -C 2 S may also be formed. At high cooling rates the
amount of these high-temperature modifications increases at the expense of -C 2 S.
Owing to the lower reactivity of belite, the overall rate of hydration, and along with it
the strength development up to about 90-180 days, is slowed down with increasing belite
and decreasing alite contents in the cement (Bei and Ludwig, 1990). At the same time the
final strength of a Portland cement with an elevated C 2 S content may exceed that of an
ordinary Portland cement, because more C-S-H and less portlandite is formed in the
hydration of dicalcium than of tricalcium silicate.
The main difference in the composition of cement pastes made from cements with
increased belite and reduced alite contents is their lower calcium hydroxide content. This
may affect positively the resistance of such hardened pastes to chemical corrosion. At the
same time the depth of carbonation increases with declining C 3 S content in cement
(Kelham and Moir, 1992).
The amount of heat liberated in the hydration declines with increasing dicalcium
silicate and decreasing tricalcium silicate contents in cement. At the same time the rate at
which the heat is released is slowed down (Yoshida, 1992). The rate of heat release may
be reduced even further by coarser grinding of the cement. A comparison of strength
development and hydration heat liberation reveals that the strength per unit quantity of
heat of hydration generally increases with increasing C 2 S and decreasing C 3 S content in
the clinker (Yoshida and Igarashi, 1992).
It has been reported (Albats and Shein, 1997) that the strength of Portland cement with
an elevated belite content may be increased by using a very high heating rate in the
clinker burning process (as high as 20 °C/min) and by simultaneously increasing the