Civil Engineering Reference
In-Depth Information
be absent in the mature paste.
Figures 4.1 and 4.2 show the effect of the amount of gypsum added to a sulfobelitic
clinker (with an SO
3
content of 10.0 wt%) on expansion and compressive strength
development.
The extent of expansion increases because of the increasing amounts of ettringite
formed after setting (Wang
et al.,
1992). Along with it the strength of the cement declines
as a consequence of excessive expansion. The available data indicate that both a non-
expansive high early strength cement and an expansive cement may be produced from the
same clinker just by varying the amount of interground calcium sulfate. To prevent
expansion, the cement must not contain excessive amounts of aluminates, calcium
sulfate, or free lime (Lan and Glasser, 1996; Beretka, 1997).
The composition of non-expansive sulfobelite cements is usually adjusted for them to
perform as high early strength binders. Such binders are characterized by a short setting
time (less than 1 hour) and a fast development of short-term strength, which is due
mainly to fast formation of the ettringite phase. Measurable strengths may be attained
within 3 hours of hydration. After most of the sulfoaluminate phase has been converted to
ettringite, the strength development slows down significantly before an additional
strength increase, associated with hydration of the belite phase, gets under way. The
strength after 28 days is relatively low, as at this time a significant fraction of dicalcium
silicate still remains non-hydrated, but a significant additional increase of strength takes
place between 28 days and 1 year.
It has been claimed (Huo
et al.,
1996) that the compressive strength of sulfobelite
cement may be increased by up to 35% by exposing the mixing water to a magnetic field
before mixing, but this claim has not so far been confirmed from other sources.
Figure 4.1
Effect of gypsum interground with a sulfobelitic clinker on the
expansion of the resultant cement. SO
3
in clinker=10.0 wt%.
Source:
Wang
et al.
(1992a)
