Page 103 - Handbook of Thermal Analysis of Construction Materials

Section 5.0 - Polymorphisom in Silicates 87

5.0 POLYMORPHISM IN SILICATES

Tricalcium silicate exists in several polymorphic forms. It can be
triclinic, monoclinic, or trigonal. The stability of polymorphs and their
transitions are temperature dependent and not easy to determine because
many forms have small transition enthalpies. The triclinic form (T )can be
I
stabilized with chromium oxide, the triclinic form (T ) by ferric oxide, the
II
monoclinic form (M ) by MgO and monoclinic (M ) form by zinc oxide. [5]
I II
The monoclinic form is present in commercial clinkers and only rarely has
the rhombohedral form been detected.
Jeffery [1][30] was one of the first to carry out the thermograms of
tricalcium silicate preparation and alites. Figure 11 compares the DTA
curves of alite and a synthetically prepared tricalcium silicate. [1][30] The C S
3
preparation exhibits as many as six endotherms at 464, 622, 750, 923, 980,
and 1465°C. The peak at 464°C is caused by the calcium hydroxide formed
from the hydration of free lime present in the preparation. Dicalcium
silicate, present as an impurity exhibits three peaks at 622, 750, and 1465°C.
The peaks at 923°C and 980°C are attributed to triclinic-to-monoclinic, and
monoclinic-to-trigonal or triclinic-to-trigonal and trigonal-to-trigonal tran-
sition plus rotation of ions, respectively. The thermogram of alite differs
from that of the synthetic silicate by having only two endotherms at 825 and
1427°C. The effect at 825°C is ascribed to monoclinic to trigonal transition
and corresponds to a similar effect in synthesized tricalcium silicate. This
effect occurs at a lower temperature owing to the solid solution effect. In the
cooling cycle, the endothermal effects show up as exothermal effects,
indicating reversibility. Regourd [31] reported DTA peaks corresponding to
tricalcium silicate transitions at 600, 920, 980, 990, and 1050°C. The
transition at 920°C was undetectable by XRD. All the transitions involve
very slight displacement of atoms without disruption of any of the first co-
ordination bonds. More recently other investigations have been carried out
on calcium silicates by applying different techniques and the temperatures
of transitions may be at some variance from other published data. The
variation in such effects is expected because the behavior of the silicate
depends on the type of stabilizer used. Foreign ions may substitute Si or Ca
ions or may occupy the interstitial sites. Depending on the nature of these
ions certain modifications may be stabilized.