Civil Engineering Reference
In-Depth Information
3
Reactive forms of dicalcium silicate and belite
cements
The two most important constituents of Portland cement are
alite,
a form of tricalcium
silicate, and
belite,
a form of dicalcium silicate. In their hydration both calcium silicates
yield—in addition to calcium hydroxide—a nearly amorphous calcium silicate hydrate
phase (C-S-H phase), and this hydration product is mainly responsible for the strength
and other physico-mechanical properties of the hardened cement paste.
In the production of Portland clinker, calcium carbonate serves as the source of the
CaO that is needed. Thus, as a first step, calcium carbonate must be decomposed to CaO
and CO
2
, before C
2
S or C
3
S can be formed in a subsequent reaction between CaO and
SiO
2
:
(3.1)
(3.2)
(3.3)
The thermal decomposition of calcium carbonate is a highly endothermic reaction, and
even though the formations of both dicalcium and tricalcium silicate from CaO and SiO
2
are exothermic, the overall formation process of these compounds from CaCO
3
and SiO
2
remains endothermic. A comparison of both reactions reveals that the overall chemical
enthalpy of C
3
S formation is distinctly higher than that of C
2
S (1268 J/kg for C
2
S and
1770 J/kg for C
3
S). Thus the potential exists to reduce the energy consumption in the
production of Portland clinker by increasing its belite content at the expense of alite.
Another difference between dicalcium and tricalcium silicate is the temperature at
which both compounds may be synthesized. Whereas dicalcium silicate is readily formed
from CaO and SiO
2
at temperatures as low as 1000-1100°C, tricalcium silicate becomes
thermodynamically stable only at about 1250°C, and is formed at a reasonable rate at
even higher temperatures. This makes it necessary to employ burning temperatures of up
to 1450-1500°C in the production of Portland clinker, and this fact is mainly responsible
for the significant thermal losses accompanying the process. These losses include heat
losses due to radiation and convection, heat lost in the clinker, heat lost in the exit gases
and dust, and heat lost in the air from the cooler.
