Page 67 - Handbook of Thermal Analysis of Construction Materials

50 Chapter 2 - Introduction to Portland Cement Concrete

are absent. The polymer content with five or more Si atoms increases as the
hydration proceeds and the amount of dimer decreases. In C S pastes the
3
disappearance of monomer results in the formation of polymers. In cement
pastes even after the disappearance of all C S and C S, some monomer is
3 2
detected possibly because of the modification of the anion structure of C-
S-H through replacement of some Si atoms by Al, Fe, or S. Admixtures can
influence the rate at which the polymerization occurs in portland cement
and C S pastes.
3
The minimum water:cement ratio for attaining complete hydration
of cement has been variously given from 0.35 to 0.40, although complete
hydration has been reported to have been achieved at a water:cement
ratio of 0.22. [13] In a fully hydrated portland cement Ca(OH) consti-
2
tutes about 20–25% of the solid content.
The ettringite group, also called AFt phase in cement paste, stands
–
for Al-Fe-tri (tri = three moles of CS) of the formula C A•3CS•H in which
3
32
Al can be replaced by Fe to some extent. The AFt phase forms in the first
few hours (from C A and C AF) and plays a role in setting. After a few days
4
3
of hydration only a little amount of it may remain in cement pastes. It
appears as stumpy rods in SEM and the length does not normally exceed a
few micrometers. The principle substitutions that exist in the AFt phase are
3+
3+
4+
-
2-
Fe and Si for Al and various anions such as OH , CO , and silicates
3
2-
for SO .
4
The monosulfate group, also known as the AFm phase, is repre-
– –
sented by the formula C ASH or C A•CS•H . AFm stands for Al-Fe-
4
12
12
3
–
mono, in which one mole of CS is present. In portland cement this phase
forms after the AFt phase disappears. This phase may constitute about 10%
of the solid phase in a mature cement paste. In SEM, this phase has a
hexagonal morphology resembling that of Ca(OH) and the crystals are of
2
sub-micrometer thickness. The principle ionic substitutions in the AFm
-
2-
-
2-
3+
3+
phase are Fe for Al , and OH , CO , Cl , etc. for SO . The density of
3 4
this phase is 2.02 g/ml. The amount of crystalline hydrogarnet present in
cement paste is less than 3%. [14] It is of the type Ca Al (OH) in which part
3
2
12
4-
3+
3+
-
of Al is replaced by Fe and 4OH by SiO [e.g., C (A F )SH ]. It
3
4
0.5 0.5
4
may be present in small amounts in mature cement pastes and is also
formed at higher temperatures. The crystal structure of this phase is related
–
to C AS (garnet). Hydrogarnet is decomposed by CO with the formation
3 3 2
of CaCO as a product.
3
It is the opinion of some workers that the lowest sulfate form of
calcium sulfohydroxyaluminate hydrate, a crystalline solid solution phase
in the system CaO-Al O -CaSO -H O, is also formed in cement pastes.
2
3
2
4