Page 66 - Handbook of Thermal Analysis of Construction Materials

48 Chapter 2 - Introduction to Portland Cement Concrete

5.0 HYDRATION OF PORTLAND CEMENT

Although hydration studies of the pure cement compounds are very
useful in following the hydration processes of portland cement itself, they
cannot be directly applied to cements because of complex interactions. In
portland cement, the compounds do not exist in a pure form but are solid
solutions containing Al, Mg, Na, etc. The rate of hydration of alites
containing different amounts of Al, Mg, or Fe has shown that at the same
degree of hydration Fe-alite shows the greatest strength. There is evidence
the C-S-H formed in different alites is not the same in composition. [12] The
hydration process of C A, C AF, and C S in cement is affected because of
2
4
3
2+
-
changes in the amounts of Ca and OH in the hydrating solution. The
reactivity of C AF can be influenced by the amount of SO 4 2- ions
4
2-
consumed by C A. Some SO ions may be depleted by being absorbed by
3 4
the C-S-H phase. Gypsum is also known to affect the rate of hydration of
calcium silicates. Significant amounts of Al and Fe are incorporated into the
C-S-H structure. The presence of alkalis in portland cement also has an
influence on the hydration of the individual phases.
It is generally believed that the rate of hydration in the first few days
of cement compounds in cements proceeds in the order of C A > C S >
3 3
C AF > C S. The rate of hydration of the compounds depends on the crystal
2
4
size, imperfections, particle size, particle size distribution, the rate of
cooling, surface area, the presence of admixtures, the temperature, etc.
After ninety days, little or no alite or aluminate phase is detectable.
Quantitative x-ray diffraction has been used to determine the
degree of reaction of individual cement compounds present in cement.
Some errors in these estimations are recognized. Figure 5 shows the
fractional amounts of alite, belite, aluminate, and ferrite phases that hydrate
[3]
in cement when hydrated for different times. These rates are not the same
when the individual compounds are hydrated.
In a mature hydrated portland cement, the products formed are
C-S-H gel, Ca(OH) , ettringite (AFt phase), monosulfate (AFm phase),
2
+
hydrogarnet phases, and possible amorphous phases high in Al and SO 4
ions. A small amount of cryptocrystalline CH may be intimately mixed
with C-S-H phase.
The C-S-H phase in cement paste is amorphous or semicrystalline
calcium silicate hydrate, the hyphens denoting that the gel does not
necessarily consist of 1:1 molar CaO:SiO . The C-S-H phase of cement
2
pastes gives powder patterns very similar to that of C S pastes. The
3