Page 63 - Handbook of Thermal Analysis of Construction Materials

46 Chapter 2 - Introduction to Portland Cement Concrete

Amorphous hydroxides of Fe and Al form in the reaction of C AF.
4
The thermodynamically stable product is C (A,F)H and this is the conver-
3
6
sion product of the hexagonal hydrates. Seldom does the formation of these
hydrates cause flash set in cements.
In cements, C AF reacts much slower than C A in the presence of
3
4
gypsum. In other words gypsum retards the hydration of C AF more
4
efficiently than it does C A. The rate of hydration depends on the compo-
3
sition of the ferrite phase, that containing higher amounts of Fe exhibits
lower rates of hydration. The reaction of C AF with gypsum proceeds as
4
follows:
– –
3C AF + 12CSH + 110H → 4[C (A,F)SH ]+ 2(A,F)H
4 2 6 32 3
The low sulfoaluminate phase can form by the reaction of excess C AF with
4
the high sulfoaluminate phase.

– –
3C AF + 2[C (A,F)SH ] → 6[C (A,F)SH ] + 2(A,F)H 3
4
4
12
32
6
The above equations involve formation of hydroxides of Al and Fe
because of insufficient lime in C AF. In these products F can substitute for
4
A. The ratio of A to F need not be the same as in the starting material.
Although cements high in C A are prone to sulfate attack, those with high
3
C AF are not. In high C AF cements, ettringite may not form from the low
4
4
sulfoaluminate possibly because of the substitution of iron in the monosulfate.
It is also possible that amorphous (A,F) prevents such a reaction. Another
3
possibility is that the sulfoaluminate phase that forms is produced in such
a way that it does not create crystalline growth pressures. The ferrite phase
in cement may behave differently from pure C AF because of the differ-
4
ences in the composition of the ferrite phases and the influence of other
compounds present in cement.

4.0 RELATIVE BEHAVIORS OF INDIVIDUAL
CEMENT MINERALS

The variation in the rate of strength development in individual
cement compounds was determined by Bogue and Lerch in 1934. [10] The
comparison of reactivities and strength development of these compounds