Page 68 - Handbook of Thermal Analysis of Construction Materials

Section 6.0 - Properties of Cement Paste 51

The mechanisms that have already been described for pure cement
compounds form a basis for a study of the hydration mechanism of portland
cement. The conduction calorimetric curves of C S and portland cement are
3
similar except portland cement may yield a third peak for the formation of
monosulfate hydrate (Fig. 1). The detailed influence of C A and C AF on
4
3
the hydration of C S and C S in cement is yet to be worked out. The delayed
3 2
nucleation models and the protective layer models, taking into account the
[8]
possible interactions, have been reviewed. Although the initial process is
not clear for C S (in cements), it appears that C A hydration products form
3
3
through solution and topochemical processes.

6.0 PROPERTIES OF CEMENT PASTE

6.1 Setting

A mixture of cement and water, mixed in certain proportions,
causes setting and hardening, and it is called a cement paste.
The stiffening times of cement paste or mortar fraction are deter-
mined by setting times. The setting characteristics are assessed by initial set
and final set. When the concrete attains the stage of initial set it can no
longer be properly handled and placed. The final set corresponds to the
stage at which hardening begins. At the time of the initial set, the concrete
will have exhibited a measurable loss of slump. Admixtures may influence
the setting times. The retarders increase the setting times and accelerators
decrease them.
At the time of the initial set of cement paste, the hydration of C S
3
will have just started. According to some investigators, the crystallization
of ettringite is the major contributing factor to the initial set. The final set
generally occurs before the paste shows the maximum rate of heat develop-
ment, i.e., before the end of the third stage in conduction calorimetry.
False or flash set is known to occur in concrete. When stiffening
occurs due to the presence of partially dehydrated gypsum, false set is
noticed. Workability is restored by remixing. False set may also be caused
by excessive formation of ettringite especially in the presence of some
retarders and an admixture such as triethanolamine. The formation of
–
syngenite (KCS H) is reported to cause false set in some instances. The
2
setting times of cement can be determined by Gillmore (ASTM C 266) or
the Vicat apparatus (ASTM 191).