PORTLAND CEMENT - Materials for Civil and Construction Engineers - page 209

can be removed when humidity is less than 11%, resulting in considerable
shrinkage.
Capillary voids are the result of the hydrated cement paste having a
lower bulk specific gravity than the cement particles. The amount and size
of capillary voids depends on the initial separation of the cement particles,
which is largely controlled by the ratio of water to cement paste. For a highly
hydrated cement paste in which a minimum amount of water was used, the
capillary voids will be on the order of 10 nm to 50 nm. A poorly hydrated
cement produced with excess water can have capillary voids on the order of
3 mm to 5 mm. Capillary voids greater than 50 nm decrease strength and in-
crease permeability. Removal of water from capillary voids greater than 50
nm does not cause shrinkage, whereas removal of water from the smaller
voids causes shrinkage.
In addition to the interlayer space and capillary voids, air can be trapped
in the cement paste during mixing. The trapped air reduces strength and in-
creases permeability. However, well-distributed, minute air bubbles can great-
ly increase the durability of the cement paste. Hence, as described later in this
chapter, admixtures are widely used to entrain air into the cement paste.

6.7 Properties of Hydrated Cement

The proper hydration of portland cement is a fundamental quality control
issue for cement producers. While specifications control the quality of the
portland cement, they do not guarantee the quality of the concrete made
with the cement. Mix design, quality control, and the characteristics of the
mixing water and aggregates also influence the quality of the concrete. Prop-
erties of the hydrated cement are evaluated with either cement paste (water
and cement) or mortar (paste and sand).

■ Setting

Setting refers to the stiffening of the cement paste or the change from a plas-
tic state to a solid state. Although with setting comes some strength, it
should be distinguished from hardening, which refers to the strength gain in
a set cement paste. Setting is usually described by two levels: initial set and
final set. The definitions of the initial and final sets are arbitrary, based on
measurements by either the Vicat apparatus (ASTM C191) or the Gillmore
needles (ASTM C266).
The Vicat test (Figure 6.4) requires that a sample of cement paste be pre-
pared, using the amount of water required for normal consistency according
to a specified procedure. The 1 mm (0.04 in.) diameter needle is allowed to
penetrate the paste for 30 seconds and the amount of penetration is mea-
sured. The penetration process is repeated every 15 minutes (every 10 min-
utes for Type III cement) until a penetration of 25 mm (1 in.) or less is
obtained. By interpolation, the time when a penetration of 25 mm occurs is

6.7.1