Civil Engineering Reference
In-Depth Information
space initially taken up by water in a cementitious mixture
is partially or completely replaced over time as the hydra-
tion reactions proceed (Table 2-5). If more than about 35%
water by mass of cement—a water to cement ratio of 0.35—
is used, then porosity in the hardened material will remain,
even after complete hydration. This is called capillary por-
osity. Fig. 2-30 shows that cement pastes with high and low
water to cement ratios have equal masses after drying
(evaporable water was removed). The cement consumed
the same amount of water in both pastes resulting in more
bulk volume in the higher water-cement ratio paste. As the
water to cement ratio increases, the capillary porosity
increases, and the strength decreases. Also, transport prop-
erties such as permeability and diffusivity are increased,
allowing detrimental chemicals to more readily attack the
concrete or reinforcing steel.
Water is found in cementitious materials in several
forms.
Free water
is mixing water that has not reacted with
the cement phases.
Bound water
is chemically combined in
the solid phases or physically bound to the solid surfaces.
A reliable separation of the chemically combined from the
physically adsorbed water is not possible. Therefore
Powers (1949)
distinguished between
evaporable
and
nonevaporable water
. The nonevaporable water is the
amount retained by a sample after it has been subjected to
a drying procedure intended to remove all the free water
(traditionally, by heating to 105°C). Evaporable water was
originally considered to be free water, but it is now recog-
nized that some bound water is also lost upon heating to
this temperature. All nonevaporable water is bound water,
but the opposite is not true.
For complete hydration of portland cement, only
about 40% water (a water-to-cement ratio of 0.40) is
needed. If a water-to-cement ratio greater than about 0.40
is used, the excess water not needed for cement hydration
remains in the capillary pores or evaporates. If a water-to-
Table 2-7. Nonevaporable Water Contents for Fully
Hydrated Major Compounds of Cement
Nonevaporable
Hydrated cement
(combined) water content
compound
(g water/g cement compound)
C
3
S hydrate
0.24
C
2
S hydrate
0.21
C
3
A hydrate
0.40
C
4
AF hydrate
0.37
Free lime (CaO)
0.33
cement ratio less than about 0.40 is used, some cement will
remain unhydrated.
To estimate the degree of hydration of a hydrated ma-
terial the nonevaporable water content is often used. To
convert the measured nonevaporable water into degrees of
hydration, it is necessary to know the value of nonevap-
orable water-to-cement ratio (w
n
/c) at complete hydration.
Experimentally, this can be determined by preparing a high
water-to-cement ratio cement paste (for example, 1.0) and
continuously grinding while it hydrates in a roller mill. In
this procedure, complete hydration of the cement will typi-
cally be achieved after 28 days.
Alternatively, an estimate of the value of nonevap-
orable water-to-cement ratio (w
n
/c) at complete hydration
can be obtained from the potential Bogue composition of
the cement. Nonevaporable water contents for the major
compounds of portland cement are provided in Table 2-7.
For a typical Type I cement, these coefficients will generally
result in a calculated w
n
/c for completely hydrated cement
somewhere between 0.22 and 0.25.
PHYSICAL PROPERTIES OF CEMENT
Specifications for cement place limits on both its physical
properties and often chemical composition. An under-
standing of the significance of some of the physical proper-
ties is helpful in interpreting results of cement tests. Tests of
the physical properties of the cements should be used to
evaluate the properties of the cement, rather than the
concrete. Cement specifications limit the properties with
respect to the type of cement. Cement should be sampled
in accordance with ASTM C 183 (AASHTO T 127). During
manufacture, cement is continuously monitored for its
chemistry and the following properties:
Particle Size and Fineness
Portland cement consists of individual angular particles
with a range of sizes, the result of pulverizing clinker in the
grinding mill (Fig. 2-31 left). Approximately 95% of cement
particles are smaller than 45 micrometers, with the average
particle around 15 micrometers. Fig. 2-31 (right) illustrates
the particle size distribution for a portland cement. The
Fig. 2-30. Cement paste cylinders of equal mass and equal
cement content, but mixed with different water to cement
ratios, after all water has evaporated from the cylinders. (1072)
