Civil Engineering Reference
In-Depth Information
Cements do not generate heat at a constant rate. The
heat output during hydration of a typical Type I portland
cement is illustrated in Fig. 2-45. The first peak shown in
the heat profile is due to heat given off by the initial hydra-
tion reactions of cement compounds such as tricalcium
aluminate. Sometimes called the heat of wetting, this first
heat peak is followed by a period of slow activity known as
the induction period. After several hours, a broad second
heat peak attributed to tricalcium silicate hydration
emerges, signaling the onset of the paste hardening
process. Finally, there is the third peak due to the renewed
activity of tricalcium aluminate; its intensity and location
normally dependent on the amount of tricalcium alumi-
nate and sulfate in the cement.
In calorimetry testing, the first heat measurements are
obtained about 7 minutes after mixing the paste; as a result
only the down slope of the first peak is observed (Stage 1,
Fig. 2-45). The second peak (C
3
S peak) often occurs
between 6 and 12 hours. The third peak (renewed C
3
A peak
on conversion of AFt to AFM) occurs between 12 and 90
hours. This information can be helpful when trying to
control temperature rise in mass concrete (Tang 1992).
When heat generation must be minimized in concrete,
designers should choose a lower heat cement, such as an
ASTM C 150 (AASHTO M 85) Type II portland cement
with the optional moderate heat of hydration require-
ments. Not all Type II cements are made for a moderate
level of heat development, however, so the moderate heat
option must be specifically requested. Type IV cement can
also be used to control temperature rise, but it is rarely
available. Moderate-heat and low-heat cements are also
available in ASTM C 595 (AASHTO M 240) and C 1157 spe-
cifications. Supplementary cementitious materials can also
be used to reduce temperature rise.
ASTM C 150 (AASHTO M 85) has both a chemical
approach and a physical approach to control heat of hydra-
tion. Either approach can be specified, but not both. ASTM
C 595 (AASHTO M 240) and ASTM C 1157 use physical
limits. See
PCA (1997)
for more information.
Loss on Ignition
Loss on ignition (LOI) of portland cement is determined by
heating a cement sample of known weight to between
900°C and 1000°C until a constant weight is obtained. The
weight loss of the sample is then determined. Normally, a
high loss on ignition is an indication of prehydration and
carbonation, which may be caused by improper or
prolonged storage, or adulteration during transport. The
test for loss on ignition is performed in accordance with
ASTM C 114 (AASHTO T 105) (Fig. 2-46). LOI values range
from 0 to 3%.
Density and Relative Density
(Specific Gravity)
The density of cement is defined as the mass of a unit
volume of the solids or particles, excluding air between
particles. It is reported as megagrams per cubic meter or
grams per cubic centimeter (the numeric value is the same
for both units). The particle density of portland cement
ranges from 3.10 to 3.25, averaging 3.15 Mg/m
3
. Portland-
blast-furnace-slag and portland-pozzolan cements have
densities ranging from 2.90 to 3.15, averaging 3.05 Mg/m
3
.
The density of a cement, determined by ASTM C 188
(AASHTO T 133) (Fig. 2-47), is not an indication of the
cement's quality; its principal use is in mixture proportion-
ing calculations.
For mixture proportioning, it may be more useful to
express the density as relative density (also called specific
gravity). The relative density is a dimensionless number
Stage 1
Stage 2
Stages 3 and 4
Stage 5
C
3
S hydration
C
3
A hydration
Time
Fig. 2-45. Heat evolution as a function of time for cement
paste. Stage 1 is heat of wetting or initial hydrolysis (C
3
A
and C
3
S hydration). Stage 2 is a dormant period related to
initial set. Stage 3 is an accelerated reaction of the
hydration products that determines rate of hardening and
final set. Stage 4 decelerates formation of hydration
products and determines the rate of early strength gain.
Stage 5 is a slow, steady formation of hydration products
establishing the rate of later strength gain.
Fig. 2-46. Loss on ignition test of cement. (43814)
