Physical Properties
of Portland Cements. ASTM C 150 and
AASHTO M 85 have specified certain
physical requirements for each type of
cement. These properties include 1)
fineness, 2) soundness, 3) consistency,
4) setting time, 5) compressive
strength, 6) heat of hydration, 7)
specific gravity, and 8) loss of
ignition. Each one of these properties
has an influence on the performance of
cement in concrete. The fineness of the
cement, for example, affects the rate of
hydration. Greater fineness increases
the surface available for hydration,
causing greater early strength and more
rapid generation of heat (the fineness
of Type III is higher than that of Type
I cement) (U.S. Dept. Trans. 1990).
ASTM C 150 and AASHTO M
85 specifications are similar except
with regard to fineness of cement.
AASHTO M 85 requires coarser cement,
which will result in higher ultimate
strengths and lower early-strength gain.
The Wagner Turbidimeter and the Blaine
air permeability test for measuring
cement fineness are both required by the
American Society for Testing Materials
(ASTM) and the American Association for
State Highway Transportation Officials
(AASHTO). Average Blaine fineness of
modern cement ranges from 3,000 to 5,000
cm2/g (300 to 500 m2/kg).
Soundness, which is the
ability of hardened cement paste to
retain its volume after setting, can be
characterized by measuring the expansion
of mortar bars in an autoclave (ASTM C
191, AASHTO T 130). The compressive
strength of 2-inch (50-mm) mortar cubes
after 7 days (as measured by ASTM C 109)
should not be less than 2,800 psi (19.3
MPa) for Type I cement. Other physical
properties included in both ASTM C 150
and AASHTO M 95 are specific gravity and
false set. False set is a significant
loss of plasticity shortly after mixing
due to the formation of gypsum or the
formation of ettringite after mixing. In
many cases, workability can be restored
by remixing concrete before it is cast.
Influence of Portland
Cement on Concrete Properties.
Effects of cement on the most important
concrete properties are presented in
Table 1.2.
Cement composition and
fineness play a major role in
controlling concrete properties.
Fineness of cement affects the
placeability, workability, and water
content of a concrete mixture much like
the amount of cement used in concrete
does.
Cement composition
affects the permeability of concrete by
controlling the rate of hydration.
However, the ultimate porosity and
permeability are unaffected (ACI Comm.
225R 1985; Powers et al. 1954). The
coarse cement tends to produce pastes
with higher porosity than that produced
by finer cement (Powers et al. 1954).
Cement composition has only a minor
effect on freeze-thaw resistance.
Corrosion of embedded steel has been
related to C3A content
(Verbeck 1968). The higher the C3A,
the more chloride can be tied into
chloroaluminate complexes—and thereby be
unavailable for catalysis of the
corrosion process.
Table 1.2. Effects of
cements on concrete properties.
|
Cement Property |
Cement Effects |
|
Placeability |
Cement amount, fineness, setting
characteristics |
|
Strength |
Cement composition (C3S,
C2S and C3A),
loss on ignition, fineness |
|
Drying Shrinkage |
SO3content, cement
composition |
|
Permeability |
Cement composition, fineness |
|
Resistance to sulfate |
C3A content |
|
Alkali Silica Reactivity |
Alkali content |
|
Corrosion of embedded steel |
Cement Composition (esp. C3A
content) |
Storage of Cement. Portland cement is a moisture-sensitive
material; if kept dry, it will retain
its quality indefinitely. When stored in
contact with damp air or moisture,
portland cement will set more slowly and
has less strength than portland cement
that is kept dry. When storing bagged
cement, a shaded area or warehouse is
preferred. Cracks and openings in
storehouses should be closed. When
storing bagged cement outdoors, it
should be stacked on pallets and covered
with a waterproof covering.
Storage of bulk cement
should be in a watertight bin or silo.
Transportation should be in vehicles
with watertight, properly sealed lids.
Cement stored for long periods of time
should be tested for strength and loss
on ignition.
Cement Certification.
The current trend in state
transportation departments is to accept
certification by the cement producer
that the cement complies with
specifications. Verifications tests are
taken by the state DOT to continually
monitor specification compliance. The
cement producer has a variety of
information available from production
records and quality control records that
may permit certification of conformance
without much, if any, additional testing
of the product as it is shipped (ACI
Comm. 225R 1985).
Blended Portland Cements
Blended cement, as
defined in ASTM C 595, is a mixture of
portland cement and blast furnace slag
(BFS) or a "mixture of portland cement
and a pozzolan (most commonly fly ash)."
The use of blended
cements in concrete reduces mixing water
and bleeding, improves finishability and
workability, enhances sulfate
resistance, inhibits the
alkali-aggregate reaction, and lessens
heat evolution during hydration, thus
moderating the chances for thermal
cracking on cooling.
Blended cement types and
blended ratios are presented in Table
1.3.
Table 1.3 Blended cement
types and blended ratios.
|
Type |
Blended Ingredients |
|
IP |
15-40% by weight of pozzolan
(fly ash) |
|
I(PM) |
0-15% by weight of Pozzolan (fly
ash)
(modified) |
|
P |
15-40% by weitht of pozzolan
(fly ash) |
|
IS |
25-70% by weight of blast
furnace slag |
|
I(SM) |
0-25% by weight of blast furnace
slag
(modified) |
|
S |
70-100% by weight of blast
furnace slag |
