Faculty of Applied Engineering and Urban Planning
Civil Engineering Department
Dr. Eng. Mustafa Maher Al-tayeb
2nd Semester 2018/2019
Concrete technology
Low heat Portland cement
The rise in temperature in the interior of a large concrete mass due to
the heat development by the hydration of cement coupled with a low
thermal conductivity of concrete can lead to serious cracking.
Cement having such a low rate of heat development was first
produced for use in large gravity dams in the United States and is
known as low heat Portland cement (type IV).
Low heat Portland cement
The limits of lime content of low heat Portland cement. After
correction for the lime combined with SO3 are:
Low heat Portland cement
The rather lower content of the more rapidly hydrating compounds
C3S and C3A results in a slower development of strength of low heat
cement as compared to the ordinary Portland cement, but the ultimate
strength is unaffected. In any case to ensure a sufficient rate of gain
of strength the specific surface of the cement must be not less than
320 m2/kg
Sulfate - resisting cement
In discussing the reactions of hydration of cement, and in particular
the setting process, mention was made of the reaction between C3A
and gypsum (CaSO4.2H2O) and of the consequent formation of
calcium sulfoaluminat. In hardened cement, calcium aluminate
hydrate can react with a sulfate salt from outside the concrete in a
similar manner: the product of addition is calcium sulfoaluminat
forming within the framework of the hydrated cement paste because
the increase in the volume of the solid phase is 227 percent gradual
disintegration of concrete results.
Sulfate - resisting cement
A second type of reaction is that of base exchange between calcium
hydroxide and the sulfate resulting in the formation of gypsum with
an increase in the volume of the solid phase of 124 percent. These
reactions are known as sulfate attack. The salts particularly active are
magnesium sulfate and sodium sulfate. Sulfate attack is greatly
accelerated if accompanied by alternating wetting and drying.
Sulfate - resisting cement
The remedy lies in the use of cement with a low C3A content, and
such cement is known as sulfate-resisting Portland cement. The
British Standard for this cement stipulates a maximum C3A content
of 3.5 percent. The SO3 content is limited to 2.5 percent. In the
United State, sulfate-resisting cement is known as Type V cement
and is covered by ASTM C 150-94.This specification limits the C3A
content to 5 percent, and also restricts the sum of the content of
C4AF plus twice the C3A content to 25 percent. The magnesia
content is limited to 6 percent.
Sulfate - resisting cement
As it is often not feasible to reduce the Al2O3 content of the raw
material, Fe2O3 may be added to the mix so that the C4AF content
increases at the expense of C3A.
The low C3A content and comparatively low C4AF content of
sulfate-resisting cement mean that it has a high silicate content and
this gives the cement a high strength but, because C2S represents a
high proportion of the silicates, the early strength is low. The heat
developed by sulfate-resisting cement is not much higher than that of
low heat cement.
Sulfate - resisting cement
It could therefore be argued that sulfate-resisting cements
theoretically an ideal cement but, because of the special requirements
for the composition of the raw materials used in its manufacture,
sulfate-resisting cement cannot be generally and cheaply made.
It should be noted that the use of sulfate-resisting cement may be
disadvantageous when there is a risk of the presence of chloride ions
in the concrete containing steel reinforcement. The reason for
this is that C3A binds chloride ions, forming calcium
chloroaluminate. In consequence these ions are not available for
initiation of corrosion of the steel.
White cement and pigments
For architectural purposes, white concrete or a Pastel color is
sometimes required. To achieve best results it is advisable to use
white cement with of course a suitable fine aggregate and, if the
surface is to be treated also an appropriate coarse aggregate. White
cement has also the advantage that it is not liable to cause staining
because it has a low content o f soluble alkalis.
White cement and pigments
White Portland cement is made from raw materials containing very
little iron oxide (less than 0.3 percent by mass of clinker) and
manganese oxide. China clay is generally used, together with chalk
or limestone free from specified impurities. Oil or gas is used as fuel
for the kiln in order to avoid contamination by coal ash. Since iron
acts as a flux in clinkering its absence necessitates higher
kiln temperatures ( up to 1650'C) but sometimes cryolite (sodium
aluminum fluoride) is added as a flux.
White cement and pigments
To obtain good color white concrete of rich-mix proportions is
generally used the water/cement ratio being not higher than about
0.4.
White cement and pigments
Atypical compound composition of white Portland cement is given in
Table but the C3S and C2S contents may vary widely. White cement
has a slightly lower specific gravity than ordinary Portland cement
generally between 3.05 and 3.10. Because the brightness of the white
color is increased by a higher fineness of cement, it is usually ground
to a fineness of 400 to 450 m2/kg.
The strength of whit of Portland cement is usually somewhat lower
than that of ordinary Portland cement.
White cement and pigments
When a pastel color is required white concrete can be used as a base
for painting. Alternatively pigments can be added to the mixer; those
are powders of fineness similar to or higher than, that of cement. A
wide range of colors is available for example iron oxides can produce
yellow, red, brown and black colors chromic oxide produces green
color, and titanium dioxide produces white color. It is essential that
the pigments do not affect adversely the development of strength of
the cement or affect air entrainment.
White cement and pigments
For instance carbon black, which is extremely fine, increases the
water demand and reduces the air content of the mix. For this reason,
some pigments are marketed in the United States with an interground
air-entraining agent it is, of course essential to be aware of this at the
mix proportioning stage.
Portland bIast furnace cement
Cements of this name consist of an intimate mixture of Portland
cement and ground granulated blast furnace slag. This slag is a waste
product in the manufacture of pig iron, about 300 kg of slag being
produced for each ton of pig iron. Chemically, slag is a mixture of
lime, silica, and alumina, that is, the same oxides that make up
Portland cement but not in the same proportions.
Portland bIastfurnace cement
Blast furnace slag varies greatly in composition and physical
structure depending on the processes used and on the method of
cooling of the slag. For use in the manufacture of blast furnace
cement the slag has to be quenched so that it solidifies as glass
crystallization being largely prevented. This rapid cooling by water
results also in fragmentation of the material into a granulated form.
Portland bIastfurnace cement
Slag can make a cementations material in different ways. It can be
used together with limestone as a raw material for the conventional
manufacture of Portland cement in the dry process. Clinker made
from these materials is often used (together with slag) in the
manufacture of Portland blast furnace cement.
This use of slag, which need not be in glass form, is economically
advantageous because lime is present as CaO so that the energy to
achieve carbonation is not required.
Supersulfated cement
Super sulfated cement is made by grinding a mixture of 80 to 85
percent of granulated blast furnace slag with 10 to 15 percent of
calcium sulfate(in the form
of burnt gypsum) and up to 5 percent of Portland cement clinker.
A fineness of 400 to 500 m2/kg is usual. The cement has to be stored
under very dry conditions as otherwise it deteriorates rapidly.
Super sulfated cement is highly resistant to sea water and can
withstand the highest concentrations of sulfates normally found in
soil or ground water, and is also resistant to acids and to oils.
Supersulfated cement
Concrete with a water/cement ratio not greater than 0.45 has been
found not to deteriorate in contact with weak solutions of mineral
acids of pH down to 3.5. For these reasons super sulfated cement is
used in the construction of sewers and in contaminated ground
although it has been suggested that this cement is less resistant than
sulfate-resisting Portland cement.
The heat of hydration of super sulfated cement is low.
Pozzolanas
One of the common materials classified as cementitious in this book
(although in reality only in latent form) is pozzolana which is a
natural or artificial material containing silica in a reactive form. A
more formal definition of ASTM 618-94 describes pozzolana as a
siliceous or siliceous and aluminous material which in itself possess
little or no cementitious value but will in finely divided form and
in the presence of moisture chemically react with calcium hydroxide
at ordinary temperatures to form compounds possessing cementitious
properties.
Supersulfated cement
The main artificial pozzolanic material, fly ash.
The natural pozzolanic materials most commonly are: volcanic ashthe original pozzolana - shales and cherts, and burnt clay.
Some natural pozzolana may create problems because of their
physical properties; e.g. diatomaceous earth, because of angular and
porous form requires a high water content.
Rice husks are a natural waste product and there is interest in using
this material in concrete
Silica fume
Silica fume is a recent arrival among cementnous materials. It was
originally introduced as a pozzolana. However, its action in concrete
is not only that of a very reactive pozzolana but is also beneficial in
other respects. It can be added that silica fume is expensive.
Silica fume is also referred to as misosilica or condensed silica fume,
but the term 'silica fume' has become generally accepted. It is a byproduct of the manufacture of silicon and ferrosilicon alloys from
high-purity quartz and coal in a submerged-arc electric furnace.