CONCRETE. RAW MATERIALS
L. Dvorkin and O.Dvorkin
1.1. Concrete. General
Concrete can be classified as composite material and that is a
combination of different components which improve their performance
properties.
In general case binder component which can be in hard crystalline or
amorphous state is considered as the matrix of composite material.
In concrete matrix phase the grains of aggregates (dispersed phase) are
uniformly distributed.
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Concrete classification
Classification
Types of concrete
indication
Cement, Gypsum, Lime, Slag-alkaline, Polymer, PolymerTypes of binders
cement
Density
Normal-weight, High-weight, Light-weight
Normal-weight, Heavy-weight, Light-weight, Inorganic,
Types of aggregates
Organic
Size of aggregates
Coarse, Fine
Workability of
Stiff and Plastic consistency
concrete mixtures
Porosity of concrete
High-density, Low-density, Cel ular
High-strength, Resistance to action of acids or alkalis, Sulfate
Typical properties
resistance, Rapid hardening, Decorativeness
Structural concrete, Concrete for road and hydrotechnical
Exploitation purpose
construction, Concrete for thermal isolation, Radiationprotective concrete, White and Coloured concrete
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1.2. Binders. Classification.
Nature of binding properties
Concrete can be produced on the basis of all types of glues which have
adhesion to the aggregates and ability for hardening and strength
development.
Organic glues
Organic –
Inorganic glues
mineral glues
Molten
Solutions,
materials,
pastes
Pastes
Solutions,
bond
Pastes
solders
Binding and production of composite materials
Fig.1.1. Types of adhesives
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Periodicity of chemical compounds binding properties
Oxide of
Oxide
chemical
Al2O3
SiO2
Fe2O3
Cr2O3
Mn2O3
GeO2
SnO2
element
BeO
----- - - - -
MgO
------- - - -
CaO
++ ++ ++ ++ ++ ++ ++
ZnO
--------- - -
SrO
++ ++ ++ + + + +
CdO
----- - - - -
BaO
++ ++ ++ ++ ++ ++ ++
Note: fixed (++) and predicted (+) existence of binding properties; fixed (--) and
foreseen (-) absence of binding properties.
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1.3. Portland cement and its types
Chemical composition of portland cement clinker is as a rule within following
range, %:
СаО-63...66
MgO-0.5...5
SiO2-22...24
SO3-0.3...1
Al2O3-4...8
Na2O+K2O-0.4...1
Fe2O3-2...4
TiO2+Cr2O3-0.2...0.5
Fig. 1.2. Crystals of alite
Fig. 1.3. Crystals of belite
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а P
а P
28 days
, М
М
thg
h,
nre
trengt
3 days
ve st
ssire
ssive s
p
re
m
p
o
m
C
oC
Age of hardening, days
Amount of alite, %
Fig. 1.4. Rate of cement paste hardening
under using cements with different grain
Fig. 1.5. Relationship between amount
sizes:
of alite and compressive strength of
1–<3 µm; 2 – 3…9 µm; 3 – 9…25 µm;
cement
4 – 25…50 µm
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1.4. Hydraulic non portland cement binders
Lime binders
Hydraulic lime binders contain materials produced by grinding or
blending of lime with active mineral admixtures (pozzolans) — natural
materials and industrial byproducts. At mixing of active mineral
admixtures in pulverized form with hydrated lime and water, a paste
which hardened can be obtained.
Typical hydraulic lime binders are lime-ash binders.
Slag binders
Slag binders are products of fine grinding blast-furnace slag which
contains activation hardening admixtures. Activation admixtures must
be blended with slag at their grinding (sulfate – slag and lime – slag
binders) or mixing with water solutions (slag - alkaline binders).
Activation admixtures are alkaline compounds or sulfates which contain
ions Са2+, (ОН)-and (SO )2-.
4
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Calcium - aluminate (high-alumina) cements
Calcium - aluminate (high-alumina) cements are quickly hardening hydraulic
binders. They are produced by pulverizing clinker consisting essentially of
calcium aluminates.
h
rcent
trengt
pe
s
h,
trengt
28 day
S
of
Age, days
Fig. 1.6. Typical curves of cement strength
increase:
1 - calcium - aluminate cement; 2 – high-early strength
portland cement; 3 – ordinary portland cement
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1.5. Concrete aggregates
Classification of aggregates for concrete
Classification
Characteristics
Kind of aggregates
indication
of classification indication
Fine aggregates
≤5 mm
Grain size
Coarse aggregates
>5 mm
Gravel
Smooth particles
Particle shape
Crushed stone
Angular particles
Heavy
ρ0>1100 kg/m3
Bulk density (ρ0)
Light
ρ0≤1100 kg/m3
Normal and high - density P≤10%
Porosity (P)
Low - density
P>10%
Normal, high and low –
density concrete,
Properties of aggregates
Exploitation purpose Concrete for
must conform to the
hydrotechnical, road and concrete properties
other kinds of construction
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Percentage retained
Percentage retained
(cumulative), by mass
(cumulative), by mass
Sieve sizes, mm
Sieve sizes, mm
Fig. 1.7. Curves indicate the limits
specified in Ukrainian Standard for fine
Fig. 1.8. Curves indicate the
aggregates:
recommended limits
specified in
1,2 - Minimum possible (Fineness
Ukrainian Standard
for coarse
modulus=1.5) and recommended
aggregates
(Fineness modulus=2) limits of aggregate
size;
3,4 - Maximum recommended (Fineness
modulus=2.25) and possible (Fineness
modulus=2.5) limits of aggregate size
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1.6. Admixtures
Chemical admixtures
European standard (EN934-2) suggested to classify chemical admixtures as follows.
Admixtures by classification (Standard EN934-2)
Type of admixture
Technological effect
Reduce water required for given consistency or
Water reducer – plasticizer*
improve workability for a given water content
Essentially reduce water required for given
High water reducer –
consistency or high improve workability for a
superplasticizer**
given water content
Increase bond of water in
Prevention of losses of water caused by
concrete mixture
bleeding (water gain)
Entrainment of required amount of air in
concrete during mixing and obtaining of uniform
Air-entraining
distribution of entrained-air voids in concrete
structure
Accelerator of setting time
Shorten the time of setting
Note:
Increase the rate of hardening of concrete with
Accelerator of hardening
change of setting time or without it.
* Plasticizer reduces the
Retarder
Retard setting time
quantity of mixing water
Dampproofing and
Decrease permeability
required to produce concrete of
permeability-reducing
Water reducer/
Combination of reduce water and retard set
a given slump at 5-12%.;
retarder
effects
** Superplasticizer reduces the
High water reducer/
Combination of superplasticizer (high water
retarder
reduce) and retard set effects
quantity of mixing water at 12Water reducer/ Accelerator
Combination of reduce water and shorten the
30 % and more.
of setting time
time of setting effects
Influence on a few properties
Complex effect
of concrete mixture and concrete
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Classification of plasticizers
Plasticizer effect
Reduce the quantity of
Category
Type of plasticizer
(increase the slump
mixing water
from 2...4 sm)
for a given slump
І
Superplasticizer
to 20 sm and more
no less than 20 %
ІІ
Plasticizer
14-19 sm
no less than 10 %
ІІІ
Plasticizer
9-13 sm
no less than 5 %
ІV
Plasticizer
8 and less
less than 5 %
Air-entrained admixtures are divided into six groups (depending on
chemical composition):
1) Salts of wood resin;
2) Synthetic detergents;
3) Salts of lignosulphonated acids;
4) Salts of petroleum acids;
5) Salts from proteins;
6) Salts of organic sulphonated acids.
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As gas former admixtures silicon-organic compounds and also aluminum
powder are used basically. As a result of reaction between these admixtures
and calcium hydroxide, the hydrogen is produced as smallest gas bubbles.
Calcium chloride is the most explored accelerating admixture. Adding this
accelerator in the concrete, however, is limited due to acceleration of
corrosion of steel reinforcement and decrease resistance of cement paste in
a sulfate environment.
As accelerators are also used sodium and potassium sulfates, sodium and
calcium nitrates, iron chlorides, aluminum chloride and sulfate and other
salts-electrolytes.
Some accelerating admixtures are also anti-freeze agents which providing
hardening of concrete at low temperatures.
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In technological practice in some cases there is a necessity in retarding
admixtures.
4
Forsen has divided retarders into
em
four groups according to their
tign
influence on the initial setting
tti
2
e
time:
1
l s
itiaIn
1. CaSO ·2H O, Ca(ClO ) ,
4
2
3 2
CaS .
2
2. CaCl , Ca(NO ) , CaBr ,
2
3 2
2
3
CaSO ·0.5H O.
4
2
3. Na CO , Na SiO .
2
3
2
3
Amount of retarder
4. Na PO , Na S O , Na AsO ,
3
4
2 4
7
3
4
Ca(CH COO) .
3
2
Fig.1.9. Effect of retarding admixrures
on initial setting time (from Forsen)
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Mineral admixtures
Mineral admixtures are finely divided mineral materials added into concrete
mixes in quantity usually more than 5 % for improvement or achievement
certain properties of concrete.
As a basis of classification of the mineral admixtures accepted in the
European countries and USA are their hydraulic (pozzolanic) activity and
chemical composition.
Fly ash is widely used in concrete mixes as an active mineral admixture.
Average diameter of a typical fly ash particle is 5 to 100 µm. Chemical
composition of fly ash corresponds to composition of a mineral phase of
burning fuel (coal).
Silica fume is an highly active mineral admixture for concrete which is widely
used in recent years. Silica fume is an ultrafine byproduct of production of
ferrosilicon or silicon metal and contains particles of the spherical form with
average diameter 0,1µm. The specific surface is from 15 to 25 m2/kg and
above; bulk density is from 150 to 250 kg/m3.
The chemical composition contains basically amorphous silica which quantity
usually exceeds 85 and reaches 98 %.
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