DURABILITY OF CONCRETE STRUCTURES
CHAPTER - 7
Blended Cements (OPC + Mineral Admixtures)
7.1 Blended cement
7.1.1 The blended cements are manufactured by adding pozzolanic or
Cementitious materials like fly ash or ground granulated blast furnace slag
(GGBFS) or condensed silica fumes (CSF) to portland cement clinker and
Gypsum. Alternatively, these pozzolanic and cementitious materials can be
introduced into Portland cement concrete during concrete making operations.
Table 7.1 gives the details of mineral admixtures, being used to make
blended cement.
Table 7.1
Classification, composition, and particle characteristics of mineral admixtures
for concrete.
Classification
Cement and pozzolanic
ground granulated blast furnace slag (cementitious)
GGBFS
High- calcium fly ash
(cementitious and
pozzolanic)
Highly active pozzolanas
condensed silical fume
Chemical and mineralogical
Composition
Mostly silicate glass
containing mainly calcium,
magnesium, aluminum and
silica. Crystalline compounds
of melilite group may be
present in small quantity
Mostly silicate glass
containing mainly calcium,
magnesium, aluminum and
alkalies. The small quantity
of crystalline matter present
generally consists of quartz
and C3A ; free lime and
periclase may be present; CS
and C4A3S may be present in
the case of high- sulfur coals.
Un-burnt carbon is usually
less than 2%.
Consists essentially of pure
silica in noncrystalline form.
Particle characteristics
Un processed material is of
sand size and contains 10 15% moisture. Before use it
is dried and ground to
particles less than 45 µ m
(usually about 500 m2 / kg
Blaine). Particles have rough
texture.
Powder corresponding to 1015% particles larger than 45µ
.m (usually 300-400 m2/kg
Blaine). Most particles are
solid spheres less than
20 µ .m in diameter. Particle
surface is generally smooth
but not as clean as in low
calcium fly ashes.
Extremely fine powder
consisting of solid spheres of
0.1 µ .m average diameter
(about 20m2/g surface area
by Nitrogen adsorption).
7.1.2 The beneficial effect of the various cementitious materials are so
significant that their use in reinforced concrete liable to corrosion in hot
climates (which is the condition prevailing in entire India during most part of
the year), is virtually necessary. Portland cement alone should not be used in
future marine structures and in coastal areas i.e. within 1 Km of coast line.
7.1.3 At present in India OPC is considered as the ‘best,’ if not the sole,
cementitious material in the concrete. The other materials, primarily fly ash
DURABILITY OF CONCRETE STRUCTURES
and GGBFS are viewed as replacements or substitutes for cement, whereas
these cemetitious materials are today concrete ingredients in their own right.
7.2
General features of use of mineral admixtures: It is a fact that their
use save energy and conserve natural resources but their technical benefits
are the strongest. They affect the progress of hydration, reduces the water
demand and improves workability. The concrete containing GGBFS, on
vibration becomes ‘mobile’ and compacts well. Silica fumes greatly reduces,
or even eliminates bleeding, The particles of Pozzolanic Fly Ash (PFA) are
spherical and thus improves the workability. Their inclusion has the physical
effect of modifying the flocculation of cement, with a resulting reduction in the
water demand. The pore size in concrete is smaller. The fine particles ‘fit in’
between cement particles, thereby reducing permeability.
7.3
Properties of concrete made from blended cement
i) Strength development: The 28 days and later, compressive strengths
are same as for OPC. However, the rate of gain of strength upto 7 days is
slower, due to heat of hydration being low. The drop in early strength should
not be considered as sign of poor quality as this is often accompanied by
enhancement of other properties.
ii) Curing: Curing is a very important stage in the life of a conventional
concrete, it becomes a critical factor in concrete containing blended cement. A
good curing method is essential, because blended cement hydrates slower
than OPC. It is potentially more vulnerable to drying conditions, thus the wet
curing requirements, which is the most neglected activity in the fields, is very
important. The curing method and period must be specified.
iii) Bleeding: The PFA, GGBFS, Silica fumes being finer than OPC, less
bleeding is observed. The freshly placed concrete is very stable, being very
cohesive and having strong internal cohesion. This has a negative effect in
the form of plastic shrinkage.
iv) Workability: The workability increases, and thus water content can be
reduced by about 3 %. The ‘ball bearing’ action of cementitious particles
improves the workability. Silica fumes demand high water due to higher
fineness. The problem is circumvented by the addition of suitable super
plasticizers.
7.4 Advantages of using Blended Cement: The engineering benefits likely
to be derived from the use of mineral admixtures (blended cements and
cement + mineral admixtures can be used interchangingly ) in concrete are
improved resistance to thermal cracking because of lower heat of hydration,
enhancement of ultimate strength, reduction in permeability due to pore
refinement, and a better durability to chemical attacks such as chloride,
sulphate water, soil and alkali- aggregate expansion.
DURABILITY OF CONCRETE STRUCTURES
i) Temperature rise: In large concrete pours like bridges, foundations and
water retaining structures, it is vital to minimise the rise of early age thermal
cracking by controlling the temp rise caused by hydration. One method of
doing this is by use of concrete containing blended cements. Fig 7.1 shows
temp. rise due to cement hydration.
Example: Let the minimum dimension is 1.0m. Assuming 400kg of
cementitious material rising temperature shall be as under:
for 100% OPC
rise in temp = 48 degree centigrade
50% GGBFS +50% OPC
rise in temp = 28 -do70% GGBFS +30%OPC
rise in temp = 16 -doii) Chloride resistance: Blended cement concrete have a higher resistance
to the penetration of chlorides. The table below shows typical diffusivity.
Table 7.2
Diffusion of chloride at 25 0c in cement paste of w/c 0.5
S.N.
1.
2.
3.
4.
DIFFUSIVITY (X 10-9 cm2/s)
100.0
TYPE OF CEMENT
Sulphate Resistant
Portland Cement
OPC
70% OPC + 25% Flyash
30% OPC + 70% GGBFS
44.7
14.7
4.1
iii) The diffusivity is substantially reduced in case of blended cement. This is
due to two mechanisms. Firstly, the incorporation of slag reduces the
permeability of the concrete and secondly the hardened paste of slag cement
bind greater amounts of chlorides than that of OPC, resulting in much lower
portion of free chlorides in the pore solution. The effect of GGBFS in regard to
Cl- penetration is shown in fig. 7.2.
Age
Depth(mm)
OPC concrete
6
12
24
36
( months)
70% GGBFS + 30% OPC
Concrete
6
12
24
36
0-10
10-20
20-30
30-40
Symbol
Chloride by wt. Of
Concrete %
0.005%
0.05 to 0.10%
0.10 to 0.25%
0.25 to 0.50%
No penetration
Fig 7.2 Chloride penetration in concrete
DURABILITY OF CONCRETE STRUCTURES
DURABILITY OF CONCRETE STRUCTURES
It may be observed from Fig 7.2 that in the first 10mm layer there no
difference. However, in 10-20 mm and 20-30 mm depth Zone in case of
blended Cement Concrete Chloride penetration is significantly less (about 1/2
of OPC). In 30-40 mm depth zone, chloride penetration is much less as
compared to OPC. It also shows that first 10mm of concrete provides little
barrier to chloride ion penetration. It also brings out that cover should be in the
range of 40-50 mm so that the free chlorides shall be less than 0.1% i.e. the
threshold value for steel corrosion.
iv) Protection to steel corrosion: The blended cement concrete is more
resistant to Chloride penetration and thus provides protection in coastal areas
against corrosion many more times than OPC concrete.
v) Sulphate resistance: Blended cement with slag content more than 50%,
exhibits better sulphate resisting properties. Depending upon the severity of
the exposure to sulphate, limitations are placed on C 3A content in cement.
This is dealt under the chapter 6 Para 6.6 (vi) - Selection of cement.
vi) Alkali-silica reaction: Blended cement with high slag is a safe cement
system for the use with reactive aggregate . A comparison of expansion for
various concrete is given in table 7.3.
Table 7.3
Age
OPC
28 days
0.64%
The expansion
50%OPC+ 50%
25%OPC+75%
GGBFS
GGBFS
0.09%
0.04%
vii) Resistance to sea water: In marine exposures, concrete containing
blended cements exhibit enhanced durability. The studies done in Belgium,
Norway, Germany, England and France have found that blended cements
with more than 50% GGBFS, have a better durability.
7.5 Suggested blends : Various blends for concrete are suggested in table
7.4.
Table 7.4
Suggested blends for concrete
1.
2.
3.
Hot weather conditions (In
most parts of India except
during winter).
In cold weather concrete
Silica fumes
OPC (with revised specifications)
Gd 43 + 50% to 70% GGBFS*
or OPC Gd 43+10-25% fly ash**
Gd 53 OPC quick setting cement
8-10% Cementitious material, if the
concrete grade is above M-60. The
cost is above Rs. 30/kg. It should be
used in high rise buildings. It is an
imported material.
DURABILITY OF CONCRETE STRUCTURES
*
GGBFS- It is being manufactured and packed in 50kg bags. The exfactory rate is about Rs 60/bag of 50 Kg. To be taken from reputed
manufacturers.
** Fly ash: Should be processed and quality product. The quality of material
needs to be ascertained.
7.6 Ground granulated blast furnace slag (GGBFS) :
7.6.1 Blast furnace slag is produced as a by-product in the manufacture of
pig iron from iron ore in the blast furnace. The molten slag is tapped from the
blast furnace and quenched by pouring it over high pressure jets of water.
During this process slag is fragmented into small granules and hence called
‘granulated blast furnace slag’. Granulated slag is a latent hydraulic material
which is ground into a superfine state under controlled conditions. The end
product is called “ ground granulated blast furnace slag” which when blended
with ordinary portland cement gives extraordinary properties to concrete as
well as mortar. The use of GGBFS started abroad in late 50’s but in India it
started in 1996 on Mumbai Municipal Corporation project of 3.5 Km long
tunnel under sea to discharge sewerage, being constructed by M/S AFCONS.
Now GGBFS is being used in many important projects.
Example : The mix being used at the Mumbai Sewerage Project is :
i)
ii)
M-45
OPC T-40(30%) + GGBFS (from Andhra Cement Ltd) 70%
= 450 Kg/Cu.m.
iii) Sand (washed ) Zone II grading = 47%
iv) Crushed aggregate
= 20% agg.No.1+ 33% agg.No. 2
v) w/c ratio
= 0.33
vi) Super plasticizer
= 1% by weight of cement
vii) Workability
= 90mm.
The average compressive strength achieved is
1 day = 10.0 N/mm 2
3 days = 32.0
7 days = 38.0
28 days = 59.0
7.6.2 There is no IS: specification for GGBFS for use with OPC. BS : 6699
may be referred. Broad specification are given in table 7.5.
DURABILITY OF CONCRETE STRUCTURES
Table 7.5
Specification for GGBFS
S.
N.
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
7.7
Property
Unit
Requirement
Heat of Hydration (measured after 10 hrs.)
Glass content
Initial setting time
Final setting time
Fineness
Soundness ( 70% of GGBFS +30% OPC)
Compressive strength - (70% GGBFS +
30% OPC)
7 days
28 days
Chlorides
CaOSiO2
Manganese
Loss of ignition
Sulphur trioxide
Magnesia
Sulphur sulphide
Insoluble Residue
Sulphate expansion
Cl-diffusion Coeff n
J/gm
180 (max m )
%
70 (min m )
Minutes 180 (minm)
Minutes 230 (minm )
M2 /kg
400 (minm)
mm
5 (max m)
N/mm2 20 (minm)
N/mm2 40 (minm)
%
0.05% (max)
%
1.4% (max)
%
1%(max)
%
2%(max)
%
1.5%(max)
%
14%(max)
%
1.5%(max)
%
1.5%(max)
%
0.01(max m
X10 -12
4
2
m /Sec.
Fly Ash :
7.7.1
Fly ash is the ash precipitated electro-statically from the exhaust
fumes of coal fired power station. In India nearly 70 million tons of fly ash is
being produced every year while a very small quantity is used in
manufacturing of cement. It is an eco-friendly product. The fly ash particles
are spherical and are generally of higher fineness than cement so that the
silica is readily available for reaction. As per IS 3812 : 1981, the percentage of
silica and alumina should be minimum 70% and maximum loss on ignition 12
%. Much superior quality fly ash is available from thermal power plants than
specified in IS code.
The Portland Pozzolana Cement makes concrete more impermeable
and denser as compared to Portland Cement. The long term strength (90
days and above) of cement blended with flyash (>25%) is better compared to
OPC. The pozzolanic material reacts with calcium hydroxide liberated by the
hydrating Portland Cement and forms cementitious compounds generally
known as C-S-H gel. The reaction can be given as under:
C3S + 6H→ C3S2H3+ 3Ca(OH)2
3C3S + 4H → C3S2H3+3Ca(OH)2
Ca(OH)2 +(SiO2 +Al2 O3)→ C3S2H3+ other components
DURABILITY OF CONCRETE STRUCTURES
The flyash converts Ca(OH) 2 into useful cementitious compounds
(C3S2H3), there by increasing the properties of hardened concrete. The
blended Cement with flyash produces less heat of hydration and offers great
resistance to the attack of aggressive waters than normal Portland Cement.
Moreover it reduces the leaching of calcium hydroxide liberated during the
setting and hydration of cement.
7.7.2
The blended cement with flyash is ideally suited for the following
constructions;
• Hydraulic structures
• Mass concreting works
• Marine structures
• Masonry mortars and plastering
•
Under aggressive conditions.
7.7.3 Though blended cement with fly ash is ideally suitable under
aggressive
condition , it is necessary that quality of fly ash is properly
evaluated , as quality of Fly ash available at most of the places is doubtful.
Use of inferior quality of Fly ash with cement, may affect corrosion resistant
properties adversely. Thus, it is necessary that quality of Fly ash is evaluated
by reputed laboratory. It should be tested for chlorides, sulphate, alkalinity and
heat of hydration as per IS : 3812. The quality of fly ash to be blended with
cement and the concrete made out of it should be approved for corrosion
resistant properties by reputed organisation like CECRI, CBRI, CRRI etc. Fly
ash should be purchased from standard firms along with test certificates.
Attention:
The rate of development of strength is slow in case of blended
cement i.e. Portland pozzolana cement and Portland slag
cement etc., as compared to ordinary Portland cement. This
aspect should be taken care while planning to use blended
cement. Accordingly, stage of prestressing period of removal of
form work and period of curing etc. should be suitably increased.