Study of the substitution of limestone filler with pozzolanic additives in
mortars
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M. Katsioti , D. Gkanis , P. Pipilikaki , A. Sakellariou , A. Papathanasiou , Ch. Teas , E. Chaniotakis
1. School of Chemical Enginneering, NTUA, 9, Heroon Polytechniou St., 157 73 Athens,
Greece,email:[email protected]
2. Public Power Corporation/ Testing Research and Standards Center
3. ΤΙΤΑΝ Cement Company S.A.
Abstract
In this work several specimens of mortars were prepared with the addition of 5% fly ash and 5% perlite
and their mechanical properties and porosity were tested and compared to those of mortars with no
additives (reference sample).
Specifically, it was studied the influence that these additives have on the elastic modulus and porosity
of the mortars.
The following measurements were made: a) chemical, mineralogical and granulometric analysis of
additives, b) normal plasticity water, c) compressive and bending strength at 28 days, d) adhesion at 28
days, e) air content, f) specific gravity of mortar, g) dispersion, h) porosity and pore size distribution of
mortar were investigated by mercury porosimetry, i) the morphological character of different mortar was
measured by scanning electron microscopy (SEM). Finally, the Young’s elastic modulus was measured
in cylindrical specimens sized 50/100mm (diameter/height) according to ASTM C 469 – 02 Standard.
The results of the present study indicated a differentiation in the microstructure of the mortars that can
be contributed to the use of different additives, such as fly ash and perlite.
The elastic modulus value is related to the ratio of compressive to flexural strength and in particular,
with the increase of the value of the ratio, the value of the Young’s elastic modulus increases. The
development of the microstructure represents a major parameter to improve existing mortars and to
formulate new mortars.
Key words: perlite, fly ash, elastic modulus, porosity, cement mortar
1. Introduction
Several studies have reported that the addition of fly ash improves the mechanical
characteristics of mortars as a consequence of their pozzolanic activity and the spherical form
of their particles. [1]
Perlite has been extensively used as a lightweight aggregate material in concrete or mortar,
offering thermal insulation, fire resistance and enhancing their mechanical properties. Perlite
possesses pozzolanic properties, and has been extensively used as a replacement of common
aggregates in mortars. [1]
The addition of natural pozzolanes to mortar enhances the final resistances since it acts both
as a chemically inert filler, improving the physical structure; and as a pozzolan, reacting
chemically with the Ca(OH)2 formed during the hydration of cement. Such property variations
are of interest, since they affect the durability of structures designed with these materials. [2]
The effects of size and shape of aggregates on the development of strength, adhesion and
deformability have been established in many previous studies and were kept constant during
this study with only the addition of the pozzolanic additives making the difference. The water to
solid ratio was also kept constant as previous studies indicated that it affects the strengths of
the mortars. [3], [4], [5]
The influence these additives have in the porosity of the mortar, has been examined as the
pore size and their distribution affects compressive strength of the mortar. [2]
In the present study, the relationship between compressive and flexural strength of mortar and
the elastic modulus as well as between compressive strength of the mortars and porosity was
investigated using different additives such as fly ash and perlite. The aim of the present study
was to investigate the use of pozzolanes natural and artificial for the production of mortars and
compare the effect each type of the additives has on the properties of the mortar having a
reference sample without pozzolanic additives. [2], [6], [7], [8]
2. Experimental
2.1 Characterization of additives of mortars
The chemical analysis of the materials used is summarized in Table 1.
SiO2
Al2O3
Fe2O3
CaO
MgO
K2O
Na2O
SO3
LOI
Table 1: Major element analysis of cement and additives
LIMESTONE
PERLITE
FLY ASH
CEM I 42,5R
FILLER
74,42
19,38
2,77
48,09
13,07
4,28
1,39
21,38
3,24
0,64
1,17
8,40
64,11
50,60
1,33
13,37
3,43
0,71
0,31
2,30
2,43
0,57
0,14
2.42
4,21
0,17
0.52
3,09
2,70
3,73
40,50
2,66
1,21
From the mineralogical analysis of the additives it is obvious that they are both highly
amorphous, especially perlite and that makes them more reactive. They both have high
amounts of quartz and some anorthite while fly ash contains some anhydrite and larnite. The
XRD analysis can be seen in Figure 1.
Figure 1: Mineralogical analysis of fly ash (a) and perlite (b)
2.2 Specimen preparation
Three types of mortars were prepared according to EN1015-2/3 and EN1015-11 and cast in
prismatic moulds of 40x40x160mm from where they were demoulded 48 hours later. The first
one (REF) is a normal cement mortar and it is used as a reference. The other two have 5%
substitution of the limestone aggregates for perlite (PER) and fly ash (FA) .The composition of
the mortars samples is given in Table 2.
Table 2:Composition of solid constitutes of the mortar samples
REF
PER
FA
LIMESTONE FILLER (%)
90,0
85,0
85,0
HYDRATED LIME (%)
1,5
1,5
1,5
CEMENT (%)
8,5
8,5
8,5
PERLITE (%)
0
5
0
FLY ASH (%)
0
0
5
In order to study the hydration products and the microstructure of the produced mortars, all
mortars were tested for air content, density, compressive and flexural strength, XRD, SEM,
elastic modulus and mercury porosimetry.
X-Ray Diffraction analysis experiments were carried out using a Siemens D5000 and their
evaluation were made using Siemens DIFFRAC A.T. Search Program Software.
Microstructural investigation was conducted using a scanning electron microscope.
The Young’s elastic modulus was measured in cylindrical specimens sized 50/100mm
(diameter/height) according to ASTM C469-02 Standard.
Finally, mercury intrusion porosimetry technique was employed to measure the porosity and
pore size distribution of the mortar samples. A Micrometrics Autopore III mercury porosimeter
was used for porosimetry measurements, having a pressure range from 0-60000 psia.
3. Results and Discussion
3.1 Properties of Fresh Mortars
The results of tests in fresh state mortars are presented in Table 3 and grain size distribution in
Figure 2, which is common for all mortars.
Table 3: Properties of Fresh Mortars
REF
PER
FLOW VALUE(cm)
17,1
16,8
3
WET BULK DENSITY (Kg/m )
1830
1850
AIR CONTENT(%)
18
16
WATER DEMAND(%)
15,25
15,50
WATER RETENTION(%)
94,88
94,86
FA
17,0
1870
15
15,75
94,84
REF
PER
FA
100
90
80
Passing (%)
70
60
50
40
30
20
10
0
90μm
0,125
0,25
0,5
0,71
1
1,25
2
3,15
Grain Size (mm)
Figure 2: Grain Size Distribution of solid constitutes of mortar samples
3.2 Mechanical Properties
Compressive and flexural strength measurements were conducted at the age of 28 days on
mortar prisms 40x40x160mm according to the EN-1015-11 standards. Adhesion tests were
conducted at the age of 28 days according to the EN-1015-12 standards. The results are given
in Table 4.
Table 4: Mechanical Properties of Mortars
Mortar
Compressive Strength
(MPa)
Flexural
(MPa)
REF
PER
FA
4,9
4,8
5,3
2,2
1,8
2,1
Strength
Compressive
Flexural Strength
2,23
2,67
2,52
to
Adhesion (MPa)
0,30
0,37
0,23
Mortar containing perlite is lighter (see Table 3) and presents better adhesion than fly ash; on
the contrary, fly ash mortar has better compressive strength (see Table 4).
There seems to be a difference in the adhesion, which needs further examining.
3.3. Mineralogical analysis of Hardened Mortar
The hydration products were mineralogically determined by X-Ray Diffraction. The results are
given in Figure 3.
Figure 3: Mineralogical analysis of hydrated mortars at 28 days
From the x-ray analysis patterns of the three hydrated mortars it is determined that the mortars
have hydrated giving cement hydration products such as portlandite, calcium silicate hydrate
and ettringite. It is also noticeable that portlandite reduced in the containing perlite and fly ash
mortars due to the pozzolanic reaction and that is more obvious in the fly ash mortar.
3.4 Scanning Electron Microscopy (SEM)
The morphological characteristics on the different mortars were measured with the use
of SEM.
Typical hydration products can be identified such as ettringite (needle-like crystals),
calcium silicate hydrate (gel-like flocks) and finally calcium hydroxide (plant-like crystals).
It is also obvious that fly ash hydrates faster than perlite and that’s why perlite gives a
higher porosity that tends to close with hydration products in more mature ages.
Figure 4,Figure 5 and Figure 6 represent the images from the SEM analysis of the mortars
containing fly ash and perlite and the reference.
Figure 4: SEM image of the reference mortar
Figure 5: SEM image of the mortar containing
perlite
Figure 6: SEM image of the mortar containing fly ash
3.5 Elastic Modulus
Measurements of the elastic modulus were conducted in the three mortars in order to evaluate
their behaviour under mechanical strain. In fact that elastic modulus is a major property in the
behaviour of the mortars against distortion the compatibility of the elastic modulus of the
mortars with the additives and the reference mortar is essential. For this purpose the Young’s
elastic elastic modulus was measured in cylindrical specimens sized 50/100mm (diameter/height)
according to ASTM C 469 – 02 Standard. The results are given in Table 5.
Table 5: Results of Elastic Modulus
E(MPa)
Mortar
28d
REF
3851
PER
4464
FA
4131
The results from the elastic modulus measurements prove that this compatibility is achieved. It
is remarked that there is a good correlation between the elastic modulus and the Compressive
to Flexural Strength ratio. This relation appears in Figure 7 where the correlation coefficient is
R2=0,94.
4500
R2 = 0,9392
4400
4300
Ε(MPa)
4200
4100
4000
3900
3800
3700
2,2
2,3
2,4
2,5
2,6
2,7
Compressive to Flexural Strength Ratio
Figure 7:Relation between Elastic modulus and Compressive to Flexural Strength Ratio
3.6 Mercury Porosimetry
For the determination of pore size distribution, the cylindrical mathematical mode (Washburns
Equation) was employed to elaborate the results P = -2Kcos(h)/r, in which “r” is the cylindrical
pore radius, “P” is the pressure at which mercury enters into the pore, “K” is the surface
tension of mercury (0,48N/m at 25oC) and “h” is the contact angle between the mercury
meniscus and a flat, nonmetallic surface (~140o). The smallest size of pore in to which mercury
can be intruded is 2nm, whereas the largest one is 7500nm. The results of total cumulative
pore volume, specific surface area, pore radius average and bulk density, are summarized in
Table 6.
MORTAR
REF
PER
FA
Table 6: Results from Mercury Porosimetry
Total
Average
Total
Bulk
Pore
Pore
Intrusive
Density
Area
Radious
Volume
(g/ml)
2
(m /g)
(μm)
(ml/g)
2,222
1,123
5,691
2,314
2,335
2,362
0,131
0,250
0,049
0,146
0,140
0,138
Porosity(%)
33,78
32,72
32,58
The mortar containing fly ash presents larger total pore area and smaller average pore radius
than the perlite and has better compressive strength (see Table 4).
The pore size distribution of the tested fly ash mortars tends towards a more uniform pore size.
Figure 8 presents the Pore Size Distribution of Mortars and Figure 9 presents the Cumulative
Pore Volume of the three mortars. According to Figures, the differentiation of the large size
pore distribution of the perlite containing mortar is obvious. This mortar has in comparison with
the other two, greater amount of large pores (greater than 5μm) although the total porosity is
equal for all three mortars. This is also explained by the fact that fly ash hydrates in earlier
days than perlite and in that way it gives more gel porosity than capillary porosity.
0,01
Incremental intrusion mL/g
0,009
0,008
0,007
0,006
FA
REF
PER
0,005
0,004
0,003
0,002
0,001
0
1000
100
10
1
0,1
0,01
Pore radius μm
Figure 8:Pore Size Distribution of Mortar
0,001
0,16
cumulative intrusion mL/g
0,14
0,12
0,1
FA
REF
PER
0,08
0,06
0,04
0,02
0
1000
100
10
1
0,1
0,01
0,001
Pore radius μm
Figure 9:Cumulative Pore Volume of Mortar
4. CONCLUSIONS
The partial substitution of the aggregate with 5% perlite or 5% fly ash in mortars after 28 days
has shown that:
•
•
•
The XRD and SEM analysis evaluation of the perlite and fly ash containing mortars, have
proved the presence of hydraulic compounds: portlandite, calcium silicate hydrate and
ettringite.
As far as the strength at 28 days seems to be the same for all mortar samples with a
tendency for increase in the fly ash containing mortar samples (this is expected to be
verified from the results of the analysis of later ages samples, e.g. 90 days etc.).
The elastic modulus was measured at about 4000 MPa with no significant differences for all
mortar samples. The elastic modulus being of almost equal magnitude for perlite and fly
ash containing mortars and the reference mortar, it is estimated that it results from the
compatibility in their mechanical behaviour.
Acknowledgements
The authors are grateful to the General Secretariat for Research and Technology of Greece for
financial support in the framework of the Program “Production of blended Portland cements.
Study of the physicochemical properties and durability of the produced mortars and concretesPENED 03”.
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