A. FRANKOVI^ et al.: LIGHTWEIGHT AGGREGATES MADE FROM FLY ASH ...
267–274
LIGHTWEIGHT AGGREGATES MADE FROM FLY ASH USING
THE COLD-BOND PROCESS AND THEIR USE IN LIGHTWEIGHT
CONCRETE
LAHKI AGREGATI IZDELANI IZ ELEKTROFILTRSKEGA PEPELA
S POSTOPKOM HLADNEGA VEZANJA IN NJIHOVA UPORABA
ZA LAHKE BETONE
Ana Frankovi~1, Violeta Bokan Bosiljkov1, Vilma Ducman2
1University of Ljubljana, Faculty of Civil and Geodetic Engineering, Jamova 2, 1000 Ljubljana, Slovenia
2Slovenian National Building and Civil Engineering Institute, Dimi~eva 12, 1000 Ljubljana, Slovenia
vilma.ducman@zag.si
Prejem rokopisa – received: 2015-12-06; sprejem za objavo – accepted for publication: 2016-02-10
doi:10.17222/mit.2015.337
Aggregates made from fly ash have been developed by means of the cold-bonding process, with the addition of Portland cement
as a binder at (10, 20, and 30) % of mass fractions, and by pouring the mixtures into moulds. After curing for 28 d the samples
were processed into aggregate by crushing and sieving. An aggregate containing a weight percentage of 10 % of cement was
additionally produced by pelletization on a granulating plate. The density, water-adsorption capacity, porosity, compressive
strengths, and frost resistance of the samples were determined. The aggregates prepared by both routes were then used to make
concrete samples, whose properties were then compared to those of conventional concrete made using limestone aggregate. The
compressive strength of the concrete made with the granulated aggregate reached 16.0 MPa after 28 d, whereas that of the
concrete made with crushed aggregate amounted to 24.1 MPa, and that of the conventional concrete was 34.6 MPa.
Keywords: fly ash, lightweight aggregates, density, compressive strength, frost resistance
Agregati izdelani iz elektrofiltrskega pepela so bili razviti s postopkom hladnega vezanja, z dodatkom (10, 20 in 30) % masnega
dele`a Portland cementa kot veziva in z ulivanjem me{anice v modele. Po 28 dnevnem strjevanju vzorcev so bili vzorci
predelani v agregat z drobljenjem in sejanjem. Sestava, ki je vsebovala 10 masnih % cementa, je bila {e dodatno peletizirana na
plo{~i za granuliranje. Dolo~ene so bile: gostota, kapaciteta absorpcije vode, poroznost, tla~na trdnost in odpornost na
zamrzovanje. Sestave, izdelane na oba na}ina, so bile uporabljene za izdelavo betonskih vzorcev, katerih lastnosti so bile
primerjane z lastnostmi obi~ajno izdelanega betona z uporabo sestave apnenca. Tla~na trdnost betona, izdelanega z granulirano
sestavo, je dosegla 16,0 MPa po 28 dneh, medtem, ko je pri betonu izdelanem iz drobljenega agregata, zna{ala 24,1 MPa, pri
obi~ajnem betonu pa je bila 34,6 MPa.
Klju~ne besede: elektrofiltrski pepel, lahki agregati, gostota, tla~na trdnost, odpornost na zamrzovanje
1 INTRODUCTION
Although, if it has a suitable composition, fly ash can
be added to cement to form a "blended cement", much of
this ash is still deposited on landfill sites, causing an
environmental burden. Apart from this, according to the
European Waste Catalogue1 fly ash is labelled as hazar-
dous waste, so that even higher costs are incurred for
landfilling. Because of these facts, more and more scien-
tists have been trying to find new ways in which such ash
could be used, e.g., for the development of alkali-acti-
vated materials, which are frequently named geopoly-
mers2,3, but also for the production of artificial aggre-
gates.4–13 Artificial aggregates can be obtained simply by
the crushing and grinding of industrial waste if the basic
starting material is bulk waste. If, however, the basic
starting material is a fine powder, such as fly ash, such
aggregate can be produced by several different routes, as
follows:
1) by high-temperature procedures, e.g., foaming and/or
sintering at elevated temperatures of approx. 1200
°C,
2) by a hydrothermal process at a temperature of approx.
250 °C (autoclaving),
3) by the cold bonding process, where consolidation
takes place at room temperature.
In the last case a binding agent needs to be added to
the mixture in order to achieve such consolidation. This
agent is usually cement and/or water glass.14–18
In the first two processes a lot of energy is consumed,
so that recently more effort has been put into investiga-
tions of cold-bonding processes.
The most common method for the preparation of
aggregates from dust waste (e.g., fly ash) is granulation
on a pelletization plate. The efficiency of granulation
depends on the fineness of the fly ash, speed and time of
rotation of the plate, as well as its inclination and the
diameter of the plate 8,12,15,19. From the point of view of
costs, another more favourable method exists, in which
there is no need for a pelletization plate, or for dry waste
powder. This alternative method consists of mixing the
raw material with a binder, and pouring the mixture into
models. After the consolidation, setting, and hardening
Materiali in tehnologije / Materials and technology 51 (2017) 2, 267–274 267
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 67.017:666.96:620.173 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(2)267(2017)
processes have been completed, the samples are crushed
and sieved into different fractions.
Aggregates obtained in the above-described ways
(especially granulation) almost always have lower den-
sities. According to the definitions given in the standard
EN 13055-1:2002 20, lightweight aggregates (LWAs) are
those whose maximum particle density does not exceed
2000 kg/m3, and whose loose bulk density does not
exceed 1200 kg/m3.
For some decades lightweight aggregates have been
used in concrete instead of ordinary aggregates, mainly
because of their contribution to the improvement of
thermo-insulation properties. Apart from this, if LWAs
are used, then a reduction can be achieved in the con-
crete’s own weight, which can be important in earth-
quake-prone areas, as well as in cases where the subsoil
beneath the foundations has a low bearing capacity.
During the last decade much attention has been paid to
the role of LWAs with open porosity in the internal
curing of concrete. This is because lightweight aggre-
gates with such porosity can serve as water reservoirs,
which are available for later hydration, thus leading to
higher compressive strengths.21–23 Many authors used
lightweight aggregates made of fly ash in concreate to
investigate the strength and durability of hardened con-
crete, and the influence of a lightweight fly-ash aggre-
gate microstructure on the properties of concrete.24–29 As
expected, the compressive strength of LWAC is lower
than the one produced by natural aggregate, even though
LWAC can anyway serve as a structural concrete.25 If
fly-ash aggregate is sintered, it reaches a higher com-
pressive strength of the aggregate itself, and conse-
quently a higher compressive strength of the concrete is
achieved.26 For cold bonded (non fired) aggregate it is
also reported that chloride penetration could be some-
what higher.27 It is well recognized that by applying a
sintering process the properties of LWAs are improved
significantly,27,29 but it shall be noted that the firing pro-
cess contributes to a much higher production cost, and
on the other hand it negatively influences the environ-
mental impacts (contributing to higher CO2 footprints).
The aim of the present work was to verify the
usability of locally available fly ash (which due to the
high loss on ignition is not suitable as an additive for
cement) for the production of a lightweight aggregate,
and to compare the results of the aggregate obtained by
the granulation process to those obtained in the case
when fly ash, together with a binding agent, is first
poured into moulds and later crushed. The frost resis-
tance of such an aggregate was also tested and opti-
mised. The behaviour of both types of aggregate in the
concrete matrix was also investigated.
2 EXPERIMENTAL PART
2.1 Basic starting materials
The basic starting material that was used to develop
the investigated LWAs was fly ash, which was obtained
as a by-product in the combustion of brown Indonesian
coal. This particular type of fly ash contains only low
amounts of sulphur. The results of a chemical analysis
showed that this fly ash contains the following chemical
components in the stated mass percentages: SiO2 27.57
%, Al2O3 8.3 7%, Fe2O3 16.24 %, MgO 7.06 %, CaO
22.99 %, Na2O 0.67 %, K2O 2.67 %, and TiO2 0.5 %.
The loss on ignition amounts to 12.57 %. Because of its
chemical composition, this type of fly ash is classified,
according to EN 450-130, as an alumino-silicate fly ash.
According to the results of a mineralogical analysis, it
consists of a glassy phase (67 %), brownmillerite (9.1
%), quartz (8.3 %), lime (6.1 %), anhydrite (3.2 %),
calcite, portlandite, and hematite.
The particle size distribution was determined by
sieving the ash through sieves with diameters of 1.8 mm,
0.5 mm, 0.125 mm, 0.063 mm and 0.04 mm, as is shown
in Figure 1. Most of the particles in the fly ash were
smaller than 1 mm, 40 % of them being smaller than
0.04 mm. The Blaine finesse was determined according
to EN 196-6 and it amounted to 2989 cm2/g.
Table 1: Composition of mixtures for the preparation of the LWA
Tabela 1: Sestava me{anic za pripravo LWA
Compo-
sition Mixture
Fly Ash
(g)
Cement
(g) Water (g)
PT1 90 % FA +10 % CEM 3600 400 2000
PT2 80 % FA +20 % CEM 3200 800 2000
PT3 70 % FA +30 % CEM 2800 1200 2000
GT1 90 % FA +10 % CEM 3600 400
Added as necessary
for the needs of the
granulation process
The binding agent cement CEM I was added in
different proportions, as shown in Table 1. A Cementol
Hiperplast 179 superplasticizer was also used.
For the frost-resistance improvement a commercial
air entraining admixture Cementol ETA S was added.
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268 Materiali in tehnologije / Materials and technology 51 (2017) 2, 267–274
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 1: The sieving curve corresponding to the investigated fly ash
Slika 1: Sejalna krivulja preiskovanega elektrofiltrskega pepela
2.2 Procedures for the production of the investigated
LWA
The aggregates were produced by means of two diffe-
rent methods.
The aggregate that was designated "PT" was pro-
duced from prisms, which were made by first mixing the
fly ash with the selected binder and water, and then
pouring the mixture into standard mortar moulds. After
curing in a climatic chamber at a temperature of 21 °C
and a relative humidity of 94 % for 28 d, the prisms were
crushed and then sieved into fractions of 0–1 mm, 1–2
mm, 2–4 mm, and 4–8 mm. The aggregate that was
designated "GT" was produced by the pelletization pro-
cedure31, using a rotating plate (Figure 2).
During the granulation phase the tilting angle was
fixed at 60°, the mixer speed was 48 min–1, and the
mixing time was 2 min. The fly ash had been previously
mixed with cement, and was then poured slowly onto the
plate. During the granulation process, water droplets
were added by spraying.
Both types of aggregate are shown in Figure 3a (the
PT aggregate) and 3b (the GT aggregate).
Normal-weight aggregate, i.e., dense limestone
aggregate from the La`e quarry in SW Slovenia, was
used for the preparation of the blank concrete samples.
2.3 Characterization of the investigated aggregates
The porosity, pore size distribution, apparent density,
and bulk density of the prepared aggregates were deter-
mined by means of mercury intrusion porosimetry
(MIP). Particles having a size of approximately 1 cm3 for
each of the prepared artificial aggregates were dried in
an oven for 24 h at 110 °C, and then analysed by means
of MIP Autopore IV 9500 equipment (Micrometrics).
The microstructure of the polished cross-sections of
the aggregate samples were examined using the back-
scattered electron (BSE) image mode of a low-vacuum
scanning electron microscope (SEM) using JEOL 5500
LV equipment.
The water absorption of all three types of crushed
aggregate (PT1, PT2, PT3) and of the granulated aggre-
gate was determined by measuring the dry mass mdry and
the wet mass after immersion for 24 h in water (this is
the saturated surface-dry mass msat). It was calculated
from Equation (1):
W
m m
m
=
−
⋅
sat dry
dry
100 (1)
The mechanical properties of the prisms were
evaluated according to EN 196-1:200532, whereas the
compressive strength of the granules was determined
after curing the granulates in a climatic chamber at 21 °C
and a relative humidity of 94 % for 28 d, according to
the procedure described by C. R. Cheeseman5, where
individual granules are loaded to failure between two
parallel plates. For the calculation of the compressive
strength the following Equation (2) was used:
R
F
hc
c=
⋅
⋅
2 8
2
,
π
(2)
where Fc is the load causing failure (i.e., fracture), h is
the spherical diameter of the granule, and the value of
2.8 is a shape factor.
The frost resistance of the aggregate was determined
according to EN 13055-1:2002 20 on two parallels, where
in each parallel 400 g of dry samples (mass M1) of the
fraction 4–8 mm was immersed in water for 4 h. After
that the samples were exposed to 20 cycles of freezing-
thawing, from –18 °C to +18 °C. After 20 cycles the
samples were dried and sieved through a sieve with
apertures of 2 mm. The remains on the sieve represent
the mass M2. The percentage of mass loss due to the
freezing action (F) is calculated using Equation (3):
F
M M
M
=
−
⋅1 2
2
100 (3)
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 2: The granulating plate with a schematic presentation of the
granulation process
Slika 2: Kro`nik za granuliranje s shematskim prikazom procesa
granulacije
Figure 3: The final products: a) PT- made from crushed aggregate and
b) GT- made from granulated aggregate
Slika 3: Kon~ni proizvodi: a) PT – izdelan iz zdrobljenega agregata in
b) GT – izdelan iz granuliranega agregata
The mechanical properties of the prisms were deter-
mined by means of a 300-kN ToniNorm compression
testing machine.
The bulk densities and strength tests of the concrete
samples were determined on standard prisms for mortars
of (4 × 4 × 16) cm.
2.4 Preparation of the fly-ash aggregate concrete
mixes
The compositions of the concrete mixes that were
prepared from the two different types of artificial aggre-
gate (PT1-C, and GT1-C), and that of the mix made
using natural limestone aggregate (LMS-C), are shown
in Table 2.
Table 2: Composition of the concrete specimens made by different
types of aggregate
Tabela 2: Sestava betonskih vzorcev, izdelanih z razli~nimi vrstami
agregatov
Sample designation PT1-C GT1-C LMS-C
Type of aggregate crushed granulated crushed
Mass (g)
Aggregate 5188 3269 9595
Cement 1505 1505 1505
Fly ash 645 645 645
Water in the aggregate 917 1531 0
Added water 1110 769 1227
Super plasticizer 7.5 7.5 7.5
w/c ratio
w/c ratio (with water in
the aggregate) 0.94 1.07 0.57
w/c ratio (without
water in the aggregate) 0.52 0.36 0.57
The contents of the individual fractions of the aggre-
gates were determined according to the curves specified
in the standard SIST 1026:2008 33 (curve B for the
crushed aggregate, and curve A for the granulated aggre-
gate (Figure 4)).
Because of the high water absorption of the light-
weight aggregates, prior to mixing they were immersed
in water for 30 min and then drained in order to remove
the surface water. Taking into account the k-value con-
cept for the fly ash (according to EN 206: 201334), a
maximum of 30 % of the cement binder was replaced by
fly ash as an additive. Compressive strength tests accord-
ing to the standard EN 196-1:200532, as well as density
tests, were performed on hardened test specimens after
(7, 28, and 90) days. In all cases the test specimens were
cured in a climatic chamber at a temperature of 21 °C
and a relative humidity of 94 %.
3 RESULTS
3.1 Characterization of the aggregates
The densities and porosities of the investigated
aggregates, determined by MIP, are shown in Table 3.
The pore size distributions for the artificial aggregates
are shown in Figure 5.
Table 3: Density and porosity of the investigated aggregates, as
determined by MIP
Tabela 3: Gostota in poroznost preiskovanih agregatov, dolo~ene z
MIP
Sample
designa-
tion
Total
intrusion
volume
(cm3/g)
Total
pore
area
(m2/g)
Average
pore
diameter
(μm)
Bulk
density
(g/cm3)
Apperent
density
(g/cm3)
Porosity
(%)
PT1 0.3899 33.171 0.047 1.182 2.1923 46.08
PT2 0.3710 46.925 0.0316 1.258 2.359 46.67
PT3 0.3178 50.231 0.0253 1.333 2.313 42.37
GT1 0.6429 31.4 0.0819 0.9195 2.2487 59.11
If the results obtained for the aggregates PT1, PT2,
and PT 3 are compared, it can be seen that in the case of
a higher amount of added binder (cement), the density of
the aggregate is higher, and the porosity is lower. It can
be further seen, comparing GT1 and PT1, that the gra-
nulated aggregate has a lower density and higher
porosity than the crushed aggregate.
For instance the density of the granulated aggregate
was 0.9 g/cm3, whereas that of the aggregates obtained
by crushing was significantly higher, i.e., 1.2 g/cm3.
However, both of these two aggregates, i.e., PT1 and
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270 Materiali in tehnologije / Materials and technology 51 (2017) 2, 267–274
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 5: Measured pore size distribution for the two artificial aggre-
gates
Slika 5: Izmerjena razporeditev velikosti por v dveh izdelanih agre-
gatih
Figure 4: Recommended sieve curves for 0/8 mm aggregate mixtures
taken from SIST 1026:200833
Slika 4: Priporo~ene sejalne krivulje za 0/8 mm me{anico agregatov,
vzeta iz SIST 1026:200833
GT1, can be classified as lightweight aggregates accord-
ing to EN 13055-1:2002.20 The measured porosity of the
aggregate PT1 was found to be equal to 46.1 %, whereas
that of the aggregate GT1 amounted to 59.1 %.
From Figure 5 it can be seen that, in the case of
aggregate PT1, almost all the pores have sizes of less
than 1 μm, the average pore size being 0.047 μm,
whereas in the case of aggregate GT1, the pore sizes
ranged between 0.003 μm and 100 μm. The average pore
size of aggregate GT1 was 0.082 μm.
The measured water absorption of the crushed aggre-
gate was, as expected from the density of the aggregates,
less than that of the granulated aggregate. In the case of
the aggregate sample PT1 the water absorption amounted
to 38.4 % (in the case of the samples PT2 and PT3:
35.9 % and 30.7 % respectively), whereas in the case of
aggregate sample GT1 it amounted to 57.8 %. According
to these results the porosity of the aggregates can be
classified as being open. It is clear that the content of the
binder has an effect on the water absorption of the
aggregate.
The results of the tensile and compressive strength
tests performed on the aggregates are presented in Table
4. As could be expected, the results show that the crush-
ing strength increases with the binder content. The
tensile strength of the crushed aggregates (samples PT1,
PT2, PT3) ranged between 2.2 MPa and 4.6 MPa,
whereas their compressive strengths ranged between
12.6 MPa and 30.6 MPa. In the case of the granulated
aggregate granules (sample GT1) only the compressive
strength was determined, by measurements that were
performed on single granules. For this reason the value
cannot be directly compared to the values obtained in the
case of the PT samples. It amounted to 0.96 MPa.
Table 4: Crushing strengths of the aggregate samples
Tabela 4: Trdnost vzorcev agregatov pri drobljenju
Bending strength Compressive strength
Sample
designa-
tion
after 7 d
curing at
94 %
humidity
(MPa)
after 28 d
curing at
94 %
humidity
(MPa)
after 7 d
curing at
94 %
humidity
(MPa)
after 28 d
curing at
94 %
humidity
(MPa)
PT1 2.1 2.2 6.8 12.6
PT2 2.9 3.2 11.1 18.6
PT3 4.1 4.6 19.5 30.6
GT1 / / / 0.96
The results of the SEM investigation confirmed the
differences in the microstructure between the crushed
aggregate and the granulated aggregate. The microstruc-
ture of sample PT1 was found to be more homogeneous
than that of sample GT1 (Figure 6). In both figures
grains of fly ash can still be seen. In the case of sample
GT1 several smaller granules are bonded together to
form a larger granule, with large associated air pores
inside, which contributes to the higher porosity of the
granulated aggregate.
Comparing the properties of the granulated aggregate
to the properties of the crushed aggregate it can be noted
that the crushed aggregate has better mechanical pro-
perties and a lower porosity, which could be partially
ascribed to the process of pelletization (if not all the
parameters are optimal), but also to the fact that during
pelletization the w/c factor is not uniformly distributed
throughout the single granules of aggregate, which intro-
duce weak points at the microstructural level. By the
mixing, pouring and crushing process of aggregate pro-
duction a more uniform w/c and microstructure of
aggregate is obtained.
3.2 Characterization of the hardened concrete test
specimens
In Table 5, the results for the compressive and
bending strengths are presented for hardened test
specimens of the concrete mixes, which were made from
LWAC, whose preparation is defined in Table 2. The
strength of the concrete made with limestone aggregate
is higher than that of the concrete made using the
artificial LW crushed or granulated aggregate, despite the
highest water-to-cement ratio. However, when crushed
aggregate is used, the bending and compressive strengths
of the hardened concrete made from them are consider-
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Figure 6: BSE SEM images of samples: a) PT1 and b) GT1of the
investigated aggregates, shown at the same magnification
Slika 6: BSE SEM-posnetek vzorcev: a) PT1 in b) GT1, preiskovanih
agregatov, prikazanih pri enaki pove~avi
ably higher than those that can be achieved with
granulated aggregate. This is confirmed by the results
that are shown in Table 5, where, despite the much lower
water-to-cement ratio of test specimen GT1-C (0.36), the
obtained strengths are still much lower than those
corresponding to test specimen PT1-C, which had a
water-to-cement ratio of 0.52.
Table 5: Determined compressive and tensile strengths of the concrete
test specimens
Tabela 5: Dolo~ene tla~ne in upogibne trdnosti preizku{anih vzorcev
betona
Test specimen designation PT1-C GT1-C LMS-C
Tensile
strength
(MPa)
after 7 d curing at
94 % humidity 2.9 2.6 5.6
after 28 d curing at
94 % humidity 3.5 2.7 6.4
after 90 d curing at
94 % humidity 3.6 2.8 7.4
Compres-
sive
strength
(MPa)
after 7 d curing at
94 % humidity 19.0 13.5 28.7
after 28 d curing at
94 % humidity 24.1 16.0 34.6
after 90 d curing at
94 % humidity 25.3 17.1 40.5
The density of the concrete made with crushed aggre-
gate amounted to 1610 kg/m3, whereas that of the con-
crete made with granulated aggregate was 1490 kg/m3.
As expected, the density of the concrete made with
natural limestone aggregate (LMS-C) was much higher,
and amounted to 2290 kg/m3 (Table 6).
Generic values for the thermal properties can be
obtained from the standard EN 1745:201234,35 (Table A.9
for lightweight aggregate concrete, and Table A.2 for
dense aggregate concrete) – the assigned values for the
different types of concrete prepared within the scope of
this investigation are also given in Table 6.
Table 6: Density of the different types of concrete together with the
corresponding thermal properties, based on the generic values given in
the standard EN 174535
Tabela 6: Gostota razli~nih vrst betonov skupaj z njihovimi
toplotnimi lastnostmi, na osnovi generi~nih vrednosti danih v
standardu EN 174535
Sample designation PT1-C GT1-C LMS-C
Density
(kg/m3)
After 90 d
curing 1610 1490 2290
10,dry
(W/mK)
Acc. to EN
1745 0.84 0.73 1.36
According to the properties obtained in the case of
test specimen PT1-C (a tensile strength of 3.5 MPa, a
compressive strength of 24.1 MPa, and a density of 1610
kg/m3), this type of concrete can be classified as a me-
dium-strength concrete with a low density and improved
thermal insulation properties.
SEM investigations of the concrete did not show
significant differences in the microstructures of the
concrete that had been made with crushed aggregate or
granulated aggregate (Figure 7). The observed cement
matrix is homogeneous with relicts of portlandite, and
adhered well to the aggregate. Interlocking of the cement
matrix with the porous crushed aggregate (PT1), where
the cement paste entered the crushed grains, was more
pronounced than in the case when the rounded granu-
lated aggregate (GT 1) was used.36
3.3 Determination and optimization of aggregate frost
resistance
The frost resistance of an aggregate depends to a
large extent on the pore structure of its grains. It is
well-known that aggregates with a lower porosity and a
greater proportion of fine pores possess better frost
resistance.37
The results of the tests that were performed in order
to determine the frost resistance of the investigated
aggregate indicated a significant loss of mass upon
freezing (Table 7), which was especially severe in the
case of the granulated aggregate (GT1), where the loss of
mass amounted to 96.7 %. The very high loss of mass
(96.7 %) on freezing of the granulated fly ash aggregates
could be ascribed to the very high porosity (water
absorption of 57.8 %) and the very low crushing strength
(0.96 MPa).
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MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 7: BSE SEM-images of: a) the test specimens PT1-B and b)
GT1-B, shown at the same magnification
Slika 7: BSE SEM-posnetek preizkusnih vzorcev: a) PT1 – B in b)
GT1 – B, prikazan pri enaki pove~avi
Samples that were obtained by pouring the mixtures
into moulds, and after that crushing and sieving (they
were designated PT), resulted in a somewhat improved
frost resistance due to the lower initial porosity, as well
as better mechanical properties. However, in this case,
too, the loss of mass was significant (from 19.1 % to
31.5 %). In the case of sample PT 1 improvements in the
frost resistance were searched for in two directions. In
the first test (sample PT 1a), the fly ash was additionally
sieved onto a 0.125 mm sieve (in this way the majority of
the organic particles were removed; loss on ignition
decreased from the original value of 10.17 % to 3.3 %),
and then processed in the same way as PT 1. In second
test (sample PT 1+ ETA S) the sample of fly ash was not
additionally processed by sieving, but instead a commer-
cially available air entraining admixture (normally used
for the improvement of the frost resistance of concrete)
was added, whose mass amounted to 0.1 % of the mass
of the cement. Both approaches significantly improved
the frost resistance of the aggregate. The sieving away of
particles above 0.125 mm resulted in a mass loss of
8.1 % after freezing thawing , whereas the adding of an
air-entrapping agent resulted in a mass loss of 1.8 %
after freezing.
Table 7: Loss of mass after 20 cycles of freezing-thawing
Tabela 7: Izguba mase po 20 ciklih zmrzovanja-taljenja
Sample designation Loss of mass (%)
GT1 96.7
PT1 19.1
PT1a 8.1
PT2 31,5
PT3 25,9
PT1+ETA S 1,8
4 CONCLUSIONS
Based on the results of the performed investigations,
it can be concluded that in the case of pouring and
crushing (PT), aggregates of higher density and strength
can be obtained in comparison with aggregates that can
be obtained by granulation (GT). In the case of crushing,
polygonally shaped aggregates are obtained, which can
improve the interlocking effect with the cement matrix.
Both of the two types of investigated aggregate (PT
and GT) were highly porous (with porosities of 38.4 %,
and 57.8 %, respectively), and their porosity was of the
open type. In such cases water can be stored within the
aggregate grains, which can then be available for
subsequent hydration. Open porosity can also contribute
to a stronger interfacial zone between the aggregate and
the cement matrix.
The frost resistance of such aggregates is influenced
by their content of organic particles, but it can be signi-
ficantly improved by removing part of the organic
particles and /or by adding a commercially admixture for
frost-resistance improvement.
The application of such aggregates in concrete has
confirmed their usability in the construction sector by the
utilization of nearly 80 % of fly ash in concrete (partly as
an additive to cement, with its main share in the
aggregate).
The methodology described in the paper for the use
of fly ash could also be used for other kinds of waste
dust that are generated, for example, in the construction
industry, in agriculture, and in the refractory industry.
Acknowledgements
This work was financially supported by the Slovenian
Research Agency Programme Group P2-0273.
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