140E-ASCE Journal, Introducing a Method to Determine Nonautoclaved

Introducing a Method to Determine Nonautoclaved Aerated
Concrete Air content Based on Packing Theory
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Mohsen Mohammadi 1; Ali Akbar Shirzadi Javid 2; and Mehdi Divandari 3
Abstract: This paper presents an experimental study on fresh and hardened properties of nonautoclaved aerated concrete (NAAC) mixtures.
It also attempts to address a new method to determine the packing density and air percentage of NAAC mixtures. Different types of NAAC
mixtures with various aluminum powder percentages and water-to-cement ratios were made. Results revealed that the new method (called the
wet packing theory) is able to determine the percentage of air pores in the aerated concrete with high accuracy. Results also showed that there
is an optimum aluminum percentage of approximately 0.0934 from the perspective of minislump diameter and compressive strength. With an
increase in the aluminum percentage compared to the optimum percentage, minislump (workability and flow-ability) and compressive
strength reduced. Scanning electron microscope (SEM) images of NAAC indicated that the air voids were shaped as an artificial porosity.
The results showed that with an increase in the amount of aluminum powder, packing density reduced. It was observed that the maximum
air percentage increased with an increase in the amount of aluminum percentage from 0.0934 to 0.1869. DOI: 10.1061/(ASCE)MT.19435533.0002180. © 2017 American Society of Civil Engineers.
Author keywords: Nonautoclaved aerated concrete; Air percent; Packing theory; Aluminum powder.
Introduction
Aerated concrete refers to concrete having a high value of pores and
air voids. These air bubbles are produced to reduce the density of
the mixture and provide good thermal insulation. Aerated concrete
was first presented in the 1930s and was referred to as foamed,
cellular, or gas concrete (Neville 1981; Holt and Raivio 2005;
Elhaddad 1993; Ng et al. 2012).
Various uses of lightweight concrete are well known all over the
world. It is proposed that the reduction of concrete’s self-weight is
the best option to reduce the dead load of structure and eventually
reduces the dimension of structural elements.
Aerated concrete is classified into two categories: autoclaved aerated concrete (AAC) and nonautoclaved aerated concrete (NAAC).
Most research studies have been carried out on the autoclaved category. The aerated concrete manufacturing process involves the
creation of macroporosity in a micromortar matrix made of sand,
lime, cement, and water in the presence of an expansive admixture
(Cabrillac et al. 2006; Narayanan and Ramamurth 2000a). In this
type of concrete, aluminum powder reacts with the lime (released
by the hydration of the binder) and water (Holt and Raivio 2005;
Wittman 1983; Xia et al. 2013; Mostafa 2005; Cenk et al. 2010).
The air voids generated by this chemical reaction cause the fresh
mortar to expand and lead to an increase in air pores (Narayanan and
Romamurthy 2000b).
1
Research Assistant, School of Metallurgy and Materials Engineering,
Iran Univ. of Science and Technology, P.O. Box 16765-163, Narmak,
Tehran, Iran. E-mail: [email protected]
2
Assistant Professor, School of Civil Engineering, Iran Univ. of Science
and Technology, P.O. Box 16765-163, Narmak, Tehran, Iran (corresponding author). E-mail: [email protected]
3
Associate Professor, School of Metallurgy and Materials Engineering,
Iran Univ. of Science and Technology, P.O. Box 16765-163, Narmak,
Tehran, Iran. E-mail: [email protected]
Note. This manuscript was submitted on December 1, 2016; approved
on August 31, 2017; published online on December 29, 2017. Discussion
period open until May 29, 2018; separate discussions must be submitted for
individual papers. This paper is part of the Journal of Materials in Civil
Engineering, © ASCE, ISSN 0899-1561.
© ASCE
The aluminum reacts with calcium hydroxide or alkali, which
releases hydrogen gas and forms air bubbles, as shown in Eq. (1)
(PFU 1972) as follows:
3CaðOHÞ2 ðaqÞðsÞ þ 2AlðsÞ
þ 6H2 OðlÞ → 3CaO · Al2 O3 .6H2 OðlÞ þ 3H2 ðgÞ
ð1Þ
Holt and Raivio (2005) confirmed that adjusting the reaction
speed and duration of the aluminum reactions is important. Therefore, it is necessary to make sure that the pore formation ends up
coinciding with the initiation of AAC hardening. Hence, the adjustment of aluminum powder and water content should control the
macropores’ size and distribution (Boesten 2012).
Literature classifies the air pore system of aerated concrete with
the aim of studying the water transport theory as artificial air pores,
intercluster pores, and interparticle pores (air pore diameters in
aerated concrete are usually 0.1–1 mm).
The first general review on aerated concrete was reported
by Valore (1954a, b), Rudnai (1963), and Short and Kinniburgh
(1963). Studies have shown that lightweight concrete has a great
potential as a structural material. Furthermore, performance and
durability are other appropriate features of this concrete (Narayanan
and Ramamurthy 2000a; Shamsuddoha et al. 2011; Alexanderson
1979; Scheffler and Paolo 2005; Esmaily and Nuranian 2012;
Schwartz et al. 1998; Schapovlov 1994). In addition, Panesar
(2013) proved that an increase in air pore percentage results in
the reduction of compressive strength and elastic modulus after 7
and 28 days. Recent research studies also showed that in order
to create uniform pore distribution in aerated concrete, the aluminum powder size should be enlarged. It is evident from the results
that the amount of aluminum particles characterizes the percentage
of overlapped pores and its distribution (Liu et al. 2013). The basic
issue in this study is to calculate the percentage of air in the AAC
or NAAC.
The usual method commonly used in ordinary concrete [ASTM
C231 (ASTM 2014)] cannot accurately determine air content of
more than 7%. Therefore, there is no study that focused on the
air void content measurement in the fresh aerated concretes
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specifically. Moreover, limited research studies have been carried
out on NAAC properties. In the present study, lightweight nonautoclaved aerated concrete was prepared. The amount of porosity in
the concrete was changed by a variation of the aluminum powder
percentage. Subsequently, the compressive strength of mixtures
was measured and compared at time intervals of 3, 7, and 28 days.
Likewise, in order to examine the workability of the prepared concretes, the amount of fresh concrete’s minislump was measured.
Furthermore, a scanning electron microscope (SEM) was used
to analyze the type of concrete pores. Last and foremost, a novel
formulaic method was proposed to determine the air percentage of
NAAC. Moreover, the effects of various aluminum powders on the
fresh and hardened properties of NAAC were evaluated.
Experimental Programs
The powder is known as 1940 grade aluminum powder. As mentioned previously, aluminum powder for aerated concrete should be
flake. Hence, according to the SEM image (Fig. 2), the shape of the
aluminum powder used is flake. Moreover, the size of the used
aluminum powder is in the range of 0.1–10 μm.
Mixture Proportions
The proportions of the NAAC mixtures are summarized in Table 3.
As could be seen, six types of NAAC mixtures were prepared with
a constant water-to-cement (W/C) ratio of 0.6 but with various percentages of aluminum powder (Mixtures A1–A6). In order to
evaluate the effect of W/C ratios, Mixtures A7–A9 were sampled
with a W/C ratio of 0.5, 0.8 and 1, respectively.
Sand content, accelerator percentage, and limestone powder
content were constant in all the mixtures.
Materials
In this study, a locally made ordinary portland Type II cement
according to the requirements of ASTM C150 (ASTM 2003)
and limestone powder as filler were used. River sand with a specific
gravity of 2,550 kg=m3 was also used as aggregate. The chemical
and grading properties of cement, sand, and limestone powder are
presented in Table 1 and Fig. 1, respectively. An accelerator admixture was also used in order to decrease the setting time of mixtures.
Physical properties of accelerator admixture are presented in
Table 2. The reason for using an accelerator with the proportion
of 8% of the cement weight was to omit the autoclave process.
An industrial oily type of aluminum powder was used in this study.
Table 2. Physical Properties of Accelerator Admixture
Property
Quantity or type
Physical mode
PH
Density (g=cm3 )
Chloride (ppm)
Powder
7–8
1.10
None
Table 1. Chemical Properties of Cement and Mineral Admixtures
(% Mass)
Composition
Sand
Cement
Limestone powder
SiO2
Al2 O3
Fe2 O3
MgO
CaO
SO3
Loss on ignition (%)
66.7
9.8
5.1
2.37
8.8
0.16
—
20.74
4.90
3.50
1.20
62.95
3.00
1.56
2.80
0.35
0.50
1.80
51.22
1.24
42.06
Fig. 2. SEM image of used industrial oily aluminum powder
Table 3. Mixture Proportions of NAAC Mixtures
Limestone
Water
Al/C Accelerator/C
Cement Sand
powder
(%)
Mix (kg=m3 ) (kg=m3 ) (kg=m3 ) (kg=m3 ) W/C (%)
Fig. 1. Particle size distribution of each solid material
© ASCE
A0
A1
A2
A3
A4
A5
A6
A7
A8
A9
648
648
648
648
648
648
648
648
648
648
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821
821
821
821
821
821
821
821
821
821
389
389
389
389
389
389
389
324
518
648
77
77
77
77
77
77
77
77
77
77
0.6
0.6
0.6
0.6
0.6
0.6
0.6
0.5
0.8
1
0.0934
0.0466
0.0934
0.1869
0.2849
0.3731
0.4673
0.0934
0.0934
0.0934
0
8
8
8
8
8
8
8
8
8
J. Mater. Civ. Eng.
Mixture Preparation
The water, limestone powder, and accelerator admixture were
poured into the mixer. The mixer was turned on and allowed to
mix well for 2 min. Cement and sand were later added to the mixture and allowed to mix together for 4 min. Aluminum powder
was mixed separately in 100–200 g of water and then added to
the contents of the mixer. The mixing process was continued for
1–2 min.
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Test Procedure
Workability of NAAC Mixtures
The workability of mixtures was tested with the minislump device.
The apparatus for the minislump test of NAAC mixtures consists
of a cone 60 mm high with a diameter of 70 mm at the top and
100 mm at the base. The cone was placed at the center of a plate
and was filled with mixture. Immediately after filling, the cone was
lifted, and the average diameter (in millimeters) of spread mixture
over the table was measured.
Fig. 3. Density and minislump results of NAAC mixtures
Density and Volume Test
The density of fresh NAAC mixtures was tested according to
ASTM C188 (ASTM 2015). Volume change in the mixtures
(due to the production of air bubbles in the mixtures) was tested
at the interval of a few seconds.
Compressive Strength Test
The compressive strength of NAAC mixtures was determined
according to BS EN1881 (BSI 2013b) after 7 and 28 days.
Water Absorption Test
Typically, this test was carried out according to standard procedure
BS 1881-Part122 (BSI 2013a); however, the difference here is that
because of the high porosity of the mixture, it is filled with water in
a relatively short time. Hence, the test was performed at shorter
intervals (1, 2, 4, and 8 min).
Fig. 4. Density and compressive strength results of NAAC mixtures
Measurement of Packing Density
In order to evaluate the packing density of NAAC mixtures, a
method called wet packing is applied based on literature presented
by Wong and Kwan (2008) and Ghoddousi et al. (2014). The
process of the measurements is described subsequently.
Table 4. Workability, Density, and Compressive Strength Results of Fresh
Mixtures
Mix
Density
(kg=m3 )
Minislump
(cm)
7-day compressive
strength (MPa)
28-day compressive
strength (MPa)
A0
A1
A2
A3
A4
A5
A6
A7
A8
A9
1,020
1,477
1,298
1,170
1,140
1,085
1,027
1,500
1,550
1,600
30
28
30
31.5
29.5
24.5
23
22
40
47
1.5
8.5
6
2
1.1
0.8
0.5
7.6
4.5
3.4
2.5
11.6
8
2.9
1.7
1.3
0.9
10.1
6.5
4.8
© ASCE
Fig. 5. Compressive strength and minislump results of NAAC mixtures
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After making the NAAC mixture in the mixer, concrete is filled
into a mold of size 100 × 100 × 100 mm. If the NAAC mixtures
include various materials, the volume of the solid materials (V C )
in the mold could be calculated from Eq. (2) as follows:
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VC ¼
M
ρ w u w þ ρ g R g þ ρ s Rs þ ρ c Rc þ ρ l Rl þ ρ m Rm
ð2Þ
where M and V = mass and volume of NAAC, respectively; ρw =
water density; uw = ratio of water volume to solid volume of granular material; ρs , ρc , ρl , and ρm = densities of sand, cement, limestone powder, and accelerating admixture as filler, respectively; and
Rs , Rc , Rl , and Rm = volumetric ratios of sand, cement, limestone
powder, and accelerating admixture, respectively, to total solid
materials. Given that the amount of aluminum powder and accelerator admixture weight used in the mixtures are very low, they do
not affect the packing density and can be ignored in the equation.
Fig. 6. Effect of water-to-cement ratio on density and compressive
strength
Having obtained V C and V, packing density (ϕ) may be calculated
by Eq. (3) as follows:
ϕ¼
VC
V
ð3Þ
SEM Test
The microstructure of NAAC mixtures was observed using a SEM
(EVO 18, Zeiss, Oberkochen, Germany).
Results and Discussion
Workability, Density, and Compressive Strength of the
Fresh NAAC
The workability, density, and compressive strength results of fresh
mixtures are given in Table 4.
For a better understanding of the parameters listed in Table 4,
they are compared in Figs. 3–5. As is evident from Fig. 3, there is
an optimal aluminum percentage of 0.093 from the perspective of
minislump diameter. The results show that minislump and workability decrease with an increase in the amount of aluminum powder in comparison to optimum percentage. This could be explained
by the fact that as long as pores are separated, they are spherical and
act as bearings, but when there are many pores, they join together
and act as a network, which leads to flow-ability decrement. However, the air percentage increases with an increase in the amount
of aluminum powder, resulting in forming more pores and consequently reducing the density.
Fig. 4 shows that the increase in aluminum percentage leads to
a decrease in compressive strength of the NAAC mixture. This is
in agreement with the results presented by Muthu Kumar and
Ramamurthy (2015). It seems that the optimum percentage of aluminum can be considered as 0.0934. By increasing the amount of
aluminum powder from 0.0934 to 0.1869, compressive strength
reduced significantly. However, increasing the percentage of aluminum powder caused a density decrement, but with regard to
compressive strength, the optimum percentage (0.0934) would
Fig. 7. Artificial pores produced in the NAAC
© ASCE
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Fig. 8. SEM images of NAAC mixtures
be suitable. Fig. 5 also clearly shows that there is an optimal percentage (0.0934) from the viewpoint of suitable compressive
strength and workability.
The effect of the water-to-cement ratio on fresh and hardened
properties of NAAC mixtures is illustrated in Fig. 6. It is obvious
that with an increase in water-to-cement ratio, compressive strength
reduced, but from the perspective of both compressive strength and
density, an optimal W/C ratio (0.6) can be considered.
SEM Results
The porosity in aerated concrete is divided into three categories:
artificial, cluster, and particle porosity. Comparing the SEM images
of the three categories with images obtained in this study, it was
clearly found in Fig. 7 that the porosity formed in this study is
an artificial porosity. When aluminum powder is very high (A6
mixture), detection of the number, type, and diameter of pores is
very difficult (Fig. 8) because when the pore number increases,
pores change to pore networks. This is in agreement with the results
reported by Narayanan and Ramamurthy (2000a).
Water Absorption of Mixtures
Volumetric water absorption of NAAC mixtures at 1, 2, 4, and
8 min are shown in Fig. 9. As can be seen, an increase in the amount
of aluminum powder leads to more absorbed water. The reason is
that by increasing the aluminum powder, the size and relative value
of pores increase.
Packing Theory for Calculation of Air Percentage
Table 5 presents the measured packing density of A1 and A2 mixtures at five intervals. The results show a reduction in packing density with time. This might be attributed to the air bubbles produced
© ASCE
Fig. 9. Water absorption of NAAC mixtures at different times
during the reaction of NAAC constitutions. The results also show
that with an increase in the amount of aluminum powder from
0.0934 to 0.1869 (A1 to A2), packing density reduces. This is due
to a higher air percentage in the A2 mixture. Figs. 10 and 11 show
the results of volume change ratio and density with time for A1 and
A2 mixtures. It can be seen that by increasing air bubbles over time,
the volume increases and density reduces.
As mentioned in the Introduction, there is no study that focused
on the air void content measurement in fresh aerated concretes specifically. Fig. 12 shows the measured air percentage of NAAC mixtures A1 and A2 over time based on packing theory. It is observed
that with an increase in aluminum powder (mixture A2), the maximum aerated air percentage is higher than mixture A1 containing
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Table 5. Measured Packing Density of A1 and A2 Mixtures
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Time (s)
Mix
0
3
6
9
12
15
18
21
24
27
30
33
36
39
42
45
A1
A2
0.549
0.602
0.536
0.548
0.536
0.521
0.536
0.497
0.524
0.462
0.524
0.437
0.513
0.420
0.506
0.404
0.502
0.399
0.482
0.399
0.474
0.399
0.472
0.399
0.467
0.399
0.463
0.399
0.458
0.399
0.454
0.399
Fig. 10. Results of volume change ratio and density with time for the
A1 mixture
Fig. 12. Measured air percent of NAAC mixtures A1 and A2 with time
based on packing theory
Fig. 11. Results of volume change ratio and density with time for the
A2 mixture
Fig. 13. Relation between air percent of NAAC mixture with volume
change
less aluminum powder (21% for A2 and 16% for A1). To determine
the air percentage of NAAC having a volume change on site or in a
laboratory, the following equation is obtained from Fig. 13:
Conclusions
y ¼ 0.2484x þ 7.1436
ð4Þ
where y = air percentage produced in the NAAC mixture; and
x = percentage of final mixture volume change compared to the
initial volume.
© ASCE
The following conclusions can be drawn from the results of this
research:
• There is an optimal percentage of aluminum (0.0934) from the
viewpoint of minislump diameter and compressive strength.
After the optimum point, the increase in percentage of aluminum results in minislump (workability and flow-ability) and
compressive strength decrement.
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• Comparing the SEM images of three categories with images
obtained in this study, it is clearly found that the porosity formed
in mixtures is an artificial one.
• With an increase in aluminum content, the percentage of water
absorbed increases. The reason is that by increasing the aluminum powder, the size and relative value of pores increase.
• The results show that an increase in the amount of aluminum
powder reduces packing density in comparison to the mixture
with lower aluminum percentage.
• Increasing the water-to-cement ratio reduces the compressive
strength, but from the viewpoint of density, an optimal waterto-cement ratio (0.6) can be considered.
• This study proposes a new method of determining air percentage
of aerated concrete based on packing theory. It observes that the
air percentage of aerated concrete increases with an increase in
aluminum powder.
Notation
The following symbols are used in this paper:
M = mass;
Rc = volumetric ratio of cement to total solid materials;
Rl = volumetric ratio of limestone to total solid materials;
Rm = volumetric ratio of accelerating admixture to total solid
materials;
Rs = volumetric ratio of sand to total solid materials;
uw = ratio of water volume to solid volume of granular
material;
V = volume;
V C = volume of solid materials;
ρc = density of cement;
ρl = density of limestone;
ρm = density of accelerator admixture;
ρs = density of sand;
ρw = density of water; and
ϕ = packing density.
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© ASCE
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