Cellular concrete with addition of aluminum recycled foil powders
Edval Gonçalves de Araújo(1), Jorge Alberto Soares Tenório(1)
(1) Departamento de Engenharia Metalúrgica e de Materiais da Escola Politécnica USP
Av. Prof. Mello Moraes, 2463 – Cidade Universitária, CEP: 05508-900, São Paulo, Brasil
Key words: cellular concrete, aluminum recycling, aluminum milling
ABSTRACT
Considering its advantage of low density and favorable insulation properties, there are
several applications for lightweight autoclaved aerated concrete of uniform cellular structure.
The raw materials for the manufacturing of cellular concrete are Portland cement, finely
grounded sand and lime. These are batched and mixed with water and metallic aluminum powder
finely divided. There is a reaction between the aluminum powder and hydroxides forming millions
of hydrogen bubbles throughout the mixture.
The aluminum powder is the highest cost component, and the objective of this work is
replacing it for another gas forming agent, like recycled foil.
The foils are grinded in a high energy ball mill (attritor). Quartz sand is mixed with
aluminum foil to reduce the time required for grinding, obtaining spherical particles and ensuring a
uniform distribution of aluminum in the gas forming agent.
The activity of this gas forming agent was determined by the gas volumetric technique.
Average particle size and compressive strength of the samples were measured. The relationship
between volume of the gas released during the reaction and milling conditions are presented,
showing its viability for producing a high quality cellular concrete.
1. INTRODUCTION
The gas developing agent commonly used in manufacturing autoclaved cellular concretes is
made of aluminum powder prepared by fine particulation of pure metallic aluminum. However, in
order to reduce the cost to produce cellular concrete, a process is being developed, which replaces
aluminum powder by powder from recycled scrap. This recycled aluminum powder costs half as
much as pure aluminum powder.
2. OBJECTIVE
In the present work, the relationship between the average particle size and the H2 release
potential of powders produced from recycled aluminum foil was investigated. In order to evaluate
the possibility of replacing Al powder by recycled foil in autoclaved cellular concrete, blocks were
molded and their compressive strength was measured as a function of apparent bulk density.
3. METHODS
Scrap aluminum foil had to be reduced into granules of 10mm length and thickness less than
1mm by a knife crusher. This material was processed in an attritor mill Netzsch Molinex PE075 of
0.25 litter capacity, high density polyethylene vial, with 50g of granules, 3g Acrawax lubricant and
900g of steel spheres. The milling times were 1h, 2h, 4h, 10h and 15h.
To intensify the grinding process, the aluminum granules were mixed before grinding with
an abrasive material such as quartz sand in an aluminum-to-sand ratio of 1:2 (by weight). The
average particle size of sand was equal to 51µm. Both systems, Al foils and Al foils with silica,
were studied.
The activity of the gas developing agent was determined by the gas volumetric method, that
is, by the amount of gas released when the gas developing agent was mixed with Ca(OH)2 as
compared with the volume released from 1g of degreased aluminum powder. The volume of the gas
evolved from a 1g batch of pure aluminum powder, calculated from the equation (1) is 1245 cm3
[1], under normal conditions: 25°C and 1atm.
2Al + 3Ca(OH)2 → 3(CaO).Al2O3 + 3H2
(1)
After each milling time, a sample of 1g of Al powder from foil or another sample of 3g of
Al-SiO2 (because the ratio is 1Al:2SiO2, for measuring the effect of 1g of Al contained) was
extracted.
The parameters for analysis of effects of mill variables are: ball to powder ratio, steel ball
diameter and impeller rotation. The results are the average particle size obtained by laser diffraction
(Malvern Instruments Model 2600LC) and the volume of evolved gas.
In statistic planning, a 23 complete factorial with two variables levels was used, the higher
level with plus signal (+) and lower level with minus signal (-) [2]. Table 1 shows the notation for
each variable level and Table 2 shows the coding from a complete factorial design, for each milling
time (1h, 2h, 4h, 10h and 15h).
Table 1 – Levels of variables of 23 factorial design.
variables
ball / powder ratio, by weight
ball diameter
impeller rotation
(+)
10:1
7.50mm
1400rpm
(-)
5:1
6.35mm
800rpm
Table 2 – Coding from a 23 factorial design, for each milling time.
test number
1
2
3
4
5
6
7
8
b/p ratio
(-)
(+)
(-)
(+)
(-)
(+)
(-)
(+)
φe
(-)
(-)
(+)
(+)
(-)
(-)
(+)
(+)
rpm
(-)
(-)
(-)
(-)
(+)
(+)
(+)
(+)
For the evaluation of mechanical properties of cellular concrete specimens, these were
molded with addition of the gas forming agents (Al and Al-SiO2). Industrial tests were carried out
with a mixture comprising the following components: 249kg/m3 of sand grounded to a specific
surface of 3000 cm2/g, 84kg/m3 of cement Portland, 58kg/m3 of lime and 10kg/m3 of gypsum. This
cellular concrete was molded in blocks, autoclaved under 200°C and 10atm. Tests of apparent
density and compressive strength were carried out following ABNT 13440 [3] and ABNT 13439
[4], respectively.
4. RESULTS AND DISCUSSION
4.1.– Particle Size Analysis
The average particle size of aluminum powders obtained from foil comminution is shown in
Figure 1(A). These data indicate that in all processing conditions an accentuated decrease of particle
size of aluminum powder has occurred.
In each milling time, it was observed that the impeller rotation was more significant in
decreasing the particle size. However, this effect was not constant, reaching its maximum at 4h.
Other principal effects, b/p and φe, were significant and constant between 2 and 15h, with
prevalence of ball to powder ratio effect compared to ball diameter. The two-factor and three-factor
interactions were not significant.
70
(A)
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
50
40
30
20
10
0
2
4
6
8
10
Time (h)
12
14
16
(B)
1200
1100
1
2
3
4
5
6
7
8
1
2
3
4
5
6
7
8
1000
900
Gas volume(ml)
µm)
60
Average particle size (
1300
800
700
600
500
400
300
200
100
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
Time (h)
Figure 1 – (A) Particle Size as a function of milling time. (B) Volume of evolved gas as a function
of milling time. Al System: solid line curves, Al-SiO2 System: dot line curves. The
numbers 1 to 8 correspond to the notation presented in Table II.
The experimental procedure with the aluminum-silica system was the same used with
aluminum. The dot line curves in Figure 1(A) present the particle size as a function of milling time.
In this graphic it can be observed that with time increase there is an accentuated decrease of average
particle size until 4h, and with a lower rate, for 10 and 15h.
The Al-SiO2 particle size was smaller than in Al system. This occurred because the sand is
more fragile and inserted in the aluminum foil. This sand features have contributed to a maximum
rapid grinding of the mixture to the required degree of dispersity. Moreover, due to the bigger
grindability of the sand and its amount, the laser diffractions results of particle size of aluminumsand system could not be compared to aluminum ones, as the diffraction analysis showed a greater
number of sand particles than Al. Thus, the experiments with or without sand addition provided the
aluminum particle size and the particle size of composite aluminum-silica, respectively.
In the analysis of results of the variable effects it was noted that the impeller rotation
velocity was once more significant. The value of this effect reached its maximum at 2h and
decreased with increasing milling time. Other main effects, b/p and φe, were similar, even though
smaller, to the rpm variable. Each of these effects had a peak at 2h, and their values decreased in
higher milling times.
The analysis of influence of atritor mill variables in both systems showed the prevalence of
the impeller rotation velocity over other parameters. In fact, rpm variable was directly related to
kinetic energy of the system (spheres + powder), and so, ball size and ball quantity also contributed
to the final results. But, in this study, the rpm variation level from 800 to 1400 was more effective
for diminishing of particle size than the variation of the two other variables, in both systems.
4.2 – Volumetric Gas Analysis
The volume of hydrogen released in both systems had a similar behaviour, as shown in
Figure 1(B). In the beginning of the milling there was an increase in the quantity of released gas
and, after this peak, there was a gradual reduction of the H2 released. The volume released by AlSiO2 was greater than in the Al system and the maximum occurred between 2 and 4h for Al- SiO2
and around 10h in the Al system. Gas release increased with milling time (decreasing particle size).
However, aluminum oxidation also increased with decreasing particle size. After the peak related
above, the specific surface area was enough for oxidation to be prevalent and so gas release
decreased. Then, oxidation of aluminum particles became the process controller and probably the
greater alumina thickness contributed to the reduction of chemical reaction of the aluminum with
lime. Therefore, the particle size and oxidation are the main factors in the reaction of bubbles
production.
4.3 – Relationship between average particle size and volumetric analysis
The Figure 2 shows the results of released gas as a function of average particle size. There
was no direct correlation, but this data could be divided in three zones. In zone I an increase of gas
release with decrease of particle size occurred. In particle size between 35 and 40µm there was a
high dispersion of the values and a decrease of gas releasing rate with the decrease of particle size
was observed in zone II. The zone III, with particle size smaller than 25µm, was characterized by
decrease of gas volume with decrease of particle size.
In the Zone I the limitation of the gas released reaction was due to the high aluminum
particle size, so the milling time was not enough. In zone II, there was a balance between decrease
of the particle size and powder oxidation. Finally, in zone III a decrease of particle size continued to
occur, but the oxidation was so intense that promoted a low efficiency in the gas production
reaction.
1200
Al
Al-SiO2
Gas volume (ml)
1000
800
600
400
200
zone III
zone II
zone I
0
10
20
30
40
50
60
70
Average particle size (µm)
Figure 2 – Average particle size in function of gas volume released.
5. Mechanical Behaviour: Apparent Density and Compressive Strength
In Figure 3(A) the curves of gas released volume (by gram of contained aluminum) versus
density can be observed. In the Figure 3(B) the results of density in function of compressive
strength are illustrated. In both cases, the minimum squares method for the correlation analysis was
used. The results show an inverse correlation between density and gas released, with the lower gas
volume occurring in 1 to 2h and 10 to 15h.
1100
Al
Al-SiO2
1000
Y=1078,91-0,785X+3,08E-4 X , R=0.95
Y =-178,76437+2,72292 X-0,00181 X2, R=0,96
3
)
900
Density (kg/m
(A)
2
800
700
600
500
400
0
200
400
600
800
1000
1200
Gas Volume (ml)
9
Al-SiO2
Al
8
7
Compressive Strength (MPa)
(B)
2
Y=1,32449-0,00602 X+1,31855E-5 X , R=0,93
Y=-3,99947X+0,01178, R=0,98
6
5
4
3
2
1
0
400
500
600
700
800
900
1000
1100
1200
Density (kg/m3)
Figure 3 – (A) Gas evolved in function of apparent density for Al and Al-SiO2. (B) Density in
function of compressive strength for Al and Al-SiO2. Minimum Square Correlation.
The correlation between density and compressive strength was direct. Nevertheless, only two
points of the Al-SiO2 system processed for 2h, as shown in Table 3, reached the conditions
established by ABNT (C12: 1,2MPa; d ≤ 450kg/m3). In the majority of the other Al-SiO2 system
samples and in all Al system (Table 4), much higher densities were obtained due to the low quantity
of bubbles, which were not enough to promote the required expansion.
Some of the denser samples could even be classified in other strength levels of cellular
concrete (C15: 1,5MPa; d ≤ 500kg/m3; C25: 2,5MPa; d ≤ 550kg/m3; C45: 4,5MPa and d ≤
650kg/m3). Thus, the particle size and the aluminum powder agent oxidation level must be adjusted
by the milling variables. The other process variables, which can be altered to obtain a certain
concrete class, are the water temperature in the process and the quantity of Al agent added.
Table 3 – Data of compressive strength of blocks for Al-SiO2 gas forming agent.
condition
(-)(-)(-)
(+)(-)(-)
(-)(+)(-)
(+)(+)(-)
(-)(-)(+)
1h
4.1±0.4
3.2±0.3
3.8±0.3
3.2±0.3
2.1±0.25
2h
4.1±0.25
3.15±0.25
2.5±0.1
3.0±0.2
1.55±0.05
4h
2.6±0.3
1.7±0.17
1.9±0.2
1.3±0.05
3.35±0.35
10h
3.35±0.25
2.9±0.1
4.0±0.3
3.3±015
5.15±0.4
15h
4.7±0.35
5.0±0.4
4.1±0.7
5.1±0.8
6.2±0.85
(+)(-)(+)
0.8±0.15
1.2±0.05
3.55±0.25
4.75±0.35
5.85±0.45
(-)(+)(+)
1.7±0.2
1.45±0.1
3±0.2
6.05±0.6
6.5±0.3
(+)(+)(+)
0.6±0.15
1.35±0.1
4.2±0.27
4.9±0.5
5.8±0.8
Table 4 - Data of compressive strength of blocks for Al gas forming agent.
condition
(-)(-)(-)
(+)(-)(-)
(-)(+)(-)
(+)(+)(-)
(-)(-)(+)
(+)(-)(+)
(-)(+)(+)
(+)(+)(+)
1h
8.25±1
7.15±0.9
8.1±1.3
7.5±1
7.0±0.9
5.9±0.7
6.6±0.7
5.55±0.6
2h
6.2±0.65
5.85±0.35
5.4±0.4
6.1±0.55
5.6±0.2
4.75±0.35
4.9±0.3
4.5±0.3
4h
5.2±0.4
4.25±0.35
5.1±0.5
4.6±0.3
3.8±0.3
3.5±0.2
4.05±0.3
2.65±0.25
10h
3.1±0.25
3.05±0.3
3.8±0.2
3.5±0.2
4.5±0.5
4.35±0.35
4.1±0.7
4.2±0.45
15h
4.1±0.45
4.4±0.6
3.65±0.4
3.55±0.35
4.75±0.8
4.35±0.5
4.5±0.6
4.4±0.5
6. – Conclusion
The particle size and evolved gas volume permit the milling conditions mapping, chosen the
best values of process variables rpm, b/p, φe and milling time. The impeller rotation proportionated
the greater effect.
It is possible to produce cellular concrete blocks using powder from aluminum recycled foil.
For a required density and strength combination the main parameters are milling time, oxidation
level and hard particles addition.
Acknowledgment:
This research was supported by FAPESP under project number 00/00219-2.
References:
1. B. V. Grishin, US PATENT 4,119,476, 8p., 1978
2. G. E. P. Box, W.G. Hunter, J. S. Hunter. Statistics for Experimenters, p. 307-351, 1978
3. ABNT STANDART 13440; 4p; 1995.
4. ABNT STANDART13439, 4p., 1995.