1 UNFIRED BRICK USING FLY ASH AND RED MUD BASED ON GEOPOLYMER TECHNOLOGY

1
UNFIRED BRICK USING FLY ASH AND RED MUD BASED
ON GEOPOLYMER TECHNOLOGY

Vu Huyen Tran
1
, Nguyen Thi Thanh Thao
2
, Nguyen Van Chanh
3

Department of Construction Material, University of Technology HCM City, Vietnam
ABSTRACT
Red mud is a waste material obtained from the aluminium extraction industry with
the Bayer process and million of tons of red mud are produced annually. Fly ash is the
finely divided residue that results from the combustion of pulverized coal in coal-fired
electric and steam generating plants and a siliceous and aluminous material. The
unfired brick resulting of the chemical reaction between red mud and fly ash is an
amorphous to semi-crystalline polymeric structure and can be hardened in ambient air
temperature. Alkaline activator is also added as a structure-forming element and
increases structural stability of unfired brick. For mix proportion using 50-60% fly ash,
40-50% red mud, 6-8ml alkaline activator for 100gr powder and curing temperature is
70
o
C, compressive strength in range of 130-150 kgf/cm
2
, compressive strength of dried
sample to compressive strength of sample saturated by water ratio 0.8-0.9, water
absorption 6.3-9.7% and high resistance to water. This paper reports about the physico-
mechanical properties and manufacruring process of the unfired brick using fly ash and
red mud through geopolymerization process with the aim of finding a new material that

is appropriate to Vietnam situation. This work proved that this unfired brick has
promising properties to be used as unfired materials in the construction.
Keywords: Red mud, Fly ash, Geopolymer , Physico-mechanical properties.
1. GENERAL
Red mud is a by-product of the Bayer process and disposed as a slurry. The liquid
phase contains about 7g/l of Na
2
O and pH in the range of 13. This is a big obstacle to
recycle use but great advantage to geopolymer process. Besides, red mud is also a very
fine material in terms of particle size distribution. Typical values would account for 90
volume % below 75µm and the specific surface of red mud is around 10m
2
/g [1]. A
chemical analysis would reveal that red mud contains silica, aluminum, iron, calcium,
titanium.., etc. The variation in chemical composition between different red muds is
high.

Figure 1. Red mud from Bayer process Figure 2. Fly ash
2
Fly ash contains mostly silicon (Si) and aluminium (Al) in amorphous form is a
possible source material for the manufacture of unfired brick based on geopolymer
technology. The physical and chemical characteristics of fly ash produced from the
combustion of coal in electric utility or industrial boilers depend on the combustion
methods, coal source and particle shape [2]. The particles of fly ash are generally
spherical, finer than Portland, typically ranging in size between 10 and 100 micron.
Therefore, fly ash is also a good mineral filler in hot mix asphalt (HMA) and improves
the fluidity of flowable fill and grout [2]. The chemical compositions of various fly
ashes show a wide range, depend on the type of coal. Fly ash consists primarily of
oxides of silicon, aluminum, iron and calcium. Magnesium, potassium, sodium,
titanium, and sulfur are also present to a lesser degree.

Geopolymerization Process
According to research of Glukhovsky in the 1950s, a general mechanism for the alkali
activation of materials primarily comprising silica and reactive alumina involved
stages: destruction–coagulation, coagulation–condensation, condensation
crystallization. Afterthat, Glukhovsky theories were extended by different authors. In
this theories [3], dissolution of the solid aluminosilicate source by alkaline hydrolysis
produces aluminate and silicate species. The dissolution of solid particles at the surface
results in the liberation of aluminate and silicate most likely in monomeric form into
solution. This is the mechanism responsible for conversion of the solid particles during
geopolymerization. Once in solution the species released by dissolution are
incorporated into the aqueous phase, which may already contain silicate present in the
activating solution. A complex mixture of silicate, aluminate and aluminosilicate
species is thereby formed. Dissolution of amorphous aluminosilicates is rapid at high
pH. This creates a supersaturated aluminosilicate solution, then the formation of a gel,
as the oligomers in the aqueous phase form large networks by condensation. After
gelation, the system continues to rearrange and reorganize, as the connectivity of the
gel network increases, resulting in the three-dimensional aluminosilicate network com-
monly attributed to geopolymers.
Barbosa et al. (2000) proposed a new model for the molecular structure of geopolymer
gel as shown in Figure 3 [4].

Figure 3. A semi-schematic structure for Na-PSS from Barbosa et al. (2000)
3
Davidovits (1999) proposed the possible applications of the geopolymer material
depending on the molar ratio of Si to Al, as given in Table 1 [5].
Table 1. The possible applications of the geopolymer material
Si/Al Application
1 Bricks, ceramics, fire protection
2 Low CO
2

cements, concrete, toxic water encapsulation
3 Heat resistance composites, fibre glass composites
2. EXPERIMENTAL
Samples were prepared using red mud from Tan Binh Chemistry Factory and fly ash
from Nhon Trach Dong Nai Power Station with ratio of red mud:fly ash was 20:80,
30:70, 40:60; 50:50 and 60:40. Fly ash provides initially the geopolymeric system with
soluble Si and Al that are essential for the aluminosilicate oligomers formation and
consequently, for the progress of the whole geopolymerization process [6] [7]. Alkaline
activator comprising soluble silicon and sodium hydroxide was added. Mixing red mud
in liquid form without drying with fly ash and alkaline activator at about 100
o
C. Mixes
were formed in cubic molds 50x50x50 mm and cured in a oven for 48 hours,
temperature 80- 100
o
C. After specimens were cured, tests in compressive strength,
water absorption, resistance to water were carried out.
3. RESULTS AND DISCUSSION
3.1 Properties of red mud and fly ash
Red mud
According to the X-ray diffraction analysis (Figure 4), red mud is a crystalline
material that comprises mainly the mineralogical phases of gibbsite, goethite… The
chemical composition of red mud was detailed in Table 2. Fe
2
O
3
content is high so the
colour of the unfired brick is almost similar to the clay brick.
Table 2. Chemical composition of red mud
Al

2
O
3
(%)
Fe
2
O
3
(%)
SiO
2
(%)
TiO
2
(%)
CaO
(%)
MgO
(%)
Na
2
O
(%)
K
2
O
(%)
Cr
2
O

3
(%)
P
2
O
5
(%)
SO
3
(%)
Cl
(%)
31.26 47.44 6.17 6.73 0.41 0.06 6.64 0.01 0.22 0.24 0.44 0.15









6055504540353025201510
55,000
50,000
45,000
40,000
35,000
30,000
25,000

20,000
15,000
10,000
5,000
0
-5,000
-10,000
-15,000
-20,000
-25,000
Sodalite 1.23 %
Rutile 1.35 %
Hematite 6.45 %
Gibbsite 54.70 %
Goethite 27.22 %
Mullite 2:1 2.61 %
Gypsum 7.80 %
Anatase 1.66 %
Titanomagnetite 2.33 %
Perowskite 2.16 %

Figure 4. XRD result for red mud
4
Fly ash
The fly ash has dark grey colour. The chemical composition was determined by
XRD analysis (Figure 6) and was given in Table 3. The molar Si-to-Al ratio of fly ash
was about 2.6 and the calcium oxide content was low 6.3%. The particle size
distributions of the fly ash was given in Figures 5. Particles smaller than 60 μm was
80%.
Table 3. Chemical

composition of fly ash


















3.2 Physico-mechanical properties of unfired brick
Compressive strength







The compressive strength of unfired brick increases as content of alkaline activator
or fly ash is increased. When alkaline activator content is increased from 8 to 10

ml/100g solid mixture, compressive strength increases significantly. The increase in

Figure 7. The relationship between fly
ash content and compressive strength

Figure 8. The relationship between
compressive strength and alkaline
activator content with 60% fly ash



29_MAU_QUOC THO_5
00-046-1045 (*) - Quartz, syn - SiO2 - WL: 1.5406 - Hexagonal - a 4.91344 - b 4.91344 - c 5.40524 - alpha 90.000 - beta 90.000 - gamma 120.000 - Primitive - P3221 (154) - 3 - 113.010 - I/Ic PDF 3.4 - F30=539(0.0018,31)
00-015-0776 (I) - Mullite, syn - Al6Si2O13 - WL: 1.5406 - Orthorhombic - a 7.54560 - b 7.68980 - c 2.88420 - alpha 90.000 - beta 90.000 - gamma 90.000 - Primitive - Pbam (55) - 167.353 - I/Ic PDF 1. - F30= 60(0.0135,37)
29_MAU_QUOC THO_5 - File: 29_MAU_QUOC THO_5.raw - Type: 2Th/Th locked - Start: 10.000 ° - End: 79.990 ° - Step: 0.030 ° - Step time: 1. s - Temp.: 25 °C (Room) - Time Started: 16 s - 2-Theta: 10.000 ° - Theta: 5.000 ° - Chi: 0.00
Lin (Counts)
0
10
20
30
40
50
60
70
80
90
100
110
120
130

140
150
2-Theta - Scale
11 20 30 40 50 60 70 80
d=5.39393
d=4.26195
d=3.43584
d=3.40314
d=3.35201
d=2.88432
d=2.69550
d=2.54964
d=2.46126
d=2.28569
d=2.21060
d=2.12612
d=1.82171
d=1.54423
d=1.52469
d=1.38295
d=1.37278
d=1.22551

Figure 6. XRD result for fly ash

Figure 5. Particle size distribution of fly ash
SiO
2
(%) 41
Al

2
O
3
(%) 23.1
Fe
2
O
3
(%) 11.2
CaO(%) 6.3
MgO (%) 3.2
K
2
O (%) 0.61
Na
2
O (%) 0.98

5
compressive strength after 5 day is similar to 14 days (Figure 8). After 14 days, with fly
ash content is 60%, compressive strength of specimens that using 4 ml alkaline
activator/100g solid mixture is 120 kG/cm
2
while compressive strength of specimens
that using using 8, 9, 10 ml alkaline activator/100g solid mixture is 156, 158 and 158
KG/cm
2
.
Figure 7 shows the relationship between fly ash content and compressive strength of
specimes that using 6 and 8 ml alkaline activator/100g solid mixture. With 8 ml

alkaline activator/100g solid mixture, fly ash content is increased from 40 to 80 %,
compressive strength increases from 110 to 190 KG/cm
2
.
Water absorption
Water absorption of unfired brick decreases respectively as content of fly ash is
increased. Figure 9 shows the relationship between fly ash content and water absorption
with content of alkaline activator is 6 and 8 ml/100g solid mixture.
After 14 days, with fly ash content is 60%, specimens get lowest water absorption
6.3% (Figure 10). However, content of alkaline activator is increased more than 8
ml/100g solid mixture, water absorption increases.










Water resistance factor
As content of alkaline activator is
increased more than 7 ml/100g solid
mixture, water resistance factors are
more than 0.9. These results show the
high resistance to water ability of this
unfired brick (Figure 11).

3.3 Production process of unfired construction materials: [8]


Figure 9. The relationship between fly
ash content and water absorption with
content alkaline activator is 8 ml/100g
solid mixture

Figure 10. The relationship between alkaline
activator content and water absorption with
fly ash content is 60%


Figure 11. The relationship between alkaline
activator content and water resistance factor
with fly ash content is 60%