Strength Analysis of Aluminosilicate Soils for Use in Brick
Manufacturing
Expedito Milyaso1*, Arthur M. Omari2, Baraka Kichonge1
1
Arusha Technical College (ATC),
Mechanical Engineering Department,
P.O. Box 296 Arusha, Tanzania
2
Mbeya University of Science and Technology (MUST),
P.O. Box P. O. Box 131Mbeya, Tanzania
*E-mail of corresponding author: expeditomilyaso@gmail.com
Abstract
The present paper analyse the strength of selected aluminosilicate soils for possibility of use in brick manufacturing.
The soil samples included in the analysis were from River Natron, Lake Natron, Oldoinyo Lengai, Mto wa Mbu,
Lokii and Kisongo. Similarly, trona was used as a catalyst while Holili and Uchira blocks were for comparative
strength analysis. The analysis showed that the compressed aluminosilicate bricks are viable option for use as
construction material. The achieved compression strength is enough for construction of building blocks for
unsheltered single storey houses in low wind areas. The bricks have shown increasing compression strength
character with time, within the 7 to 90 days of investigation at room temperature. There exists a good correlation
between the maximum compressive strength and the maximum amount of combined aluminosilicates. The higher
the amounts of silica and alumina produced a better strength. Comparable results are demonstrated by the
composition of Uchira and Holili bricks. Natron aluminosilicate soils has extremely long setting times, but on
activation by an alkaline material (Trona) this time can be reduced. The SiO2/Al2O3 ratio of the fabricated bricks is
very nearly 1:1 suitable for bricks, ceramics and fire protection.
Key Words: Aluminosilicate, alkaline, geopolymer, silica, trona, compression strength
1.
Introduction
Building bricks requires a certain strength in order to
qualify for construction. Various materials are used
for manufacturing of construction bricks however
they are associated with processes which are not
environmentally friendly. The present manufacturing
processes of cement and bunt bricks have a great
effect on the environment. The emission of
greenhouse gases from high temperature techniques
used during production of cement causes depletion of
ozone layer resulting into global warming (Velders et
al., 2007).
Bricks made using clay are burnt at a temperature of
about 1000ºC to achieve the required strength
(Kreimeyer, 1987; Weng et al., 2003). According to
Building Research Unit of Tanzania (BRU 1981) the
minimum strength requirement for building blocks
for single storey houses are classified into three
classes that varies according to the wind conditions
on the site as Class A, B and C. Class A applies to
hard wind areas for buildings that are not sheltered by
other buildings with minimum brick strength of
2.5N/mm2 whereas Class B applies to hard wind
areas for buildings that are sheltered or to low wind
areas for buildings that are not sheltered with
minimum brick strength 1.5N/mm2. Class C
minimum brick strength is 1.0N/mm2 and it applies to
low wind areas for buildings that are unsheltered.
British
Standards
Compression
Strength
Requirements for Mortars (BS 5628 Part1: 1978)
recommends the minimum compressive strength for
solid bricks having a ratio of height to least
horizontal dimension of between 0.6 - 2.0 is
2.8N/mm2. British Standards Compression Strength
Requirements for Mortars (BS 5628 Part1: 1978) is
as summarized in Appendix 1.
Aluminosilicate materials are ceramic like inorganic
polymers that are based on silica and produced at low
temperatures (Barsoum, 2006). They are considered
to be strong, fire and acid resistant (Duxson et al.,
2007). They can easily be produced by casting or
extruding. Aluminosilicate based bricks use cheap
material abundantly available naturally in some areas,
e.g. lateritic clay earth (Boutterin and Davidovits,
1982). These materials mixed with a simple
geopolymer binder are compressed to give the shape
of a brick and thus can be termed as geoplymer
materials (Davidovits, 2002).
The main objective of this study is to analyse the
strength of selected aluminosilicate soils for
possibility of use in brick manufacturing. The study
output will contribute to the body of knowledge that
exists in Tanzanian soils. Furthermore, the study
results will assists in conservation of environment by
providing an alternative method in bricks
manufacturing for construction purposes thus
replacing harmful traditional methods.
2. Methodology
The study on strength analysis of aluminosilicate
soils for use in brick manufacturing followed the
following methodology in attaining its objectives:
2.1 Soil samples
The soil samples used in the analysis were undergone
a preliminary tests as reported in Milyaso et al.
(2015). These include samples from River Natron,
Lake Natron, Oldoinyo Lengai, Mto wa Mbu, Lokii
and Kisongo. Correspondingly, trona was used as a
catalyst while Holili and Uchira blocks were for
comparative strength analysis.
2.2 Soil samples sythesizing and compression
strength tests
Analysis was carried out to determine the cementing
properties of the sample mixtures using “Unconfined
Compression Strength (UCS)–test” reference BS1881
(1983), CML 2000. One hundred and eight (108)
samples were made and water was added to get the
plastic property then each mixture was compacted to
obtain a cylindrical cube. This number of samples
was determined by considering the six soil samples
and mixing them at the ratio of 10:90; 20:80; 30:70;
40:60; and 50:50. These soil samples were mixed
thoroughly with water before compaction in order to
allow for different elements or compounds to come
together. Compaction was done using a compaction
machine (Soil compactor). These cubes were then
sealed with a plastic paper to isolate from hydration
reaction. Observation was carried out from the first
day and after thirty (30) days the cubes were tested
for their compression strength.
2.3 Sample Selection
The mixtures of soil samples which showed the best
results were selected for the manufacture of bricks
based on the compression strength. The higher the
compression strength the better the mixed samples,
however other factors like the debonding behavior
during compression testing were also observed.
2.4 Bricks Manufacturing
The ratios of mixing during manufacturing were
determined based on the percentage composition of
elements of each of the selected samples. The
mixtures were homogenized and brick specimens
were uniaxially pressed manually in steel moulds
with internal dimensions of 70 × 70 × 70 mm
typically used in the laboratory test to form 70 × 70 ×
70 mm specimens at room temperature. A total of
twenty four specimens were casted. These specimens
were then air-dried at an average temperature of 25
ºC. Compression test was then carried out.
2.5 Compression
Strength
Composition Tests
and
Chemical
The compression testing machine was used to
determine the compression strength in kN, at 7, 14,
30 and 90 days. Six specimens were tested at each of
these durations and results were tabulated in an Excel
sheet. Using Equation 1, compression strength in
N/mm2 was calculated and the average value was
taken for comparison with natural and cement based
blocks.
𝜎=
𝐹
… … … … … … … … … … … … … . (1)
𝐴𝑐
σ = Compression strength
F = Applied load
Ac = Cross sectional area
The compression strength of natural blocks from
Uchira and Holili were determined earlier whereby
three specimens were tested from each site and the
results were tabulated in an excel sheet and then the
average value was calculated for comparison
purpose. The chemical composition of the fabricated
bricks was again carried out using the XRF
procedures (Norrish and Hutton, 1969).
2.6 Comparison with Natural Blocks and Cement
Based Blocks
The fabricated bricks were then compared for
compression strength with the natural blocks from
Holili and Uchira and cement based blocks.
3. Result and discussion
3.1 Results
3.1.1
Soil samples sythesizing and compression
strength tests
One hundred and eight (108) cylindrical cubes of
diameter 150mm and height 225mm were casted.
Each cube had five layers of the same composition
mixture. For every layer the machine was set to
compact with sixty two blows [BS 1377 (1990), Part
4. This is to allow for proper compaction of the cube
without air gaps. Figure 1 illustrates the test setup for
compaction of different soil mixtures. Tests results
are as tabulated in Tables 1 and 2.
Figure 1: Compaction test set-up
The result for most of cubes shows very small
compression strength of less than 2.0 N/mm2. Blocks
made with Mto wa Mbu soil sample had better
strength values. The compression strength values
increased as more Lake Natron soil sample was
added to the Mto wa Mbu sample until it reached a
maximum value of 0.8 N/mm2 whereby the
composition was 70% Mto wa Mbu and 30% Lake
Natron. Addition of more Lake Natron soil sample
resulted in the strength starting to drop.
Table 1: Compression strength of soil mixtures at different compositions (N/mm2)
SAMPLE
0
RIVER
NATRON
LAKE
NATRON
0LDOINYO
LENGAI
50
60
70
80
90
100
50
60
70
80
90
100
50
60
70
80
90
100
10
MTO WA MBU
20
30
40
50
0.69
0
10
LOKII
20
30
0.56
40
50
0.76
0
10
KISONGO
20
30
0.74
0.32
0.19
0.1
0.68
0
0
0
0
0
0
0.71
0.77
0.32
0.65
0.57
0.2
0.46
0.15
0
0.18
0.12
0
0
0
0
0
0
0
0.62
0.63
0.33
0.56
0.24
0.12
0.22
0
0
0
0
0
0
0
0
50
0.69
0.15
0.71
0
40
0
0
0
Table 2: Compression strength of soil mixtures at different compositions (N/mm2)
SAMPLE
MTO WA MBU
0
LOKII
KISONGO
50
60
70
80
90
100
50
60
70
80
90
100
50
60
70
80
90
100
10
RIVER NATRON
20
30
40
50
0.76
0
10
LAKE NATRON
20
30
40
0.79
0
OLDOINYO LENGAI
10
20
30
40
0.78
0.78
0.77
0.68
0.79
0.66
0.79
0.74
0.65
0.74
0.74
0.69
0.77
0.69
0.63
0.78
0.79
0.67
0.78
0.78
0.71
0.78
0.76
0.7
0.76
0.8
0.69
0.8
0.8
0.69
0.65
0.71
0.56
0.67
0.7
0.58
0.69
0.69
0.59
0.66
0.67
0.58
0.63
0.72
50
0.62
0.67
0.8
0.75
0.72
50
0.71
0.57
0.72
Fabrication, Observation and Testing of
Brick Specimens
area in contact with the compression load Ac = 70 ×
70 = 4900 mm2.
Brick specimens were fabricated according to the
procedures explained earlier in the methodology. In
order to obtain a ratio of Si:Al = 1, then 4 units of
Mto wa Mbu soil sample and 1 unit of Lake Natron
soil sample was used or 1 unit of Mto wa Mbu and 4
units of Lake Natron soil samples were used. In
addition 5 units of Trona were used in order to
complete the chemical reaction. The results are as
presented in Table 3. These values were again
calculated based on Equation1, whereas the surface
In order to follow the in-situ morphological changes
with time, the samples were being observed daily
until the end of observation. In view of the very rapid
hydration reaction for the fabricated blocks, 80% or
higher compressive strength can be achieved after 14
days and thereafter the strength development is very
slow. In this study observations started one day after
mixing, and continued until 90 days had elapsed.
3.1.2
Table 3: Compression Strength of Casted Bricks at Different Duration (N/mm2)
DURATION
7 Days
14 Days
30 Days
90 Days
3.1.3
SAMPLE
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
1
2
3
4
5
6
STRENGTH
(N/mm2)
Ratio: 4:1:5.(Brick
01)
Still wet
Still wet
Still wet
Still wet
Still wet
Still wet
1.43
1.27
1.31
1.41
1.59
1.53
2.02
2.12
2.27
2.02
2.10
2.14
2.53
2.43
2.55
2.12
2.37
2.61
Chemical Analysis of the Fabricated
Bricks
Weight
(Kg)
0.89
0.8
0.85
0.76
0.76
0.77
0.7
0.72
0.71
0.68
0.66
0.7
0.59
0.67
0.59
0.59
0.64
0.62
0.5
0.56
0.54
0.59
0.57
0.52
STRENGTH
(N/mm2)
Ratio: 1:4:5 (Brick
02)
Still wet
Still wet
Still wet
Still wet
Still wet
Still wet
1.41
1.35
1.35
1.37
1.51
1.47
1.51
1.80
1.65
1.76
1.57
1.57
Collapsed
Collapsed
Collapsed
Collapsed
Collapsed
Collapsed
Weight
(Kg)
0.73
0.69
0.66
0.69
0.77
0.7
0.61
0.6
0.65
0.58
0.63
0.64
0.55
0.55
0.53
0.61
0.52
0.5
Not done
Not done
Not done
Not done
Not done
Not done
Chemical analysis was carried out on the
manufactured blocks in order to determine the
composition of elements after synthesis. The two
bricks whose relative soil compositions are indicated
in Table 4 (brick 01 and brick 02) were tested. The
test results as shown in Table 4 and Figure 2,
indicates that the amounts of compositions of almost
all the compounds have dropped in brick 02 when
compared to brick 01. SiO2 has dropped from 17.6%
to around 2.6% and Al2O3 as well has dropped to
around 2.3% from 17.4%. Also Na2O in the two
bricks has changed from 84.7% (for Trona /Magadi)
to around 40.9%. The ratio between SiO2 and Al2O3
has become 1:1 which was the desired ratio
according to the literature review on page 16.
However brick 01 showed better compressive
strength than brick 02 as sown in Table 5.
Table 4: Fabricated Bricks Chemical Composition
Sample
Brick
01
Brick
02
SiO2
Al2O3
Fe2O3
K2O
Na2O
17.55
17.41
4.50
1.11
36.80
2.61
2.27
4.58
0.63
44.90
Na2O
Elements
K2O
Fe2O3
Al2O3
SiO2
0
10
20
30
% Composition
Brick 02
40
50
Brick 01
Figure 2: Chemical composition of the fabricated bricks
3.1.4
(i)
Comparison with Natural Blocks and
Cement Based Blocks
Composition of Uchira Blocks
Visual characterization indicates that Uchira blocks
are reddish brown volcanic gravel soil. Physical
analysis indicates that the binding effect that occurs
on self-cemented reddish brown volcanic gravel soil
is due to the presence of naturally occurring
pozzolanic soil. Analysis on mechanical properties of
this material indicates that this material has a
saturated bulk density of 1.38 gm/cm3; dry density of
1.14 g/cm3; specific gravity of 2.52 porosity of 0.57;
and void ratio of 1.34.
(ii)
Unconfined Compression Strength of
Uchira Blocks
The vertical compression load was applied on these
blocks to evaluate the compression strength of this
self-cemented reddish brown volcanic gravel soil that
occur at Uchira. Test results indicates that block has
an average compression strength of 9.1 N/mm2.
(iii) Composition of Holili Blocks
Visual characterization indicates that Holili blocks
are grayish volcanic gravel soil. Physical analysis
indicates that the binding effect that occurs on selfcemented grayish volcanic gravel soil is due to the
presence of naturally occurring alkaline soil.
Analysis on mechanical properties of this cemented
grayish volcanic gravel soil indicated the following
average values: saturated bulk density of 1.968
gm/cm3; dry density of 1.659 g/cm3; specific gravity
of 2.65, porosity of 0.37 and void ratio of 0.60.
(iv) Unconfined Compression Strength of
Holili Blocks
The vertical compression load was applied on these
blocks to evaluate the compression strength of this
self-cemented grayish volcanic gravel soil that occur
at Holili. Test results indicates that Holili self
cemented grayish volcanic gravel soil has an average
compression strength of 55.6 N/mm2.
Therefore when three samples of Uchira volcanic
blocks whose average thickness was 140 mm and
three samples of Holili blocks whose average
thickness was 120 mm were tested their strength is
much higher than the recommended minimum values
of 1.0, 1.5 and 2.5 N/m2.
3.2 Discussion of Results
3.2.1 Visual Analysis
The surface of the developed geopolymer block is
characterized by a rougher texture than that of clay or
cement bricks. According to D’Ayala (2004), this
roughness characteristic is believed to be responsible
for the increased bond strength with mortar. During
compression testing it was observed that blocks did
not crack either vertically or horizontally, they failed
like a spongy material by debonding from the sides
representing semi-crystalline properties (McNulty,
2009).
Lake Natron soil is greatly dominated by volcanic silt
with little clay content, its particles are moderately
fine-grained particles, it does not have adequate
cohesion to bind soils together, that means it is non
plastic. Mto wa Mbu soil is greatly dominated by
gravel and volcanic ashes, it has adequate cohesion to
bind soils together, it is plastic. The optimum
moisture content (OMC) was determined for both
Lake Natron and Mto wa Mbu soil samples. Lake
Natron sample had 32.46%, whereas Mto wa Mbu
sample had 26.37%. This is due to the fact that finer
soil particles absorb more water than course particles
because of higher particle surface area and hence
higher moisture requirement. The maximum dry
density (MDD) determined for Lake Natron had 1.69
g/cm3 and Mto wa Mbu had 1.85 g/cm3. This shows
that the values of MDD did not have great difference.
Hence according (D’Ayala, 2004), these values could
produce light weight bricks as revealed from the
results that the average weight of the casted brick 01
is 0.55 kg after 90 days of investigation compared to
0.80 kg of the first 7 days; thus the weight is getting
less as the number of days increases.
3.2.2 Strength and Time
From the chemical analysis, in order to obtain a ratio
of SiO2/Al2O3 = 1 with constant amount of Na2O, 4
units of Mto wa Mbu, 1 unit of Lake Natron and 5
units of Trona were synthesized and blocks were
casted. Or else 1 unit of Mto wa Mbu, 4 units of Lake
Natron and 5 units of Trona were used. After 30 days
the casted blocks reached an average compression
strength value of 2.14 N/mm2 and after 90 days the
value rose to 2.44 N/mm2 as seen in Table 5. This
shows that the blocks were hardening with time.
However the natural occurring stone blocks from
Uchira and Holili had very high compression strength
of about 9.1 N/mm2 and 55.6 N/mm2 respectively.
This higher strength is the result of longer hardening
time which took over for a number of years.
According to Building Research Unit of Tanzania
(BRU) (1981), the recommended minimum
compression strength is 1.0 N/mm2 which is
applicable to low wind areas for buildings that are
unsheltered and the British Standards (BS) (1978)
compression strength requirements for mortars,
proposes the minimum compression strength to be
1.0 N/mm2 as a site test and not the laboratory test.
Hence, since the obtained value of 2.44 N/mm2 as a
laboratory test of brick 01 as shown in Table 5, the
manufactured block is suitable for use as class B and
C (BRU, 1981).
3.2.3 Chemical Composition
There is great difference in the amounts of SiO2,
Al2O3 and Fe2O3 between the casted block and the
blocks from Uchira and Holili. The casted block
(Brick 01) has the following chemical composition:
SiO2 :17.55% ; Fe2O3 : 4.50% and Al2O3 : 17.41%.
However Uchira block has the following
compositions: SiO2 : 45.3%; Fe2O3 : 15.9% and
Al2O3: 8.8%, and Holili block has: SiO2 : 46.2%;
Fe2O3 :17.2%; Al2O3 : 8.6%. The ratio between SiO2
and Al2O3 of the casted blocks became 1:1 (for both
bricks, although the actual values of silica and
alumina for brick 01 are about six to seven times
those of brick 02 which was the desired ratio. The
amounts of compounds from the original soil samples
are higher and they approach the amounts of
compounds in blocks from Uchira and Holili, if these
amounts could be maintained during synthesis; it
could result to better casted block properties.
Table 5: Average Compression Strength for the Casted Brick 01 and Brick 02
Compression strength at different durations and different mixtures
Ratio:4:1:5; Mto wa Mbu:Lake Natron:Trona (Brick 01)
Duration(Days)
Average strength (N/mm2)
Comments
7
Still wet
Cannot be used
14
1.42
Low strength
30
2.14
Recommended
90
2.44
Recommended
Ratio:1:4:5; Mto wa Mbu:Lake Natron:Trona (Brick 02)
7
Still wet
Cannot be used
14
1.41
Not recommended
30
1.64
Not recommended
90
Collapsed
Cannot be used
3.2.4 Chemical Composition
There is great difference in the amounts of SiO2,
Al2O3 and Fe2O3 between the casted block and the
blocks from Uchira and Holili. The casted block
(Brick 01) has the following chemical composition:
SiO2 :17.55% ; Fe2O3 : 4.50% and Al2O3 : 17.41%.
However Uchira block has the following
compositions: SiO2 : 45.3%; Fe2O3 : 15.9% and
Al2O3: 8.8%, and Holili block has: SiO2 : 46.2%;
Fe2O3 :17.2%; Al2O3 : 8.6%. The ratio between SiO2
and Al2O3 of the casted blocks became 1:1 (for both
bricks, although the actual values of silica and
alumina for brick 01 are about six to seven times
those of brick 02 which was the desired ratio. The
amounts of compounds from the original soil samples
are higher and they approach the amounts of
compounds in blocks from Uchira and Holili, if these
amounts could be maintained during synthesis; it
could result to better casted block properties. Case in
point is that of Lake Natron soil sample which had
SiO2 : 37.11%; Al2O3 : 8.99% and Mto wa Mbu soil
sample had SiO2 : 35.29% ; Al2O3 9.89%. Brick 01
showed better compressive strength than brick 02 as
shown in Table 5. This could be attributed to the
higher values of Silica and Alumina although the
ratio in both bricks is 1:1. Table 6 and Figure 3
depicts the comparison of the chemical compositions
between the casted bricks and the natural occurring
self cemented blocks
commonly bound as part of the cement stone. As the
water evaporates, it leaves the salt behind, which
forms a white, fluffy deposit as seen in Figure 4 of
the casted bricks. These deposits can normally be
brushed off easily and they are referred to as
"efflorescence" or "saltpetering."
Efflorescence effect causes weak bonding of the
casted geopolymer brick. There is a tendency of
animals like goats and cows to swallow the walls
built out of these bricks because they test salty, this
causes walls to be thinner and may lead the walls to
collapse.
Table 6: Chemical Compositions Comparison
Sample
SiO2
Al2O3
Fe2O3
K2O
Na2O
Brick 01
Brick 02
Uchira
Block
Holili
Block
17.55
17.61
2.41
2.27
4.5
4.58
1.11
0.63
36.8
44.9
45.3
8.8
15.9
0.5
2.7
46.2
8.6
17.2
0.2
2.3
3.2.5 Efflorescence Effect
When water moving through a brick wall or other
structures, or water being driven out as a result of the
release of the heat of hydration e.g. in cement stone,
the process brings salts to the surface that are not
Figure 4: Efflorescence observed on the casted
bricks
Na2O
Elements
K2O
Fe2O3
Al2O3
SiO2
0
5
10
15
20
25
30
35
40
45
50
% Composition
Holili Block
Uchira Block
Brick 02
Brick 01
Figure 3: Comparison of the Chemical Compositions between the Casted Bricks and the Natural Occurring
Self Cemented Blocks
4.
Conclusion
From the findings presented in this study it can be
conclude that:
 The compressed geopolymer bricks produced in
this study seem to be a viable option for use as
construction material. The achieved compression
strength is enough for construction of building blocks
for unsheltered single storey houses in low wind
areas. The bricks have shown increasing compression
strength character with time, within the 7 - 90 days of
investigation at room temperature.
 There exists a good correlation between the
maximum compressive strength and the maximum
amount of combined aluminosilicates. The higher
the amounts of silica and alumina produce better
strength, as shown in brick 01 as compared to brick
02. Similar results are proved by the composition of
Uchira and Holili blocks.
 Natron geopolymers has extremely long setting
times, but on activation by an alkaline material
(Trona) this time can be reduced.
 The SiO2/Al2O3 ratio of the fabricated bricks is
very nearly 1:1 suitable for bricks, ceramics and fire
protection.
 The study has shown that addition of Trona to the
mixture significantly reduces setting time.
5.
Recommendations
The followings are the recommendations resulted
from the study:
 Brick based on aluminosilicates and alkaline soil
produced in this study was seen to be porous. It is
therefore recommended for further studies on
porosity on soil samples of Arusha.
 Further studies on how to eliminate the
efflorescence effect on the casted bricks is
recomemended.
6.
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Appendix 1: British Standards Compression Strength Requirements for Mortars
Type of Mortar (Proportion by volume)
Increasing ability to
accommodate
movement e.g.
temp 
Increasing strength

Mortar
designation
Mean
compressive
strength at 28
days (N/mm2)
Cement:Lime:
Sand
Masonry
Cement:Sand
Cement :
Sand with
plasticizers
Lab
tests
Site
tests
i
1 : 0 to ¼ : 3
-
-
16.0
11.0
ii
1 : ½ : 4 to 4½
1 :2½ to 3½
1 : 3 to 4
6.5
4.5
iii
1 : 1 : 5 to 6
1 : 4 to 5
1 : 5 to 6
3.6
2.5
iv
1 : 2 : 8 to 9
1 : 5½ to 6½
1 : 7 to 8
1.5
1.0
Increasing resistance to frost attack during
construction 
Improvement in bond and consequent resistance to
rain 