Clay Minerals (1998)33,453-465
New uses for brick-making clay materials
from the Bail6n area (southern Spain)
I. G O N Z A L E Z ,
E. G A L A N ,
A. M I R A S
AND P. A P A R I C I O
Departamento de Cristalografia, Mineralogia y Q. Agricola, Universidad de Sevilla, Apdo. 553, 41071 Sevilla, Spain
(Received 15 February 1997; revised 8 September 1997)
ABSTRACT: An attempt has been made to assess new potential applications for the Bail6n clays,
traditionally used for manufacturing bricks, based on mineralogical, chemical, particle size, plasticity
and firing results. Raw materials and mixtures used by the local factory were selected and tested with
the addition of some diatomite, feldspar or kaolin. Based on their properties, clay materials from
Bail6n might be suitable for making porous red wall tiles, clinker, vitrified red floor tiles and porous
light-coloured wall tiles by pressing; the first could be manufactured from the raw materials and
mixtures currently used by the local manufactures. On the other hand, stoneware shaped by
extrusion, such as perforated bricks, facing bricks and roofing tiles, can be also manufactured from
the mixtures used at the factory if they contain 20-25% carbonate and small amounts of iron oxides;
lightweight bricks require black and yellow clays with diatomite.
Tertiary clays from the Bail6n area (southern Spain)
have traditionally been used for manufacturing
bricks and other ceramic building materials
(GonzSJez et al., 1985, 1992). Except for some
specific clays, the Bail+n reserves appear to be
virtually unlimited. More than 90% of the industrial
products obtained from these clays are perforated
bricks and only two factories manufacture higher
grade products such as clinker facing bricks and
roofing tiles. Production in the area amounts to
nearly two million pieces per day.
The purpose of this work was: (a) the investigation of new potential uses for these raw materials
on the basis of their mineralogical, chemical, drying
and forming properties; and (b) the preparation of
mixtures with other minerals and rocks (e.g. kaolin,
feldspar, diatomite) in order to make higher-grade
products.
MATERIALS
AND METHODS
The clays exploited in the Bail6n area belong to
Neogene sediments in the Guadalquivir basin which
were unconformably deposited over Mesozoic rocks
(Fig. 1). The Tertiary lithological sequence from
bottom to top is as follows: (a) Lower Formation
(20--40 m thick), consisting of conglomerates,
breccia, and sandstone; (b) Marly Formation
(about 200 m thick), dated as T o r t o n i a n Messinian, with interbedded sands and a carbonate
content that increases toward the top; and (c) Upper
Formation, dated as Messinian (IGME, 1977),
consisting of limestone, sandstone and sandy
limestone with interstratified marly levels.
The raw materials for brickmaking are obtained
from the intermediate formation (Fig. 1), the
detailed composition of which is as follows:
(a) black marls locally known as 'barro negro'
(black clay); (b) yellow marly clays with sandy
beds called 'barro rubio' (yellow clay); and (c)
white marls with fossil remains, named 'barro
blanco' (white clay). Manufacturers use these
marls mixed in different proportions, in addition
to 'barro rojo' (red clay) from Triassic quarries in
the Guarrom~in area or Los Lentiscares (a place
north of Bail~n) (Gonz~dez et al., 1985).
Thirteen samples representative of the clays used
by the local manufacturers were selected, including
raw clays and mixtures. The samples were labelled
as follows (Table 1): 1, 2, 3, 6, 7 and 9 (raw clays),
and ECG, LAG, SRG, MPG1, MPG2, MPG3 and
TRG (mixtures). They were characterized miner-
~:3 1998 The Mineralogical Society
454
I. Gonzdlez et al.
410m
MARL
~
SANDYMARL
SILTYMARL
SAND
0
w
1OKra
CLAYEYSAND
PALAEOZOIC
F~
TRIASSIC
~
PLIOCENE
AGIDICROCK8
r~
MIOCENE
~
QUATERNARY
-- --342m
FIG. 1. Location of the Bail6n area and representative section.
alogically by X-ray diffraction (XRD) analysis of
the bulk and <2 lam fractions, using a Phillips PW
1130/90 X-ray diffractometer at 20 rnA at 40 kW,
with Ni-filtered Cu-K~ radiation. The quantitative
mineralogical composition of the samples was
determined using standard methods and data
reported by Schultz (1964), Biscaye (1965) and
Martin Pozas (1975) on random (bulk) and
glycolated oriented samples (<2 ~m fraction). The
major elements were analysed by gravimetry (SiO2)
or atomic absorption/emission spectrometry (all
other elements). The grain-size distribution was
d e t e r m i n e d on <100 lxm samples using a
M i c r o m e r i t i c s 5100 Sedigraph instrument.
Atterberg limits were also calculated.
Before the firing properties of the materials were
studied, their chemical and mineralogical data were
plotted on the triangular charts of Fabbri & Fiori
(1985) and Fiori et al. (1989), in order to assess
potential ceramic applications for the clays.
Samples with a chemical or mineralogical composition not falling within the defined application fields
were amended by adding other raw materials,
namely washed kaolin from Burela (Spain),
commercially available feldspar from Otavi and
diatomite from Porcuna (Jaen, Spain), to obtain
appropriate compositions for making new products.
The mixtures were ground for 30 min, homogenized
and sieved through <100 lam mesh before pressing
at 2000 kg/cm 2.
Brick-making clay materials
455
TABLE 1. Loams and mixtures used by Bail6n factories.
Loams
Samples
Black
Blonde
Red
White
1, 2
3
6, 9
7
Mixtures from factories:
Samples
ECG
LAG
TRG
SRG
MPG1
MPG2
MPG3
Black
40%
60%
45%
-
Blonde
White
Red
Others
20%
15%
25%
55%
5%
25%
75%
25%
10%
20%
25%
70%
-
15%
15%
10%
20%
75%
25%
20%
5%
-
Mixtures were assessed for ceramic properties
including grain-size distribution, plasticity and
firing characteristics (linear shrinkage, water
absorption capacity, bulk density and open porosity
as determined according to Spanish standard UNE
61-033-75)
o v e r the t e m p e r a t u r e
range
750-1200~
X-ray diffraction and scanning
electron microscopy (SEM) were used to characterize the high-temperature phases formed in the
fired bodies and microstructures. Mineral phases
were semi-quantitatively determined with the aid of
the calibration curves reported by Huertas et al.
(1991).
RESULTS
AND
DISCUSSION
R a w materials characterization and ceramic
suitability
The materials used at the local brick factories
contain a maximum of 35% carbonate (mainly
calcite), 1 0 - 4 5 % quartz and some feldspar (<7%);
some samples include hematite and/or goethite, at
levels <5%. Phyllosilicates typically account for
3 0 - 6 0 % of the overall sample weight. The clay
minerals in the materials consist essentially of illite
and smectite, in addition to some kaolinite
(Table 2).
The chemical compositions of the raw materials
are well correlated with their mineralogical
composition (Table 3). Note the high proportions
of Fe203 in the mixtures containing red clay (6, 9
Use
Hollow block
Perforated brick
Perforated brick
Facing brick
Clinker
Facing brick
Roofing tile
and MPG2) by virtue of the presence of hematite.
On the other hand, the A1203 content in most of the
samples was quite low.
Based on the results obtained by SchmidtReinholz & Essen-Kray (1986), and on the charts
proposed by Fiori et al. (1989), the chemical data
for the samples studied suggest that they are
suitable for making red stoneware by pressing or
extrusion; sample 7, however, is unsuitable for
ceramic uses because of its high lime content. In
fact, white clay (sample 7), because of its high
carbonate and smectite contents, is never used
alone, but in mixtures with other types of clay that
improve plasticity. Some samples (MPG2, 6 and 9)
are suitable for making vitrified red floor tiles.
However, most of the studied raw materials require
the addition of some SiO2 or A1203 if they are to be
used for making high-grade products such as
clinker, light-coloured wall tiles or vitrified red
floor tiles (Figs. 2a,b,c).
The m i n e r a l o g i c a l data for the samples
(Figs. 3a,b) also suggest that most can be used
for making red stoneware. However, if specific
products such as clinker, porous or vitrified
pieces, and similar products, are to be obtained,
their composition must be slightly altered, as
revealed by the chemical analyses. Some samples
could be used to make porous red wall tiles
(SRG, LAG, 1 and 3), facing bricks (MPG1,
MPG3) and roofing tiles (TRG) ( S c h m i d t Reinholz & Essen-Kray, 1986). Based on these
results, mixtures MPG1, MPG3 and TRG are
456
I. Gonz6lez et al.
TABLE 2. Mineralogical composition of samples.
Samples
Q
Ph
1
45
40
30
27
13
27
37
40
26
26
20
38
27
36
27
52
62
45
58
34
34
56
41
58
53
38
2
3
6
7
9
ECG
LAG
TRG
SRG
MPGI
MPG2
MPG3
Bulk composition
Ca
Do
12
24
10
0
35
0
24
17
10
30
18
4
32
Fd
Hm
2
4
1
6
4
0
4
8
4
4
4
4
4
4
4
3
4
6
3
2
3
5
5
3
3
7
1
0
0
5
0
5
0
0
0
0
0
0
0
Phyllosilicates <2 ~tm fraction
Sm
!
C1/K
Pyr
36
28
53
0
58
0
22
5
4
21
30
0
46
50
65
42
100
34
98
68
85
96
70
65
89
40
14
7
5
0
8
2
10
10
0
9
5
5
8
0
0
0
0
0
0
0
0
0
0
0
6
6
Quartz (Q), phyllosilicates (Ph), calcite (Ca), dolomite (Do), feldspars (Fd), hematite (Hm), smeetites (Sin), illite
(I), chlorite + kaolinite (Ct/K), pyrophyllite (Pyr).
c u r r e n t l y b e i n g used c o r r e c t l y by the local
manufacturers; however, others such as SRG
and L A G could also be employed for making
higher-grade products.
Based on clay mineralogy, samples 3 and 7 lie
outside the theoretical composition field for bricks
because o f their high smectite contents. Samples 6,
9, MPG2 and SRG possess high illite contents and
are thus theoretically unsuitable for making red
floor tiles, consistent with the chemical composition
results. The other samples have suitable compositions for making red stoneware and, again, require
s o m e m i x i n g i f t h e y are to be u s e d for
manufacturing higher-grade products.
The particle-size distribution as plotted on a
Winkler chart reveals that samples 6, 7 and MPG1
are unsuitable for making ceramic products. The
others fall inside the field of roofing tiles (SRG,
TRG, MPG2 and MPG3) or hollow blocks (1, 2, 3,
9, LAG and ECG).
TABLE 3. Chemical composition of samples.
Samples
SiO2
AIzO3
1
2
3
6
7
9
ECG
LAG
SRG
TRG
MPG1
MPG2
MPG3
57.3
50.8
56.6
55.8
42.9
54.8
56.9
54.8
53.3
56.6
57.7
56.6
50.2
15.2
9.3
12.1
18.1
9.2
17.0
10.1
10.2
9.3
11.0
12.2
16.3
9.3
LOI = loss on ignition.
Fe203
4,67
4.33
4.23
8.61
4.11
7,97
4.21
4.84
4.26
5.65
5.47
7.48
4.54
CaO
MgO
Na20
K20
LOI
Total
6.58
14.17
9.11
2.37
19.69
3.56
I0.30
12.09
13.18
8.88
10.41
3.59
14.28
1.69
2.56
2.24
2.78
2.40
3.08
1.63
2.07
220
2.35
2.22
1.82
1.63
1.42
1.40
1.74
1.65
0.80
0.92
1.19
1.77
1.34
1.40
1.36
0.83
1.04
3.12
2.74
3.10
5.10
1.79
5.07
2.72
3.I7
2.63
3.76
3.39
3.95
2.20
9.80
14.59
10.79
5.47
18.88
7.35
12.97
11.20
13.65
10.15
7.08
7.42
13.49
99.78
99.89
99.91
99.88
99.77
99.75
100.02
100.14
99.86
99.79
99.83
97.99
96.68
Brick-making clay materials
a]
457
SiO 2
50% SiO2
AI203
I
I
I
Fe203+MgO+
CaO+Na20+K20
@ Porous light-coloured
~ Vitrified red floor tiles
[ ] Clinker
~
Fe203+CaO+MgO
Red bodies
CaO+MgO
b]
AI203
Fe203
~]]) Porous light-coloured
Na20+K20
~ Vitrified red floor tiles
[] Clinker
( ~ Red bodies
F[o. 2. Triangular charts of Fiori et al. (1989) for the raw materials and mixtures studied. (a) SiO2/A1203/total
oxides. (b) (Fe203 + CaO + MgO)/A1203/(Na20 + K20). (e) (CaO + MgO)/Fe2OJ(Na20 + K20).
458
I. Gonzhlez et al.
a)
Ca+Do
Q+Fd
(El) Porous light-coloured
Ph
[ ] Clinker
~ Vitrified red floor tiles
Sm
b)
m7
",
o3
MPG1,
K
(1~ Porous light-coloured
.
~ Clinker
.
.
.
MPG
"z~l
~ Vitrified red floor tiles
FIG. 3. Triangular charts of Fiori et al. (1989) for the raw materials and mixtures studied. (a) Quartz (Q) feldspars (Fd)/[calcite (Ca) + dolomite (Do)]/phyllosilicates (Ph). (b) Smectites (Sm)/kaolinite (K)/illite (1).
Brick-making clay materials
459
Regarding plasticity, the loams (samples 6 and 7
excluded) are suitable for making ceramic bodies.
As a rule, they are inadequately plastic for
extrusion moulding but are suitable for pressure
moulding.
In summary, all the marls, clays and clay
mixtures used in the Bail6n area except white
clay (sample 7) are suitable for manufacturing red
stoneware products. Mixtures SRG and LAG,
currently employed for making perforated bricks,
and clays 1 and 3, can also be used for
manufacturing red wall tiles. Mixtures MPG1,
MPG3 and TRG make appropriate raw materials
for their present uses (facing bricks and roofing
tiles). Manufacturing higher grade products from
them would require modification of their compositions by mixing with other raw materials.
based on their mineralogical and chemical
compositions. Samples TRG1, 6, 9, and 10 could
also be useful for this purpose but were discarded
because of their high iron oxide contents
(Figs. 2b,c). The selected samples were mixed
with 25% kaolin.
Lightweight bricks were manufactured from
samples 2 and 3, which were chosen on account
of their suitable mineralogy (carbonate content
15-24%), high smectite contents (30-50%) and
appropriate particle-size distribution. They were
mixed with diatomite in a 30% proportion.
Table 4 shows the mineralogical composition of
the mixtures tested which are theoretically suitable
for the various intended uses in the coarse-grained
ceramic manufacture (Schmidt-Reinholz & EssenKray, 1986).
New formulations and products
Technological properties of the proposed
mixtures
Diatomite, feldspar and beneficiated mediumgrade kaolin were tested as additives for the
original clays and mixtures in order to make
higher-grade products.
Clinker was manufactured from samples MPG2,
6 and 9, which have low carbonate contents
(Fig. 3a) and near ideal chemical compositions.
We also used sample 1, i.e. black clay, which is the
most abundant in the area and could thus be opened
to new markets. This was mixed with 20% kaolin in
order to obtain the required amount of alumina in
the mixture (Fig. 2b).
With regard to vitrified red floor tiles, the results
for the above-mentioned samples (MPG2, 6 and 9)
were different. Based on chemical composition,
these samples should be suitable for making this
type of product, but their mineralogical composition, which, according to Kolkmeier (1990), is more
directly related to the technical properties, is not
quite consistent with this use. In addition, according
to Gonzhlez et al. (1985), the water absorption
( 2 0 - 2 2 % at 950-1000~
and open porosity
( 3 4 - 3 5 % ) of samples 6 and 9 make them
unsuitable for manufacture of vitrified bodies.
Thus it seems that there is little potential in these
samples for making vitrified red floor tiles. We
prefer to mix them with feldspar and this test was
performed by mixing sample 9 (red clay) with 20%
feldspar.
Samples 2, LAG and ECG were selected for
making porous light-coloured wall tiles as they
were the closest to the theoretical application field
Based on the Atterberg plasticity index, the
mixtures are not very plastic. Also, based on the
Clay Workability Chart of Bain & Highley (1978),
the samples LAG+K, ECG+K, I+K, 2+K, 2+D
and 3+D exhibit optimal moulding properties
(Fig. 4a). The others are unsuitable because of
their poorer cohesion. Based on the Casagrande
chart (Fig. 4b), the samples that lay in the zone of
suitable moulding properties in the Bain &
Highley chart should be optimal or suitable for
extrusion shaping; the others are appropiate for
press-moulding.
The linear shrinkage, bulk density, open porosity
and water absorption results were plotted on firing
diagrams. The firing tests performed on the
mixtures provided the optimum vitrification
temperature. A temperature of 850~ is the most
suitable for making perforated bricks, porous red
wall tiles and facing bricks from the natural and
mixed samples currently used by the local
manufacturers (SRG, LAG, 1, 3, MPG1, MPG2
and MPG3). Making roofing tiles (from TRG),
however, entails firing at a higher temperature
(1000~
The mixtures used to make clinker (Table 4)
exhibited a differential behaviour. Thus, the
ceramic bodies made with sample MPG2 broke
above 950-1000~
with high linear shrinkage
values. Therefore, the modified mixture was no
better than that currently used by the manufacturers.
The other samples tested provided the best water
460
I. G o n z f i l e z
acceptable
moulding
properties
50 - m
(3}
et al.
increasingshrinkage
45- 40--
35--
.~
poorer
30- --cohesion
25--
~ t..,.q
* v...~
~
stickyconsistency
20--
r~
c~
15---
~
,.
pottery
bricks ~
10---
optimum
~ moulding
properties
5----
I
0
0
I
10
I
20
plasticity
|
MPG2+K
LAG+K
b)
}
30
9+Fd
9+K
6+K
ECG+K
I+K
2+K
[
40
50
index
[]
2+D
3+D
50--
X 40-d~
13
r 30--
,i,[
) m
0 20-Or)
~,. 10--
o
o
I
I
lO
I
20
I
30
I
40
I
4-
50
60
I
70
I
80
liquid limit
9*K
(]]~)
MPG2+K 9+Fd
O
LAG+K ECG+K I+K
6+K
2+K
[~]
2+D
3§
A: Optimum zone to the shaped by extrusion
B: Suitable zone to the shaped by extrusion
FIG, 4. (a) Clay workability chart (after Bain & Highley, 1978). (b) Casagrande chart.
I
90
Brick-making clay materials
461
TABLE 4. Mineralogical composition of the mixtures studied (%).
Sample
Bulk sample
Ph
Ca+Do
Q
2+K(25%)
ECG+K(25%)
LAG+K(25%)
a
9+Fd(20%)
1+K(20%)
6+K(20%)
9+K(20%)
MPG2+K(20%)
b
2+D (30%)
3+D (30%)
d
c
Phyllosilicates <2 p.m fraction
Sm
I
CI/K
Pyr
Fd
35
30
32
35
40
45
24
20
18
6
10
5
10
10
20
25
20
30
65
70
50
0
0
0
20
35
18
12
25
38
45
80
86
57
14
15
tr
tr
8
28
5
0
0
10
0
18
0
0
0
100
23
60
60
14
0
59
40
40
70
0
0
0
0
16
30
25
37
56
30
15
3
4
39
60
55
35
6
5
0
0
(a) Porous light-coloured wall tiles; (b) vitrified red floor tiles; (c) clinker; (d) light-weight bricks.
K: kaolinite, Fd: feldspar and D: diatomite.
SAMPLE 6 + KAOLINITE
SAMPLE 9 + FELDSPARS
2,~
40
2,5
30
2,~
o
2,1
20 ~
2,(~
"0
1o
--Ib-- OP
i:i
25
o
v2.2
20
~,
15
~:
-~ 2,1
~
~
lo
zo
/
~n 1,~
i
1,8
x
J
9
900
x~x~X'"-,.~
1,9
s
850
1000
950
J
9
1050
,
1100
9
1,8
,
1150
i
850
-
,
900
Temperature ~
-....
8-
-
,
9
\
5
//
I
.
950
lOOO
lO5O
Temperature *C
-
i
11oo
--O--
2O
9
n
115o
WA
--X--LFS
9
o-
"~"
\\
/
1s
WA
LFS
30
15
6
~8
o-
.~5
.F=
10
~4
.2-
~4-
5
x.._._.-~xt
i
050
-
i
900
5
~
o
.3
S
o
0
10
-
l
-
950
1000
1050
1100
1150
0
850
900
950
1000
1050
1100
1150
Temperature ~
Temperature ~
FI~. 5. Sintering diagrams for a representative mixture
for clinker making. OP: open porosity, BD: bulk
density, WA: water absorption, LFS: linear firing
shrinkage.
FIG. 6. Sintering diagrams for a representative mixture
for making vitrified red floor tiles. OP: open porosity,
BD: bulk density, WA: water absorption, LFS: linear
firing shrinkage.
1. Gonz6lez et al.
462
SAMPLE 2 + KAOLINITE
i
2,4 o
--e--
OP
- - X - - 8D
2,2
42
41
-
E
2,0
.o
v
/
"~ 1,8
/
r'-
x~~....--q------__~
40 O
"13
.--s
39 "13
O
38 _.
~Z
]: .............................~ , _ ._ / . ~ . _ _ ~ x
1,6-
37
m~ 1 , 4 36
1,2
i
750
9
i
9
800
i
9
850
i
9
900
i
35
950
Temperature ~
i
i
i
..........................,
13
O~
--~---Z~--
25
WA
LFS
24
0
r
23
g
if)
0
fi
E
o_
2 - -2
22 5
. ~ -3
.J
21
-4
Cso
d00
850
9o0
9'60
20
Temperature ~
FIG. 7. Sintering diagrams for a representative mixture for making light-coloured wall tiles. OP: open porosity,
BD: bulk density, WA: water absorption, LFS: linear firing shrinkage.
absorption and linear firing shrinkage values over
the temperature range 1070-1100~ (Fig. 5). The
mineral phases formed consisted essentially of
plagioclase and mullite.
The ceramic performance of the mixture tested
for making vitrified red floor tiles (Table 4) was
very good. Mixing with feldspar supplied the SiO2
required to decrease linear shrinkage and water
absorption; in fact, water absorption was -5% and
linear shrinkage 2 - 4 % over the temperature range
studied (1000-1050~
Fig. 6). The occurrence of
large amounts of Fe203 and K20 in the sample
accelerated vitrification through the fluxing effect
of these oxides. The microstructure of the fired
bodies is typical of predominantly amorphous
materials. The new phases formed consisted of
mullite and K-feldspar. In addition, firing at 10501100~ would probably be better in order to obtain
a water absorption <5%, also if the linear shrinkage
would increase up to -7%.
Brick-making clay materials
463
Fla. 8. SEM image of new mineral phases formed on heating at 950~
Mixtures 2+K and LAG + K were the best in
ceramic terms among those tested for making
porous light-coloured wall tiles. There was slight
expansion at low firing temperatures (750~
probably because there is a recarbonation
phenomena of unreacted CaO and/or MgO from
carbonate decomposition. However, body volumes
remained quite stable over the range 800-1000~
(Fig. 7). In this temperature range, preceding
vitrification, open porosity (35-37%) and water
absorption (20-23%) are especially suitable for
making this type of product. The new phases
formed are calcium plagioclase and wollastonite
(Fig. 8), consistent with the high carbonate contents
in the samples (Gonz~,lez Garcia et al., 1990).
Finally, the mixtures tested for making lightweight bricks (Table 4) exhibited almost constant
bulk density, water absorption and porosity
throughout the firing temperature range (Fig. 9).
Shrinkage was slight (1-2%). As can be seen from
Fig. 10, no vitrification was observed in the
temperature range studied; consequently the materials can be used for making very low-temperature
refractory bodies.
CONCLUSIONS
Based on the mineralogical, chemical, particle size,
plasticity and firing results obtained, marl and clay
materials from Bail6n might be suitable for making
porous red wall tiles, vitrified red floor tiles, porous
light-coloured wall tiles and clinker by pressing.
The first of these may be made from the raw
materials and mixtures currently used by the local
manufactures and the last by extrusion from
mixtures with black clays. Perforated bricks,
facing bricks and roofing tiles can be also
manufactured from the mixtures used at the
factory with the addition of 20-25% carbonate
and small amounts of iron oxides. Lightweight
bricks may be manufactured using black and yellow
clays mixed with diatomite.
464
I. Gonzhlez et at.
SAMPLE 3 + DIATOMITE
2,0.
9
i
i
45
....e - - O P
- - x - - BD
40
~
o ~
E
o
o')
J
o
_ _ X ~
xj
v
x
x
"o
35
l
"10
O
r.-
30
nl
~
25 " "
r
8'0o
820
900
~so
1500
20
Temperature ~
4~ - - - ' - ' , - ' - - - - r ~ r ~ --e-- WA
,
28
.
22 g
.__.,.. 1
._
_1
20
o
18 o~
0
"~"
16
-1
750
800
850
900
950
1000
Temperature~
FIG. 9. Sintering diagrams for a representative mixture for making lightweight bricks. OP: open porosity, BD:
bulk density, WA: water absorption, LFS: linear firing shrinkage.
ACKNOWLEDGMENTS
REFERENCES
The authors are grateful to J. Konta and B. Fabbri for
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comments. This work was partially supported by the
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465
FIG. 10. SEM image of diatomite remains on heating at 900~
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