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Some notes on the firing colour of clay bricks
Applied Clay Science, 2 (1987) 175-183 Elsevier Science Publishers B.V., Amsterd,m - - Printed in The Netherlands
175
Short Communication
S o m e N o t e s on the F i r i n g Colour of Clay B r i c k s ROLF KREIMEYER
Bundesanstalt fiir Geowissenschaften und Rohstoffe, StiUeweg 2, D-3000 Hannover 51 (Federal Republic of Germany) (Received January 15, 1986; accepted after revision December 18, 1986)
ABSTRACT Kreimeyer, R., 1987. Some notes on the firing colour of clay bricks. Appl. Clay Sci., 2: 175-183. There is a close relationship between the occurrence of high-temperature crystalline phases and the colour of clay bricks after being fired under oxidizing conditions at 1000°C. Firing colours in various red shades expected on the basis of the relatively high Fe-content (3-7 wt. % ) may fail to appear due to the incorporation of Fe in specific high-temperature crystalline phases rather than its occurrence as free iron oxide in the form of hematite. X-ray investigations show that these minerals could be one or the other of mullite or metakaolinite and a fassaitic pyroxene in which the iron is present in its trivalent form. Yellow and beige to light brown colours result in the formation of these minerals. Mullite or metakaolinite are formed from kaolinitic raw materials containing only little CaO and alkali oxides. Fassaitic pyroxene is formed from CaCO3-rich materials in which the CaC03 is fine grained and homogeneously dispersed. If the conditions are not favourable to form these minerals the firing colour of the bricks will be in various shades of red due to fine grained, dispersed hematite. INTRODUCTION
It is t h e aim o f this s t u d y to i n v e s t i g a t e some o f t h e factors b e h i n d t h e firing colour o f brickclays. It is well k n o w n t h a t t h e m a i n c o l o u r i n g a g e n t in brickclays is iron oxide. R e d or r e d b r o w n colours are usually o b t a i n e d due to t h e a p p e a r a n c e o f fine grained, dispersed h e m a t i t e , w h i c h develops d u r i n g firing f r o m iron m i n e r a l s (goethite, siderite, p y r i t e etc. ) u n d e r oxidizing firing conditions. T h e f o r m a t i o n o f yellow or beige firing colours f r o m iron c o n t a i n i n g brickclays has b e e n i n v e s t i g a t e d several times, b u t t h e results o b t a i n e d are c o n t r a d i c t i o n a r y . As early as 1874 Seger (1908) divided t h e light b u r n i n g clays into two groups. A c c o r d i n g to S e g e r t h e first g r o u p is c h a r a c t e r i z e d b y a high A1203 c o n t e n t a n d a low CaO c o n t e n t ; t h e s e c o n d b y a high CaCO3 c o n t e n t . T h e i n f l u e n c e o f t h e Ca c o n t e n t on t h e firing c o l o u r has b e e n i n v e s t i g a t e d several times. A light
0169-1317/87/$03.50
© 1987 Elsevier Science Publishers B.V.
176
17.
(~)
16 15 ~4 13. 12, 11.
10. mmm m
9. 8.
• .A
-.- ". •
7.
•
• •
" o
o
o OD
6,
0 0
5.
o
0
c O
4, 3. 2, I. 0i
" ~ ~ ~i
~ ~ 8'~
~ d g
Fig. 1. Clays of the Lower Cretaceous (KBT). Firing colour in relationship to CaO + MgO- and Fe20:~-contents. firing colour has been considered to be due to the formation of dicalciumferrite ( Sandfort and Liljegren, 1963), the incorporation of iron in gehlenite (Dal, 1956) or pyroxene (Klaarenbeek, 1961; Peters and Jenni, 1973). TEST MATERIALS Test materials for this study were raw materials and test bricks from various regions of Lower Saxony. These clays are described in an earlier publication {Stein et al., 1981 ). Their chemical compositions, mineralogical compositions, grain size analyses and various ceramic, physical and rheological properties are reported in detail. Test bricks from these raw materials were produced in the Ziegelforschungsinstitut ( institute for brick research) in Essen. They were fired in an electric kiln under oxidizing conditions and a temperature of 1000 ° C. OCCURRENCE OF YELLOW FIRING COLOURS Yellow firing colours were obtained from raw materials which are characterized by low contents of CaO (less than 2 wt. %, see Fig. 1 ) Main components of these raw materials are quartz and kaolinite, but illitic muscovite may occur
177 TABLEI Chemicalcompositionofthe Schnaittenbacherkaolinand a yellowburningclay Sample
SiO~
AI20~,
Fe20:~
TiO2
CaO
MgO
K20
Na20
LOI
SBK
48.22 56.52
36.73 21.05
0.25 5.29
0.47 1.11
0.08 0.21
0.09 1.21
1.52 1.81
0.08 1.15
12.47 11.40
KTH 5
Allanaly~esin wt. %. as well. The iron content of the raw materials varies between 3 and 10 wt. % and averages 4.5 wt. %. It was found t h a t in some fo the yellow firing clays not all of the iron is evenly distributed, but some coarse grained siderite or siderite accumulations are present, which during firing convert to hematite resulting in some small red spots in a yellow brick. However, solution of the hematite by hydrochloric acid demonstrates, t h a t up to 3-4 wt. % iron are incorporated in other minerals which are not soluble in hydrochloric acid.
Firing tests and qualitative X-ray investigations on artificial mixtures For the following tests a Schnaittenbacher kaolin was used (chemical composition see Table I, sample S B K ) . For comparison the chemical composition of a typical yellow burning clay is given as well ( sample K T H 5). After firing at 1300°C 55 wt. % of mullite are obtained from the Schnaittenbacher kaolin, the remainder being SiO2 (quartz and crystobalite) and glass. Mixtures of kaolin and very fine grained Fe203 were produced in such a way, t h a t after firing definite mixtures of mullite and Fe203 were obtained (see Table II). All samples were fired in an electric kiln for 24h at temperatures of 1000°C and 1300°C. After cooling the firing colour was determined visually and the product was investigated by X-ray (Guinier camera, Fe-radiation). In addition the lattice constants of the mullite obtained from the samples fired at 1300 °C were determined. The results of the firing tests are recorded in Table III and the lattice constants of the mullite are recorded in Table IV. TABLE II Samples with definite mullite-Fe203ratios Sample K 1 Sample K2 Sample K3 Sample K4 Sample K5 Sample K6
mullite+ 0.5 wt. % Fe203 mullite+ 3.33 wt. % Fe203 mullite+ 6.66 wt. % Fe2O3 mullite+ 8.5 wt. % Fe203 mullite+ 11.0 wt. % Fe203 mullite+14.0 wt. % Fe203
178 TABLE III Crystalline phases and firing colour of samples K1-K6 fired at 1000°C and 1300 °C Sample
Crystalline phases identified
Firing temperature 1000 ° C: K1 Quartz K2 Quartz K3 Quartz K4 Quartz K5 Quartz K6 Quartz, hematite Firing temperature 1300 ° C K1 Mullite, quartz K2 Mullite, quartz K3 Mullite, quartz K4 Mullite, quartz K5 Mullite, quartz K6 Mullite, quartz, hematite
Firing colour
yellow yellow yellow yellow yellow light redbrown
yellow yellow yellow yellow yellow light redbrown
Variation of the firing colour by addition of alkali oxides and calcium oxide Further tests were designed to change the firing colour of kaolinitic raw materials by addition of alkali oxides and calcium oxide. Kaolin-FE203 mixtures according to sample K3 were mixed with various amounts of Na2CO3 + K2CO3 and CaC03 ( see Table V). The samples were heated in an electric kiln for 24 h at a temperature of 1000 ° C only. After cooling the firing colour of samples was determined visually and the product was investigated by X-ray diffraction. TABLE IV Lattice constants 1 (A) of mullite in samples K1-K6 fired at 1300 °C Sample
a
b
c
Fee03 (wt.%)
K1 K2 K3 K4 K5 K6
7.5491 7.5526 7.5597 7.5629 7.5653 7.5659
7.6953 7.7084 7.7192 7.7268 7.7319 7.7318
2.8847 2.8900 2.8957 2.8983 2.9017 2.9019
0.5 3.33 6.66 8.5 11.0 14.0
1Lattice constants of pure mullite: a = 7.5456/~; b = 7.6898/~; c = 2.8842]k.
179 TABLE V Kaolin-Fe203 mixtures with various amounts of alkali oxidesand calciumoxide Sample S 1 Sample $2 Sample $3 Sample C1 Sample C2 Sample C3 Sample C4
K3+0.5 wt. % Na2C03+0.5 wt. % K2CO 3 K3 + 1.0 wt. % Na2C03 + 1.0 wt. % K2CO3 K3 + 2.0 wt. % Na2C03+ 2.0 wt. % K2C03 K3 + 1.0 wt. % CaC03 K3 + 2.0 wt. % CaC03 K3 + 3.0 wt. % CaC03 K3 +4.0 wt. % CaC03
OCCURRENCE OF LIGHT BROWN AND BEIGE FIRING COLOURS Light brown or beige test bricks were obtained from raw materials characterized by high contents of alkaline earth elements (CaO + MgO contents higher t h a n or equal to 10 wt. % ). One of the main components of the raw materials is calcite, others might be quartz, mica and clay minerals in various amounts. The iron content of the raw materials averages 6 wt. %. Tests using the method of Peters (1968) were made to determine the valency state of the iron in the outer part of the beige coloured test bricks. It was found, t h a t nearly all the iron is present in its trivalent state. Although some clays (especially the Wealden clays, see Fig. 1) are CaOrich, the expected beige or light brown colour fails to appear and redbrown colours with only few yellow spots are obtained. Factors, other t h a n mineralogical or chemical composition must therefore be responsible for the appearance of light colours. A significant difference of the Wealden clays compared to other raw materials (Lower Cretaceous clays, see Fig. 2) is the coarse grain size of the carbonates.
Firing tests with artificial clay-CaC03 mixtures Clay-CaCO3 mixtures were produced using two clays with similar chemical composition but different grain size distribution (see Tables VII and VIII). T A B L E VI
Crystallinephases and firing colourof samples $1-$3 and C1-C4 Sample
Crystallinephases identified
Firing colour
S1 $2 $3 C1 C2
Quartz, mullite? Quartz,hematite None (whollyamorphous) Quartz Quartz,anorthite,hematite
yellow orange-yellow dark brown yellow lightredbrown
C3 C4
Quartz, anorthite, hematite Quartz, anorthite, hematite
redbrown redbrown
180
("/.) 11?O,
98" 7.
6. 5.
O.o °o
4.
0
0 •
•
II
O0
J.
0
2-
c¢0 + ~ j o (~.)
t
Fig. 2. Wealden-clays ( K T H ) . Firing colour in relationship to CaO + MgO- a n d Fe203-contents.
The CaC03 addition to the clays was 10 and 20 wt. %, respectively. Two different forms of CaCO3 were used, coarse grained (grain size 200-20/~m) and fine grained (grain size < 20/Lm). All samples are described in Table IX. Test bricks were formed and fired in an electric kiln at a temperature of 1000 ° C for 2 h. The mineralogical composition of the test bricks was determined semiquantitatively by X-ray diffraction. The firing colour of the test bricks was determined visually. The results of the firing tests are recorded in Table X. TABLE VII Chemical composition of the clays used Sample
SiO~
AI20:~
Fe~O:l
Ti02
CaO
MgO
Na20
K~O
LOI
KBT 9 KTR 4
49.5 53.5
20.1 20.6
8.0 7.7
0.81 1.02
1.90 1.90
1.40 2.14
1.02 0.53
2.85 2.96
12.8 8.6
T A B L E VIII G r a i n size ( i n / l m ) distribution of the clays used (in % ) Sample
200-63
63-20
20-6.3
6.3 -2
2
KBT 9 KTR 4
4.9 3.2
3.4 15.1
12.3 23.0
16.5 22,7
62.8 35.9
181
T A B L E IX Sample description of clay-CaCOa mixtures M1 M2 MS1 MS2 S1 $2 SS1 SS2
KBT 9 " " " KTR 4 " " "
+ 10 +20 + 10 +20 + 10 +20 + 10 + 20
wt. % . . . . . . . . . . . . . .
. .
.
CaCOa .
(coarse grained) .
.
.
{fine grained)
. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
(coarse grained)
. .
.
(fine grained)
. .
TABLE X
Results of the firing tests with clay-CaC0:~ mixtures
Sample
Main components
Secondary components
Traces
M1 M2 MS1 MS2 S1 $2 SS1 SS2
aK, Q Q, Gehl. Q, Plg. Q, Pig., Pyr. aK, Q Gehl., Q aK, Q Q
Gehl., Hem. aK, Pig., Hem. Hem.
Plg. Pyr., Wo. Pyr., Gehl. Gehl., Hem. Plg. Pyr. Wo., Pyr. Wo., Hem
Gehl., Hem Plg., Hem Gehl., Pig., Hem Plg., Gehl., Pyr.
Colour redbrown redbrown*l redbrown
beige redbrown redbrown .1
redbrown beigebrown
*~Few yellow spots visible in an almost redbrown sample. Legend. aK=amorphous component; Q=quartz; Plg = plagioclase; Gehl = gehlenite; H e m = hematite; Pyr = pyroxene.
The mineral equilibrium of the fired clay-CaCOa mixtures and the chemical composition of the newly formed Ca-silicates The grain size distribution of the carbonate as well as the grain size distribution of the silicates influences the mineralogical composition of the fired samples. Fine grained raw materials favour the formation of plagioclase in samples containing 10 wt. % CaCO3 and the formation of plagioclase and pyroxene in samples containing 20 wt. % CaCO3. Coarse grained raw materials on the other hand favour the formation of gehlenite. The presence of gehlenite together with quartz is a mineral unequilibrium. According to the following reaction they should convert to anorthite and wollastonite. Gehlenite + 2 quartz
, anorthite-{- wollastonite
The presence of gehlenite indicates, that not all siliceous material, especially the clay minerals rather than quartz grains, reacted with CaO that develops from the CaCO3 during firing. A crystalline phase (gehlenite) with higher CaO
182
contents than expected from the chemical composition, developed locally enriched. The lattice constants of gehlenite occurring in the artificial clay-CaCO3 mixtures could not be determined accurately. Only few peaks are without doubt attached to gehlenite. But normally they are diffuse which may be due to fine grain size, lattice defects or unsufficient crystallization. The observable peaks, however, are shifted to lower 20 values, which results in larger lattice constants. This is expected, when the bigger Fe 3+ ion is incorporated in the structure instead of the smaller A13+ ion. Moreover, previous investigations (Dal, 1956 ) have shown, that gehlenite may incorporate Fe 3+ in its structure. The chemical composition of the pyroxene occurring in Ca-rich bricks was determined. Gel syntheses of pyroxenes with known chemical composition were carried out. The X-ray diffraction patterns of the synthesized pyroxenes were compared with the X-ray diffraction patterns of the beige coloured bricks and samples MS2 and SS2. The pyroxene in the beige coloured bricks and in samples MS2 and SS2 was found to be a pyroxene of the composition CaMgo.5 Feo.~ Alo.5 Si,.5 O6. CONCLUSIONS
Kaolinitic, Ca-poor clays No hematite can be detected by X-ray diffraction in the samples K 1 - K 5 aider firing at 1000°C and 1300°C, respectively. Increasing amounts of Fe203 in the samples are coupled with increasing lattice constants of mullite in samples fired to 1300 ° C. The only explanation for this fact is the incorporation of Fe 3+ for A13+ in the mullite structure. The occurrence of free Fe203 in the form of hematite in sample K6 shows that the lattice cell dimensions reach their maximum at 11 wt. % Fe203 and no more iron can be incorporated in the mullite structure. Since no hematite can be detected in samples K 1 - K 5 fired at a temperature of 1000 ° C either, it is concluded that the iron is already incorporated in the amorphous substance metakaolinite, which forms by subsolidus reactions. No fine dispersed hematite occurs in bricks as a colouring agent when the raw material consists mainly of kaolinite and quartz. Hematite, however, occurs as a colouring agent when fairly small amounts of impurities such as alkalies or CaO are present in the raw material. Alkalies induce the early formation of a glassy matrix and CaO in the raw material leads to the formation of anorthite, which does not incorporate Fe 3+ in its structure. In both these cases the amount of metakaolinite in the fired product is considerably reduced. As a result "free" Fe203 in the form of hematite occurs as a colouring agent. The firing colour of kaolinitic clays is therefore not only due to the amount
183 of finely dispersed FeeO3 in the raw material, but it depends also on the amount of impurities ( alkalies and CaO ), which themselves are non-colouring at all.
Ca-bearing clays The beige firing colour of CaO-rich clays is favoured by the formation of a fassaitic pyroxene. This pyroxene incorporates Fe 3+ in its structure and prevents the formation of finely dispersed hematite as a colouring agent. The formation of this pyroxene starts in clays containing at least 10 wt. % CaO, provided the CaO in the raw material is fine grained and homogeneously dispersed. Coarse grained and locally enriched CaCO3 in the raw material favours the formation of gehlenite. Although this mineral might incorporate Fe 3+ in its structure it is enriched locally in the final product and causes dingy colours. Contents of CaO lower than 10 wt. % in the raw material favour the formation of plagioclase, which does not incorporate Fe 3+ in its structure.
REFERENCES
Dal, P. H., 1956. Mineralogische Zusammensetzung des Farbstoffesim gelbenTon. Klei, 6:572-580 and 616-617. Klaarenbeek, F. W., 1961. The development of yellow colours in calcareous bricks. Trans. Brit. Ceram. Soc., 60: 739-771. Kreimeyer, R., 1983. Die Abhiingigkeit der Brennfarbe von Ziegelnvonder Zusammensetzung der Ausgangssubstanz. 5th Mtg. European Clay Groups, Prague, pp. 515-521. Kreimeyer, R., 1983. Zur Entstehung hellerBrennfarben bei eisenhaltigenZiegelrohstoffen.Geol. Jahrb., D 75, pp. 91-122. Kreimeyer, R. and Eckhardt, F. J., 1980. Relations between the firingcolour of bricks and the raw material composition. 4th Mtg. European Clay Groups, Freising, Abstr.,p. 85. Peters, A., 1968. Ein neues Verfahren zur Bestimmung von Fe-II-Oxid in Mineralen und Gesteinen. Neues Jahrb. Mineral., Monatsh., 119-125. Peters, T. J. and Jenni, J.-P.,1973. Mineralogische Untersuchungen fiberdas Brennverhalten von Ziegeltonen. Beitr. Geol. Schweiz, Geotech. Ser.,50. Sandfort, F. and Liljegren, B., 1963. The formation of colour in red and yellow brick. Trans. Chalmers Univ. Tech., 282: 3-16. Seger, H. A., 1908. Gesammelte Schriften. H. Hecht und E. Cramer, Berlin, 85 pp. Stein, Eckhardt, Hilker, Irrlitz,Kosmahl, Mattiat, Piltz,Raschka and RSsch, 1981. Die ziegeleitechnischen Eigenschaften niedersa'chsischerTone und Tonsteine. Geol. Jahrb., D 45.
175
Short Communication
S o m e N o t e s on the F i r i n g Colour of Clay B r i c k s ROLF KREIMEYER
Bundesanstalt fiir Geowissenschaften und Rohstoffe, StiUeweg 2, D-3000 Hannover 51 (Federal Republic of Germany) (Received January 15, 1986; accepted after revision December 18, 1986)
ABSTRACT Kreimeyer, R., 1987. Some notes on the firing colour of clay bricks. Appl. Clay Sci., 2: 175-183. There is a close relationship between the occurrence of high-temperature crystalline phases and the colour of clay bricks after being fired under oxidizing conditions at 1000°C. Firing colours in various red shades expected on the basis of the relatively high Fe-content (3-7 wt. % ) may fail to appear due to the incorporation of Fe in specific high-temperature crystalline phases rather than its occurrence as free iron oxide in the form of hematite. X-ray investigations show that these minerals could be one or the other of mullite or metakaolinite and a fassaitic pyroxene in which the iron is present in its trivalent form. Yellow and beige to light brown colours result in the formation of these minerals. Mullite or metakaolinite are formed from kaolinitic raw materials containing only little CaO and alkali oxides. Fassaitic pyroxene is formed from CaCO3-rich materials in which the CaC03 is fine grained and homogeneously dispersed. If the conditions are not favourable to form these minerals the firing colour of the bricks will be in various shades of red due to fine grained, dispersed hematite. INTRODUCTION
It is t h e aim o f this s t u d y to i n v e s t i g a t e some o f t h e factors b e h i n d t h e firing colour o f brickclays. It is well k n o w n t h a t t h e m a i n c o l o u r i n g a g e n t in brickclays is iron oxide. R e d or r e d b r o w n colours are usually o b t a i n e d due to t h e a p p e a r a n c e o f fine grained, dispersed h e m a t i t e , w h i c h develops d u r i n g firing f r o m iron m i n e r a l s (goethite, siderite, p y r i t e etc. ) u n d e r oxidizing firing conditions. T h e f o r m a t i o n o f yellow or beige firing colours f r o m iron c o n t a i n i n g brickclays has b e e n i n v e s t i g a t e d several times, b u t t h e results o b t a i n e d are c o n t r a d i c t i o n a r y . As early as 1874 Seger (1908) divided t h e light b u r n i n g clays into two groups. A c c o r d i n g to S e g e r t h e first g r o u p is c h a r a c t e r i z e d b y a high A1203 c o n t e n t a n d a low CaO c o n t e n t ; t h e s e c o n d b y a high CaCO3 c o n t e n t . T h e i n f l u e n c e o f t h e Ca c o n t e n t on t h e firing c o l o u r has b e e n i n v e s t i g a t e d several times. A light
0169-1317/87/$03.50
© 1987 Elsevier Science Publishers B.V.
176
17.
(~)
16 15 ~4 13. 12, 11.
10. mmm m
9. 8.
• .A
-.- ". •
7.
•
• •
" o
o
o OD
6,
0 0
5.
o
0
c O
4, 3. 2, I. 0i
" ~ ~ ~i
~ ~ 8'~
~ d g
Fig. 1. Clays of the Lower Cretaceous (KBT). Firing colour in relationship to CaO + MgO- and Fe20:~-contents. firing colour has been considered to be due to the formation of dicalciumferrite ( Sandfort and Liljegren, 1963), the incorporation of iron in gehlenite (Dal, 1956) or pyroxene (Klaarenbeek, 1961; Peters and Jenni, 1973). TEST MATERIALS Test materials for this study were raw materials and test bricks from various regions of Lower Saxony. These clays are described in an earlier publication {Stein et al., 1981 ). Their chemical compositions, mineralogical compositions, grain size analyses and various ceramic, physical and rheological properties are reported in detail. Test bricks from these raw materials were produced in the Ziegelforschungsinstitut ( institute for brick research) in Essen. They were fired in an electric kiln under oxidizing conditions and a temperature of 1000 ° C. OCCURRENCE OF YELLOW FIRING COLOURS Yellow firing colours were obtained from raw materials which are characterized by low contents of CaO (less than 2 wt. %, see Fig. 1 ) Main components of these raw materials are quartz and kaolinite, but illitic muscovite may occur
177 TABLEI Chemicalcompositionofthe Schnaittenbacherkaolinand a yellowburningclay Sample
SiO~
AI20~,
Fe20:~
TiO2
CaO
MgO
K20
Na20
LOI
SBK
48.22 56.52
36.73 21.05
0.25 5.29
0.47 1.11
0.08 0.21
0.09 1.21
1.52 1.81
0.08 1.15
12.47 11.40
KTH 5
Allanaly~esin wt. %. as well. The iron content of the raw materials varies between 3 and 10 wt. % and averages 4.5 wt. %. It was found t h a t in some fo the yellow firing clays not all of the iron is evenly distributed, but some coarse grained siderite or siderite accumulations are present, which during firing convert to hematite resulting in some small red spots in a yellow brick. However, solution of the hematite by hydrochloric acid demonstrates, t h a t up to 3-4 wt. % iron are incorporated in other minerals which are not soluble in hydrochloric acid.
Firing tests and qualitative X-ray investigations on artificial mixtures For the following tests a Schnaittenbacher kaolin was used (chemical composition see Table I, sample S B K ) . For comparison the chemical composition of a typical yellow burning clay is given as well ( sample K T H 5). After firing at 1300°C 55 wt. % of mullite are obtained from the Schnaittenbacher kaolin, the remainder being SiO2 (quartz and crystobalite) and glass. Mixtures of kaolin and very fine grained Fe203 were produced in such a way, t h a t after firing definite mixtures of mullite and Fe203 were obtained (see Table II). All samples were fired in an electric kiln for 24h at temperatures of 1000°C and 1300°C. After cooling the firing colour was determined visually and the product was investigated by X-ray (Guinier camera, Fe-radiation). In addition the lattice constants of the mullite obtained from the samples fired at 1300 °C were determined. The results of the firing tests are recorded in Table III and the lattice constants of the mullite are recorded in Table IV. TABLE II Samples with definite mullite-Fe203ratios Sample K 1 Sample K2 Sample K3 Sample K4 Sample K5 Sample K6
mullite+ 0.5 wt. % Fe203 mullite+ 3.33 wt. % Fe203 mullite+ 6.66 wt. % Fe2O3 mullite+ 8.5 wt. % Fe203 mullite+ 11.0 wt. % Fe203 mullite+14.0 wt. % Fe203
178 TABLE III Crystalline phases and firing colour of samples K1-K6 fired at 1000°C and 1300 °C Sample
Crystalline phases identified
Firing temperature 1000 ° C: K1 Quartz K2 Quartz K3 Quartz K4 Quartz K5 Quartz K6 Quartz, hematite Firing temperature 1300 ° C K1 Mullite, quartz K2 Mullite, quartz K3 Mullite, quartz K4 Mullite, quartz K5 Mullite, quartz K6 Mullite, quartz, hematite
Firing colour
yellow yellow yellow yellow yellow light redbrown
yellow yellow yellow yellow yellow light redbrown
Variation of the firing colour by addition of alkali oxides and calcium oxide Further tests were designed to change the firing colour of kaolinitic raw materials by addition of alkali oxides and calcium oxide. Kaolin-FE203 mixtures according to sample K3 were mixed with various amounts of Na2CO3 + K2CO3 and CaC03 ( see Table V). The samples were heated in an electric kiln for 24 h at a temperature of 1000 ° C only. After cooling the firing colour of samples was determined visually and the product was investigated by X-ray diffraction. TABLE IV Lattice constants 1 (A) of mullite in samples K1-K6 fired at 1300 °C Sample
a
b
c
Fee03 (wt.%)
K1 K2 K3 K4 K5 K6
7.5491 7.5526 7.5597 7.5629 7.5653 7.5659
7.6953 7.7084 7.7192 7.7268 7.7319 7.7318
2.8847 2.8900 2.8957 2.8983 2.9017 2.9019
0.5 3.33 6.66 8.5 11.0 14.0
1Lattice constants of pure mullite: a = 7.5456/~; b = 7.6898/~; c = 2.8842]k.
179 TABLE V Kaolin-Fe203 mixtures with various amounts of alkali oxidesand calciumoxide Sample S 1 Sample $2 Sample $3 Sample C1 Sample C2 Sample C3 Sample C4
K3+0.5 wt. % Na2C03+0.5 wt. % K2CO 3 K3 + 1.0 wt. % Na2C03 + 1.0 wt. % K2CO3 K3 + 2.0 wt. % Na2C03+ 2.0 wt. % K2C03 K3 + 1.0 wt. % CaC03 K3 + 2.0 wt. % CaC03 K3 + 3.0 wt. % CaC03 K3 +4.0 wt. % CaC03
OCCURRENCE OF LIGHT BROWN AND BEIGE FIRING COLOURS Light brown or beige test bricks were obtained from raw materials characterized by high contents of alkaline earth elements (CaO + MgO contents higher t h a n or equal to 10 wt. % ). One of the main components of the raw materials is calcite, others might be quartz, mica and clay minerals in various amounts. The iron content of the raw materials averages 6 wt. %. Tests using the method of Peters (1968) were made to determine the valency state of the iron in the outer part of the beige coloured test bricks. It was found, t h a t nearly all the iron is present in its trivalent state. Although some clays (especially the Wealden clays, see Fig. 1) are CaOrich, the expected beige or light brown colour fails to appear and redbrown colours with only few yellow spots are obtained. Factors, other t h a n mineralogical or chemical composition must therefore be responsible for the appearance of light colours. A significant difference of the Wealden clays compared to other raw materials (Lower Cretaceous clays, see Fig. 2) is the coarse grain size of the carbonates.
Firing tests with artificial clay-CaC03 mixtures Clay-CaCO3 mixtures were produced using two clays with similar chemical composition but different grain size distribution (see Tables VII and VIII). T A B L E VI
Crystallinephases and firing colourof samples $1-$3 and C1-C4 Sample
Crystallinephases identified
Firing colour
S1 $2 $3 C1 C2
Quartz, mullite? Quartz,hematite None (whollyamorphous) Quartz Quartz,anorthite,hematite
yellow orange-yellow dark brown yellow lightredbrown
C3 C4
Quartz, anorthite, hematite Quartz, anorthite, hematite
redbrown redbrown
180
("/.) 11?O,
98" 7.
6. 5.
O.o °o
4.
0
0 •
•
II
O0
J.
0
2-
c¢0 + ~ j o (~.)
t
Fig. 2. Wealden-clays ( K T H ) . Firing colour in relationship to CaO + MgO- a n d Fe203-contents.
The CaC03 addition to the clays was 10 and 20 wt. %, respectively. Two different forms of CaCO3 were used, coarse grained (grain size 200-20/~m) and fine grained (grain size < 20/Lm). All samples are described in Table IX. Test bricks were formed and fired in an electric kiln at a temperature of 1000 ° C for 2 h. The mineralogical composition of the test bricks was determined semiquantitatively by X-ray diffraction. The firing colour of the test bricks was determined visually. The results of the firing tests are recorded in Table X. TABLE VII Chemical composition of the clays used Sample
SiO~
AI20:~
Fe~O:l
Ti02
CaO
MgO
Na20
K~O
LOI
KBT 9 KTR 4
49.5 53.5
20.1 20.6
8.0 7.7
0.81 1.02
1.90 1.90
1.40 2.14
1.02 0.53
2.85 2.96
12.8 8.6
T A B L E VIII G r a i n size ( i n / l m ) distribution of the clays used (in % ) Sample
200-63
63-20
20-6.3
6.3 -2
2
KBT 9 KTR 4
4.9 3.2
3.4 15.1
12.3 23.0
16.5 22,7
62.8 35.9
181
T A B L E IX Sample description of clay-CaCOa mixtures M1 M2 MS1 MS2 S1 $2 SS1 SS2
KBT 9 " " " KTR 4 " " "
+ 10 +20 + 10 +20 + 10 +20 + 10 + 20
wt. % . . . . . . . . . . . . . .
. .
.
CaCOa .
(coarse grained) .
.
.
{fine grained)
. .
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
.
(coarse grained)
. .
.
(fine grained)
. .
TABLE X
Results of the firing tests with clay-CaC0:~ mixtures
Sample
Main components
Secondary components
Traces
M1 M2 MS1 MS2 S1 $2 SS1 SS2
aK, Q Q, Gehl. Q, Plg. Q, Pig., Pyr. aK, Q Gehl., Q aK, Q Q
Gehl., Hem. aK, Pig., Hem. Hem.
Plg. Pyr., Wo. Pyr., Gehl. Gehl., Hem. Plg. Pyr. Wo., Pyr. Wo., Hem
Gehl., Hem Plg., Hem Gehl., Pig., Hem Plg., Gehl., Pyr.
Colour redbrown redbrown*l redbrown
beige redbrown redbrown .1
redbrown beigebrown
*~Few yellow spots visible in an almost redbrown sample. Legend. aK=amorphous component; Q=quartz; Plg = plagioclase; Gehl = gehlenite; H e m = hematite; Pyr = pyroxene.
The mineral equilibrium of the fired clay-CaCOa mixtures and the chemical composition of the newly formed Ca-silicates The grain size distribution of the carbonate as well as the grain size distribution of the silicates influences the mineralogical composition of the fired samples. Fine grained raw materials favour the formation of plagioclase in samples containing 10 wt. % CaCO3 and the formation of plagioclase and pyroxene in samples containing 20 wt. % CaCO3. Coarse grained raw materials on the other hand favour the formation of gehlenite. The presence of gehlenite together with quartz is a mineral unequilibrium. According to the following reaction they should convert to anorthite and wollastonite. Gehlenite + 2 quartz
, anorthite-{- wollastonite
The presence of gehlenite indicates, that not all siliceous material, especially the clay minerals rather than quartz grains, reacted with CaO that develops from the CaCO3 during firing. A crystalline phase (gehlenite) with higher CaO
182
contents than expected from the chemical composition, developed locally enriched. The lattice constants of gehlenite occurring in the artificial clay-CaCO3 mixtures could not be determined accurately. Only few peaks are without doubt attached to gehlenite. But normally they are diffuse which may be due to fine grain size, lattice defects or unsufficient crystallization. The observable peaks, however, are shifted to lower 20 values, which results in larger lattice constants. This is expected, when the bigger Fe 3+ ion is incorporated in the structure instead of the smaller A13+ ion. Moreover, previous investigations (Dal, 1956 ) have shown, that gehlenite may incorporate Fe 3+ in its structure. The chemical composition of the pyroxene occurring in Ca-rich bricks was determined. Gel syntheses of pyroxenes with known chemical composition were carried out. The X-ray diffraction patterns of the synthesized pyroxenes were compared with the X-ray diffraction patterns of the beige coloured bricks and samples MS2 and SS2. The pyroxene in the beige coloured bricks and in samples MS2 and SS2 was found to be a pyroxene of the composition CaMgo.5 Feo.~ Alo.5 Si,.5 O6. CONCLUSIONS
Kaolinitic, Ca-poor clays No hematite can be detected by X-ray diffraction in the samples K 1 - K 5 aider firing at 1000°C and 1300°C, respectively. Increasing amounts of Fe203 in the samples are coupled with increasing lattice constants of mullite in samples fired to 1300 ° C. The only explanation for this fact is the incorporation of Fe 3+ for A13+ in the mullite structure. The occurrence of free Fe203 in the form of hematite in sample K6 shows that the lattice cell dimensions reach their maximum at 11 wt. % Fe203 and no more iron can be incorporated in the mullite structure. Since no hematite can be detected in samples K 1 - K 5 fired at a temperature of 1000 ° C either, it is concluded that the iron is already incorporated in the amorphous substance metakaolinite, which forms by subsolidus reactions. No fine dispersed hematite occurs in bricks as a colouring agent when the raw material consists mainly of kaolinite and quartz. Hematite, however, occurs as a colouring agent when fairly small amounts of impurities such as alkalies or CaO are present in the raw material. Alkalies induce the early formation of a glassy matrix and CaO in the raw material leads to the formation of anorthite, which does not incorporate Fe 3+ in its structure. In both these cases the amount of metakaolinite in the fired product is considerably reduced. As a result "free" Fe203 in the form of hematite occurs as a colouring agent. The firing colour of kaolinitic clays is therefore not only due to the amount
183 of finely dispersed FeeO3 in the raw material, but it depends also on the amount of impurities ( alkalies and CaO ), which themselves are non-colouring at all.
Ca-bearing clays The beige firing colour of CaO-rich clays is favoured by the formation of a fassaitic pyroxene. This pyroxene incorporates Fe 3+ in its structure and prevents the formation of finely dispersed hematite as a colouring agent. The formation of this pyroxene starts in clays containing at least 10 wt. % CaO, provided the CaO in the raw material is fine grained and homogeneously dispersed. Coarse grained and locally enriched CaCO3 in the raw material favours the formation of gehlenite. Although this mineral might incorporate Fe 3+ in its structure it is enriched locally in the final product and causes dingy colours. Contents of CaO lower than 10 wt. % in the raw material favour the formation of plagioclase, which does not incorporate Fe 3+ in its structure.
REFERENCES
Dal, P. H., 1956. Mineralogische Zusammensetzung des Farbstoffesim gelbenTon. Klei, 6:572-580 and 616-617. Klaarenbeek, F. W., 1961. The development of yellow colours in calcareous bricks. Trans. Brit. Ceram. Soc., 60: 739-771. Kreimeyer, R., 1983. Die Abhiingigkeit der Brennfarbe von Ziegelnvonder Zusammensetzung der Ausgangssubstanz. 5th Mtg. European Clay Groups, Prague, pp. 515-521. Kreimeyer, R., 1983. Zur Entstehung hellerBrennfarben bei eisenhaltigenZiegelrohstoffen.Geol. Jahrb., D 75, pp. 91-122. Kreimeyer, R. and Eckhardt, F. J., 1980. Relations between the firingcolour of bricks and the raw material composition. 4th Mtg. European Clay Groups, Freising, Abstr.,p. 85. Peters, A., 1968. Ein neues Verfahren zur Bestimmung von Fe-II-Oxid in Mineralen und Gesteinen. Neues Jahrb. Mineral., Monatsh., 119-125. Peters, T. J. and Jenni, J.-P.,1973. Mineralogische Untersuchungen fiberdas Brennverhalten von Ziegeltonen. Beitr. Geol. Schweiz, Geotech. Ser.,50. Sandfort, F. and Liljegren, B., 1963. The formation of colour in red and yellow brick. Trans. Chalmers Univ. Tech., 282: 3-16. Seger, H. A., 1908. Gesammelte Schriften. H. Hecht und E. Cramer, Berlin, 85 pp. Stein, Eckhardt, Hilker, Irrlitz,Kosmahl, Mattiat, Piltz,Raschka and RSsch, 1981. Die ziegeleitechnischen Eigenschaften niedersa'chsischerTone und Tonsteine. Geol. Jahrb., D 45.