Electron microprobe analyses of Nigerian clinkers

CEMENT and CONCRETE RESEARCH. Vol. 15, pp. 662-668, 1985. Printed in the USA. 0008-8846/85 $3.00+00. Copyright (c) 1985 Pergamon Press, Ltd.

ELECTRON MICROPROBEANALYSES OF NIGERIAN CLINKERS S.L. Sarkar Department of Geology and Mineral Sciences University of l l o r i n l l o r i n , Nigeria D.M. Roy Materials Research Laboratory The Pennsylvania State U n i v e r s i t y U n i v e r s i t y Park, PA 16802

(Refereed) (Received Feb. 4, 1984; in final form March 19, 1985) ABSTRACT Accurate EMPA results were obtained for a l i t e and b e l i t e phases in seven Nigerian c l i n k e r s , though the level of accuracy dropped appreciably for the i n t e r s i t i a l phases owing to the l i m i t a t i o n of probe analyses of minute grains (< 5um). A p a r t i t i o n of the 12% Fe203 was used for d i s t i n c t i o n between aluminate and f e r r i t e phases. The l a t t e r exhibited preference for MgO, TiO 2 and Mn203, whereas the aluminate was enriched in K20. A l i t e and b e l i t e showed s l i g h t concentration of P205 and S03. Na20 and Cr203 were evenly d i s t r i b u t e d in minor amounts in all the four principal phases. Introduction There are 8 cement works in Nigeria (West A f r i c a ) producing OPC. Clinker samples were collected from all these works for a complete i n v e s t i g a t i o n of t h e i r microstructures. Their c h a r a c t e r i s t i c s from reflected l i g h t microscopy and also under SEM have described elsewhere by SLS ( I - 3 ) . The present work outlines the results of microprobe analyses of the four principal phases, which not only r e f l e c t the amount, type and location of minor impurities in these c l i n k e r s , and t h e i r possible s u b s t i t u t i o n s in these phases, but also e n t a i l s the v a r i a t i o n of these phases from the stoichiometric compositions as a r e s u l t of multicomponent s u b s t i t u t i o n s . Method The analyses were performed under an ISI-DSI30 SEM f i t t e d with a Kevex 700 micronalyzer unit. The usual experiment parameters, specimen preparation and standardization techniques were adopted (4). The c l i n k e r specimen from 662

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663 CLINKERS, SEM, EMPA, ALITE, BELITE, ANALYSES

Calabar works, however, shattered into minute particles before i t could even be resin impregnated. Hence i t s analysis was not possible. Results and Discussion A comparison between the theoretical values of some of the principal phases in clinker (Table I) and the experimental values (Table I I ) obtained from EMPAshows the composition of the phases in some Nigerian clinkers to be significantly different from the theoretical in some phases. This is not unusual, because of the multicomponent substitutions taking place in the impure clinker phases. Slight inadvertent errors may also have arisen from sampling. TABLE I Theoretical Values of Oxides in Some of the Phases of Cement Phase

CaO

SiO2

Al203

Fe203

C3S*

73.686

26.314

C2S*

65.118

34.882

C3A*

62.256

37.744

C4AF*

45.686

20.774

33.540

C6AF2*

43.820

13.280

42.890

C6A2F*

47.720

28.930

23.350

C2F*

40.514

C2A*

52.372

59.486

47.628

* Cement chemists notation: C = CaO, S = SiO2, A = Al203, F = Fe203 The alite and belite phases could be easily distinguished from their morphology (Figure l) using etched specimens (4). The peripheral dendritic feature of belites could also be recognized (Figure 2), and in some instances the presence of secondary belite (1) in large alite grains (Figure 3) was also observed. Distinction between the i n t e r s t i t i a l phases from EMPA, however, proves extremely d i f f i c u l t (4), owing to the minute size of these grains. The packing of calc-silic~e grains is sometimes so compact that very l i t t l e (< l~m) i n t e r s t i t i a l space remains between them for probe analysis to be performed satisfactorily. These two factors contributed towards the loss of accuracy in the analytical results, as evidenced in Table I I . A partition at around 12% Fe203 was used as the control point for distinction beween C3A and C4AF phases. This is in contra~t to the American clinkers, where a much lower partition point (7% Fe203) was used (5). The A1203 content of C3A in all the analyses was found to be much lower ( < 24%) than the theoretical value of 37.74%. The CaO percentage, however, was very close to the theoretical value, though in some i t exceeded the theoretical (value) by up to 7%. The low A1203 content in C~A suqqests that most of the minor element substitution occurs here.

0.76

Nkalagu

1

Nkalagu

0.79

Nkalagu

i

19.31

not

not

not

not

32.20

27.03

27.42

26.83

28.43

32.27

25.32

21.80

23.87

22.69

22.47

23.12

22.91

SiO 2

-

be

be

0.08

0.07

0.20

be

be

0.56

0.67

0.72

0.33

0.73

O.ll

0.18

0.II

0.67

0.13

0.12

P205

i

I

]

2.18 0.55

2.47

0.53

0.29

0.45

0.88

0.35

0.81

0.45

0.49

l .56

0.59

O. 84

0.32

0.71

0.54

1.32

0.24

64.77 66.50

64.38

51.05

52.50

48.17

53.80

48.53

61.24

66.79

66.13

63.46

66.90

62.19

70.47

75.48

74.18

68.63

71.80

74.59

67.44

i 033 r 6g61

I

analysed ]

analysed ]

0.02

0.01

analysed

analysed

0.07

0.17

0.31

0.26

-

0.36

-

-

0.25

0.58

CaO

Phases in Nigerian Clinkers ]

0.52 -

0.28

l .29

1.37

l .92

1.09

l .46

0.19

0.14

O.ll

O.Ol

0.40

TiO 2

0.24

0.02

0.02

0.19

0.14

O. 04

0.21

0.16

0.02

0.26

Cr203

. . . . . . . . . . . . . . . . . . L * by weight percent. Oxide content calculated by the pro ram FRAMEC (by courtesy of Dr. A. Sandborg, Edax L a b o r a t o r i e s , U.S.A.)

0.52

23,23

23.44

0,38

Ewekoro

i

0.66

2,09

Ashaka

0.41

20.65

0.84

Yandev

18.28

[Could

13.96

12.41

[Could

0.45

1.33

1.41

Sokoto

Ukpilla

1.46

0.90

Ewekoro

Shagamu

0.92

Ashaka 17.36

1.36

Yandev

1.46

[6oul d 12.62

0.95

Sokoto

1.67

8.77

1.78

1.44

1.51

2.54

1.41

2.41

[Could

1.18

0.52

1.38

1.03

0.42

0.17

1.12

0.67

0.77

0.72

0.12

3.00

1.07

0.71

3.09

Al203

O.lO

0.55

MgO

Ukpilla

Aluminate

1.21

Ewekoro

0.65

I.II

Ashaka

Shagamu

0.54

Yandev

Ferrite

0.14

Sokoto

0.89

Nkalagu

0.90

0.50

Ewekoro

Shagamu

0.41

Ashaka

Belite

1.31

Sokoto

Yandev

0.23

Na20

0.87

~

Ukpilla

Alite

Phase

Shagamu

Works

i

TABLE I I

Mean Values o f Oxides* Present in the Four P r i n c i p a l

0.02

0.19

1.51

0.68

0.66

0.86

0.81

0.25

O. 08

0.05

0.26

MnO2

i

8.09

6.05

6.74

t l .03

12.26

29.30

30.40

28.37

27.93

38.10

l .85

O. 88

0.67

3.95

l .79

0.74

0.94

0.71

0.07

4.29

1.24

0.65

0.75

Fe203

p~ o

E~

cL

c~

-s

cz~ pJ

z o

< o

Vol. 15, No. 4

665 CLINKERS, SEM, EMPA, ALITE, BELITE, ANALYSES

FIG. 1 Ukpilla clinker showing a l i t e (A), belite (B) and the i n t e r s t i t i a l phases (AF). A l i t e crystals are mostly euhedral to subhedral, whereas belite crystals are usually rounded. The aluminate and f e r r i t e phases cannot be distinguished in the i n t e r s t i t i a l matrix.

FIG. 2 Rounded belite crystals exh i b i t i n g peripheral dendrites in Shagamu clinker.

FIG. 3 A secondary belite (>>B) in a large a l i t e crystal in Ewekoro clinker. The a l i t e crystals are separated by the i n t e r s t i t i a l matrix (AF).

666

Vol, 15, No. 4 S.L. Sarkar and D.Mo Roy

According to Woermann et. al (6) AI, Mg and/or Fe is found in the greatest q u a n t i t y in the a l i t e in c l i n k e r . The present study, however, indicates that maximum amount of Mg is present in the f e r r i t e phase. This is possible, cons i d e r i n g t h a t Mg can s u b s t i t u t e f o r e i t h e r Fe or Ca (7), i . e . , greater l a t i t u d e of s u b s t i t u t i o n a l f a c i l i t i e s are a v a i l a b l e in the f e r r i t e l a t t i c e . From Figure 4 i t is observed t h a t Mg is d e f i n i t e l y incorporated in the i n t e r s t i t i a l matrix. Supportive x - r a y mapping of A1 and also of Fe+Mg (in combinat i o n ) proves Mg to be disseminated in regions of low A1 concentration. This confirms the presence of Mg in the f e r r i t e phase. Some Mg is also found in the matrix as f i n e l y dispersed p e r i c l a s e c r y s t a l s . Two types of s o l i d solution s u b s t i t u t i o n s f o r C2(AF) + MgO have been proposed (6), i . e . f o r Ca and Fe.

Fig. 4.

Elemental X-ray Mapping o f Shagamu c l i n k e r . Fe + Mg, Mg, A1 and Si.

Clockwise -

T i t a n i a was the highest (up to 2%) in the f e r r i t e phase. Bucchi (8) claims t h i s phase is capable of accommodating up to 18.5% mass percent Ti02 in the l a t t i c e , whereas a l l the other phases can take < 5%. Owing to the strong Ti - 0 bond, Ti+4 can s u b s t i t u t e f o r Fe+3 in the FeO4 tetrahedra and also f o r A1+3.

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667 CLINKERS, SEM, EMPA, ALITE, BELITE, ANALYSES

Phosphate and sulphate were predominantly high in the a l i t e and belite phase. Salge and Thormann(9) consider P205 to concentrate preferentially in the belite l a t t i c e to the detriment of a l i t e and i n t e r s t i t i a l phases. Fundamental studies (I0) show that C2S can coexist with CaSO4 in the ternary system CaO - C2S - CaS04, and the l i m i t of S03 saturation in C2S is 2.9%. The existence of alkali compounds as sulphates in Nigerian clinkers has already been discussed by SLS(3). The maximum amount of P205 encountered (< I%) in the a l i t e phase is well below the saturation l i m i t set by Gutt and Smith (lO). The C3Aphasein three clinkers was enriched in K20 (up to 2.5%). According to Guinier andRegourd (7), K20 plays the same role as Na20 in altering the l a t t i c e symmetry (of this phase) from cubic to orthorhombic, monoclinic and tetragonal. No evidence of K20 rich belite was found from potassium x-ray imaqining. I t was found to be concentrated mainly in the i n t e r s t i t i a l matrix. There was a s l i g h t tendency for Mn203 to concentrate in the f e r r i t e phase (0.66 - 1.5%), in which case i t s substitutional trend could be expected to be similar to that of Ti02. I t i s , however, sparsely di stributed in the other phases ( < 0.4%). The d i s t r i b u t i o n of both Na20 and Cr20~ was f a i r l y even in all the phases, though the amount of Cr203 was appreclably lower (< 0.3%) than Na20, which is in excess of 2% in the C3A phase of theAshakaclinker.

A comparison of the two tables SugQests that the composition of the f e r r i t e phase in the clinkers varies between C4AF, C6AF2 and C6A2F. Conclusions Quantitative EMPAof the four principal phases in Nigerian clinkers, representing the seven d i f f e r e n t cement works in the country, indicates a f a i r l y high amount of minor elemental substitutions in these phases. Owing to instrumental l i m i t a t i o n s , accuracy in probe analyses of the minute grains in the i n t e r s t i t i a l phases proves d i f f i c u l t to achieve, though reasonably accurate results on the oxidic compositions could be derived (from computation) for the a l i t e and the b e l i t e phases in the clinkers. The a l i t e phase is characterized by Na20, MgO, AI203, P205, SO3, K20, Cr203, Mn203 and Fe203 substitutions, without preference for a particular element; whereas the belite phase additionally contains TiO2, though Mn203 is v i r t u a l l y nonexistent in this phase. The aluminate phase is enriched in K20, in addition to Na2O, MgO, Ti02 and Mn203 substitutions. The f e r r i t e phase exhibits a strong a f f i n i t y for MgO, Ti02 and Mn203. A fixed composition for this phase, however, could not be assigned, as the composition varies between C4AF, C6AF2 and C6A2F. Lack of analysis of valence state of these minor ions precludes any d e f i n i t e conclusions as to t h e i r substitutional trends, but the results give an insight into the compositional variations arising from multicomponent subs t i t u t i o n s . I t also suggests that s o l u b i l i t i e s of these foreign ions are not independent of each other. Acknowledgement SLS wishes to thank the University of l l o r i n , Nigeria for granting him leave and financial support for carrying out this work with Professor D.M.

668

Vol. 15, No. 4 S.L. Sarkar and D.M. Roy

Roy at the Materials Research Laboratory, The Pennsylvania State University, U.S.A. D.M. Roy was supported by NSF Grant CPE-8112821. References I.

S.L. Sarkar, The Microscope, 2_9_9,1981, 127.

2.

S.L. Sarkar, Nigerian Jour. Sci. Tech., 2 ( I ) , 1984.

3.

S.L. Sarkar, Cem. Concr. Res., 13 (2), 1983, 431-434.

4.

S.L. Sarkar and D.M. Roy, Cem. Concr. Res., 14, 1984, 83.

5.

S.L. Sarkar and D.M. Roy, Proc. Vlth Annual Conf. Cem. Micros., Albuquerque, Ed., J. Bayles, I n t l . Cem. Microscopy Assn., 1206 Coventry Lane, Duncanville, TX 75137, March 1984, 37-46.

6.

E. Woermann, W.E. Eysel and T. Hahn, Zem. Kalk Gips, No. 9, 1969, 414.

7.

M. Regourd and A. Guinier, Proc. Vlth Int. Symp. Chem. Cem., Moscow, Stroyizdat, Moscow, USSR, Vol. I, 1974, 25.

.

R. Bucchi, Proc. Vllth Int. Symp. Chem. Cem., Paris, Paris, France, 1980, I - I / 3 .

Editions Septima,

9.

H. Salge and P. Thormann, Zem. Kalk Gips, 2__6_6,1973, 532.

I0.

W. Gutt and M.A. Smith, Trans. B r i t . Ceram. Soc., 66, 1968, 557.