A two-stage reaction sequence for C3S formation

CEMENT and CONCRETE RESEARCH. Vol. 19, pp. 837-847, 1989. Printed in the USA. 0008-8846/89. $3.00+00. Copyright (c) 1989 Pergamon Press plc.

A TWO-STAGE REACTION SEQUENCE FOR C3S FORMATION

J. A. Chesley* and G. Burnet Ames Laboratory, U.S.D.O.E. and Department of Chemical Engineering Iowa State University, Ames, IA 50011

(Refereed) (Received August 23, 1988; in final form May 19, 1989)

ABSTRACT Reaction mechanism and kinetics were investigated for the formation of C3S from C2S obtained as a byproduct from the Ames Lime-Soda Sinter Process for the recovery of AI203 from the mixed oxides found in power plant fly ash. A change in the rate of reaction was found to occur after about 25 minutes at burning temperatures of 1300 to 1500 C. The change was attributed to a two-stage reaction sequence in which the reaction rate for the first stage is phase boundary controlled and for the second phase diffusion controlled. A modified version of the Ginstling-Brounshtein solid-state reaction rate equation was found to describe the diffusion-controlled portion of the process. Apparent activation energies determined for this part of the process agreed with reported activation energies for Ca diffusion in clinker melt and for the self-diffusion of Ca in CaO. Introduction An understanding of the reaction mechanism and kinetics for the formation of cement clinker is required for reliable process design and plant operation. Of particular importance is the formation of Ca3SiO 5 (C3S) from Ca2SiO 4 (C2S) and CaO.

(i)

C2S + CaO C3S

The study of C3S reaction kinetics in cement clinker has typically been based on solid-state reaction schemes (I). One of the difficulties of such a study is the complex nature of cement clinker formation. For this reason, the extent of reaction is normally indexed by measuring the amount of unreacted lime.

* Chemical/Physical Research Department, Construction Inc., 5420 Old Orchard Road, Skokie, IL 60077-4321.

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In this study, which is described in more detail in reference (2), a byproduct from the Ames Lime-Soda Sinter Process was used as the principle raw material a~d source of C2S. The process begins with a well-mixed and finely divided feed of fly ash, limestone, and soda ash which is heated to about 1200 C in a rotary kiln. Lime (CaO) supplied by the disassociation of CaCO 3 combines with SiO 2 derived from the fly ash to form calcium silicates primarily C2S. Alumina, also from the fly ash, reacts with the lime and soda to form calcium and sodium aluminates, respectively, which are extracted from the ground clinker using a dilute solution of Na2CO 3. After separation from the sinter residue, the extract is contacted with CO 2 to precipitate AI(OH)3, which is then calcined to produce metallurgical grade AI203. Nearly one and one-half tons of sinter residue are produced for every ton of fly ash processed and the residue possesses unique characteristics, e.g., fine particle size, low AI203 content, and large amounts of chemically bound lime, which make it an attractive raw material for the manufacture of sulfate-resistant cement. Utilization of the residue, which consists chiefly of beta-C2S, eliminates the need for its disposal and improves the economic feasibility of the lime-soda sinter process. Data are reported for the burning at various times and temperatures of cement raw mixtures containing the sinter residue, thus providing an opportunity to investigate independently the reaction between CaO and C2S. Solid state reaction models that best describe the reaction are determined and activation energies are calculated. Experimental An experimental method was developed for obtaining the reaction rate data required for a kinetic study. The lime content was held constant at a 0.90 LSF (lime saturation factor -- the actual weight of CaO in the clinker divided by the maximum CaO content which can be combined by the acidic oxides under normal operating conditions) while the alumina flux content was varied. The latter was set at a 3.3 or a 4.3 SR (silica ratio -- the weight of SiO 2 in the cement divided by the sum of the AI203 and Fe203). Burning the resulting formulations for different periods of time at constant temperature gave the data required. Raw Materials The sinter residue was produced from the processing of a fly ash genera=ed by the burning of a Powder River Basin, Wyoming subbituminous coal. The normalized composition of the residue (2) is shown in Table i. The crystalline phases present in the residue were beta-C2S, C3A , CaCO3, MgO, and a ferrite phase. A sieve analysis of the residue showed that 82 wt% was finer than 45 microns. In addition to the sinter residue, the cement raw mixes consisted of reagent grade CaCO 3 and specially-prepared AI203. The latter was produced in the laboratory by the slow heating of pure AI(OH) 3 to i000 C. Parameters and Procedures The two SR's used spanned

the possible range of AI203 flux content.

An SR of

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839 C2S , C3S , LIME-SODA SINTER,

KINETICS

TABLE 1 Comparison of the Composition of the Sinter Residue with Typical Cement Raw Materials

Concentration,

wt% Sinter Residue

Element

Clay b

Limestone b

Raw Mix b

XRF Analysis

Normalized

SiO 2

60.48

2.16

14.30

27.51

27.18

A1203 a

17.79

1.09

3.03

2.36

2.33

Fe203

6.77

0.54

I.Ii

3.98

3.93

CaO

1.61

52.72

44.38

55.72

55.06

MgO

3.10

0.68

0.59

3.71

3.67

SO 3

0.21

0.02

0.07

0.01

0.01

Na20

0.74

0.Ii

0.13

1.51

1.49

K20

2.61

0.26

0.52

0.09

0.09

TiO 2

0.92

0.91

P205

1.43

1.41

Moisture

0.48

0.48

L.O.I. Total

6.65

42.39

35.86

3.44

3.44

99.96

i00.00

99.99

101.16

I00.00

a For clay, limestone and raw mix, includes P205 , TiO2, and Mn205. b From reference (3).

the

A1203

concentration

as

reported

4.3 results when the only flux materials present are those introduced with the sinter residue. The SR of 3.3 corresponds to the maximum amount of A1203 that can be added while still meeting Type V portland cement specifications. In the laboratory, the appropriate amounts of sinter residue, CaC03, and A1203 were proportioned to I00 g batches. Following blending of the dry reagents, two samples of I0 g each containing I0 wt% water as a binder (wet basis) were pressed into cylindrical pellets 2.54 cm (i inch) in diameter under a pressure of 578 kg/m 2 (I000 Ibs) using a Carver hydraulic press and stainless steel die set. The green pellets were then dried at I00 C for at least two hours to remove free water. The dried pellets were burned using a rapid temperature furnace (C/M Inc., Model No. 1720DBL) equipped with a programmable controller. The controller allowed for simulation for the time/temperature relation of an actual cement

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kiln. The pellets were placed in the furnace on a platinum sheet and were burned in an air atmosphere. The heating sequence consisted of a 15 C/min heat-up period to the clinkering temperature (1300, 1400, 1450, and 1500 C were used) followed by a holding time of 1-4 hours. The burned pellets (clinker) were then uniformly cooled in 15 minutes to 1250 C and removed from the furnace to cool rapidly to ambient temperature. The pellets were then placed in a desiccator to avoid hydration. The cooled clinker was crushed by use of an alumina mortar and pestle. Determination of unreacted CaO in the crushed clinker was accomplished using the procedure developed by Javellena and Jawed (4). The free lime amount thus obtained could be expressed as a fraction of reaction completed by comparison to the amount of unbound CaO present in the unburned pellets (i.e., that supplied by the CaCO 3 raw material plus that found as a phase in the sinter residue). The initial amount of unbound CaO which would react was estimated to be 19.8 wt% for the SR=4.3 mixes and 21.3 wt% for the SR=3.3 mixes. Kinetic Model Chemical reactions in a mixture of crystalline reagents such as exists in the unburned pellets exhibit a number of special characteristics. For instance, a heterogeneous reaction will take place at the interface between the two coexisting solid phases involved. Therefore, regardless of the number of reactants in the system, a given reaction will involve only two components and will consist of the breaking and reforming of chemical bonds. Jander (5) has developed a kinetic model for a solid-state system that consists of two reacting phases and that is diffusion controlled. In this model, as illustrated in Figure I, a product layer forms between phases A and B through which the reactants must diffuse to sustain the reaction. Jander's model has been verified for a number of solid-state systems but has been found inadequate for others due to its many simplifying assumptions (6). Though many of the kinetic models used to describe the reaction forming C3S, such as Jander's model, are solid-state, the presence of a liquid phase has been found to have a strong influence on the rate of formation. The effect of the liquid phase has been investigated by Kondo and Choi (7), Jawed and associates (8), Mackenzie and Hadipour (9), and Christensen and Jepsen (I0). Presence of the melt phase has been shown to be a precursor for the formation of C3S. Numerous microstructure investigations of quenched clinkers have shown that the cement melt wets the solid phase grains, thus forming a continuous network connecting most of the clinker mass. This melt network, comprising 25-30 wt% of the entire mass, enables movement of species to occur more rapidly than could be possible in the solid-state. However, information gained by more complex and complete reaction models have confirmed the results obtained by the simple solid-state binary models (I). The solid-state model most commonly used for cement clinker kinetics is a modification of Jander's model developed by Ginstling and Brounshtein (Ii). The limiting assumptions used in deriving the modified model are the following: •

The reaction considered is an additive reaction.

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19, No. 6

841 C2S , C3S , LIME-SODA SINTER, KINETICS

Phase B

\

A

i ~"~ u

- Phase

A

/

Phase B

= Product

,

FIG. i Schematic of Jander solid-state reaction geometry

•

• •

• • • • • • •

Nucleation, followed by surface diffusion, occurs at a temperature below that needed for bulk diffusion such that a coherent product layer is present when bulk diffusion does begin. The chemical reaction is diffusion controlled. The surface of the component in which the reaction is taking place is completely and continuously covered with particles of the other component. Bulk diffusion is unidirectional. The product phase is not miscible with any of the reactants. The reacting particles are all spheres of uniform radii. The ratio of the volume of the product layer to volume of the material reacted is unity. The increase in thickness of the product layer follows Barrer's relation. The diffusion coefficient of the transported species is not a function of time. The activity of the reacting species remains constant on both sides of the reaction interface.

The Ginstling-Brounshtein model is given by the following relation between time and fraction of reaction having occurred.

kGBt = 2kD-----!t= I - 2 x)2/3 r2 ~ x - (i -

(2)

o kGB is the Ginstling-Brounshtein rate constant, t is the time of reaction, D is the diffusivity of the migrating species, r o is the initial diameter of the reacting particle, and x is the fraction of the reaction completed. where

842

Vol. J.A.

Chesley

and

Results

Experimental

19, No. 6

G. Burner

and D i s c u s s i o n

Results

The

results of the factoral designed experiments are Figures 2 and 3 for a SR of 4.3 and 3.3, respectively.

~

9

'

I

;'~'

'

I

'

'

'

shown

'

I

I

graphically

I

I

in

I

/~-.e E

8 7

W t..J

6

UJ n~ 0 n" LJ

X

5

Z C)

4

t-LJ mn,h_

3

_

2

~

o BURNING TEMP=I500C o BURNING TENP=1450C BURNING TEMP=1400C x BURNING TEMP=I300C

1

l,,,

0

I,

,,,

50

I, 100

~

I

I

I I 150

TIME AT BURNING TEMPERATURE,

Fraction temperature

Analysis

T

L

I

I , 200

,

,

, 250

MINUTE5

FIG. 2 of CaO reacted as a function of time and burning for r e s i d u e - c e m e n t f o r m u l a t i o n SR = 4.3, LSF = 0.90

of Data

Equation 2 assumes that initially no reaction has occurred and that isothermal conditions exist throughout the period of reaction. Neither of these conditions were met by the e x p e r i m e n t a l method used in this investigation. The dried pellets were placed in a furnace p r e h e a t e d to 400 C and then brought to burning temperature at w h i c h time the period of constant temperature burning began. While heating up was occurring, a significant amount of reaction took place as can be seen in Figures 2 and 3. To correct for this, the GinstlingB r o u n s h t e i n model was rederived using new b o u n d a r y conditions of x = x o at t = O, where zero denotes the b e g i n n i n g of the constant temperature burning period. This d e r i v a t i o n yielded the following: kGBt

= 2kDt r2

2 = [I - ~ x

- (i -

O

= fGB(X)

- fGB(Xo)

x)2/3

2 Xo)2/3 ] - [i - --x°3 - (I ]

(3)

Vol.

19, No. 6

843

C2S , C3S , LIME-SODA SINTER, KINETICS

1 -

,-,

;

J

_~

I

I

I

I

I

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I

[

i

.7

ILl

E; .6 bJ rY

o

.5

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oz

.4

I-LJ h

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i_ i_

o BURNING TEMP:1500C o BURNING TEMP:I450C

BURNING TEMP:1400C BURNING TEMP=I300C

.2 .1

i_ i

,

i

,

I

i

i

50

,

,

I,

I

I

I

100

I

I

I

I

I 200

150

TIME lqT BURNING TEMPERATURE, MINUTE5 FIG. 3 Fraction of CaO reacted as a function of time and burning temperature for residue-cement formulation SR = 3.3, LSF = 0.90

The fit of the experimental data to the modified Ginstling-Brounshtein equation can be tested by constructing a ploc of fGB(X) versus t. If a fit exists, the plot will be linear with a slope equal to the rate constant kGB and a y-intercept of fGB(Xo). The degree of fit will be determined from the coefficient of correlation.

TABLE 2 Fit of Experimental Results to the Modified Ginstling-Brounshtein Reaction Model(2)

Reaction Rate Coefficient of

Burning Formulation

Temperature,

°C

Correlation

Constant, kGB min -I x I04

SR = 3.3, LSF = 0.90

1300 1400 1450

0.9857 0.9800 0.9148

4.3 14.3 II.0

SR = 4.3, LSF = 0.90

1300 1400 1450 1500

0.9853 0.9909 0.9582 0.8820

7.0 5.5 9.2 15.6

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C h ; s l e y and G. Burner

When this technique was applied to the experimental data, the results shown in Table 2 were obtained. Not all the experimental data were used in this analysis. The 1500 C burn using the SR = 3.3 formulation (Figure 3) was so near completion after about 5 or i0 minutes at the burning temperature that there was insufficient information to determine any correlation. Also, the 1300 C burn using the SR = 4.3 formulation (Figure 2) appeared to reach a constant level of completion after about 30 minutes. Consequently, only the data taken up to 30 minutes were used for this run.

.3

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.2

o F{X} EXPERIMENTFIL

I

--

/

.15

I

0

I

I

I 20

K

I

i

I 40

I

i

i

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z

x

m

60

I

z

I

BO

I

oo

TIME RT BURNING TEMPERRTURE. MINUTES FIG. 4 Experimental points from the analysis of SR = 3.3 residue-cement formulation burned at 1450 C fit into the Ginstling-Brounshtein rate equation

As shown in Table 2, the correlation coefficient is significantly lower at the higher burning temperatures. Further, as illustrated in Figure 4 for the 1450 C run at SR = 3.3, the experimental points in a plot of fGB(X) versus t display a break corresponding to two linear regions of differing slope. This two-stage reaction sequence became more pronounced as the burning temperature was increased, accounting for the lower correlation coefficients. In an attempt to model the early stages of reaction, use was made of a relationship developed by Cohn (12) which takes phase boundary phenomena into consideration. Working with spheres, Cohn assumes that the driving force for reaction consists of two parts, that associated with phase boundary resistance, and that resulting from diffusion. In the first stages of reaction for such a system, the diffusion (product) layer is thin and offers little resistance, so the reaction is primarily phase boundary controlled. As the product layer increases, a point is reached where resistance due to diffusion predominates. Under these conditions, the initial rate of reaction will be proportional to

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19, No. 6

845 C2S , C3S , LIME-SODA SINTER, KINETICS

the rate of increase in the thickness of the product layer and the subsequent rate will be proportional to the square of the product layer growth rate. The Cohn follows:

relationship

for

phase

boundary

(PB)

control

kpBt = k_tt = i - (i - x) I/3 ro

for

spheres

is

as

(4)

As was the case for the Ginstling-Brounshtein analysis, equation 4 assumes isothermal conditions and an initial condition of no reaction. In a treatment similar to that used for the Ginstling-Brounshtein relationship, equation 4 was rederived to fit the experimental conditions used in this investigation by changing the boundary conditions to x = x o at t = 0. kpBt = k~t = [i - (i - x) I/3] - [I - (I - Xo )I/3] ro

(5)

= fpB (x) - fpB(Xo ) Table 3 shows the coefficients of correlation and reaction rate constants calculated assuming that equation 5 describes the initial stage of the burning reaction and that equation 3 describes the second stage. The coefficients of correlation show significant improvement over those in Table 2. At a burning temperature of 1300 C and an SR of 4.3 where only data for the first 30 minutes of burning time were used, no break in the plot of f(x) versus t was observed. This suggests an explanation for the anomalous behavior shown in Figure 2 for this run. In the early stages of reaction, the system is phase boundary controlled and the mobility of Ca is not important. As the product layer develops, however, further reaction requires diffusion of Ca through the layer. When this does not occur because of the low temperature and, more importantly, because of the minimal amount of melt that is present, reaction essentially ceases.

Apparent Activation Energy The Arrhenius equation rate with temperature. following.

is commonly used to express the variation of reaction An integrated form of this equation is given by the

(6) k = A exp(-E/RT) where k is the reaction rate constant, A is the collision factor, E is the Arrhenius activation energy, R is the gas constant, and T is the absolute temperature. Taking the logarithm of both sides of the equation results in a linear equation. E I In(k) . . . . . + in(A) (7) R

T

In this form, the apparent activation energy can be determined from a plot of in(k) versus I/T. The resulting line will have a slope of -E/R and y-intercept of In(A). This method of analysis was used to determine the apparent activation energies based on the revised Ginstling-Brounshtein reaction rate constants for the latter stages of reaction. The activation energy determined for the SR = 4.3 formulation was 103 k~/mole (24.6 kcal/mole) and for the

846

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J.A. Chesley and G. Burner

TABLE 3 Fit of Experimental Results to Reaction Models for the Burning of Cement Formulations Based on the Two-Stage Reaction Scheme (2)

Reaction Rate Burning Temp.,

LSF

=

1300 1400 1450 1500

°C

0.90, SR

Coeff. of Correlation

=

Constant kpB min -I x 103

Reaction Rate Coeff. of

Constant, kpB

Correlation

min -I x 104

4.3 0.9845 0.9421 0.9982 0.9976

1.9 2.35 5.5 10.6

0.9939 1.000 0.9960

4.5 6.8 6.8

1.4 5.1 6.4

0.9956 0.9863 0.9920

3.6 12.1 6.7

LSF = 0.90, SR = 3.3 1300 1400 1450

0.9729 0.9975 0.9999

SR = 3.3 ~ormulation 121 kJ/mole (28.9 kcal/mole). These computed values are reasonably close to the activation energy associated with the diffusion of Ca through clinker melt, reported to be 164 kJ/mole (13) and with the activation energy for self-diffusion of Ca in CaO, reported to be 142-268 kJ/mole (i). The difference in activation energies found in this investigation and those reported may arise from the low amount of melt found in the raw mix formulations, especially the SR = 4.3 design. Kondo and Choi (7) showed that the apparent activation energy for the formation of cement clinker could vary from 175 kJ/mole to 515 kJ/mole in cement feeds producing a melt amount of 15 wt% and 30 wt%, respectively. Furthermore, it is doubtful whether the particle size for either the C2S particles or the reagent grade calcite was uniform, which could affect the reaction rate constant. The C2S particles in the raw material sinter residue were actually found to be thick-shelled hollow spheres, a result of the diffusion controlled mechanism by which the C2S was formed (2). Conclusions Investigation into the formation of C3S from the C2S found in a byproduct from the Ames Lime-Soda Sinter Process indicates that the reaction follows a two-stage sequence. This observation can be explained by a mechanism postulated by Cohn whereby early reaction is phase boundary controlled and subsequent reaction, after the product layer has grown sufficiently thick, is diffusion controlled. Apparent activation energies were determined by assuming that the diffusion controlled reactions could be modeled by a rederived Ginstling-Brounshtein solid-state rate equation. The activation energies were found to approximate

Vol. 19, No. 6

847 C2S, C3S, LIME-SODA SINTER, KINETICS

those reported in CaO.

for Ca diffusion

in clinker

melt

and

for self-diffusion

of Ca

Acknowledsment Ames Laboratory is operated for the U. S. Department of Energy by lowa State University under Contract No. W-7405-Eng-82. This work was supported by the Assistant Secretary for Fossil Energy through the Morgantown Energy Technology Center. The authors wish to thank Dr. Turgut Demirel for helpful suggestions throughout the investigation. References i. 2. 3. 4. 5. 6. 7. 8. 9. I0. ii. 12. 13.

T. K. Chatterjee, A. K. Chatterjee, and S. W. Ghosh, Sil. Ind. 45(9), 171 (1980). J. A. Chesley, Low-Alumina Portland Cement from Lime-Soda Sinter Residue, Ph.D. Dissertation, Iowa State University, Ames, IA (1987). Fo M. Lea, The Chemistry of Cement and Concrete, 3rd edition, Chemical Publishing Co., Inc., New York (1971). M. P. Javellana and I. Jawed, Cem. Concr. Res. 12(3), 399 (1982). W. Jander, Ziet. Anorg. Allegemeine Chemie 163, i (1927). W. D. Kingery, H. K. Bowen, and D. R. Uhlmann, Introduction to Ceramics, 2nd edition, John Wiley and Sons, New York (1976). R. A. Kondo and S. Choi, Proc. 5th Int. Congr. Chem. Cem., Tokyo, Vol. i, 163 (1968). I. Jawed, J. F. Young, A. Ghose, and Jo Skalny, Cem. Concr. Res. 14(1), 99 (1984). K. J. D. Mackinzie and N. Hadipour, Trans. J. British Ceram. Soc. 77(6), 168 (1976). N. H. Christensen and O. L. Jepsen, J. Amer. Ceram. Soc. 54(4), 208 (1971). A. M. Ginstling and B. I. Brounshtein, J. Applied Chem. of the U.S.S.R. (English Translation) 23, 1327 (1950). G. Cohn, Chem. Rev. 42(3), 527 (June 1948). G. C. Bye, Portland Cement Composition, Production, and Properties, Pergamon Press, London (1983).