ELSEV I ER Applied Clay Science 9 ( 1995 ) 337-354 '
Thermal stability and pozzolanic activity of calcined illite
Changling He a,., Emil Makovicky a, Bjarne Osb~eck b Department of Mineralogy, Institute of Geology, University of Copenhagen. ¢)ster Voldgade 10,
D K-1350 Copenhagen, Denmark h R&D Division, Laboratory. F.L. Smidth and CO. A/S, VigerslevAlle 77, DK-2500 Valby, Denmark
Received 18 May 1994; accepted after revision 21 October 1994
Abstract
Standard illite (American Clay Society, IMt- 1 ) was calcined at 650,790 and 930°C for 100 minutes. Both the raw and calcined samples, before and after being mixed with Ca(OH)2 in the presence of simulated cement pore solution, were studied by DTA, TG (for raw illite), XRD, SEM, EMPA and chemical solubility. The reaction rate of the mixtures was monitored by a chemical shrinkage test. Technological properties of the untreated and calcined illite-cement mortars were studied by rheology (flow) and by a compressive strength test after reaction for 2, 7, 28 and 91 days. The current investigation revealed that illite has low pozzolanic activity. Dehydroxylation at 650°C does not upgrade it significantly. Further calcination at 790°C brings about considerable activation but it still does not qualify as a pozzolan. Calcination at 930°C produces the highest pozzolanic activity. Com- pressive strength of the mortar with 930°C illite is 79% of that of reference ordinary portland cement.
I. Introduction
Fired clays, after being ground and mixed with lime, were the first man-made hydraulic cement (Cook, 1985). Although today they encounter considerable competition from indus- trial waste products such as fly ash, fired clays are still a good energy saving admixture and
blending material for cement and concrete, especially in those countries devoid of suitable industrial waste or natural pozzolan. Industrial grade clays or shales are usually a mixture of several clay minerals associated with non-clay minerals. The present study attempts to characterize the influence of individual clay species in these mixtures. Illite is a very common component in bulk clays. However, studies on its pozzolanic activity and its chemical, physical and mineralogical behaviour in the cement and concrete chemistry are rather scarce.
338 Changling He et al. /Applied Clay Science 9 (1995) 337-354
A literature survey shows that some data, which are mainly technical tests on mixtures of illite and lime mortar, are reported in the papers by Mielenz et al. (1950), Forrester (1974) and Ambroise et al. (1985).
In the present investigation, rather pure illite samples, untreated and thermally activated at three different temperatures, were extensively studied by means of:
( 1 ) Laboratory investigations of chemical reactions of untreated and calcined illite with laboratory grade calcium hydroxide Ca(OH) 2(CH) and with ordinary Portland cement (OPC).
(2) Technological tests to determine flow characteristics and strength development of mortars using 30 wt% pozzolan as a replacement for cement.
2. Materials and experimental method
The illite sample investigated was obtained from the Source Clays Repository of the Clay Mineral Society of America (Standard No. IMt-1 ). Its origin is Silver Hill, Montana and it has been previously studied by e.g. Hower and Mowatt (1966), Srodon (1984) and Srodon and Eberl (1984). Details of mineralogical and technological methods used in the present investigation as well as data on the instruments used are given by He et al. (1994).
3. Results and discussion
3.1. Mineralogical characterization and thermal reactions of illite
The illite sample is grayish green in color and shows lath-looking fragments ( l to 2 cm in length) in hand-specimens. The results of bulk chemical analysis are shown in Table 1 together with data from Hower and Mowatt (1966) and Srodon and Eberl (1984). The structure formula of the investigated illite, based on microprobe analysis results is ( IgN).69Nao.02Cao.03 ) ( All.45Fe 3 + o.23 Fe2 + 0.08Mg0.28) ( S i3.48A10.52 ) O10 (O H ) z. Agreement with the references is generally good.
The XRD pattern of the bulk sample (Fig. 1 ) shows dominant illite with minor admixtures of quartz, K-feldspar and calcite. The main K-feldspar reflection at 3.24 A is partially overlapped by illite and quartz, but is discernible as a peak shoulder. Kubler's illite crys- tallinity index (Kubler, 1964) is 0.42 ° (20) , measured on the XRD pattern of oriented, air-dried clay fraction ( < 2/~m). Srodon's Ir value (Srodon, 1984), which is defined as the ratio of intensity ratios (001)/(003) of the air-dry and the glycolated samples, was estimated as 1.30. This suggests that an expanding component is present. Further investi- gation applying Srodon's method, showed that the expanding layers are ISII ordered and constitute less than 15% of the clay fraction. This is in agreement with Srodon and Eberl (1984) who report an Ir value of 1.51 and < 15% of ISII admixture.
Modal mineralogical composition for the bulk sample, based on the combination of XRD identification data, the bulk chemical analysis and microprobe analyses of individual phases, is given in Table 1.
( l ) Bulk sample: X-ray fluorescence, atomic absorption and potenfiometric titration for Fe 2+ . (2) Illite component; electron microprobe analysis (EDS, average of 4 analyses) (3) Hower and Mowatt (1966), < 5/xm fraction; methods used as ( 1 ). (4) Srodon and Eberl (1984), < 2/xm fraction; X-ray fluorescence. "Total iron oxide. bWeight loss on heating to 980°C. CThe modal mineralogical composition, based on XRD, bulk chemical analysis and EPMA.
D T A and TG curves are presented in Fig. 2. Analysis of gases released during DTA shows
that a small endothermic reaction at about 100°C and a weight loss of 2.5% represent
evaporation of absorbed moisture and interlayer water. A broad endothermic reaction with
a peak at about 580°C and a major weight loss of 5.4 wt% represents dehydroxylation of
illite. The third H20 peak, partly overlapping with the main H20 peak, at about 680°C, is tentatively attributed to dehydroxylation of mixed smectite layers (Mackenzie, 1970).
Dissociation of calcite (detected as CO2 release) takes place at this temperature as well. An extended high temperature endothermic reaction from below 900°C to over 1100°C, without an obvious weight loss, is due to destruction of illite structure. The extraordinarily
large convex area between about 100 to 500°C on the DTA curve in is most probably caused
by the thermal conductivity changes connected with textural changes upon dehydration.
3.2. Calcination effect
Based on the DTA results, the following calcination temperatures were selected: (a)
650°C, the end of the dehydroxylation reaction; (b) 930°C, the peak temperature of illite destruction, and (c) 790°C, an intermediate temperature between these two processes. The calcination conditions were as follows, heating rate: 7°C/min; time at maximum tempera-
d(001 ) of oriented <2/zm fraction is 10.158/~. aPercentage change is given in Fig. 3. bThis serves as 20 internal reference.
ture: 100 min; clay spread in a layer of about 7 mm thick; cooling to room temperature in a closed furnace at a rate of about 10°C/h (He et al., 1994).
Phase transformations on calcination were monitored by XRD of cooled products. The recorded patterns (Fig. 1 ) indicate that dehydroxylation did not result in a collapse of illite structure. The main illite peaks persisted even after the sample had been heated at 930°C although they diminished considerably. Measurements of the (001) peak area of illite at 10 ,~ revealed that about 17% of illite survived calcination to 930°C (Table 2). A broad diagnostic peak of y-AIzO3 at 1.98 ,~ appears at 790°C and 930°C indicating the formation of this topotactic phase ( Brindley and Lemaitre, 1987). The calcite peak at 3.03/~ decreases with increasing temperature and disappears completely at 930°C. The main K-feldspar reflection at about 3.24 ,~ is getting more pronounced as reflections of other crystalline components become reduced on temperature increase.
Destruction of crystalline phases by calcination results in formation of amorphous sub- stances causing an increased background in XRDpatterns. The very pronounced background increase that was observed during calcination of kaolinite (He et al., 1994) does not occur for illite. Still, the increase of background between about 20-35 ° (20 ) , expressed as the area of a circle segment that describes the shape of the background under the illite (003) peak, is appreciable (Table 2). It represents an increase of over 53% for the whole calci- nation process, dominantly taking place in the last heating interval from 790 to 930°C. This increase is well correlated with the decrease in the intensity of the (001) reflection (Fig. 3).
Illite calcined at different temperatures and subsequently subjected to the same grinding procedure (He et al., 1994) shows systematic variations in the density and specific surface area (Fig. 4). The density was almost unchanged after the sample was calcined at 650°C, in spite of dehydration and dehydroxylation that caused 7.9% weight loss. Further calci- nation to 930°C reduces the density steadily from 2.78 to 2.64 g/cm 3. The specific surface area (BET) decreased drastically on calcination from 650 to 930°C due to increasing agglomeration of illite particles and closing of pores upon sintering at higher temperature.
The particle size distribution was measured by laser diffractometry after the samples have been dispersed by ultrasonic treatment (UST) for 4 minutes. The upper maximum of the bimodal curve shifts markedly to about 7 times the original particle size (Fig. 5) after calcination at 650°C and 790°C. Thermal expansion and contraction on the crystal structure level caused by dehydroxylation was estimated for pure muscovite as + 1.05% for d(001 )
342 Changling He et al. /Applied Clay Science 9 (1995) 337-354
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Fig. 3. Calcination effects on illite expressed by XRD characteristics. Connection lines between data points in Figs. 3 to 14 do not have physical meaning.
(about +3% for the unit cell volume) by Eberhart (1963) . For illite (with < 15% ISII mixture), it was estimated as - 1.4% for d(001) and as + 0.9% for b in the present study (Table 2); the difference for d(001 ) can be explained by the collapse of expandable layers in IMt-1. These changes are apparently of no significance in the observed tremendous increase of particle size. Observing that the particle distribution curves of calcined illite are rather similar to that of untreated illite without UST (Fig. 5), it can be assumed that agglomeration of original particles is responsible for the increase in the particle size. Calcination obviously strengthened the particle clusters produced by agglomeration so that they efficiently resist UST. The measured particle size increase with calcination temperature is reversed after calcination at 930°C: a considerable decrease is observed.
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Fig. 4. Density and specific surface area (BET) of the untreated and calcined illite.
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Changling He et al. /Applied Clay Science 9 (1995) 337-354 343
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Fig. 5. Particle size distribution for untreated and calcined illite after ultrasonic dispersion.
Insight into these micro-structural changes was provided by scanning electron microscopy of untreated and calcined samples (Fig. 6). Untreated illite appears as a loose packing of small particles, about 0.5/zm in diameter and 0.2/xm thick. Shapes of individual particles are rather irregular (Fig. 6a). After calcination at 650°C, individual particles agglomerate into compact lumps (Fig. 6b). The 790°C sample displays similar agglomerated lumps and individual particles appear less individualized than before (Fig. 6c). In the sample calcined at 930°C, the individual illite particles are hardly discernible, and small crystallites, perhaps the newly formed T-AI203 grains, appeared on the lump surfaces (Fig. 6d). These changes are parallel to the particle size decrease observed at this temperature. The development of density with calcination (Fig. 4) results from the combination of the observed agglomera- tion, development of microporosity and recrystallization.
According to Moran and Gilliland (1950), Lea (1970) and Surana and Joshi (1990), the quantity of alkali or acid soluble Si and AI reflects the content of active aluminosilicates ready to react with cement or cement pore solution, i.e., the pozzolanic activity of the material.
In the present investigation, parallel testing for both alkali and acid soluble Al and Si in the pozzolans and reaction products was carried out. Na(OH) and HC1 solutions were respectively used (as described by He et al., 1994).
As shown in Table 3 and Fig. 7a, b, the amount of acid and alkali soluble Si and AI from illite significantly increased on calcination. Most of the increase takes place during the first step (dehydroxylation) and it levels off during further calcination. The amount of acid soluble A1 decreases after calcination at 790°C due to the formation of "y-AI203, while soluble Si still keeps increasing at 930°C due to the liberation of Si from illite during continuous dissociation. The increase in soluble Si can be expected to continue until illite disappears completely or until the formation of high-temperature silicates starts. Total
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Changling He et al. /Applied Clay Science 9 (1995) 337-354
Table 3 Alkali and acid solubility of untreated and calcined illite
alkaline solubility of (Si + A1) from illite calcined at 930°C represents only 27% of that from kaolinite calcined at 950°C (He et al., 1994). These tests indicate a critical point around 930°C in activating illite for a pozzolanic material since crystallization - - which reduces the amorphous constituent - - will considerably reduce the pozzolanic activity. For materials in which crystallization immediately follows destruction great caution must be taken in controlling the calcination temperature in order to obtain an optimum pozzolan.
3.3. Physical and chemical properties of illite-cement mortars
Compressiue strength of cement-pozzolan mortars is one of the most important and reliable parameters for evaluating a pozzolanic material (Mehta, 1984). In the present study, Mini-RILEM 2 X 2 X 15 cm 3 mortar prisms (F.L. Smidth Denmark A/S, 1969) were tested to study the compressive strengths and their development with reaction time. The ratio of OPC:pozzolan:sand:water was 0.7:0.3:3.0:0.6. The preparation procedure is described in detail by He et al. (1994). The results of compressive strength tests are shown in Fig. 8a and Table 4. It is apparent that dehydroxylation of illite calcined at 650°C practically does not contribute to the compressive strength of resulting mortars. However, a considerable improvement is observed for the samples calcined at 790°C and a dramatic one for those from 930°C. This improvement becomes apparent only after curing for 28 days and develops further with curing up to 91 days, when the experiment was terminated.
The hydraulic index, H, was calculated for mortars based on untreated or calcined illite as follows:
H = °'x - °'x' inert X 100 O'0 - - O'x, inert
where ox is the compressive strength of the tested sample, O'o is that of reference OPC and ox. incr, is the strength of the sample blended with an inert component in the same replacement percentage as for the tested samples. Here the latter value was assumed to be equal to 63% of pure reference OPC. When H is equal to 0, the blended component is completely inert, whereas when it is equal to 100, the blended component has the same activity as reference cement (Keil, 1963). The results are shown in Table 4 and Fig. 8b. They indicate that untreated and 650°C illite are still not better than the inert material. Illite calcined at 790°C and 930°C has, respectively, a low, and a moderately high hydraulic index.
The rheological behaviour of mortar and concrete is an important issue when evaluating pozzolanic materials. This factor is studied by measuring the flow of the mixture with a
346 Changling He et al. / Applied Clay Science 9 (1995) 337-354
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Fig. 7. (a) Acid soluble Si and A1 in untreated and calcined illite. (b) Alkali soluble Si and AI in untreated and calcined illite.
specific water content. In the present investigation flow tests were carried out on the same mortars as used for compressive strength tests. The flow curve in Fig. 9 is nearly the inverse of the density curve in Fig. 4, suggesting that they reflect the same structural and textural changes. Calcination of illite at 650°C decreases but at higher temperatures increases the flow of resulting mortars. The practical consequence is obvious: calcination of illite at 790°C (and beyond) reduces the water demand (compared to the untreated and low-temperature treated illite) thus improving the technological properties of cement.
Since the total volume of water and cement-pozzolan paste will decrease during hydration reaction - - the so-called chemical shrinkage-- (Taylor, 1990), measurement of the shrink-
Changling He et al. /Applied Clay Science 9 (1995) 337-354 347
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Fig. 8. (a) Compressive strength of illite-cement mortars (Mini-RILEM), with the untreated and calcined illite component. Curing time at 40°C is indicated. (b) Hydraulic index of the same illite-cement mortars.
age reveals the reaction rate of the pozzolanic admixture (Knudsen, 1985). In the present investigation, the untreated or calcined illite was thoroughly mixed with CH in a 1:1 weight ratio in the presence of simulated cement pore solution (He et al., 1994). The pastes were kept in a water bath at 40°C for 28 days. The total volume change of the mixture was periodically monitored. Fig. 10 shows that rates of chemical shrinkage for various untreated and calcined illite-containing pastes are considerably lower than for the reference fly ash. Up to 28 days, variation between the samples was insignificant. Only the paste with illite calcined at 930°C displayed a slightly faster reaction rate during the first 300 hours of
348 Changling He et al. / Applied Clay Science 9 (1995) 337-354
Table 4 Compressive strength and hydraulic index of cement-illite mortars (Mini-RILEM)
Curingdays Compressive strength (MPa) Hydraulic index
Each value is an average of 3 measurements; the mean (resp. maximum) coefficient of variation is 2.3% (3.9%). Conditions: 30 wt% replacement of OPC by raw/calcined illite; water/(cement + pozzolan) = 0.6; curing at 40°C.
hydration but it was overtaken afterwards by the samples heated at lower temperatures. Untreated illite did not differ significantly from the rest of the samples.
After reaction for 150 days at 40°C, the pastes of untreated and calcined illite mixed with CH for chemical shrinkage tests were studied for their reaction products. The pastes were freeze-dried and slightly pulverized for X-ray diffraction analyses, electron microprobe analysis (EPMA), and SEM.
The XRD patterns of the mixtures are dominated by portlandite (Fig. 11). The only unambiguous reaction product is C4A~HI2 (C=CaO, A=AI203, H=H20, U=CO2, S=SiOz) with the peaks (0001) and (0002) at about 7.6 and 3.8 ,~. It is formed by carbonation of C4AH~3 (Taylor, 1990). This phase was nearly absent in the sample with untreated illite.
In general, the XRD patterns confirm the low reactivity of illite. However, the different mixtures can be classified into two groups based on the amounts of remanent CH which is
o LI_
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135
125
/ - /
/ , / /
/, / ,
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/ /
115 2'0 ~ 650 700 9:30 Calcination T (°C)
Fig. 9. Flow of illite-cement mortars with untreated and calcined illite components,
Changling He et al. /Applied Clay Science 9 (1995) 337-354 349
6 -
Q)
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0
c-
I 0 I O0 1000
HOUFS
Fig. 10. Chemical shrinkage of untreated and calcined illite-Ca(OH)z mixtures reacting at 40°C. Corresponding mix with fly ash shown as reference.
monitored by the intensity of its strongest ( 101 ) peak at 2.26 ,~. The mixtures with untreated and 650°C illite display the peak intensity about 113 cps whereas illite calcined at 790 and 930°C yields about 75 cps (Table 5). These two groups can also be recognized by studying the XRD curve between d-values of 3.03 and 3.1 t ,~ for a peak or a diffuse band of C-S- H at 3.07 ,~ (Murat, 1983). A small diffraction peak of about 13 cps appeared in this region after 650°C (Table 5). It reaches about 20 cps for the second group of the calcined samples. The XRD analysis of the reaction products confirms that calcination at 650°C is not sufficient to activate illite but a more pronounced activation takes place at 790°C and 930°C.
The identity of the XRD patterns of untreated and calcined illite-CH mixtures reacted for 5 months and for 18 months suggests that practical phase equilibrium, or a "steady state" (De Silva and Glasser, 1993) has been achieved. Chemical composition of individual phases in the mixtures, especially of the amorphous ones, was studied by EPMA. A JEOL Superprobe with a "rRACOR energy dispersive system with 10 kV accelerating voltage and 10 nA current was used. Analytical results are presented in the ternary diagram SiO2-A1203- CaO in Fig. 12.
For the sample containing untreated illite and largely also for that with illite calcined at 650°C, the majority of compositional points cluster around the CH and illite poles. Reaction products were hardly observed. Points spread randomly along the SiO2-CH line refer to physical mixtures of quartz and CH. For the samples containing illite calcined at 790°C and 930°C, an accumulation of points at the Ca/Si ratio of about 4, is largely a mixture of micrometer sized C-S-H with CH, as suggested by Taylor (1990). Its intergrowth with illite explains most of compositional points in Fig. 12. For the sample with illite calcined at 930°C, the relative AI concentration in individual analysis is consistently higher than that
350 Changling He et al. / Applied Clay Science 9 (1995) 337-354
Fig. 11. XRD patterns of untreated and calcined i l l i te-Ca(OH)2 pastes (150 days at 40°C). l= i l l i t e ;
P = portlandite; Q = quartz, C = calcite and A = calcium carboaluminate.
at lower temperatures. The K20/SiO2 molar ratio of about 0.1, similar to that of illite, suggests that they represent illite mixed with fine grained y-AI203 which was detected by XRD and SEM. Mixtures of C-S-H and/or CH with CnAHx and illite were detected in the samples calcined at 790 and 930°C. The analytical results in the SiO2-A1203-CaO system suggest that an assemblage of C-S-H, CH, C4AHx and illite residues can be expected when
Table 5 X-ray diffraction intensities for calcined illite-Ca(OH)2 mixtures
calcined illite reacts with surplus CH. No gehlenite hydrate or/and hydrogarnet, known from most other pozzolans (Serry et al., 1984; De Silva and Glasser, 1993) were detected.
SEMphotographs of the mixtures of untreated/calcined illite with CH (Fig. 13) show very different degrees of hydration reactions. Samples containing untreated and 650°C illite have a flaky character. The proportion of secondary fibrous phases is low. In the samples containing illite calcined at 790°C a much more profound alteration of original grains is observed and in the sample from 930°C, secondary fibrous phases completely mask the original components.
Analytical data on acid soluble Al and Si of the illite-cement mixture reacted for 120 days at 40°C are presented in Fig. 14. Indicating the amount of acid soluble hydration products they serve as a measure of pozzolanic activity of calcined clay (Turriziani, 1964). Soluble Si and A1 both increase substantially with increasing calcination temperature of illite but the increase in soluble AI ceases after the calcination temperature of 790°C, similar to pure illite.
4. Conclusions
The current investigation revealed that illite has low pozzolanic activity. Untreated illite is a practically inert component from the point of view of cement and concrete chemistry. The structural damage to illite inflicted by dehydroxylation at 650°C does not upgrade it
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Changling He et al. /Applied Clay Science 9 (1995) 337-354 353
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significantly. Further calcination at 790°C considerably activates illite, but because of the moderate compressive strength results it does not qualify as a pozzolan.
Calcination at 930°C produces the highest pozzolanic activity. Still, after 28 days it reaches only 79% of the compressive strength of reference OPC and only 65% of the strength of the completely analogous metakaolin (950°C) - cement mortar. In commercial clays, illite can be considered as a low to moderate pozzolan.
Acknowledgements
The authors wish to thank Assoc. Prof. J. Ronsbo and Mr. J. Flyng for the keen interest and assistance at the electron microprobe analyses and Assoc. Prof. E. Leonardsen and Mrs. T. Poulsen for the X-ray diffraction analyses. Mr. H.B. Jensen in F.L. Smidth and CO. A/S kindly provided assistance in the technical tests. Material and apparatus needed for the project were financed by the University of Copenhagen, F.L. Smidth and CO. A/S and Danish Natural Research Council. Kind assistance of Assoc. Prof. J. Rensbo was crucial in setting up this project, Reviews by Drs J. Decleer and M. Murat improved substantially the quality of the paper.
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