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Assessment of pozzolanic potential in lime–water systems of raw and calcined kaolinic clays from the Donnigazza Mine (Sardinia–Italy)
Applied Clay Science 33 (2006) 66 – 72 www.elsevier.com/locate/clay
Technical Note
Assessment of pozzolanic potential in lime–water systems of raw and calcined kaolinic clays from the Donnigazza Mine (Sardinia–Italy) Stefano Cara a , Gianfranco Carcangiu a,⁎, Luigi Massidda b , Paola Meloni b , Ulrico Sanna b , Massimo Tamanini a a b
Istituto di Geologia Ambientale e Geoingegneria del C.N.R., Sezione di Cagliari, Piazza d'Armi, 1, Italy Dipartimento di Ingegneria Chimica e Materiali, Università degli Studi di Cagliari, Piazza d'Armi, 1, Italy Received 12 June 2003; received in revised form 14 April 2005; accepted 30 December 2005 Available online 28 February 2006
Abstract Raw and calcined kaolinic clays from a mid-western Sardinia (Italy) sector were investigated in order to evaluate the feasibility of lime-activated materials. The great reserves of clays in this mining district, which are not well suited to traditional ceramics because of their low kaolinite content, excess of quartz, iron oxides, alunite, and the open pit exploitation, suggest an alternative use for these materials. The kaolinic clay samples and a reference kaolin were thermally activated in a laboratory oven at 530, 630 and 800 °C temperatures. Pastes of thermally activated and non-activated materials with lime, in the ratio of 1 : 1 by weight and water / solid ratio 1 : 2, were prepared and cured at different times. The reactions that took place in the metakaolin–lime–water systems were monitored using thermogravimetric analysis, differential thermal analysis and X-ray diffraction analysis. The results of pozzolanic tests and laboratory analyses were combined and offered very interesting property relations compared with a high-grade kaolin used as reference sample, even though a slower hydration kinetics. The large availability of the Donnigazza kaolinic clay reserves encourages the use of these materials in the binders sector and particularly in the Cultural Heritage sector as pozzolanic additives for restoration mortars. © 2006 Elsevier B.V. All rights reserved. Keywords: Kaolinite clay; Thermal treatment; Metakaolin; Pozzolanic activity; Lime consumption
1. Introduction Thermal treatment of clays between 500 and 800 °C is a well-known process to obtain lime-activated materials. ⁎ Corresponding author. Fax: +39 070 675 5511. E-mail address: [email protected] (G. Carcangiu). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.12.005
The great availability of industrial by-products with pozzolanic characteristics, like fly ash, has decreased the importance of this thermal technology. Nevertheless it is still interesting not only for sparely industrialized regions lacking natural pozzolana, but also because of the remarkable properties related to the pozzolanic activity of products, lower than silica fume but generally better than fly ash (Frias et al.,
S. Cara et al. / Applied Clay Science 33 (2006) 66–72
1993). The main objective of this study is to evaluate the feasibility of lime-activated materials using some low-grade kaolinic clays from the Romana– Cossoine area, mid-western Sardinia. In this note we consider both the classical pozzolanic test, based on the determination of the silica solubility after acidbase attack (UNI EN 196/2), and a quick spectrophotometric experimental method (Surana and Joshi, 1988, 1990). Our investigation has been finally completed with the study of raw and calcined kaolinic materials/lime reaction kinetics carried out with pastes.
67
flows and domes of the Upper Andesitic Series (SA2 Auct., Fig. 1, northern sector); rhyolitic pyroclastic flows series, hosting the mineralisation (SI2 Auct.) and lava flows sequence ranging from rhyodacites to andesite–basalts (SA3 Auct.). A NE–SW and NW–SE fault network characterizes the area where an intensive kaolinic clay exploitation has been carried out during the last decades. The Donnigazza Mine is located into the rhyolite deposits nearby the tectonic contact with the andesitic volcanites. The reserves, evaluated by recent direct investigations (test holes and trenches), are about 1.200.000 t within the actual exploitation area.
1.1. Geological setting 2. Materials and methods
Fig. 1 shows the geological setting of the investigated area and the location of the Donnigazza open pit mine in the Logudoro region (mid-west Sardinia). This region is well-known for the occurrence of kaolinisation of calcalkaline Oligo-Miocene volcanics. In this region the Tertiary volcanism consists of andesitic lava. The geological sequence in the investigated area is characterized, from bottom to top, by lava
2.1. Selection of materials Three samples (MK1, MK2 and MK3) of commercial kaolinic materials were collected from the Donnigazza Mine, one of the most important in Sardinia. The lime used for the preparation of pastes was an analytical grade Ca(OH)2. A high grade kaolin (KP) was supplied by Carlo Erba Reagenti; it was used as a reference sample.
Fig. 1. Geological sketch map of the Donnigazza mine area (after Ligas et al., 1997, modified).
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S. Cara et al. / Applied Clay Science 33 (2006) 66–72
Table 1 Chemical composition (wt.%) of the Sardinian kaolinic clays (MK) and of the reference kaolin (KP)
MK1 MK2 MK3 KP
SiO2
Al2O3
Fe2O3
TiO2
MgO
CaO
Na2O
K2O
MnO2
P2O5
H2O
68.50 77.12 79.18 48.47
18.94 16.48 13.39 35.97
0.90 0.40 0.17 0.91
0.45 0.26 0.30 0.10
0.30 0.06 0.07 0.16
0.20 0.12 0.15 0.05
0.15 0.16 0.14 0.21
0.14 0.05 0.06 1.30
0.01 0.01 0.01 0.02
0.13 0.05 0.12 0.29
7.92 5.80 6.42 12.56
2.2. Experimental techniques Chemical analyses were performed by X-ray fluorescence (XRF) using a Philips PW1400 spectrometer operating with a Rh Tube at 30 kV and 60 mA. The fluorescence data were processed according to the Franzini et al. (1972) procedure. A Rigaku-Geigerflex D-max diffractometer with a CuKα radiation was used for identification of the mineralogical phases by comparison with the Powder Diffraction File (JCPDS, 1985). Chemical analyses were combined with XRD data to obtain a normative mineralogical composition of the materials. Granulometric analyses were performed with the Micromeritics SediGraph 5100 Particle Size Analysis System. The kaolinic clays and the reference kaolin were thermally activated at three different temperatures that arose from the thermal behaviour proposed by Smykatz-Kloss (1974). The thermal treatment was carried out in a laboratory gradient oven. The materials were heated at a constant rate (7 °C/min) from room temperature up to 400 °C, then at 2 °C/min up to the different calcination temperatures, where they remained for 100 min. Two different methods were applied in order to predict the behaviour (reactivity) of the materials in the presence of Ca(OH)2. Data from the first test, carried out according to the classical UNI-EN 196/2 procedure, refer to the quantity of insoluble residue after acid-base attack. The second evaluation test was carried out according to the spectrophotometric method proposed by Surana and Joshi (1988, 1990). Our samples were dissolved in a NaOH 0.5 N solution, heated in a microwave oven at fixed operating conditions of power (500 W) and time (6 min). Thermogravimetric analysis (TGA), differential thermal analysis (DTA) and X-ray diffraction analysis (XRD) were carried out to observe the reactions that took place in the metakaolin–lime system. TGA and DTA were performed with a Stanton STA780 simultaneous analyser. The samples were heated from room temperature up to 1100 °C at a rate of 15 °C/ min, under static atmospheric conditions: α-Al2O3 was used as reference material.
The pastes so obtained were cured for 1, 3, 7, 14, 28, 90 and 180 days in sealed Coulter containers to avoid carbonation. After the different curing times, hydration was stopped through water removal from the samples after grinding and dispersing the powder in pure ethanol. The samples were filtrated then dried at 80 °C. The rate of pozzolanic reaction progress was evaluated by monitoring by XRD and DTA the residual lime content in the different pastes.
3. Results and discussion Table 1 gives the chemical composition of raw kaolinic clays (MK) and reference kaolin (KP) used for the preparation of pastes. The XRD analyses of MK samples show a mineralogical composition characterized by the constant presence of kaolinite, tridymite and cristobalite: quartz was detected only in the MK1 sample. The mineralogical composition of the reference KP kaolin is given by kaolinite and illite. A normative mineralogical calculation of the studied kaolinic clays was also performed (Table 2), by combining the bulk chemical analyses of the samples with XRD data and by assuming the whole H2O content to be related to the kaolinite phase (the only hydrate phase in our samples). On the other hand, the presence of illite in the KP reference sample did not permit correct determination of the kaolinite content from normative calculation; however, the chemical data (Table 1) allows us to estimate a kaolinite content higher than in the MK samples, but lower than 88.16 wt.% (calculated value by assuming all the H2O in the KP to be related to the kaolinite phase). Table 2 Mineralogical normative composition of Sardinian kaolinic clays samples (wt.%)
2.3. Metakaolin–lime systems preparation Pastes of the different materials (activated and nonactivated kaolinic clays) with lime in a 1 : 1 ratio by weight with a water / solid ratio 1 : 2 were prepared.
Kaolinite Silica a a
MK1
MK2
MK3
46 44
38 60
31 67
The term Silica corresponds to the sum of the various silica phases (quartz, tridymite, cristobalite and opal).
S. Cara et al. / Applied Clay Science 33 (2006) 66–72
The granulometric distribution of the three kaolinic clay samples were compared with the curve of the KP reference sample (Fig. 2); the reference KP kaolin shows lower particle sizes and consequently a higher surface area than the Sardinian kaolinic clays. The activation temperatures selected according to DTA and XRD data analyses were: 530 °C the maximum temperature for kaolinite dehydroxylation and formation of metakaolinite; 630 °C the end of the kaolinite dehydroxylation process and finally 800 °C, a temperature which avoided mullite formation. As shown in Fig. 3, the kaolinite XRD peaks disappear at temperature higher than 530 °C while the peaks of various silica phases appear unaltered.
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Fig. 3. XRD (CuKα radiation) patterns of each samples (from top to bottom) to raw and heated material at 530, 630, 800 °C. I = illite; K = kaolinite; Q = quartz; Si = silica metastable phases.
3.1. Pozzolanic activity Tables 3 and 4 show the results of the two pozzolanic tests performed. The pozzolanic activity tests point out the reactivity of the materials when the solubility of the sample (as silica soluble phases) is assumed as pozzolanic index. The untreated Sardinian clays show higher values of silica soluble phases than pure kaolin. This behaviour could be related to the presence of metastable silica phases, particularly tridymite, and of other poorly crystalline reactive silica phases. Broad XRD peaks of poorly crystallized cristobalite or βcristobalite (White, 1965; Fournier, 1973) centered at 4,1 and 2,5 Å were observed in our samples. It is wellknown that amorphous silica is commonly found in hydrothermal ore deposits, and it is attributed to relatively quick changes of the physical and chemical composition of the hydrothermal solution (Fournier, 1985). The crystallinity of kaolinite phase appears to play a secondary role in the reactivity of samples. In fact the
crystallinity, evaluated by measuring the width at half the maximum height of the diffraction peak (FHWM) at d = 3.58 Å (Hinckley, 1963; Brindley and Brown, 1984) after the background removal, is quite similar for the kaolinic clays in Table 5. 3.2. Kinetics of metakaolin–lime reaction DTA and XRD data enable the evolution of the pozzolanic reaction to be monitored by the decrease of the Ca(OH)2 endothermic peak and the appearance of new hydrated phases in the metakaolin/lime system. The pure kaolin KP, not thermally activated, does not exhibit lime reactivity (Fig. 4): the endothermic lime peak keeps the same shape and amplitude and no new phases thermally active are formed. On the contrary, the unheated sample MK1 shows reactivity, with a progressive reduction of the Ca(OH)2 endothermic peak during curing (Fig. 5). Similar behaviour was obtained for the other kaolinic Sardinian clays. As regards the thermally activated samples, the kinetics of metakaolin–lime systems below depicted (Figs. 6 and 7) is referred to the 630 °C for KP and the only MK1. Figs. 6 and 7 show the rate of lime consumption. A progressive reduction of the Ca(OH)2 endothermic peak and the formation, after 7 days of curing, of new
Table 3 Fraction (wt.%) of insoluble residual after acid-basic treatment (UNI 196-2)
Fig. 2. Cumulative granulometric curves of the materials: 1) KP; 2) MK1; 3) MK2; 4) MK3.
Sample
NC
530 °C
630 °C
800 °C
KP K1 MK2 MK3
52 39 27 11
14 3 3 3
13 2 2 2
12 2 2 2
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S. Cara et al. / Applied Clay Science 33 (2006) 66–72
Table 4 Fraction (wt.%) of soluble phases as determined by spectrophotometric method (Surana and Joshi, 1988) Sample
NC
530 °C
630 °C
800 °C
KP MK1 MK2 MK3
4.1 34.1 55.0 67.0
24.8 49.3 77.3 71.8
35.8 59.0 84.8 72.5
37.7 62.3 89.5 76.0
hydrated phases can be observed. The pozzolanic reaction can be considered to complete after 28 days in case of the KP–lime system. The kinetics of MK1– lime systems appears slower: the whole of the lime was only consumed after 90 days. It can be also observed that, apart from kaolinite content, Table 2, the difference in the pozzolanic activity is very slight between the MK samples. The characterisation by XRD of the hydrated phases was fairly difficult because the occurrence of some overlapping peaks. Moreover, recognition of C–S–H by XRD was very difficult due to low crystallinity. In XRD patterns of KP–lime and MK1–lime systems (calcined at 630 °C) the pozzolanic reaction evolution can be clearly observed through the progressive reduction of portlandite peaks. It was also observed that illite, identified in the KP sample, participates to some extent in the pozzolanic reaction: its diffraction peaks decreased as the reaction progressed. As demonstrated by other authors, illite remains a generally poor pozzolan, even though its activity considerably increases at calcination temperature over 900 °C (He et al., 1995). The hydration products were essentially C2ASH8 (strätlingite), tetracalcium aluminate hydrate, C4AH13, and C–S–H gel, according to the reaction schemes proposed by Murat (1983), for metakaolin–lime systems cured at T = 20 °C, on the basis of a complete hydration without carbonation or water evaporation. The C4AH13 phase seems more stable with short curing times than longer ones. A similar behaviour has been observed for the other Sardinian metakaolin–lime systems.
Fig. 4. DTA curves of the raw KP kaolin–lime system at different curing times (in days): P = portlandite.
DTA curves and XRD patterns show similar trends for all MK systems: the lime peaks progressively decreased and at 180 days disappeared in every case. The amounts of residual lime of different systems at different curing times (determined through TGA measurements) are reported in Table 6. It can be pointed out that: • the untreated Sardinian kaolinic clays, unlike pure kaolin, show a certain lime reactivity, increasing as the reactive silica content increases; • the activated reference kaolin exhibits a strong lime reactivity when the activation temperature is high enough to completely transform the kaolin to metakaolin;
Table 5 FHWM index of the reference KP sample and the untreated Sardinian kaolinic clays Sample
FHWM a
KP MK1 MK2 MK3
0.205 0.219 0.204 0.177
a
Measurements referred to the (002) reflection (d = 3.58 Å).
Fig. 5. DTA curves of raw MK1 sample–lime system at different curing times (in days): P = portlandite.
S. Cara et al. / Applied Clay Science 33 (2006) 66–72
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Table 6 Residual lime (wt.%) a in the pastes of untreated and treated specimens as a function of the ageing time (in days)
Fig. 6. DTA curves of KP kaolin (calcined at 630 °C)–lime system at different curing times (in days): P = portlandite; ST = strätlingite C2ASH8; CAH = tetracalcium aluminate hydrate C4AH13.
Sample
1
3
7
14
28
90
180
KP-NC KP-530 KP-630 KP-800 MK1-NC MK1-530 MK1-630 MK1-800 MK2-NC MK2-530 MK2-630 MK2-800 MK3-NC MK3-530 MK3-630 MK3-800
95.90 86.10 82.50 86.10 97.20 71.90 79.50 83.00 89.90 73.00 75.60 82.10 86.00 85.10 72.80 91.10
91.20 74.10 71.60 77.30 89.10 63.00 55.90 69.60 83.90 56.70 70.70 64.00 83.30 75.20 58.60 65.50
90.40 42.00 41.80 49.60 71.10 45.70 44.40 45.80 57.90 49.60 46.50 57.00 51.50 50.60 48.40 56.50
87.50 29.10 12.60 14.80 58.40 41.00 27.70 28.50 54.40 29.80 28.80 27.00 43.90 42.20 38.00 49.00
78.90 28.9 0.00 0.00 56.30 27.10 15.70 16.80 44.00 19.70 22.00 17.70 39.80 29.40 15.30 32.90
79.00 27.00 0.00 0.00 43.20 12.20 16.48 11.30 35.50 14.10 14.40 17.00 16.40 14.80 15.10 17.30
73.00 0.00 0.00 0.00 14.70 0.00 0.00 0.00 10.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00
a
• the activated Sardinian kaolinic clays, even with a slow hydration kinetics, are able to consume all the lime present in the system especially after long curing times; • calcination at 630 and 800 °C gives similar results. 4. Final remarks The assessment of the pozzolanic activity of the Donnigazza kaolinic clays encourages possible nontraditional applications for these materials. Pozzolanic tests carried out by different chemical attacks provided good results for untreated kaolin too. The silica
Fig. 7. DTA curves of MK1 sample (calcined at 630 °C)–lime system at different curing times (in days): P = portlandite.
Values calculated through TGA measurements.
metastable phases, mostly tridymite and to a lesser extent cristobalite and opal, can be considered to play an important role in the hydration process of the untreated samples. However, the presence of these metastable phases strongly affects the results of the classic pozzolanic tests based on the acid-base attack of the materials (UNI EN 196/2). These tests can produce misleading data about the pozzolanic activity evaluation. It is in our opinion that the only investigations that can shed light upon the pozzolanic properties are those based on the determination of the effective Ca(OH)2 consumption; that is the essential measure of the pozzolanic reaction. The proposed pozzolanic tests are not completely effective for determination of the pozzolanic activity of metakaolin–lime–water systems. Further laboratory tests are in progress in order to demonstrate an improvement of the mechanical strength and durability of mortars and concrete formulated with these materials. The thermal treatment of kaolinic samples at 630 °C is considered to be a good compromise between activation and economics: the reactivity of samples calcined at 630 °C and 800 °C gives similar results at long curing times. The use of these materials in the binders sector appears to be encouraging by taking into account the large availability of the Donnigazza kaolinic clay reserves evaluated, by recent direct investigations (test holes and trenches) within the actual exploitation area, of about 1.200.000 t.
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These materials could find an appropriate role particularly in the Cultural Heritage sector as pozzolanic additives for restoration mortars. Acknowledgments Authors are grateful to Mr. G. De Giorgi, owner of the Donnigazza mining title, for supplying the commercial materials tested in this work. References Brindley, G., Brown, G., 1984. Crystal Structures of Clay Minerals and their X-ray Identification. The Mineralogical Society, London. Fournier, R.O., 1973. Silica in thermal waters: laboratory and field investigations. Proceed. of Intern. Symp. on Hydrogeochemistry and Biogeochemistry, vol. 1. J.W. Clark, Washington, D.C., pp. 122–139. Fournier, R.O., 1985. The behaviour of silica in hydrothermal solutions. In: Berger, B.R., Bethke, P.M. (Eds.), Reviews in Economic GeologyGeology and Geochemistry of Epithermal Systems. Soc. Econ. Geol., Socorro, NM 87801, vol. 2, pp. 45–61. Franzini, M., Leoni, L., Saitta, M., 1972. A simple method to evaluate the matrix effects in X-ray fluorescence analysis. X-ray Spectrom. 1, 151–154. Frias, M., Sànchez de Rojas, M.I., Cabrera, J., 1993. The effect that the pozzolanic reaction of metakaolin has on the heat evolution in MKcement mortars. Cem. Concr. Res. 23, 627–639.
He, C., Osbaeck, B., Makovicky, E., 1995. Pozzolanic reactions of six principal clay minerals: activation, reactivity assessments and technological effects. Cem. Concr. Res. 25, 1691–1702. Hinckley, D.N., 1963. Variability in “crystallinity values” among the kaolin deposits of the coastal plain of Georgia and South Carolina. Clays Clay Miner. 2, 229–235. JCPDS FILE, 1985. Inorganic Phases. JCPDS International Centre for Diffraction Data, Swarthmore PA, USA. Ligas, P., Uras, I., Dondi, M., Marsigli, M., 1997. Kaolinitic materials from Romana (north-west Sardinia, Italy) and their ceramic properties. Appl. Clay Sci. 12, 145–163. Murat, M., 1983. Hydration reaction and hardening of calcined clays and related minerals: I. Preliminary investigation on metakaolinite. Cem. Concr. Res. 13, 259–266. Smykatz-Kloss, W., 1974. Differential thermal analysis — application and results in mineralogy. Springer-Verlag, Berlin, Heidelberg, New York. Surana, M.S., Joshi, S.N., 1988. Spectrophotometric method for estimating the reactivity of pozzolanic materials. Adv. Cem. Res. 1, 238–242. Surana, M.S., Joshi, S.N., 1990. Estimating activity of pozzolanic materials by spectrophotometric method. Adv. Cem. Res. 10, 81–83. UNI EN 196 Parte II, 1991. Metodi di Prova dei Cementi — Analisi Chimica dei Cementi. White, D.E., 1965. Saline waters of sedimentary rock. Am. Assoc. Pet. Geol., Mem. 4, 342–366 The assessment of the pozzolanic activity of the Donnigazza kaolinic clays encourages possible nontraditional.
Technical Note
Assessment of pozzolanic potential in lime–water systems of raw and calcined kaolinic clays from the Donnigazza Mine (Sardinia–Italy) Stefano Cara a , Gianfranco Carcangiu a,⁎, Luigi Massidda b , Paola Meloni b , Ulrico Sanna b , Massimo Tamanini a a b
Istituto di Geologia Ambientale e Geoingegneria del C.N.R., Sezione di Cagliari, Piazza d'Armi, 1, Italy Dipartimento di Ingegneria Chimica e Materiali, Università degli Studi di Cagliari, Piazza d'Armi, 1, Italy Received 12 June 2003; received in revised form 14 April 2005; accepted 30 December 2005 Available online 28 February 2006
Abstract Raw and calcined kaolinic clays from a mid-western Sardinia (Italy) sector were investigated in order to evaluate the feasibility of lime-activated materials. The great reserves of clays in this mining district, which are not well suited to traditional ceramics because of their low kaolinite content, excess of quartz, iron oxides, alunite, and the open pit exploitation, suggest an alternative use for these materials. The kaolinic clay samples and a reference kaolin were thermally activated in a laboratory oven at 530, 630 and 800 °C temperatures. Pastes of thermally activated and non-activated materials with lime, in the ratio of 1 : 1 by weight and water / solid ratio 1 : 2, were prepared and cured at different times. The reactions that took place in the metakaolin–lime–water systems were monitored using thermogravimetric analysis, differential thermal analysis and X-ray diffraction analysis. The results of pozzolanic tests and laboratory analyses were combined and offered very interesting property relations compared with a high-grade kaolin used as reference sample, even though a slower hydration kinetics. The large availability of the Donnigazza kaolinic clay reserves encourages the use of these materials in the binders sector and particularly in the Cultural Heritage sector as pozzolanic additives for restoration mortars. © 2006 Elsevier B.V. All rights reserved. Keywords: Kaolinite clay; Thermal treatment; Metakaolin; Pozzolanic activity; Lime consumption
1. Introduction Thermal treatment of clays between 500 and 800 °C is a well-known process to obtain lime-activated materials. ⁎ Corresponding author. Fax: +39 070 675 5511. E-mail address: [email protected] (G. Carcangiu). 0169-1317/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.clay.2005.12.005
The great availability of industrial by-products with pozzolanic characteristics, like fly ash, has decreased the importance of this thermal technology. Nevertheless it is still interesting not only for sparely industrialized regions lacking natural pozzolana, but also because of the remarkable properties related to the pozzolanic activity of products, lower than silica fume but generally better than fly ash (Frias et al.,
S. Cara et al. / Applied Clay Science 33 (2006) 66–72
1993). The main objective of this study is to evaluate the feasibility of lime-activated materials using some low-grade kaolinic clays from the Romana– Cossoine area, mid-western Sardinia. In this note we consider both the classical pozzolanic test, based on the determination of the silica solubility after acidbase attack (UNI EN 196/2), and a quick spectrophotometric experimental method (Surana and Joshi, 1988, 1990). Our investigation has been finally completed with the study of raw and calcined kaolinic materials/lime reaction kinetics carried out with pastes.
67
flows and domes of the Upper Andesitic Series (SA2 Auct., Fig. 1, northern sector); rhyolitic pyroclastic flows series, hosting the mineralisation (SI2 Auct.) and lava flows sequence ranging from rhyodacites to andesite–basalts (SA3 Auct.). A NE–SW and NW–SE fault network characterizes the area where an intensive kaolinic clay exploitation has been carried out during the last decades. The Donnigazza Mine is located into the rhyolite deposits nearby the tectonic contact with the andesitic volcanites. The reserves, evaluated by recent direct investigations (test holes and trenches), are about 1.200.000 t within the actual exploitation area.
1.1. Geological setting 2. Materials and methods
Fig. 1 shows the geological setting of the investigated area and the location of the Donnigazza open pit mine in the Logudoro region (mid-west Sardinia). This region is well-known for the occurrence of kaolinisation of calcalkaline Oligo-Miocene volcanics. In this region the Tertiary volcanism consists of andesitic lava. The geological sequence in the investigated area is characterized, from bottom to top, by lava
2.1. Selection of materials Three samples (MK1, MK2 and MK3) of commercial kaolinic materials were collected from the Donnigazza Mine, one of the most important in Sardinia. The lime used for the preparation of pastes was an analytical grade Ca(OH)2. A high grade kaolin (KP) was supplied by Carlo Erba Reagenti; it was used as a reference sample.
Fig. 1. Geological sketch map of the Donnigazza mine area (after Ligas et al., 1997, modified).
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S. Cara et al. / Applied Clay Science 33 (2006) 66–72
Table 1 Chemical composition (wt.%) of the Sardinian kaolinic clays (MK) and of the reference kaolin (KP)
MK1 MK2 MK3 KP
SiO2
Al2O3
Fe2O3
TiO2
MgO
CaO
Na2O
K2O
MnO2
P2O5
H2O
68.50 77.12 79.18 48.47
18.94 16.48 13.39 35.97
0.90 0.40 0.17 0.91
0.45 0.26 0.30 0.10
0.30 0.06 0.07 0.16
0.20 0.12 0.15 0.05
0.15 0.16 0.14 0.21
0.14 0.05 0.06 1.30
0.01 0.01 0.01 0.02
0.13 0.05 0.12 0.29
7.92 5.80 6.42 12.56
2.2. Experimental techniques Chemical analyses were performed by X-ray fluorescence (XRF) using a Philips PW1400 spectrometer operating with a Rh Tube at 30 kV and 60 mA. The fluorescence data were processed according to the Franzini et al. (1972) procedure. A Rigaku-Geigerflex D-max diffractometer with a CuKα radiation was used for identification of the mineralogical phases by comparison with the Powder Diffraction File (JCPDS, 1985). Chemical analyses were combined with XRD data to obtain a normative mineralogical composition of the materials. Granulometric analyses were performed with the Micromeritics SediGraph 5100 Particle Size Analysis System. The kaolinic clays and the reference kaolin were thermally activated at three different temperatures that arose from the thermal behaviour proposed by Smykatz-Kloss (1974). The thermal treatment was carried out in a laboratory gradient oven. The materials were heated at a constant rate (7 °C/min) from room temperature up to 400 °C, then at 2 °C/min up to the different calcination temperatures, where they remained for 100 min. Two different methods were applied in order to predict the behaviour (reactivity) of the materials in the presence of Ca(OH)2. Data from the first test, carried out according to the classical UNI-EN 196/2 procedure, refer to the quantity of insoluble residue after acid-base attack. The second evaluation test was carried out according to the spectrophotometric method proposed by Surana and Joshi (1988, 1990). Our samples were dissolved in a NaOH 0.5 N solution, heated in a microwave oven at fixed operating conditions of power (500 W) and time (6 min). Thermogravimetric analysis (TGA), differential thermal analysis (DTA) and X-ray diffraction analysis (XRD) were carried out to observe the reactions that took place in the metakaolin–lime system. TGA and DTA were performed with a Stanton STA780 simultaneous analyser. The samples were heated from room temperature up to 1100 °C at a rate of 15 °C/ min, under static atmospheric conditions: α-Al2O3 was used as reference material.
The pastes so obtained were cured for 1, 3, 7, 14, 28, 90 and 180 days in sealed Coulter containers to avoid carbonation. After the different curing times, hydration was stopped through water removal from the samples after grinding and dispersing the powder in pure ethanol. The samples were filtrated then dried at 80 °C. The rate of pozzolanic reaction progress was evaluated by monitoring by XRD and DTA the residual lime content in the different pastes.
3. Results and discussion Table 1 gives the chemical composition of raw kaolinic clays (MK) and reference kaolin (KP) used for the preparation of pastes. The XRD analyses of MK samples show a mineralogical composition characterized by the constant presence of kaolinite, tridymite and cristobalite: quartz was detected only in the MK1 sample. The mineralogical composition of the reference KP kaolin is given by kaolinite and illite. A normative mineralogical calculation of the studied kaolinic clays was also performed (Table 2), by combining the bulk chemical analyses of the samples with XRD data and by assuming the whole H2O content to be related to the kaolinite phase (the only hydrate phase in our samples). On the other hand, the presence of illite in the KP reference sample did not permit correct determination of the kaolinite content from normative calculation; however, the chemical data (Table 1) allows us to estimate a kaolinite content higher than in the MK samples, but lower than 88.16 wt.% (calculated value by assuming all the H2O in the KP to be related to the kaolinite phase). Table 2 Mineralogical normative composition of Sardinian kaolinic clays samples (wt.%)
2.3. Metakaolin–lime systems preparation Pastes of the different materials (activated and nonactivated kaolinic clays) with lime in a 1 : 1 ratio by weight with a water / solid ratio 1 : 2 were prepared.
Kaolinite Silica a a
MK1
MK2
MK3
46 44
38 60
31 67
The term Silica corresponds to the sum of the various silica phases (quartz, tridymite, cristobalite and opal).
S. Cara et al. / Applied Clay Science 33 (2006) 66–72
The granulometric distribution of the three kaolinic clay samples were compared with the curve of the KP reference sample (Fig. 2); the reference KP kaolin shows lower particle sizes and consequently a higher surface area than the Sardinian kaolinic clays. The activation temperatures selected according to DTA and XRD data analyses were: 530 °C the maximum temperature for kaolinite dehydroxylation and formation of metakaolinite; 630 °C the end of the kaolinite dehydroxylation process and finally 800 °C, a temperature which avoided mullite formation. As shown in Fig. 3, the kaolinite XRD peaks disappear at temperature higher than 530 °C while the peaks of various silica phases appear unaltered.
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Fig. 3. XRD (CuKα radiation) patterns of each samples (from top to bottom) to raw and heated material at 530, 630, 800 °C. I = illite; K = kaolinite; Q = quartz; Si = silica metastable phases.
3.1. Pozzolanic activity Tables 3 and 4 show the results of the two pozzolanic tests performed. The pozzolanic activity tests point out the reactivity of the materials when the solubility of the sample (as silica soluble phases) is assumed as pozzolanic index. The untreated Sardinian clays show higher values of silica soluble phases than pure kaolin. This behaviour could be related to the presence of metastable silica phases, particularly tridymite, and of other poorly crystalline reactive silica phases. Broad XRD peaks of poorly crystallized cristobalite or βcristobalite (White, 1965; Fournier, 1973) centered at 4,1 and 2,5 Å were observed in our samples. It is wellknown that amorphous silica is commonly found in hydrothermal ore deposits, and it is attributed to relatively quick changes of the physical and chemical composition of the hydrothermal solution (Fournier, 1985). The crystallinity of kaolinite phase appears to play a secondary role in the reactivity of samples. In fact the
crystallinity, evaluated by measuring the width at half the maximum height of the diffraction peak (FHWM) at d = 3.58 Å (Hinckley, 1963; Brindley and Brown, 1984) after the background removal, is quite similar for the kaolinic clays in Table 5. 3.2. Kinetics of metakaolin–lime reaction DTA and XRD data enable the evolution of the pozzolanic reaction to be monitored by the decrease of the Ca(OH)2 endothermic peak and the appearance of new hydrated phases in the metakaolin/lime system. The pure kaolin KP, not thermally activated, does not exhibit lime reactivity (Fig. 4): the endothermic lime peak keeps the same shape and amplitude and no new phases thermally active are formed. On the contrary, the unheated sample MK1 shows reactivity, with a progressive reduction of the Ca(OH)2 endothermic peak during curing (Fig. 5). Similar behaviour was obtained for the other kaolinic Sardinian clays. As regards the thermally activated samples, the kinetics of metakaolin–lime systems below depicted (Figs. 6 and 7) is referred to the 630 °C for KP and the only MK1. Figs. 6 and 7 show the rate of lime consumption. A progressive reduction of the Ca(OH)2 endothermic peak and the formation, after 7 days of curing, of new
Table 3 Fraction (wt.%) of insoluble residual after acid-basic treatment (UNI 196-2)
Fig. 2. Cumulative granulometric curves of the materials: 1) KP; 2) MK1; 3) MK2; 4) MK3.
Sample
NC
530 °C
630 °C
800 °C
KP K1 MK2 MK3
52 39 27 11
14 3 3 3
13 2 2 2
12 2 2 2
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Table 4 Fraction (wt.%) of soluble phases as determined by spectrophotometric method (Surana and Joshi, 1988) Sample
NC
530 °C
630 °C
800 °C
KP MK1 MK2 MK3
4.1 34.1 55.0 67.0
24.8 49.3 77.3 71.8
35.8 59.0 84.8 72.5
37.7 62.3 89.5 76.0
hydrated phases can be observed. The pozzolanic reaction can be considered to complete after 28 days in case of the KP–lime system. The kinetics of MK1– lime systems appears slower: the whole of the lime was only consumed after 90 days. It can be also observed that, apart from kaolinite content, Table 2, the difference in the pozzolanic activity is very slight between the MK samples. The characterisation by XRD of the hydrated phases was fairly difficult because the occurrence of some overlapping peaks. Moreover, recognition of C–S–H by XRD was very difficult due to low crystallinity. In XRD patterns of KP–lime and MK1–lime systems (calcined at 630 °C) the pozzolanic reaction evolution can be clearly observed through the progressive reduction of portlandite peaks. It was also observed that illite, identified in the KP sample, participates to some extent in the pozzolanic reaction: its diffraction peaks decreased as the reaction progressed. As demonstrated by other authors, illite remains a generally poor pozzolan, even though its activity considerably increases at calcination temperature over 900 °C (He et al., 1995). The hydration products were essentially C2ASH8 (strätlingite), tetracalcium aluminate hydrate, C4AH13, and C–S–H gel, according to the reaction schemes proposed by Murat (1983), for metakaolin–lime systems cured at T = 20 °C, on the basis of a complete hydration without carbonation or water evaporation. The C4AH13 phase seems more stable with short curing times than longer ones. A similar behaviour has been observed for the other Sardinian metakaolin–lime systems.
Fig. 4. DTA curves of the raw KP kaolin–lime system at different curing times (in days): P = portlandite.
DTA curves and XRD patterns show similar trends for all MK systems: the lime peaks progressively decreased and at 180 days disappeared in every case. The amounts of residual lime of different systems at different curing times (determined through TGA measurements) are reported in Table 6. It can be pointed out that: • the untreated Sardinian kaolinic clays, unlike pure kaolin, show a certain lime reactivity, increasing as the reactive silica content increases; • the activated reference kaolin exhibits a strong lime reactivity when the activation temperature is high enough to completely transform the kaolin to metakaolin;
Table 5 FHWM index of the reference KP sample and the untreated Sardinian kaolinic clays Sample
FHWM a
KP MK1 MK2 MK3
0.205 0.219 0.204 0.177
a
Measurements referred to the (002) reflection (d = 3.58 Å).
Fig. 5. DTA curves of raw MK1 sample–lime system at different curing times (in days): P = portlandite.
S. Cara et al. / Applied Clay Science 33 (2006) 66–72
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Table 6 Residual lime (wt.%) a in the pastes of untreated and treated specimens as a function of the ageing time (in days)
Fig. 6. DTA curves of KP kaolin (calcined at 630 °C)–lime system at different curing times (in days): P = portlandite; ST = strätlingite C2ASH8; CAH = tetracalcium aluminate hydrate C4AH13.
Sample
1
3
7
14
28
90
180
KP-NC KP-530 KP-630 KP-800 MK1-NC MK1-530 MK1-630 MK1-800 MK2-NC MK2-530 MK2-630 MK2-800 MK3-NC MK3-530 MK3-630 MK3-800
95.90 86.10 82.50 86.10 97.20 71.90 79.50 83.00 89.90 73.00 75.60 82.10 86.00 85.10 72.80 91.10
91.20 74.10 71.60 77.30 89.10 63.00 55.90 69.60 83.90 56.70 70.70 64.00 83.30 75.20 58.60 65.50
90.40 42.00 41.80 49.60 71.10 45.70 44.40 45.80 57.90 49.60 46.50 57.00 51.50 50.60 48.40 56.50
87.50 29.10 12.60 14.80 58.40 41.00 27.70 28.50 54.40 29.80 28.80 27.00 43.90 42.20 38.00 49.00
78.90 28.9 0.00 0.00 56.30 27.10 15.70 16.80 44.00 19.70 22.00 17.70 39.80 29.40 15.30 32.90
79.00 27.00 0.00 0.00 43.20 12.20 16.48 11.30 35.50 14.10 14.40 17.00 16.40 14.80 15.10 17.30
73.00 0.00 0.00 0.00 14.70 0.00 0.00 0.00 10.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00
a
• the activated Sardinian kaolinic clays, even with a slow hydration kinetics, are able to consume all the lime present in the system especially after long curing times; • calcination at 630 and 800 °C gives similar results. 4. Final remarks The assessment of the pozzolanic activity of the Donnigazza kaolinic clays encourages possible nontraditional applications for these materials. Pozzolanic tests carried out by different chemical attacks provided good results for untreated kaolin too. The silica
Fig. 7. DTA curves of MK1 sample (calcined at 630 °C)–lime system at different curing times (in days): P = portlandite.
Values calculated through TGA measurements.
metastable phases, mostly tridymite and to a lesser extent cristobalite and opal, can be considered to play an important role in the hydration process of the untreated samples. However, the presence of these metastable phases strongly affects the results of the classic pozzolanic tests based on the acid-base attack of the materials (UNI EN 196/2). These tests can produce misleading data about the pozzolanic activity evaluation. It is in our opinion that the only investigations that can shed light upon the pozzolanic properties are those based on the determination of the effective Ca(OH)2 consumption; that is the essential measure of the pozzolanic reaction. The proposed pozzolanic tests are not completely effective for determination of the pozzolanic activity of metakaolin–lime–water systems. Further laboratory tests are in progress in order to demonstrate an improvement of the mechanical strength and durability of mortars and concrete formulated with these materials. The thermal treatment of kaolinic samples at 630 °C is considered to be a good compromise between activation and economics: the reactivity of samples calcined at 630 °C and 800 °C gives similar results at long curing times. The use of these materials in the binders sector appears to be encouraging by taking into account the large availability of the Donnigazza kaolinic clay reserves evaluated, by recent direct investigations (test holes and trenches) within the actual exploitation area, of about 1.200.000 t.
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These materials could find an appropriate role particularly in the Cultural Heritage sector as pozzolanic additives for restoration mortars. Acknowledgments Authors are grateful to Mr. G. De Giorgi, owner of the Donnigazza mining title, for supplying the commercial materials tested in this work. References Brindley, G., Brown, G., 1984. Crystal Structures of Clay Minerals and their X-ray Identification. The Mineralogical Society, London. Fournier, R.O., 1973. Silica in thermal waters: laboratory and field investigations. Proceed. of Intern. Symp. on Hydrogeochemistry and Biogeochemistry, vol. 1. J.W. Clark, Washington, D.C., pp. 122–139. Fournier, R.O., 1985. The behaviour of silica in hydrothermal solutions. In: Berger, B.R., Bethke, P.M. (Eds.), Reviews in Economic GeologyGeology and Geochemistry of Epithermal Systems. Soc. Econ. Geol., Socorro, NM 87801, vol. 2, pp. 45–61. Franzini, M., Leoni, L., Saitta, M., 1972. A simple method to evaluate the matrix effects in X-ray fluorescence analysis. X-ray Spectrom. 1, 151–154. Frias, M., Sànchez de Rojas, M.I., Cabrera, J., 1993. The effect that the pozzolanic reaction of metakaolin has on the heat evolution in MKcement mortars. Cem. Concr. Res. 23, 627–639.
He, C., Osbaeck, B., Makovicky, E., 1995. Pozzolanic reactions of six principal clay minerals: activation, reactivity assessments and technological effects. Cem. Concr. Res. 25, 1691–1702. Hinckley, D.N., 1963. Variability in “crystallinity values” among the kaolin deposits of the coastal plain of Georgia and South Carolina. Clays Clay Miner. 2, 229–235. JCPDS FILE, 1985. Inorganic Phases. JCPDS International Centre for Diffraction Data, Swarthmore PA, USA. Ligas, P., Uras, I., Dondi, M., Marsigli, M., 1997. Kaolinitic materials from Romana (north-west Sardinia, Italy) and their ceramic properties. Appl. Clay Sci. 12, 145–163. Murat, M., 1983. Hydration reaction and hardening of calcined clays and related minerals: I. Preliminary investigation on metakaolinite. Cem. Concr. Res. 13, 259–266. Smykatz-Kloss, W., 1974. Differential thermal analysis — application and results in mineralogy. Springer-Verlag, Berlin, Heidelberg, New York. Surana, M.S., Joshi, S.N., 1988. Spectrophotometric method for estimating the reactivity of pozzolanic materials. Adv. Cem. Res. 1, 238–242. Surana, M.S., Joshi, S.N., 1990. Estimating activity of pozzolanic materials by spectrophotometric method. Adv. Cem. Res. 10, 81–83. UNI EN 196 Parte II, 1991. Metodi di Prova dei Cementi — Analisi Chimica dei Cementi. White, D.E., 1965. Saline waters of sedimentary rock. Am. Assoc. Pet. Geol., Mem. 4, 342–366 The assessment of the pozzolanic activity of the Donnigazza kaolinic clays encourages possible nontraditional.