30) clay

CEMCON-05016; No of Pages 11 Cement and Concrete Research xxx (2015) xxx–xxx

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Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay Nishant Garg, Jørgen Skibsted ⁎ Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, DK-8000 Aarhus C, Denmark

a r t i c l e

i n f o

Article history: Received 29 May 2015 Accepted 25 August 2015 Available online xxxx Editor: Keith Baldie Keywords: Thermal treatment (A) Blended cement (D) Amorphous material (B) Spectroscopy (B) Quartz

a b s t r a c t Calcined clays can be potential supplementary cementitious materials if effects of heat-treatment on their structure and reactivity are understood. This work reports structural characterization of an interstratified illite/smectite clay, including a quartz impurity, upon heating using 27Al and 29Si MAS NMR spectroscopy and ICP-OES analysis. During dehydroxylation (600–900 °C) the Q3-type SiO4 sites become disordered and octahedral AlO6 sites transform into tetrahedral sites, resulting in an amorphous material with substantial pozzolanic properties, as demonstrated by reactivity tests and hydration studies of a Portland cement–calcined clay blend. At higher temperatures (above 950 °C), inert Q4-type phases crystallize which radically reduce the reactivity. At optimum calcination temperature (900 °C), the amorphous material contains highly dissolvable elemental species as seen from complementary ICP-OES analysis. The quartz impurity exhibits a unique variation in 29Si spin–lattice relaxation times upon heat-treatment which is ascribed to changes in the concentration of impurity ions in quartz. © 2015 Elsevier Ltd. All rights reserved.

1. Introduction Presently, there is an increasing global interest in promoting supplementary cementitious materials (SCMs) [1,2] in cement blends to substitute a substantial part of the traditional ordinary Portland cement (OPC) [3]. This interest is driven primarily by two obvious factors, firstly, the need to control the overall OPC production which contributes roughly with 5–7% of the anthropogenic CO2 emissions [4] and secondly, the environmental as well as financial incentive in employing waste products from other industries, such as slags, fly ashes, and silica fume. To minimize transportation costs, contractors and builders are often forced to utilize whatever by-product is available in their vicinity. The performance and supply of such an SCM may not necessarily be adequate and thus, there is a growing demand for new SCMs that are both reactive as well as readily available. Calcined clays may be an ideal and attractive candidate since clays are highly abundant in young sedimentary deposits in the Earth's crust and they are a rich source of alumina and silica which can drive the pozzolanic reaction in a blended cement [5]. Clay minerals or layered hydrous phyllosilicates are essentially made of repeated, twodimensional tetrahedral (T-) and octahedral (O-) sheets. A variety of cations (Si4 +, Al3 +, Fe3 +, Mg2 + etc.) populate the T- and O-sheets which share a common plane of oxygen atoms. Clay minerals are often classified based on the alternate arrangement of these T- and

⁎ Corresponding author. Tel.: +45 8715 5946; fax: +45 8619 6199. E-mail address: [email protected] (J. Skibsted).

O-sheets. Most naturally occurring clays can thus be classified as 1:1 (TO), 2:1 (TOT), and mixed-layer (interstratified) minerals [6]. Upon heating, these rather inert, semi-crystalline clays can transform into reactive, amorphous SCMs. The calcined clay known as ‘metakaolin’, which is derived from the 1:1 clay kaolinite (Si2Al2O5(OH)4), has been widely studied both in terms of its dehydroxylated structure as well as its pozzolanic reactivity [7,8]. However, 2:1 clays and other mixed-layer variations of these minerals have received lesser attention — probably because of their intrinsically lower reactivity and subsequently lower performance as SCMs in cement blends [9,10]. Moreover, while the physical and chemical properties of metakaolin in mortars and concrete have been the subject for a large number of studies [11,12] (and references therein), the potential of interstratified 2:1 clays has not been fully explored. Several studies have examined illite [13], smectite [14–16], and a set of clays including mixtures of these 2:1 clays [9,10,17–20] but only a handful of studies have focused specifically on ‘interstratified’ illite/smectite clays [21–23]. For calcined clays, it is a principal challenge to understand the atomic-scale changes in the structure of the clay undergoing thermal treatment. In addition, information on which structural phases correspond to maximum reactivity/dissolution and at what point the amorphous clay is rendered unreactive are questions of primary importance. Recently, a structurally pure montmorillonite has been characterized and its pozzolanic reactivity as a function of its thermal decomposition sequence has been reported [16]. The maximum reactivity for this montmorillonite corresponded to an amorphous structure that is completely dehydroxylated and free from condensed

http://dx.doi.org/10.1016/j.cemconres.2015.08.006 0008-8846/© 2015 Elsevier Ltd. All rights reserved.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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Q4 phases which will crystallize out at high heating temperatures. It is of interest to determine if similar mechanisms govern the behavior of interstratified clays. This work reports an examination of an interstratified illite/smectite clay (ISCz-1) where layers of illite (non-swelling clay) alternate with that of a smectite (swelling clay). The primary difference between illite and smectite is that the former has non-hydrated, interlayer cations in well-defined positions (primarily K+), which results in isomorphous substitution of silicon by aluminum in the tetrahedral sheet to maintain the charge balance. ISCz-1 is an ordered, illite-rich clay (~70% illite, 30% smectite), which has been chosen for the present investigations since it may serve as a model system for illite-dominant interstratified, mixedlayer 2:1 clays found in nature. Changes in short-range order of the clay undergoing thermal treatment are followed by solid-state magic-angle spinning (MAS) NMR spectroscopy, which has the ability to detect crystalline and amorphous components in an equal manner. The pozzolanic reactivities of the calcined clays are gauged by 27Al and 29Si MAS NMR, inductively coupled plasma-optical emission spectroscopy (ICP-OES), and thermogravimetric analysis (TGA), and relations between short-range structure and reactivity are investigated. Finally, the performance of the clay-based SCM is investigated in a blended cement system, where a considerable change in the composition and structure of the C–S–H phase is induced upon addition of such an SCM. 2. Materials and methods 2.1. Clay mineral heat-treatment A sample of illite/smectite clay from the former Czechoslovakia, known as ISCz-1, was obtained from the Source Clays Repository managed by the Clay Minerals Society (CMS), Purdue University (Indiana, USA). This standard clay is an illite/smectite (70/30) interstratified type of clay according to the baseline reports published by CMS [24]. The clay has the following oxide composition, 51.6 wt.% SiO2, 25.6 wt.% Al2O3, 1.11 wt.% Fe2O3, 0.039 wt.% TiO2, 2.46 wt.% MgO, 0.67 wt.% CaO, 0.32 wt.% Na2O, 5.36 wt.% K2O, 0.04 wt.% P2O5, and 10.2 wt.% H2O [25], which gives the estimated structural formula: (Mg0.03Ca0.1Na0.09K0.95)[Al3.39Mg0.48Fe(III)0.12][Si7.19Al0.81]O20(OH)4. In addition to the clay, the samples contain an impurity of quartz, which constitutes 43 wt.% of the sample according to a 29Si MAS NMR spectrum acquired with full relaxation. An attempt to remove quartz by discarding the clay residue on a 40 μm sieve was unsuccessful (no intensity reduction for the quartz peak in the 29Si NMR spectrum), suggesting that quartz exists in the ISCz-1 clay as a fine-grained, phase impurity. For heat-treatment, 1.00 g of finely ground clay was spread as a thin layer in a ceramic container and transferred into the oven preheated to the desired temperature (ranging from 200 to 1100 °C, with an interval of 50–100 °C). This broad temperature range was chosen in order to encompass the entire solid-state thermal transformation sequence that occurs prior to melting. After 2 h, the clay was rapidly cooled to room temperature by storing the ceramic container on a brass plate in a desiccator for half an hour, and subsequently the heated clay was transferred into a sealed glass container. 2.2. Reactivity test The reactivity test employed here is a modification of the European Standard EN 196-5, the principal difference being that the degree of reaction is quantified by the amounts of anhydrous silicate phases and silicate hydration products from 29Si MAS NMR rather than by a determination of the Ca2+ ion concentration in the solution or the residual amount of Ca(OH)2 in the solid residue. The size fraction of 20 to 40 μm of the heated clay was obtained by sieving and 0.10 g of this fraction was mixed with an excess of calcium hydroxide (0.30 g) in 50 mL ultra-pure water (Milli-Q® water by Millipore). The mixture in

a conical flask was placed in an oil bath at 40 °C and stirred with a magnetic stirrer (200 rpm). After seven days, a solid residue was obtained after filtering which was subsequently dried in a desiccator over silica gel under slightly reduced pressure for one day. 29Si MAS NMR spectra of the solid residues were deconvolved to quantify the reactivity of the calcined clay and to obtain information about the formed hydration products.

2.3. Calcined clay–Portland cement blend A white Portland cement (wPc) from Aalborg Portland A/S, (Aalborg, Denmark) was used to cast a blended cement paste with 30 wt.% replacement of wPc by calcined ISCz-1 (900 °C). The wPc has the following bulk-oxide content: 67.3 wt.% CaO, 22.3 wt.% SiO2, 3.60 wt.% Al2O3, 0.21 wt.% Fe2O3, 1.40 wt.% MgO, 3.00 wt.% SO3, 0.52 wt.% K2O, 0.01 wt.% Na2O, 0.025 wt.% P2O5, a loss of ignition of 1.43 wt.%, a gypsum content of 3.66 wt.%, and a limestone content of 0.56 wt.%. A water/powder ratio of 0.50 and ultra-pure water was used to make the paste, using a motorized stirrer equipped with a custom-made paddle for mixing for 5 min. 50 mL polypropylene vials were used to cast the paste which were demoulded after 24 h and finally sealed in plastic containers (75 mL) filled with ultra-pure water. The hardened pastes were stored at 20.0 ± 0.1 °C in a climate chamber, and a small fraction of the paste sample (~ 1.5 g) was extracted at appropriate hydration times (1, 3, 7, 28, and 105 days). The hydration was stopped by grinding the sample, followed by stirring the sample in 10 mL isopropyl alcohol (min. 99%) for 1 h. The solid residue was filtered and subsequently dried in a desiccator over silica gel under slightly reduced pressure for one day. Afterwards, to avoid CO2 contamination, the sample was transferred to an airtight container and stored in a desiccator at slightly reduced pressure.

2.4. NMR measurements Single-pulse and saturation-recovery (SR) 29Si MAS NMR spectra for the clay calcined at different temperatures were recorded on a Varian Unity-plus 200 (4.7 T) spectrometer, using a homebuilt CP/MAS NMR probe for 7 mm o.d. zirconia (PSZ) rotors and a spinning speed of vR = 7.0 kHz. The single-pulse spectra employed a pulse width of 3.0 μs (42° pulse) for an rf field strength of γB1/2π = 38 kHz, a relaxation delay of 10 s, and typically 8192 scans. The same 29Si rf field strength was used in the SR experiments for the saturation-train and 90° pulses. Spectra of the calcium hydroxide and calcined clay mixtures (reactivity experiments) as well as the Portland cement blends were acquired on a Varian INOVA-400 spectrometer (9.39 T), employing a homebuilt CP/MAS NMR probe for 7 mm o.d. zirconia (PSZ) rotors, vR = 6.0 kHz, a pulse width of 3.0 μs (45° pulse) for an rf field strength of γB1/2π = 42 kHz, relaxation delays of 15–30 s, and typically 5600 scans. The 29Si chemical shifts were referenced to neat tetramethylsilane (TMS), using a natural sample of β-Ca2SiO3 (δiso = −71.33 ppm) [26] as a secondary reference. The Varian VnmrJ software was used for the deconvolutions of the 29Si MAS NMR spectra, following the method for analysis of cement blends developed in our laboratory and summarized recently in studies of limestone–Portland cements [27] and Portland cement–metakaolin blends [28]. The 27Al MAS NMR spectra were recorded on a Varian Direct-Drive VNMR-600 (14.09 T) spectrometer using a home-built CP/MAS probe for 4 mm o.d. zirconia (PSZ) rotors. A spinning speed of vR = 13.0 kHz, singlepulse excitation with a pulse width of 0.5 μs (~ 11° pulse) for an rf field strength of γB1/2π = 61 kHz, a relaxation delay of 2.0 s, and typically 4096 scans were used. A 1.0 M aqueous solution of AlCl3·6H2O was used as the 27Al chemical shift reference. For comparison, the peak intensities in the final spectra have in all figures been normalized to the actual sample mass packed in the NMR rotors.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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2.5. ICP-OES and TGA experiments The measurements were performed on a Spectro Arcos instrument (Model: ARCOS FHS12, Power: 5000 VA). The filtrates were obtained by filtering the reaction mixtures from the reactivity tests after seven days of curing. Then they were passed through a 0.2 μm syringe filter, diluted and acidified with 1% HNO3 for solution elemental analysis by ICP-OES. The inbuilt semi-quant method was used for the measurements and each sample was analyzed thrice. Thermogravimetric analysis was performed by a PerkinElmer STA 6000 simultaneous thermal analyzer. The sample (~23 mg) was investigated in the temperature range 35–1100 °C at a heating rate of 10 °C per minute and in an argon flow of 65 mL per minute. 3. Results and discussion 3.1. Thermal transformation of illite/smectite (ISCz-1) The 29Si MAS NMR spectrum of the as-received ISCz-1 clay (Fig. 1, 25 °C) contains a broad component between − 80 to − 100 ppm and a sharp peak at −107.8 ppm, the latter originating from the impurity of quartz. The broad component consists of at least two overlapping resonances, originating from the silicate sheets in smectite and illite [29]. The low-frequency part, centered around − 93 ppm, is ascribed to Q3(0Al)-type SiO4 tetrahedra in the smectite sheets, having three Si–O–Si bonds in the silicate sheet and one Si–O–M2 bond to two M

Fig. 1. 29Si MAS NMR spectra (4.7 T, νR = 7.0 kHz) of illite/smectite (ISCz-1) calcined for 2 h at the listed temperatures in the range 200–1100 °C (25 °C corresponds to the untreated sample).

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sites in the octahedral sheet through a tri-coordinated oxygen (M = Al3+, Fe3+, Mg2+) [30,31]. The high-frequency part around −88 ppm corresponds to a Q3(1Al)-type SiO4 environment from the illite sheets where the silicon atoms undergo isomorphic substitution by aluminum atoms, and the excess negative charge thus produced is balanced by the potassium ions in the interlayer [32,33]. Upon heating at 200 °C, the overall center of gravity (Fig. 2) of the broad component in the 29Si NMR spectra shifts from − 93.4 to − 94.5 ppm. This low-frequency shift of ~1 ppm has been reported earlier for montmorillonites [16,34] and it reflects the removal of interlayer water between the smectite sheets. The center of gravity of the 29Si Q3 peaks is almost constant for the samples heated between 200 and 500 °C, although some additional broadening is observed. This broadening most likely reflects that the overall structure becomes more rigid at higher calcination temperatures. Upon heating to 600 °C, a radical change in the chemical shift from − 94.7 (500 °C) to − 98.1 (600 °C) shows that an amorphous alumino-silicate structure begins to form, corresponding to the onset of the dehydroxylation process. This partial breakdown of the structure is in agreement with thermo-gravimetric analysis data (Fig. 3) which shows a mass loss for the clay in the range 600–700 °C as a result of the removal of hydroxyl groups. The thermogravimetric analysis also shows that a significant part of interlayer water molecules are gradually released from the structure in the temperature range from 100 °C to approx. 500 °C. Moreover, no further mass reduction is observed above 800 °C, reflecting that all interlayer water and all hydroxyl groups have been removed from the structure. The mass loss of 6.6 wt.% agrees well with the structural formula for the illite/smectite (c.f., Section 2.1), when the impurity of quartz in the sample is taken into account. The wide temperature range of roughly 500–800 °C for the dehydroxylation, as indicated by the 29Si MAS NMR data (Fig. 2), suggests the existence of OH groups in various local environments, owing to the interstratification of two 2:1 minerals. The 29Si chemical shift of the resonance from the disordered silica shifts steadily towards lower frequency and undergoes further broadening when the clay is calcined between 600 and 900 °C, suggesting that the majority of the clay structure has undergone amorphization at 900 °C. The second major transformation in the structure of the heated clay occurs at 950 °C, which is ascribed to recrystallization of the dominant Q3-type of amorphous silica into a condensed Q4-type silica with four Si–O–Si bonds in a segregated, framework silica network. This transformation is reflected by the low-frequency shift of the 29Si center of gravity from −100.7 ppm (900 °C) to −105.4 ppm (950 °C). For heat treatment above 1000 °C, the Q4-type silica is dominant and the structure is stable up to at least 1100 °C, the maximum calcination

Fig. 2. 29Si chemical shifts (centers of gravity, δcg(29Si)) plotted as a function of the calcination temperature for illite/smectite (ISCz-1). The data are obtained from the 29Si MAS NMR spectra in Fig. 1 and the error limits for the individual δcg(29Si) values are approximately ±0.20 ppm.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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Fig. 3. Thermogravimetric mass-loss curves for the illite/smectite (ISCz-1) clay. A similar measurement covering a temperature range up to 1100 °C shows no further mass loss from 900 °C to 1100 °C.

temperature used in this work. This thermal transformation sequence involving dehydration, dehydroxylation, amorphization, and finally recrystallization has been reported for several 2:1 type clay minerals using tools like solid-state NMR [16,33–36], thermal analysis [37], a combination of EPR, XRD, SEM [38] and TEM [39]. The studied mixedlayer illite/smectite clay also follows such a transformation scheme, and thus it is expected that its pozzolanic behavior as function of calcination temperature will be similar to that recently reported for a pure montmorillonite [16]. Supplementary information on the thermal transformation sequence can be obtained from 27Al MAS NMR, as illustrated by the spectra of the smectite/illite samples heated at different temperatures in Fig. 4. The spectrum of the as-received clay contains centraltransition resonances for one Al site in octahedral coordination (δcg(27Al) = 4.1 ppm) and two Al sites in tetrahedral coordination (δcg(27Al) = 58.6 and 70.2 ppm). The 27Al chemical shift of the lowfrequency tetrahedral resonance (~58.6 ppm) is in accordance with an Al(–OSi)4 site, e.g., as found in zeolitic aluminosilicate structures. The resonance is saturated in the 27Al MAS spectrum acquired with a very short relaxation delay (0.01 s, Fig. 4), which indicates that it originates from a phase with a very low concentration of paramagnetic ions (e.g., Fe3+). Thus, this resonance is ascribed to a small fraction of Al3+ guest-ions incorporated in quartz, following an ongoing 27Al MAS NMR study of pure quartz samples, which reports a tetrahedral resonance at δcg(27Al) = 57.5 ppm (14.1 T) for Al3 + ions in quartz [40]. The high-frequency tetrahedral peak at ~70 ppm corresponds to the aluminum ions incorporated in the tetrahedral silicate sheets of illite by isomorphic substitution for silicon. Upon heating, the aluminum environments do not undergo any significant changes up to 500 °C, which is ascribed to the fact that the octahedral aluminate sheet is sandwiched between two tetrahedral silicate sheets and thereby less affected by the dehydration taking place in the interlayer in this temperature range. When the dehydroxylation onsets at ~ 600 °C and at the heating temperatures above, the aluminum environments transform from dominant octahedral to pentahedral (δcg(27Al) = 23 ppm) and tetrahedral (δcg(27Al) = 61 ppm) coordination. Subsequently, at higher temperatures (800–900 °C), the resonance from octahedral Al has completely disappeared and the spectra are dominated by tetrahedrally coordinated Al. For the sample heated at 900 °C, the 27Al MAS spectrum contains a broad, tetrahedral centerband resonance with a tail to low-frequency, which is indicative of a highly distorted aluminum environment/structure, characterized by a distribution of 27Al quadrupole coupling parameters and chemical shifts. Thus, this spectrum is similar to 27Al MAS NMR spectra reported for aluminosilicate glasses [41] and blast-furnace slags [42]. It is also noted that the 27Al MAS NMR spectra do not give indications of the presence of any

Fig. 4. 27Al MAS NMR spectra (14.09 T, vR = 13.0 kHz) showing the central-transition region for illite/smectite (ISCz-1) calcined for 2 h at the listed temperatures in the range of 25–1100 °C. A spectrum of the untreated sample (25 °C) was also acquired with a short relaxation delay of 0.01 s (lower spectrum) whereas the other spectra employed a relaxation delay of 2.0 s. The asterisks indicate spinning sidebands.

metastable intermediate phase during the dehydroxylation process of the illite/smectite, as opposed to thermal treatment of kaolinite which forms metakaolin as a metastable, dehydroxylated phase [43]. Thus, it is expected that the illite/smectite clay will have a narrow range of calcination temperatures where it is most reactive, as opposed to metakaolin which is usually reactive over a much wider temperature range. Based on the characterization of the transformations induced by heat-treatment so far, it can be seen that major structural events like dehydroxylation and recrystallization occur at ~ 600 °C and ~950 °C, respectively. Therefore, the pozzolanic reactivity of the heated clay is tested over a broad temperature range from 500 to 1000 °C in Section 3.3 to evaluate the effect of calcination on reactivity. 3.2. 29Si spin–lattice relaxation of the quartz impurity The 29Si MAS NMR spectra following the heat-treatment of the illite/smectite sample from ambient temperature up to 1100 °C show a gradual decrease in intensity for the quartz peak up to 600 °C and a subsequent continuous increase in intensity upon heat-treatment up to 1100 °C. This intensity variation, with a minimum around 600 °C, is ascribed to changes in the spin–lattice relaxation rate for the 29Si spins in quartz for the different heat-treated samples. This phenomenon is examined in more detail by 29Si saturation-recovery (SR) NMR experiments for selected samples studied in Fig. 1. The 29Si SR NMR spectra reveal that the resonances from the illite/smectite clay are fully relaxed for relaxation delays above 0.5 s for the samples at ambient

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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temperature up to roughly 800 °C and for delays above 5 s at higher calcination temperatures. Thus, full relaxation is achieved for the resonances originating from illite/smectite in all of the spectra shown Fig. 1, all acquired with a relaxation delay of 10 s. This fast relaxation is ascribed to the significant amount of Fe2+/Fe3+ ions in the clay structure, which are primarily present in the octahedral sheets before calcination, since the 29Si spin–lattice relaxation in such systems is dominated by dipole–dipole couplings between the 29Si nuclear spins and the electron spins of the unpaired electron in paramagnetic Fe centers [44]. In contrast, a strong saturation is observed for the quartz peak in all spectra, reflecting that the amount of paramagnetic impurities in this phase is very low. 29Si SR NMR spectra, employing 15–20 different recovery times in the range 0.5 s–8000 s, have been acquired for some of the heat-treated samples. When the spin–lattice relaxation is governed by dipole–dipole interactions with paramagnetic centers and spin-diffusion is absent, the spin–lattice relaxation behavior for the 29Si nuclei can be described by a stretched exponential relationship [45–47]. The quartz peak overlaps with the resonances from the clay calcined at high temperatures and thus, two components have been assumed in the analysis of the quartz-peak intensities from the 29Si SR NMR experiments, corresponding to the expression "

sffiffiffiffiffiffiffiffi!# t

Mz ðt Þ ¼ M0; c 1− exp −

0

T 1;c

" þ M 0; q 1− exp −

sffiffiffiffiffiffiffiffi!# t 0

T 1;q

: ð1Þ

Here, Mz(t) is the magnetization at the recovery time (t), M0, i is the equilibrium longitudinal magnetization, and T'1,i is the stretched exponential spin–lattice relaxation time for the contribution from the calcined clay (i = c) and the quartz phase (i = q). Least-squares fitting of this equation to the experimental quartz intensities, observed for selected samples, are illustrated in Fig. 5 and the resulting parameters are summarized in Table 1 along with data for the samples calcined at 700 °C and 1100 °C. For the untreated clay (25 °C) and the samples heated up to 700 °C, the overlap between the quartz peak and the clay component is minimal and the intensities can be analyzed by consideration of the quartz component of Eq. (1) only. The T'1,q values (Table 1) show clearly that the relaxation times increases for heat-treatment temperatures up to 600 °C and thereafter decreases when the temperatures are further increased. The initial increase in T'1,q values is ascribed to migration of Fe3+ ions out of the quartz structure, forming separate Fe2O3 particles, following earlier

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Table 1 Time constants for the 29Si spin–lattice relaxation (T'1,q) of the quartz component in selected calcined illite/smectite samples.a Temperature (°C)

M0,q

' T1,q (s)

25 500 600 700 850 1000 1100

6.88 ± 0.23 5.27 ± 0.23 3.14 ± 0.23 4.62 ± 0.43 4.99 ± 0.13 7.84 ± 0.13 4.62 ± 0.24

745 ± 100 1282 ± 199 1556 ± 481 1329 ± 431 870 ± 129 257 ± 27 147 ± 35

M0,c

0.74 ± 0.07 2.32 ± 0.13 2.95 ± 0.27

' T1,c (s)

0.45 ± 0.27 0.32 ± 0.12 0.92 ± 0.31

R2b 0.996 0.996 0.967 0.958 0.996 0.997 0.991

a The parameters M0,q, T'1,q, M0,c, and T'1,c describe a two-component stretched exponential relationship (c.f., Eq. (1)) and are determined from saturation-recovery 29Si MAS NMR spectra, acquired at 4.7 T using recovery times up to 8000 s. b Correlation coefficients from the least-squares fits of Eq. (1) to the experimental data.

studies of iron centers in amethyst crystals of quartz using optical absorption spectroscopy and electron spin resonance (ESR) [48,49]. The color of amethyst is largely determined by iron in tetrahedral coordination substituting for silicon. Heat-treatment of amethyst crystals at 225–480 °C for 1 h resulted in a decrease in optical absorption with increasing temperature (and a bleaching of the crystals) [48], which is ascribed to the migration of iron out of the crystal lattice, precipitating as a separate Fe2O3 phase [49]. The subsequent decrease in T'1,q values above 600 °C may arise from effects related to the structural displasive α–β phase transition that occurs at 573 °C for quartz and the release of iron from the illite/smectite clays at high temperatures. The 29 Si spin–lattice relaxation analysis of heated montmorillonite in our recent study [16] revealed that an iron oxide phase segregates at high calcination temperatures. Most of the iron in the actual untreated sample is present in the clay and Fe2O3 separated out from this phase during heating may become available for incorporation in quartz. The high-temperature β-form of quartz may more easily adopt guest-ions as compared to the α-form, and these ions will be fixed in the crystal lattice upon the rapid cooling of the samples after heat-treatment. Thus, an increased fraction of iron guest ions in quartz at heattreatment temperatures above 600 °C may account for the decrease in 29 Si spin–lattice relaxation times. The 29Si spin–lattice relaxation in quartz (amethyst crystal) has been studied by in-situ 29Si NMR at 9.4 T from room temperature up to 870 °C [50]. That study revealed a decrease in T1 values from roughly 3500 s to 1000 s in the temperature range from room temperature up to 550 °C for the α-form and nearly a constant value of T1 = 800 s for 600 to 870 °C for the β-form of quartz. This variation in T1 values cannot be compared directly with the present 29 Si SR NMR study at 4.7 T where all experiments are performed at ambient temperature for quenched samples of the heat-treated composite material. This is due to the fact that changes in the electron correlation times of the paramagnetic centers with temperature will have an important impact on the relaxation rates at elevated temperatures. 3.3. Pozzolanic reactivity of calcined illite/smectite

Fig. 5. Variations in the intensities of the quartz resonance at −107.8 ppm as a function of the recovery time in saturation-recovery 29Si MAS NMR spectra (4.7 T). The calcination temperatures of the analyzed clays are listed next to their stretched-exponential fits and the resulting relaxation data are summarized in Table 1. Data for the as-received clay is indicated by squares, clays calcined at 500 °C by circles, 600 °C by upward pointing triangles, 850 °C by downward pointing triangles and 1000 °C by diamonds.

The 29Si MAS NMR spectra of the dried, solid residues collected after seven days of reaction for the calcined clay and excess of calcium hydroxide contain resonances from three distinct components as seen in Fig. 6. One constituent is the unreacted calcined clay which appears as a broad component in the approximate chemical shift ranges −80 to −100 ppm (25–600 °C) and of −85 to −115 ppm (700–1000 °C). The spectra also contain the narrow peak from quartz (−107.8 ppm), and its changing intensity is ascribed to variations in 29Si spin–lattice relaxation times (Table 1) rather than chemical reactivity of quartz in the calcium hydroxide medium [40]. The third component is observed in the range, − 75 to −90 ppm, and reflects hydration products formed by the reaction of dissolved clay species and calcium hydroxide in water. This hydrated phase bears a clear resemblance to the 29Si resonances observed for the less-ordered C–S–H type phases in 29Si

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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Fig. 6. 29Si MAS NMR spectra (9.39 T, vR = 6.0 kHz) of the residue after the seven days reaction of calcium hydroxide and illite/smectite calcined at the listed temperatures.

MAS NMR spectra of hydrated Portland cements. For such phases, three types of resonances at approx. −79, −81, and −85 ppm are generally observed, corresponding to chain-end SiO4 sites (Q1, −79 ppm), chain SiO4 sites adjacent to an AlO4 tetrahedron (Q2(1Al), − 81 ppm), and pure SiO4 chain units with two Si–O–Si bonds (Q2, −85 ppm) [51–53]. The individual spectra in Fig. 6 have been deconvolved, using resonances/subspectra for three components. As a starting point, the 29 Si MAS NMR spectrum of the residue from the clay calcined at 900 °C was analyzed (Fig. 7) and the best fitting of the resonances from the C–S–H phase was established. The subsequent analyses of the other spectra employed fixed linewidths and frequencies for the set of resonances belonging to the C–S–H phase and only their intensities were kept unrestrained in the fitting procedure. Subspectra for the unreacted, calcined clays were initially established by fitting the spectra in Fig. 1 and using these in the fits of the residues from the reactivity tests. Some minor modifications of the linewidths and intensities of the peaks were allowed to achieve the best fits. Obviously, this step adds some uncertainty to the calculated values but it can hardly be avoided because of the overlap of resonances from the unreacted calcined clay and the formed C–S–H phase. Moreover, it is also likely that the calcined clay is undergoing incongruent dissolution, resulting from preferential dissolution of less condensed phases, and this may justify the minor modifications made in the subspectra for the calcined clays in the analysis of the residues. The results from deconvolution of the 29Si MAS NMR spectra in Fig. 6 are summarized in Table 2. Assuming that the consumption of silicate

Fig. 7. The (a) experimental and (b) simulated 29Si MAS NMR spectrum of the reactivity test residue of calcium hydroxide and illite/smectite calcined at 900 °C. The individual components of the simulated spectra are shown as follows: (c) the quartz peak, (d) and (e) the Q1, Q2 and Q2(1Al) sites of the C–S–H phase, and (f) the subspectrum for the unreacted calcined clay. Multiple resonances were used for the construction of the subspectrum shown in (f) and two overlapping resonances were used for each component of the C–S–H shown in (d).

Table 2 Relative intensities for the calcined clay (Iclay), quartz (Iquartz) and C–S–H (IC–S–H) components, obtained from deconvolutions of the 29Si MAS NMR spectra of the solid residues from the clay reactivity tests (Fig. 7).a Calcination temperature

Iclay (%)

Iquartz (%)

IC–S–H (%)

corr b IC−S−H (%)

C–S–H Al/Si ratioc

25 500 600 700 800 850 900 950 1000

77.1 81.1 78.5 69.2 52.0 48.8 34.0 55.4 53.0

19.0 12.0 7.2 9.2 11.9 16.0 23.0 19.2 27.7

4.0 ± 3.3 7.0 ± 1.8 14.3 ± 0.9 21.7 ± 4.4 36.1 ± 2.3 35.3 ± 4.4 43.0 ± 4.2 25.4 ± 0.3 19.3 ± 0.1

4.9 ± 4.0 8.0 ± 2.1 15.4 ± 0.9 23.8 ± 4.8 41.0 ± 2.6 42.0 ± 5.2 55.8 ± 5.4 31.4 ± 0.4 26.7 ± 0.1

0.09 0.07 0.09 0.08 0.10 0.13 0.12 0.10 0.07

a Iclay, Iquartz, and IC–S–H represent the output from a deconvolution. Error limits are only corr ) as this is the principal parameter of interest (c.f., Fig. 8). included for IC–S–H (and IC−S−H b Intensity for the C–S–H phase, corrected for the contribution from quartz to the corr spectral intensities, IC−S−H = IC–S–H / (1 − Iquartz). c Al/Si ratio for the C–S–H phase, calculated from the simulated intensities, I(Q1), I(Q2(1Al)), and I(Q2), constituting the C–S–H phase in the deconvolutions of the 29Si NMR spectra, Al/Si = ½I(Q2(1Al)) / [I(Q1) + I(Q2(1Al)) + I(Q2)]. Estimated error limits: ±0.02.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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species from the clay is proportional to the overall reaction, the degree of reaction for the calcined clay will be equal to the fraction of formed C–S–H phase, after correction for the contribution to the spectral intensities from quartz, which is considered as an inert phase in the present system. Quartz, which consists of silicon nuclei in a condensed Q4-type arrangement, is expected to act as an inert filler in cementitious systems. “Inert filler” refers to a material which is chemically inert but exhibits the filler effect in a hydrating cement system, i.e., it reduces the cement content and often provides additional sites for the nucleation of hydration products [54]. In a recent study of pozzolanic reactivity of calcined kaolinites, quartz was used as an inert filler in cement pastes to serve as a reference for the cement–clay blends [55]. It was found that the presence of quartz in cement paste generated a slightly higher amount of cumulative heat as compared to the pure cement paste but it was overall much lower than the heat generated by kaolinites calcined at different temperatures. Thus, quartz only serves as a filler in such a system and does not participate in the pozzolanic reaction. In addition, only the resonances from the calcined clay and the C–S–H phase are observed with full relaxation, hence only those are taken into consideration. Fig. 8 reveals that the reactivity of the clay increases with increasing calcination temperature but only to a certain extent (~ 900 °C) after which the reactivity begins to decline, suggesting the formation of inert phases in the calcined material. Highest pozzolanic reactivity, above 50%, is observed for the calcination temperature of 900 °C, whereas it is significantly lower at 950 °C. A similar decrease in reactivity with heat-treatment temperature was also observed for a pure montmorillonite, where it was attributed to the formation of condensed, inert Q4-type phases [16]. The present results for the calcined illite/smectite can be interpreted in a similar manner, since the center of gravity of the resonances in the 29Si MAS NMR spectra (Fig. 2) decreases from − 100.7 ppm (900 °C) to − 105.4 ppm (950 °C), which suggests a condensation of silicon tetrahedra, resulting in a structural crystallization of a dominant Q4-type phase. Moreover, the corresponding 27Al NMR spectra (Fig. 4) indicate that the clay calcined at 950 °C includes a more ordered phase, as seen by the dominance of Al in tetrahedral coordination and the absence of pentahedral and octahedral resonances. Thus, once an inert phase forms at a given calcination temperature, all clay calcined at that critical temperature (950 °C for the actual sample) and at temperatures above have reduced reactivity. The low resolution of the 29Si NMR resonances from the Q1, Q2(1Al) and Q2 sites of the C–S–H phase prevents a detailed analysis of the

Fig. 8. Percentage of reacted clay based on the deconvolution of the spectra in Fig. 5. Reacted clay (%) corresponds to the relative fraction of the clay transformed into a hydration product. Error bars for the two data points on the right are smaller than the symbol used.

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average alumino-silicate chain lengths and pure silicate chain lengths of the C–S–H phases in the different reactivity experiments since these measures are associated with large error limits. Although, this is also the situation for the calculated Al/Si ratios of the C–S–H phases, these data have been included in Table 2 for reference. Nevertheless, it is interesting to note that they indicate an increase in Al/Si ratio at the heat-treatment temperatures resulting in high reactivity. The highest incorporation of Al in the C–S–H phase for the heated clay with optimum reactivity was more clearly observed in our recent study of heat-treated montmorillonite [16], and it may reflect that high reactivity is associated with a structure of the heat-treated material from which Al3+ species are easily dissolved. 27 Al MAS NMR spectra have also been acquired for the reactivity test residues to gain information about the types of aluminate hydrate phases formed in the mixtures as shown in Fig. 9. All the spectra have a prominent resonance with δcg(27Al) = 9.3 ± 0.1 ppm which is assigned to a hydroxy-AFm calcium aluminate hydrate phase of the type Ca4[Al(OH)6]2(OH)2·xH2O (x = 4–12) [56], formed as a hydration product. The intensity of this peak increases with increasing heattreatment temperature until the highest reactivity is achieved at 900 °C after which it decreases significantly. This is in accordance with the interpretation of the 29Si NMR data, which suggest that high reactivity is associated with a heat-treated material from which Al3+ ions are easily dissolved. In addition, the small tetrahedral peak at 58 ppm, ascribed to Al3+ guest ion in quartz, is observed in all spectra, as expected. The spectrum of the clay calcined at 900 °C includes three additional

Fig. 9. 27Al MAS NMR spectra (14.09 T, vR = 13.0 kHz) showing the central-transition region for the residue after the seven days reaction of calcium hydroxide and illite/smectite calcined at the listed temperatures. The asterisks indicate spinning sidebands.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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distinct centerband resonances at ~ 73 ppm , ~ 35 ppm, and ~ 5 ppm which all can be associated with the C–S–H phase in the form of tetrahedral Al sites in the silicate chains, five-fold AlO5 units in the C–S–H interlayer, and octahedrally coordinated aluminum most likely present as a nanostructured aluminate phase formed on the surface of the C–S–H, respectively [57,58]. The 27Al MAS NMR spectra of the samples calcined at 950 and 1000 °C (Fig. 9) clearly reveal that only a small amount of the material has reacted, as seen by the small octahedral Al resonances from the hydroxy-AFm phase, and they bear a clear resemblance to the spectra of the unreacted, calcined materials (Fig. 4). Overall, the 27Al MAS NMR spectra support the results on reactivity from 29Si NMR and the relation between reactivity and heat-treatment temperature shown in Fig. 7 for the illite/smectite clay.

Fig. 10. Solution concentrations of (a) potassium (K), (b) aluminum (Al), and (c) silicon (Si) in the filtrates collected from the calcined clay and calcium hydroxide mixtures after seven days of reaction at a temperature of 40 °C as a function of the calcination temperature for the illite/smectite clay. The error bars represent the standard deviation based on three measurements and in most cases they are smaller than the used data symbols.

Further insight into the reactivity of the calcined clays can be gained by analysis of solution concentrations for key elements present in the filtrate from the reactivity test flasks, as shown in Fig. 9. The solution concentrations for both potassium (Fig. 10a) and aluminum (Fig. 10b) exhibit dependencies with the calcination temperatures which are in good agreement with the degree of reaction for the calcined clay derived for the solid residues shown in Fig. 8. The highest concentrations of aluminum and potassium in the solutions from the reactivity tests are observed for the clay calcined at 900 °C which indicates that the calcined clay is highly dissolvable at this calcination temperature. The significant decrease in the amount of dissolved aluminum for the clay calcined at 950 °C is in good agreement with a lower amount of calcium aluminate hydrate phase formed at this temperature (Fig. 9). A similar, remarkable decrease in the release of aluminate monomers from a commercial illite/smectite calcined at 950 °C has also been reported by other researchers [23] who suggested that for 950 °C and higher heat-treatment temperatures, the aluminum of the clay is stabilized in an inert, spinel-type phase. The silicon concentration in the solutions (Fig. 10c) for the clays heat-treated at 25–700 °C and 1000 °C is 6–7 μM, which is in agreement with the expected value from the [Si]–[Ca] solubility curve for C–S–H(I) [59] when the solution is saturated with respect to Ca(OH)2. The increased Si concentrations of 8–12 μM for calcination temperatures of 800–950 °C are in agreement with the increased solubility of the calcined clays observed by the measurements of the K and Al concentrations (Fig. 10a and b). However, the reactivity solutions for all calcined clays are expected to be saturated in lime and thus, C–S–H with a high Ca/Si ratio should be formed in all reaction mixtures, resulting in a Si concentration of ~6 μM in the solution under equilibrium conditions. Thus, the increased Si concentrations for the samples calcined at 800–950 °C may reflect either kinetic effects of the C–S–H formation or changes in the C–S–H solubility as a result of the presence of several other ionic species (e.g., K+ ions: 500–800 μM) in the solution. Overall, the solution concentration data in Fig. 10 are in good agreement with the observed reactivities measured for the solid phases by 29Si and 27Al MAS NMR (Figs. 7–9). Thus, the interstratified illite/smectite has a narrow range of optimum calcination temperatures (800–900 °C), and it is prone to the risk of overheating which can greatly reduce its pozzolanic reactivity. This sensitivity to calcination temperature is similar to recent observations for a pure montmorillonite [16] whereas it is in contrasts with the reactivity of heat-treated kaolinite, whose metastable phase (metakaolin) is reactive over a wider range of calcination temperatures [9,10].

Fig. 11. 29Si MAS NMR spectra (9.39 T, vR = 6.0 kHz) of (a) 100% wPc, and (b) the wPc– calcined (900 °C) illite/smectite blend (70–30 wt.%) hydrated up to 105 days.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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3.4. Performance of calcined illite/smecite in a cement blend The previous section has shown that the illite/smectite clay exhibits optimum reactivity when calcined at 900 °C and thus, this calcined clay is investigated in a blend with white Portland cement (wPc) to gauge its chemical properties as a SCM in a hydrated material. 29Si MAS NMR spectra of both the pure wPc and the wPc–calcined clay blend (70/30 w/w), following the hydration for up to 105 days, are shown in Fig. 11. In addition to the resonances from quartz (− 107.8 ppm), the calcined clay (− 85 to − 120 ppm) and the C–S–H phase ( − 79 to − 85 ppm) mentioned earlier (Fig. 6), the spectra also include the well-known pattern of overlapping resonances at higher frequencies from alite (Ca3SiO5, − 65 to − 75 ppm) and belite (β-Ca2SiO4, −71.3 ppm) [47]. The spectra of the paste containing 30 wt.% calcined illite/smectite (Fig. 11b) clearly reveal that the calcined clay is partly consumed during hydration whereas quartz is inert, as expected. Comparison of the Q2(1Al) and Q2 resonances of the C–S–H phase formed in the pure wPc and the blend with calcined clay shows that the intensities of these peaks increase during hydration in comparison to the Q1 peak intensity for the calcined clay blend. This increase suggests a C–S–H with longer average alumino-silicate chains in the wPc–calcined clay blend — an observation that is supported by the calculated chain lengths listed in Table 3. The 29Si NMR spectra of the cement pastes (Fig. 11) have been deconvolved using the same approach as for the spectra in Fig. 6 and with similar subspectra for the C–S–H and the unreacted calcined clay component as shown in Fig. 7. Best fitting resonances for the C–S–H were derived from simulating the spectrum of the wPc–calcined clay blend after 28 days of hydration, and then they were consistently used (fixed positions and line widths) for the remaining spectra. The results from the deconvolutions are used to calculate the degree of calcined clay reaction along with the Al/Si ratio, CL, and CLSi parameters for the formed C–S–H phases. For the pure white Portland cement these parameters are comparable to C–S–H data for similar white Portland cements reported previously [53]. The deconvolutions also provide intensities for the remains of anhydrous alite and belite. Comparison of these intensities for the pure wPc and the wPc–calcined clay blend shows no appreciable difference and thereby shows that the actual calcined clay does not affect the hydration kinetics for alite and belite. The degrees of clay reaction (Table 3) reveal that the calcined clay reacts slowly during early hydration. After 28 days of hydration, only ~18% of the calcined clay has reacted, suggesting a lower pozzolanic performance in comparison to a calcined pure montmorillonite whose degree of reaction was 25% at the same age, determined in a similar wPc–calcined clay hydration experiment [16]. This observation of low reactivity for the calcined illite/smectite clay supports previous studies where illite-rich clays were found to be the least reactive among other clay minerals [9,10]. However, at longer hydration times of more than three months the degree of reaction of this interstratified

Table 3 Average chain lengths of the alumino-silicate chains (CL), pure silicate chain lengths (CL Si), and Al/Si ratios of the C–S–H phase formed in the hydrated white Portland cement– calcined clay blends calculated from deconvolutions of the 29Si MAS NMR spectra.a Hydration time

Pure wPc

wPc–calcined illite/smectite (900 °C)

CL

CLSi

Al/Si ratio

CL

CLSi

Al/Si ratio

Degree of clay reaction (%)

1 day 3 days 7 days 28 days 105 days

3.30 3.00 3.00 3.42 3.67

2.53 2.48 2.52 2.85 3.02

0.071 0.052 0.048 0.046 0.046

3.02 2.98 3.13 3.89 4.80

2.41 2.51 2.57 3.00 3.41

0.063 0.047 0.053 0.061 0.071

1.0 3.5 5.3 17.9 32.6

a The blend includes 70 wt.% wPc and 30 wt.% calcined clay. The chain lengths are defined as: CL = 2[I(Q1) + I(Q2) + 3/2I(Q2(1Al))] / I(Q1) and CLSi = 2[I(Q1) + I(Q2) + 2 I(Q (1Al))] / [I(Q1) + I(Q2(1Al))]. The Al/Si ratio is defined as Al/Si = ½I(Q2(1Al)) / [I(Q1) + I(Q2(1Al)) + I(Q2)]. The estimated error limits are ±0.25 for CL and CL Si and ±0.015 for the Al/Si ratios.

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illite/smectite (~33%) becomes comparable to that of the calcined pure montmorillonite clay (~30% [16]). Comparison of the data for the pure cement and the blended cement shows that both the CLSi and CL parameters for the wPc–calcined clay blend are slightly higher than for the pure cement paste after prolonged hydration. A longer mean aluminosilicate chain length (CL) testifies the pozzolanic reaction of the calcined illite/smectite clay where Al(OH)− 4 ions released from the calcined clay structure enter the bridging sites in the dreierketten structure of silicate tetrahedra in the C–S–H phase. This effect is also seen by the slight increase in Al/Si ratio of the wPc–calcined clay blend whose Al/Si ratio after 105 days of hydration is slightly higher than that of the pure cement paste. Most calcined clays represent a significant source of aluminum and thus longer mean aluminosilicate chain lengths (CL ) and higher Al/Si ratios in the formed C–S–H phase are one of the primary effects of addition of such materials. This has been most clearly detected for wPc–metakaolin blends with substitution levels ranging from 5 to 30 wt.% [28] and in our recent study of calcined montmorillonite [16]. The 27Al MAS NMR spectra of the pure wPc and wPc–calcined clay blend hydrated for up to 105 days are shown in Fig. 12. Aluminum in octahedral coordination is present in three distinct phases of which two are structurally similar to the phases reported in spectra of the reactivity test residues (Fig. 9). The resonance at 13.3 ± 0.1 ppm is assigned to ettringite (AFt, Ca6[Al(OH)6]2(SO4)3·26H2O) and the centerband at 9.9 ± 0.1 ppm to monosulfate (AFm, Ca4[Al(OH)6]2SO4·6H2O) or a hydroxy-AFm phase (Ca4[Al(OH)6]2(OH)2·xH2O, x = 4–12) [56]. The peak at 4.9 ± 0.1 ppm is ascribed to the so-called third aluminate hydrate phase which most likely originates from a nanostructured aluminate layer formed at the surface of the C–S–H phase [57,58]. Additionally, tetrahedral aluminum present in the C–S–H aluminosilicate chains is seen by the broad peak around ~ 73 ppm and fivefold coordinated aluminum present as AlO5 units in the interlayer of the C–S–H by the low-intensity resonance around ~ 35 ppm. Lastly, the unreacted calcined clay gives rise to the broad resonance around ~60 ppm in the wPc–calcined clay blend, which is the dominating component in the spectral region for tetrahedral aluminum at early hydration times. There is no significant difference between the spectra of the two pastes for the first three days of hydration (Fig. 12), in accordance with a slow pozzolanic reaction of the clay as also seen by 29Si NMR (Fig. 11). However, after seven days of hydration and further on, the wPc–calcined clay blend clearly contains a larger proportion of the AFm phase in comparison to ettringite and the corresponding pastes of hydrated pure wPc. This increased intensity for the AFm phase originates from aluminum released from the calcined clay which either stabilizes the aluminum-rich monosulfate phase at the expense of ettringite or in the absence of sulfate ions leads to the formation of hydroxy-AFm. The intensity of the Al(IV) resonance from the unreacted calcined clay decreases with increasing hydration time, supporting that the degree of clay reaction increases as obtained from the deconvolution of the 29Si MAS NMR spectra (Table 3). Thus, 27Al MAS NMR confirms the overall conclusion that illite/smectite calcined at 900 °C participates in the pozzolanic reaction by dissolving aluminum and silicon from its structure, a process which consequently modifies the structure and composition of the C–S–H phase and results in a slightly larger amount of formed AFm phases. 4. Conclusions The thermal decomposition sequence and pozzolanic reactivity for an interstratified illite/smectite (70/30) clay, including a quartz impurity, have been investigated principally by solid-state NMR. It has been found that the clay part of the composite material follows a four-step transformation sequence, corresponding to dehydration (25–200 °C), dehydroxylation (600–800 °C), amorphization (800–900 °C), and recrystallization (950 °C and above). Thus, the illite/smectite clay undergoes continuous transformations with increasing temperatures,

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

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Fig. 12. 27Al MAS NMR spectra (14.09 T, vR = 13.0 kHz) of (a) pure wPc and (b) the wPc–heated (900 °C) illite/smectite blend (70 wt.%/30 wt.%) hydrated from one to 105 days. The resonances in the region for octahedrally coordinated Al are assigned to ettringite (AFt, E), monosulfate (AFm, M), and a nanostructured third aluminate hydrate phase (T), formed at the surface of the C–S–H. The asterisks indicate spinning sidebands and the open circles show the centerband from Al(IV) incorporated in the silicate chains of the C–S–H phase.

which does not lead to the formation of any metastable phase similar to the well-known metakaolin phase in the heat-treatment of kaolinite. The pozzolanic reactivity of the calcined material is found to be dependent on the degree of disorder as well as the induced dehydroxylation in the heated clay. Most importantly, the illite/smectite clay is found to exhibit optimum reactivity at 900 °C, and heating above this temperature results in the crystallization of inert, condensed Q4-type phases which significantly hamper the pozzolanic reaction. The amounts of dissolvable elemental species (K, Al, and Si), determined from ICP-OES analysis, are found to be in good agreement with the pozzolanic reactivity of the calcined clay as determined from analysis of the 29Si MAS NMR spectra, acquired for the solid residues from the Ca(OH)2–calcined clay reactivity blends. The quartz component remains inert upon heat-treatment and does not contribute to the pozzolanic potential of the heated material. A unique variation in 29Si spin–lattice relaxation times with maximum values around 600 °C has been observed for the quartz component with the heat-treatment temperature. This variation is ascribed to the migration of Fe3 + ions from the quartz lattice in to a separate Fe2O3 phase at temperatures below the α–β phase transition (573 °C) for quartz and on the contrary, incorporation of iron released from the calcined illite/smectite clay into the β-form of the quartz lattice at temperatures above 600 °C. Finally, the performance of the optimally calcined clay has been studied for a white Portland cement–calcined clay blend (70/30 w/w) which reveals a small degree of clay reaction at early hydrations times but a significant degree of reaction after prolonged hydration, the latter comparable to similar observations for a white Portland cement– calcined, pure montmorillonite blend. It is found that the reaction of the calcined clay results in an increase of the mean alumino-silicate chain length for the formed C–S–H phase which also exhibits a slightly higher Al/Si ratio as compared to the C–S–H formed by hydration of a pure white Portland cement.

The present results also illustrate the capability of solid-state NMR as a powerful tool for understanding the disordered structure of calcined clays, allowing unambiguous distinction between reactive and inert (Q4-type) phases. Further understanding of dissolution kinetics of such calcined clays and improved knowledge of structural factors affecting the dissolution process are some of the potential directions that need to be pursued in future research. Acknowledgment The use of the facilities at the Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry, Aarhus University, sponsored by the Danish Natural Science Research Council, the Danish Technical Science Research Council, and the Carlsberg-Foundation (No. CF140138) is acknowledged. The Danish National Advanced Technology Foundation is thanked for financial support to the SCM project (No. 095-2010-1). Support from FLSmidth A/S, Research Dania and Aalborg Portland A/S has been important for this work. References [1] B. Lothenbach, K. Scrivener, R.D. Hooton, Supplementary cementitious materials, Cem. Concr. Res. 41 (2011) 1244–1256. [2] R. Snellings, G. Mertens, J. Elsen, Supplementary cementitious materials, Rev. Mineral. Geochem. 74 (2012) 211–278. [3] M.C.G. Juenger, F. Winnefeld, J.L. Provis, J.H. Ideker, Advances in alternative cementitious binders, Cem. Concr. Res. 41 (2011) 1232–1243. [4] J.S. Damtoft, J. Lukasik, D. Herfort, D. Sorrentino, E.M. Gartner, Sustainable development and climate change initiatives, Cem. Concr. Res. 38 (2008) 115–127. [5] J. Ambroise, M. Murat, J. Péra, Hydration reaction and hardening of calcined clays and related minerals V. Extension of the research and general conclusions, Cem. Concr. Res. 15 (1985) 261–268. [6] F. Bergaya, G. Lagaly, Handbook of Clay Science, 2nd ed. Newnes, Elsevier, 2013. [7] J. Rocha, J. Klinowski, Solid-state NMR studies of the structure and reactivity of metakaolinite, Angew. Chem. Int. Ed. 29 (1990) 553–554. [8] C. He, E. Makovicky, B. Osbæck, Thermal stability and pozzolanic activity of calcined kaolin, Appl. Clay Sci. 9 (1994) 165–187.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006

N. Garg, J. Skibsted / Cement and Concrete Research xxx (2015) xxx–xxx [9] R. Fernandez, F. Martirena, K.L. Scrivener, The origin of the pozzolanic activity of calcined clay minerals: a comparison between kaolinite, illite and montmorillonite, Cem. Concr. Res. 41 (2011) 113–122. [10] C. He, B. Osbæck, E. Makovicky, Pozzolanic reactions of six principal clay minerals: activation, reactivity assessments and technological effects, Cem. Concr. Res. 25 (1995) 1691–1702. [11] A.M. Rashad, Metakaolin as cementitious material: History, scours, production and composition — a comprehensive overview, Constr. Build. Mater. 41 (2013) 303–318. [12] N. Garg, K. Wang, Comparing the performance of different commercial clays in fly ash-modified mortars, J. Sustain. Cem. Mater. 1 (2012) 111–125. [13] C. He, E. Makovicky, B. Øsbæck, Thermal stability and pozzolanic activity of calcined illite, Appl. Clay Sci. 9 (1995) 337–354. [14] E. Pomakhina, D. Deneele, A.-C. Gaillot, M. Paris, G. Ouvrard, 29Si solid state NMR investigation of pozzolanic reaction occurring in lime-treated Ca-bentonite, Cem. Concr. Res. 42 (2012) 626–632. [15] C. He, E. Makovicky, B. Osbæck, Thermal treatment and pozzolanic activity of Naand Ca-montmorillonite, Appl. Clay Sci. 10 (1996) 351–368. [16] N. Garg, J. Skibsted, Thermal activation of a pure montmorillonite clay and its reactivity in cementitious systems, J. Phys. Chem. C 118 (2014) 11464–11477. [17] A. Tironi, M.A. Trezza, A.N. Scian, E.F. Irassar, Assessment of pozzolanic activity of different calcined clays, Cem. Concr. Compos. 37 (2013) 319–327. [18] S.C. Taylor-Lange, F. Rajabali, N.A. Holsomback, K. Riding, M.C.G. Juenger, The effect of zinc oxide additions on the performance of calcined sodium montmorillonite and illite shale supplementary cementitious materials, Cem. Concr. Compos. 53 (2014) 127–135. [19] G. Habert, N. Choupay, G. Escadeillas, D. Guillaume, J.M. Montel, Clay content of argillites: influence on cement based mortars, Appl. Clay Sci. 43 (2009) 322–330. [20] S.C. Taylor-Lange, E.L. Lamon, K.A. Riding, M.C.G. Juenger, Calcined kaolinite– bentonite clay blends as supplementary cementitious materials, Appl. Clay Sci. 108 (2015) 84–93. [21] T. Seiffarth, M. Hohmann, K. Posern, C. Kaps, Effect of thermal pre-treatment conditions of common clays on the performance of clay-based geopolymeric binders, Appl. Clay Sci. 73 (2013) 35–41. [22] C. He, E. Makovicky, B. Osbæck, Thermal stability and pozzolanic activity of raw and calcined mixed-layer mica/smectite, Appl. Clay Sci. 17 (2000) 141–161. [23] A. Buchwald, M. Hohmann, K. Posern, E. Brendler, The suitability of thermally activated illite/smectite clay as raw material for geopolymer binders, Appl. Clay Sci. 46 (2009) 300–304. [24] S. Chipera, D. Bish, Baseline studies of the clay minerals society source clays: powder X-ray diffraction analyses, Clays Clay Miner. 49 (2001) 398–409. [25] C. Vogt, J. Lauterjung, R.X. Fischer, Investigation of the clay fraction (b2 μm) of the clay minerals society reference clays, Clays Clay Miner. 50 (2002) 388–400. [26] J. Skibsted, J. Hjorth, H.J. Jakobsen, Correlation between 29Si NMR chemical shifts and mean Si–O bond lengths for calcium silicates, Chem. Phys. Lett. 172 (1990) 279–283. [27] T.F. Sevelsted, D. Herfort, J. Skibsted, 13C chemical shift anisotropies for carbonate ions in cement minerals and the use of 13C, 27Al and 29Si MAS NMR in studies of Portland cement including limestone additions, Cem. Concr. Res. 52 (2013) 100–111. [28] Z. Dai, T.T. Tran, J. Skibsted, Aluminum incorporation in the C–S–H phase of white Portland cement–metakaolin blends studied by 27Al and 29Si MAS NMR spectroscopy, J. Am. Ceram. Soc. 97 (2014) 2662–2671. [29] S.P. Altaner, C.A. Weiss, R.J. Kirkpatrick, Evidence from 29Si NMR for the structure of mixed-layer illite/smectite clay minerals, Nature 331 (1988) 699–702. [30] C.A. Weiss, S.P. Altaner, R.J. Kirkpatrick, High-resolution 29Si NMR spectroscopy of 2: 1 layer silicates; correlations among chemical shift, structural distortions, and chemical variations, Am. Mineral. 72 (1987) 935–942. [31] S. Cadars, R. Guégan, M.N. Garaga, X. Bourrat, L. Le Forestier, F. Fayon, T.V. Huynh, T. Allier, Z. Nour, D. Massiot, New insights into the molecular structures, compositions, and cation distributions in synthetic and natural montmorillonite clays, Chem. Mater. 24 (2012) 4376–4389. [32] G.E. Roch, M.E. Smith, S.R. Drachman, Solid state NMR characterization of the thermal transformation of an illite-rich clay, Clays Clay Miner. 46 (1998) 694–704. [33] D.L. Carroll, T.F. Kemp, T.J. Bastow, M.E. Smith, Solid-state NMR characterisation of the thermal transformation of a Hungarian white illite, Solid State Nucl. Magn. Reson. 28 (2005) 31–43. [34] S.R. Drachman, G.E. Roch, M.E. Smith, Solid state NMR characterisation of the thermal transformation of fuller's earth, Solid State Nucl. Magn. Reson. 9 (1997) 257–267.

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[35] K. Mackenzie, I. Brown, The thermal reactions of muscovite studied by highresolution solid-state 29-Si and 27-Al NMR, J. Mater. Sci. 22 (1987) 2645–2654. [36] J.J. Fitzgerald, A.I. Hamza, S.F. Dec, C.E. Bronnimann, Solid-state 27Al and 29Si NMR and 1H CRAMPS studies of the thermal transformations of the 2:1 phyllosilicate pyrophyllite, J. Phys. Chem. 100 (1996) 17351–17360. [37] V. Fajnor, K. Jesenák, Differential thermal analysis of montmorillonite, J. Therm. Anal. Calorim. 46 (1996) 489–493. [38] F. Presciutti, D. Capitani, A. Sgamellotti, B.G. Brunetti, F. Costantino, S. Viel, A. Segre, Electron paramagnetic resonance, scanning electron microscopy with energy dispersion X-ray spectrometry, X-ray powder diffraction, and NMR characterization of iron-rich fired clays, J. Phys. Chem. B 109 (2005) 22147–22158. [39] C.J. McConville, W.E. Lee, Microstructural development on firing illite and smectite clays compared with that in kaolinite, J. Am. Ceram. Soc. 88 (2005) 2267–2276. [40] J. Skibsted, C. Ruiz-Santaquiteria, Characterization of aluminium guest ions in quartz and of quartz reactivity in Portland cement blends by 27Al and 29Si MAS NMR spectroscopy, Manuscript in Preparation. [41] M. Moesgaard, D. Herfort, J. Skibsted, Y. Yue, Calcium aluminosilicate glasses as supplementary cementitious materials, Glas. Technol.: J. Glas. Sci. Technol. Part A 51 (2010) 183–190. [42] J. Schneider, M.A. Cincotto, H. Panepucci, 29Si and 27Al high-resolution NMR characterization of calcium silicate hydrate phases in activated blast-furnace slag pastes, Cem. Concr. Res. 31 (2001) 993–1001. [43] K.J.D. MacKenzie, I.W.M. Brown, R.H. Meinhold, M.E. Bowden, Outstanding problems in the kaolinite-mullite reaction sequence investigated by 29Si and 27Al solid-state nuclear magnetic resonance: I, Metakaolinite, J. Am. Ceram. Soc. 68 (1985) 293–297. [44] A. Labouriau, Y.-W. Kim, W.L. Earl, Nuclear-spin-lattice relaxation in natural clays via paramagnetic centers, Phys. Rev. B 54 (1996) 9952–9959. [45] D. Tse, S.R. Hartmann, Nuclear spin-lattice relaxation via paramagnetic centers without spin diffusion, Phys. Rev. Lett. 21 (1968) 511–514. [46] J. Skibsted, L. Hjorth, H.J. Jakobsen, Quantification of thaumasite in cementitious materials by 29Si{1H} cross-polarization magic-angle spinning NMR spectroscopy, Adv. Cem. Res. 7 (1995) 69–83. [47] S.L. Poulsen, V. Kocaba, G. Le Saoût, H.J. Jakobsen, K.L. Scrivener, J. Skibsted, Improved quantification of alite and belite in anhydrous Portland cements by 29Si MAS NMR: effects of paramagnetic ions, Solid State Nucl. Magn. Reson. 36 (2009) 32–44. [48] R.T. Cox, Optical absorption of the d4 ion Fe4+ in pleochroic amethyst quartz, J. Phys. C Solid State Phys. 10 (1977) 4631. [49] G. Lehmann, Yellow color centers in natural and synthetic quartz, Phys. Kondens. Mater. 13 (1971) 297–306. [50] D.R. Spearing, I. Farnan, J.F. Stebbins, Dynamics of the α–β phase transitions in quartz and cristobalite as observed by in-situ high temperature 29Si and 17O NMR, Phys. Chem. Miner. 19 (1992) 307–321. [51] I.G. Richardson, G.W. Groves, The structure of the calcium silicate hydrate phases present in hardened pastes of white Portland cement/blast-furnace slag blends, J. Mater. Sci. 32 (1997) 4793–4802. [52] I. Richardson, The nature of the hydration products in hardened cement pastes, Cem. Concr. Comp. 22 (2000) 97–113. [53] M.D. Andersen, H.J. Jakobsen, J. Skibsted, Incorporation of aluminum in the calcium silicate hydrate (C–S–H) of hydrated Portland cements: a high-field 27Al and 29Si MAS NMR investigation, Inorg. Chem. 42 (2003) 2280–2287. [54] T. Oey, A. Kumar, J.W. Bullard, N. Neithalath, G. Sant, The filler effect: the influence of filler content and surface area on cementitious reaction rates, J. Am. Ceram. Soc. 96 (2013) 1978–1990. [55] A. Alujas, R. Fernández, R. Quintana, K.L. Scrivener, F. Martirena, Pozzolanic reactivity of low grade kaolinitic clays: influence of calcination temperature and impact of calcination products on OPC hydration, Appl. Clay Sci. 108 (2015) 94–101. [56] J. Skibsted, E. Henderson, H.J. Jakobsen, Characterization of calcium aluminate phases in cements by 27Al MAS NMR spectroscopy, Inorg. Chem. 32 (1993) 1013–1027. [57] M.D. Andersen, H.J. Jakobsen, J. Skibsted, A new aluminium-hydrate species in hydrated Portland cements characterized by 27Al and 29Si MAS NMR spectroscopy, Cem. Concr. Res. 36 (2006) 3–17. [58] G.K. Sun, J.F. Young, R.J. Kirkpatrick, The role of Al in C–S–H: NMR, XRD, and compositional results for precipitated samples, Cem. Concr. Res. 36 (2006) 18–29. [59] H.M. Jennings, Aqueous solubility relationships for two types of calcium silicate hydrate, J. Am. Ceram. Soc. 69 (1986) 614–618.

Please cite this article as: N. Garg, J. Skibsted, Pozzolanic reactivity of a calcined interstratified illite/smectite (70/30) clay, Cem. Concr. Res. (2015), http://dx.doi.org/10.1016/j.cemconres.2015.08.006