Thermal Activation of a Pure Montmorillonite Clay and Its Reactivity in

Article pubs.acs.org/JPCC

Thermal Activation of a Pure Montmorillonite Clay and Its Reactivity in Cementitious Systems Nishant Garg and Jørgen Skibsted* Instrument Centre for Solid-State NMR Spectroscopy, Department of Chemistry and Interdisciplinary Nanoscience Center, Aarhus University, Langelandsgade 140, DK-8000 C Aarhus, Denmark S Supporting Information *

ABSTRACT: Clay minerals are potential candidates as raw materials for new supplementary cementitious materials (SCMs) that can partly replace Portland cement and thereby significantly reduce CO2 emissions associated with cement production. We present the characterization of the complex, disordered structure of a pure montmorillonite clay heated at various temperatures (110−1100 °C), by solid-state 27Al and 29Si MAS NMR methods. The SiO4 tetrahedra and AlO6 octahedral sites become progressively more distorted, exhibit a significant degree of disorder upon dehydroxylation (600−800 °C), and do not lead to the formation of any metastable phase. At high temperatures (1000−1100 °C), the layer structure of the clay breaks down, forming stable crystalline phases. The chemical reactivity, measured as the degree of dissolution/precipitation in an alkaline solution, is found to be proportional to the degree of disorder/dehydroxylation. The maximum reactivity as a function of the heating temperature is achieved at 800 °C prior to the formation of inert, condensed Q4-type phases in the material. At maximum reactivity the calcium silicate hydrate (C-S-H) phase contains silicate chains with the highest aluminum incorporation, leading to blended cements containing a C-S-H phase with longer chain lengths. Most importantly, by exploiting the differential spin−lattice relaxation behavior of the 29Si spins, evidence of multiple sites and components in both the naturally occurring and heated montmorillonite is being reported for the first time.

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INTRODUCTION

methods where maximum compressive strength or reactivity has been the principal goal. Clay minerals, specifically “planar hydrous phyllosilicates”, are composed of repeated tetrahedral (T) and octahedral (O) two-dimensional sheets.5 The T-sheet is commonly occupied by corner-sharing tetrahedral cations like Si4+, Al3+, and Fe3+, while the O-sheet is usually formed by edge-sharing octahedral cations such as Al3+, Fe3+, Mg2+, and Fe2+ arranged in a pseudohexagonal symmetry. The T- and O-sheets share a plane of oxygen atoms (apical oxygen), whereas the O-sheet also has nonapical oxygen atoms that form hydroxyl groups that exist in either trans- or cis-vacant configuration. Clay minerals are usually classified as 1:1 (TO), 2:1 (TOT), or mixed-layer minerals depending upon the alternate arrangement of these Tand O-sheets. The 2:1 clay minerals usually exhibit isomorphic substitution, often resulting in a net negative charge that is balanced by interlayer cations (Ca2+, Na+, or K+) coordinated to H2O molecules in the interlayer region. Kaolinite (Si2Al2O5(OH)4), a 1:1 clay mineral with widespread industrial applications, has been extensively studied in

Increasing mass production of Portland cement is a growing concern for the increasing greenhouse gas emissions, since cement production accounts today for roughly 7% of the anthropogenic CO2 emission.1 Thus, current research in cement science is to a large extent focused on replacing the carbon intensive Portland cement component in concrete with alternative or supplementary cementitious materials (SCMs).2,3 Alumina (Al2O3) and silica (SiO2) are the principal constituents in most naturally occurring clay minerals, which makes them potential SCM candidates for cement replacement. However, clays usually have a fixed lattice structure in their original state where the aluminate and silicate sites have minimal reactivity. It is well-known that thermal treatment can induce disorder in their structure and potentially lead to enhanced pozzolanic reactivity.4 (The “pozzolanic reaction” of a SCM is a reaction that takes place under alkaline conditions in cement blends and involves the consumption of excess calcium hydroxide produced as one of the hydration products by the calcium silicate phases of Portland cement.) However, determination of the optimum heating temperature and an understanding of its structural background are not straightforward and often the optimum temperature has been reported from trial and error © 2014 American Chemical Society

Received: March 13, 2014 Revised: May 8, 2014 Published: May 8, 2014 11464

dx.doi.org/10.1021/jp502529d | J. Phys. Chem. C 2014, 118, 11464−11477

The Journal of Physical Chemistry C

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terms of both its thermal transformation sequence6 and its reactivity in the dehydroxlated form of a metastable phase.7,8 There is a comparative lack of knowledge in the case of montmorillonite, a common 2:1 mineral of the smectite group, probably as a result of the complex and disordered nature of the raw mineral itself.9 Similarly, while a dehydroxylated structure for kaolinite has been proposed at the atomistic scale,10,11 the dehydroxylation mechanism for smectites has not been established, as it is further complicated by the presence of interlayer cations and isomorphic substitutions.12,13 Nevertheless, there has been a few solid-state NMR studies on thermal transformation of 2:1 clay minerals including montmorillonite,6,14 illite,15 and pyrophyllite16,17 that have established a generalized thermal decomposition sequence of dehydration, dehydroxylation, amorphization, and crystallization - a sequence that has also been verified by thermogravimetric methods18 and combinations of other analytical techniques.19,20 The principal aim of the present work is to study the reactivity of a structurally pure montmorillonite clay as a function of the structural changes induced by the thermal treatment. The dehydroxylated form of kaolinite known as metakaolin has been widely studied as a SCM in cement systems21 because of its supposedly highest reactivity among the dehydroxylates of all clay minerals.22,23 While the reactivity of unheated montmorillonite has been explored in recent literature,24,25 structural studies on the pozzolanic reaction of dehydroxylated or disordered montmorillonites are limited.26 This paper serves to establish a better link between the thermally transformed structure and reactivity for montmorillonite, in particular for cementitious blends where aluminate and silicate ions from the heated material, leached in the alkaline pore solution, will react with the excess of calcium hydroxide, forming additional quantities of the calcium silicate hydrate (C-S-H) phase, the principal binding component in hydrated Portland cement systems.

Reactivity Test. The reactivity test involved mixing 0.100 g of heated clay with 0.300 g of calcium hydroxide in 50 mL of triple-distilled water. The heated clay was sieved to get the size fraction between 20 and 40 μm. The mixture was stirred for 7 days in a closed, conical flask immersed in an oil bath maintained at 40 °C. After filtration, the solid residue was dried under slightly reduced pressure in a desiccator and subsequently stored in a sealed glass container prior to the NMR experiments. The reactivity was determined from deconvolution of the 29Si MAS NMR spectra, providing information about the relative fractions of unreacted clay material and the formed hydration products. Portland Cement Blend. The performance of the heated SAz-2 montmorillonite in a Portland cement blend was investigated for a white Portland cement (wPc) from Aalborg Portland, Cementir Holding SpA (Aalborg, Denmark). The wPc has the following bulk-oxide content: 69.2 wt % CaO, 24.6 wt % SiO2, 2.11 wt % Al2O3, 0.32 wt % Fe2O3, 0.65 wt % MgO, 2.23 wt % SO3, 0.003 wt % K2O, 0.15 wt % Na2O, 0.39 wt % P2O5, a loss of ignition of 1.02 wt %, and a Blaine fineness30 of 413 m2/kg. A blend of 70 wt % wPc and 30 wt % heated montmorillonite (800 °C) and a paste of pure wPc were hydrated for up to 1 year using a water/powder ratio of 0.50 and triple-distilled water. The pastes were mixed using a motorized stirrer equipped with a custom-made paddle. The pastes were cast into 25 mL propylene vials, demolded after 24 h, and subsequently sealed in plastic containers (75 mL) filled with distilled water and stored at 20.0 ± 0.1 °C in a climate chamber. At appropriate times (1, 7, 28, 90, 180, and 365 days) a small fraction of the paste sample was ground and the hydration stopped by stirring the ground powder in 10 mL of isopropyl alcohol (min., 99%), using a magnetic stirrer, for approximately 1 h. After filtration, the samples were subsequently dried under slightly reduced pressure in a desiccator over silica gel at room temperature and kept in airtight containers prior to the NMR investigations to prevent CO2 contamination. NMR Measurements. Single-pulse and inversion−recovery (IR) 29Si MAS NMR spectra for the clays heated at different temperatures were recorded on a Varian Unity-plus 200 (4.7 T) spectrometer, using a home-built 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 IR experiments for the 180° and 90° pulses; however, longer and optimized relaxation delays (10−80 s) were employed, in particular for the samples heated at the high temperatures. The single-pulse 29Si MAS NMR spectra of the calcium hydroxide and heated clay mixtures as well as the Portland cement blends were acquired on a Varian INOVA-400 spectrometer (9.39 T), employing a home-built 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, a relaxation delay of 15 s, and typically 5600 scans. The 29Si{1H} CP/MAS NMR of the heated clays were obtained using the same setup at 9.39 T, using rf field strengths of γB1/(2π) ≈ γB2/(2π) = 35 kHz for the Hartmann−Hahn match, a CP contact time of 1.0 ms, a 4 s relaxation delay, and 4096 scans. The 29Si chemical shifts were referenced to neat tetramethylsilane (TMS), using a sample of β-Ca2SiO3 (δiso = −71.33 ppm)31 as a secondary reference. The Varian VnmrJ software was used for the deconvolutions of the

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MATERIALS AND METHODS Clay Mineral Heat Treatment. A structurally pure sample of montmorillonite clay, devoid of unwanted secondary phases or clays, from Arizona (Cheto mine, Apache County), U.S., known as SAz-2 was purchased from Source Clays Repository under the Clays Minerals Society (CMS), Purdue University (IN, U.S.). SAz-2 is equivalent to SAz-1 except that it is available in a chunk form at the repository. This standard clay is a high magnesium dioctahedral montmorillonite that according to the baseline reports published by CMS27 consists of 98 wt % smectite, 1 wt % quartz, and 1 wt % other minor phases. Elemental analysis based on atomic absorption and flame absorption spectrometry28 has established its composition: 59.65 wt % SiO2, 19.98 wt % Al2O3, 1.77 wt % Fe2O3, 0.25 wt % TiO2, 6.73 wt % MgO, 3.15 wt % CaO, 0.06 wt % Na2O, 0.19 wt % K2O, 0.01 wt % P2O5, and loss of ignition of 4.58 wt % (110−550 °C) and 3.59 wt % (550−1000 °C), giving the estimated structural formula (Ca0.44Na0.015K0.03)[Al2.73Mg1.29Fe(III)0.17Ti0.02][Si7.69Al0.31]O20(OH)4. For the heat treatment experiments, 1.00 g of clay was ground in an agate mortar and spread as a thin layer in a ceramic container to be stored in a furnace at the desired temperature for 2 h. After the 2 h, the container with the clay was allowed to cool in a separate oven maintained at 100 °C for half an hour to avoid spontaneous rehydroxylation.29 Following this cooling, the clay powder was stored and sealed in a glass container kept in a desiccator. 11465



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Si MAS NMR spectra, following the approach for analysis of cement blends developed in our laboratory and summarized recently in a study of limestone−Portland cement blends.32 The 27Al MAS NMR spectra were recorded on a Varian DirectDrive VNMR-600 (14.09 T) spectrometer using a homebuilt CP/MAS probe for 4 mm O.D. zirconia (PSZ) rotors. A spinning speed of vR = 13.0 kHz, single-pulse 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. The MQMAS spectra employed the standard two-pulse sequence with 180° (8.2 μs) and 60° (2.7 μs) “liquid” pulses for the triple-quantum excitation and conversion, respectively, a 1 s relaxation delay, 48 t1 increments, and 2352 scans for each increment. The 27Al chemical shifts were referenced to a 1.0 M aqueous solution of AlCl3·6H2O. The signal intensities in the reported spectra have in all figures been normalized to the actual sample mass in the packed NMR rotors.

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RESULTS AND DISCUSSION Thermal Decomposition of Montmorillonite. 29Si MAS NMR spectra following the heat treatment of the montmorillonite SAz-2 clay from 25−1100 °C are shown in Figure 1. Figure 2. (a) 29Si chemical shifts (centers of gravity, δcg(29Si)) and (b) line widths (fwhm) as a function of the heat treatment temperature for montmorillonite. The data are obtained from the 29Si MAS NMR spectra in Figure 1.

(δcg(29Si)) and the half widths (fwhm) as a function of temperature in Figure 2. The shift from −93.3 to −94.4 ppm and the increase in line width from fwhm = 4.8−8.0 ppm, observed for heating up to 200 °C, reflect the initial dehydration where interlayer water molecules are removed from the structure. This process may be associated with the diffusion of interlayer cations (Na+, K+, and Ca2+) into the octahedral vacancies, thereby reducing the layer charge, resulting in a pyrophyllite-like structure,35,36 as reflected by the ∼1 ppm change in 29Si chemical shift to lower frequency. A very similar shift from −93.4 to −94.7 ppm on going from room temperature to heat treatment at 400 °C has been reported in a 29Si NMR study of a Ca montmorillonite.14 The 29 Si chemical shift of the Q3 resonance does not change in the temperature range from 200 to 500 °C, while a small increase in fwhm is observed in this interval (Figure 2). The latter may reflect that the overall structure becomes more rigid when the heating temperature is increased. An abrupt change in chemical shift between 500 °C (−94.8 ppm) and 600 °C (−97.1 ppm) is observed and reflects the onset of the dehydroxylation process. Moreover, the line shape for the resonance at 600 °C (Figure 1) indicates the presence of two different Q3 sites at −97.6 and −101.5 ppm (cf., Figure 3, vide infra), which may originate from SiO4 tetrahedra connected to hydroxylated Me(VI) sites, as present in the layer structure at low temperature, and dehydroxylated Me(VI) sites in the octahedral sheet, respectively. The intensity for the resonance at −101.5 ppm from the dehydroxylated sites increases in intensity for the samples heated at 700 and 750 °C. A significant increase in line width is observed for the samples heated at 800−900 °C along with a continuous shift of the center of gravity (δcg) toward lower frequency. Thus, a number of different SiO4 environ-

Figure 1. 29Si MAS NMR spectra (4.7 T, νR = 7.0 kHz) of montmorillonite SAz-2 heated for 2 h at temperatures in the range 25−1100 °C.

The spectrum of the untreated clay contains a single resonance at −93.3 ppm, which corresponds to a Q 3 -type SiO 4 environment including three Si−O−Si bonds in the silicate layer and one Si−O−Me2 bond to two Me sites in the octahedral sheet (Me = Mg2+, Al3+, Fe3+, Ti4+) via a tricoordinated oxygen. This is in agreement with the chemical shifts (−94.1 to −93.0 ppm) reported in earlier 29Si MAS NMR studies of natural montmorillonites.9,33,34 Upon heating, the resonance shifts to lower frequency and its line width increases, as most clearly seen by the plots of the center of gravity 11466



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ison of the resonances observed by 29Si MAS and CP/MAS NMR for the sample heated at 600 °C (Figure 3) indicates that this sample contains two distinct 29Si sites, since 1H−29Si polarization transfer is only observed for the high-frequency part of broadened resonance (the distinction of multiple sites is investigated in more detail in the section on 29Si inversion− recovery NMR studies; vide infra). Almost no intensity is observed by 29Si{1H} CP/MAS NMR for the sample heated at 700 °C, and the resonance at roughly −96 ppm is completely absent at 800 °C, reflecting that the sample is fully dehydroxylated at this temperature. The 27Al MAS NMR spectrum of the unheated montmorillonite (Figure 4) contains central-transition resonances for one octahedrally

Figure 3. 29Si{1H} CP/MAS NMR spectra (9.39 T, νR = 5.0 kHz, τCP = 1.0 ms) of samples of montmorillonite heated at different temperatures. For comparison, the dotted line represents the singlepulse 29Si MAS NMR spectrum (9.39 T, νR = 6.0 kHz) of the sample at 600 °C, scaled to match the CP spectrum.

ments are present in these samples, reflecting an amorphization of the layer structure of the clay. The low-frequency shift in this temperature range reflects a partial condensation of the SiO4 tetrahedra to Q4 sites. At 1000 °C a narrow component at −111.4 ppm emerges, and at 1100 °C, this resonance dominates the 29Si NMR spectrum. The −111.4 ppm resonance is assigned to the SiO2 polymorph, cristobalite, following earlier 29Si NMR studies of this mineral37 and opal samples.38,39 Thus, the 29Si NMR spectra of the hightemperature samples (Figure 1) reveal that the layer structure breaks down and SiO4 tetrahedra across the collapsed interlayer space recrystallize to form cristobalite. In addition to cristobalite, small amounts of cordierite ((Mg,Fe)2Al4Si5O8) and anorthite (CaAl2Si2O8) are also formed, as evidenced by the powder XRD pattern (Figure 1S in Supporting Information) of the material heated at 1000 °C and in agreement with earlier studies of the high-temperature reactions for montmorillonite.40,41 These silicate phases can also be identified in the 29Si NMR spectra of the 1000 and 1100 °C samples (Figure 2S) by broadened resonances in the range −80 to −100 ppm. The dehydration and dehydroxylation reactions for montmorillonite can also be followed by 29Si{1H} CP/MAS NMR as illustrated in Figure 3. The 29Si{1H} CP/ MAS NMR spectrum of the untreated sample is very similar to the single-pulse 29Si MAS NMR spectrum (Figure 1), demonstrating that all Si atoms have hydroxyl groups/water molecules in their near vicinity. When the sample is heated to 200 and 400 °C, the 29Si{1H} CP/MAS resonance (−94 ppm) becomes broader and decreases slightly in intensity, which reflects that interlayer water molecules are removed from the structure, resulting in a more rigid structure. The 29Si{1H} CP/ MAS intensity further decreases for the spectra of the samples heated at 500 and 600 °C, indicating the onset of the dehydroxylation in this temperature range. Moreover, compar-

Figure 4. 27Al MAS NMR spectra (14.09 T, νR = 13.0 kHz) illustrating the central transition region for montmorillonite heated for 2 h at the listed temperatures. The asterisks indicate spinning side bands.

coordinated Al site (δcg(27Al) = 0.3 ppm) and two Al sites in tetrahedral environments (δcg(27Al) = 59 and 69 ppm). The low-frequency tetrahedral resonance at ∼59 ppm is assigned to a small amount of an impurity phase, present in a form with a low content of Fe3+ guest ions, as revealed by its significantly slower spin−lattice relaxation compared to the resonance at δcg(27Al) = 69 ppm. This is observed in 27Al MAS NMR spectra acquired with different, short relaxation delays (Figure 3S). The low-frequency tetrahedral resonance at ∼59 ppm has been assigned to a small impurity of analcime (NaAlSi2O6·nH2O) in a previous study on montmorillonites.42 However, considering the low bulk Na2O content (0.06 wt %) of the present montmorillonite sample, this assignment seems unlikely to be valid here. The 27Al chemical shift of at ∼59 ppm strongly suggests that the impurity resonance originates from an Al(OSi)4 site, indicating the presence of a minor amount of a zeolitic aluminosilicate phase. Similar uncertainty about the assignment of this resonance was also expressed for another natural montmorillonite.9 The high-frequency tetrahedral resonance reflects a small degree of isomorphic substitution 11467



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of aluminum into the tetrahedral silicon layer. The center band from octahedrally coordinated Al exhibits an asymmetric line shape with a tail to lower frequency that is characteristic for Al sites experiencing a small distribution in quadrupole coupling parameters and chemical shifts. This distribution may reflect the presence of multiple Al sites, [Al3−x3+Rx2+(OH)2O4]−1−x, R = Mg2+, Fe2+, in the Al(VI) layer having different surrounding environments as a result of the partial substitution of Al3+ by Mg2+, Fe3+, and Fe2+.43 For the heat treatment below 500 °C, the 27Al MAS NMR spectra (Figure 4) show that the aluminum sites are largely unaffected by the dehydration process. This shows that the dehydration does not take place in the nearest coordination spheres to Al, since the AlO6 layer is sandwiched between the two silicon sheets. Significant changes in the 27Al MAS NMR spectra are apparent from 500 to 600 °C, reflecting the partial dehydroxylation of the Al layer, which is observed by a structural coordination rearrangement for Al where octahedrally coordinated Al is transformed into 5-fold and tetrahedrally coordinated Al, as evidenced by the resonances at 26 and 59 ppm, respectively. At higher temperatures, the fraction of Al(VI) decreases and almost all Al(VI) has transformed into 5-fold and tetrahedrally coordinated Al at 800 °C, showing that a less-ordered/amorphous material has formed at this temperature. The significant changes in Al coordination environments on going from heat treatment at 700 to 800 °C are also apparent from the 27Al MQMAS NMR spectra acquired for these temperatures (Figure 5). Separate resonances from Al(IV), Al(V), and Al(VI) are observed in the 27Al MQMAS spectrum of the sample at 700 °C, whereas only the resonance from Al in tetrahedral coordination is apparent for the sample heated to 800 °C. The Al(IV) resonances at 700 and 800 °C and the peak from 5-fold Al at 700 °C exhibit a substantial line broadening in the isotropic (F1) dimension of the MQMAS spectra, which originates from a distribution in 27Al NMR interaction parameters for these sites, thereby reflecting the amorphous nature of these samples. For the samples heated at 850−1000 °C, the 27Al MAS NMR spectra (Figure 4) are strongly dominated by the Al(IV) center band and there is no clear evidence for the presence of either 5fold or octahedrally coordinated Al. The Al(IV) center band is significantly broadened with a tail to low-frequency, reflecting a distribution in quadrupole coupling and chemical shift parameters. Thereby, the 27Al MAS NMR spectra of the samples heated at 850−1000 °C resemble similar spectra reported for the Al(IV) sites in slags44 and aluminosilicate glasses.45 A clear reduction in line width of the Al(IV) center band is observed from 1000 to 1100 °C, which suggests the formation of a material with a higher degree of local order for the AlO4 sites at very high temperatures, e.g., cordierite and anorthite as indicated by 29Si NMR (Figures 1 and 2S). The structural coordination rearrangements for the Al sites associated with heat treatment to temperatures of 1100 °C resemble those observed in the kaolinite−mullite transformation.46 However, a major difference is that the montmorillonite dehydroxylate does not form a metastable phase like that of dehydroxylated kaolinite, “metakaolin”. A characteristic structural feature for metakaolin is the mutual presence of Al in tetrahedral, 5-fold, and octahedral coordination in similar amounts, whereas the Al(VI) sites in montmorillonite transform into tetrahedral Al sites upon heat treatment in the temperature range 500−850 °C.

Figure 5. 27Al MQMAS NMR spectra (14.09 T, νR = 13.0 kHz) of montmorillonite heated for 2 h at (a) 700 °C and (b) 800 °C. The projections onto the F1 and F2 axes correspond to summations over the 2D plot. The asterisks indicate spinning side bands.

Pozzolanic Reactivity of Heated Montmorillonites. 29Si MAS NMR spectra of the solid phases from the reactivity tests, where the heated clays have been cured in a saturated solution of calcium hydroxide (see experimental section), are shown in Figure 6. Generally, the resonances in the range −90 to −120 ppm originate from the heated clay material, which has not reacted in the reactivity test, whereas resonances between −75 to −90 ppm reflect hydration products formed by reaction with Ca(OH)2 and water. Distinct peaks at roughly −79, −81.4, and −85 ppm are observed for the hydration products, demonstrating the formation of a less-ordered calcium silicate hydrate (CS-H) phase, including a minor fraction of AlO4 sites in the chains of silicate tetrahedra, similar to the principal hydration product resulting from Portland cement hydration. The three peaks observed for the C-S-H phase are assigned to dimers of SiO4 tetrahedra or chain end groups (Q1), chain SiO4 sites connected to an AlO4 tetrahedron Q2(1Al), and pure SiO4 chain units, following earlier studies of C-S-H phases formed in hydrated Portland cements.47−49 C-S-H hydration products are observed at all heat treatment temperatures, and it is noteworthy that the untreated clay also partly reacts with the excess of calcium ions in the high pH solution. This observation is in agreement with a 27Al and 29Si MAS NMR study of montmorillonite samples cured for up to 3 months in a Ca(OH)2 solution, which indicated that a small part of 11468



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Figure 6. 29Si MAS NMR spectra (9.39 T, vR = 6.0 kHz) of the reactivity test mixtures of calcium hydroxide and montmorillonite heated at the listed temperatures.

montmorillonite dissolves congruently under highly alkaline conditions.42 Information on the degree of reaction for the clay materials and on the C-S-H structure is obtained from deconvolution of the 29Si MAS NMR spectra (Figure 6), employing subspectra for the anhydrous clay material and fixed resonances for the CS-H phase. Subspectra for the individual anhydrous clay samples were obtained by deconvolving the 29Si NMR spectra of the anhydrous materials (Figure 1) by a number of peaks and keeping the relative ratio of these peaks constant in the deconvolution of the samples from the reactivity tests. Thus, this procedure assumes a congruent dissolution of the heated clay materials, in agreement with the result mentioned above for untreated montmorillonite.42 The resonances from the C-SH phase were simulated using two overlapping peaks for the Q1 and Q2 types of SiO4 tetrahedra and a single resonance for the Q 2 (1Al) site. As an example, Figure 4S shows the deconvolution of the 29Si NMR spectrum for montmorillonite heated at 850 °C and exposed to the Ca(OH)2 solution. Fixed values for the frequencies and line widths for the peaks constituting the C-S-H phase were used consistently for the quantification of the Q1, Q2(1Al), and Q2 intensities in the analysis of the spectra shown in Figure 6. An exception was made in the deconvolution of the spectra for the samples heated at 750 and 800 °C, where the frequency for the Q2(1Al) peak was shifted by 0.4 ppm (−81.4 to −81.0 ppm) to improve the deconvolutions. The fraction of reacted clay material as a function of the heat treatment temperature is shown in Figure 7a and clearly reveals that maximum reactivity is obtained for the clay heated at 800 °C. Moreover, the temperature range giving a high degree of reaction is rather narrow (750−800 °C), which may reflect that a metastable phase is not formed upon dehydroxylation. In a recent review and on the basis of earlier studies, Snellings et al.3

Figure 7. (a) Degree of reaction for heated montmorillonite in reactivity tests with calcium hydroxide and (b) the Al/Si ratio of the formed C-S-H phase in these experiments, both as a function of the heat treatment temperature. The data are derived from deconvolution of the 29Si MAS NMR spectra, using the approach illustrated in Figure 4S. The intensities for the individual components for the different samples are summarized in Table 1S.

also summarized a narrow temperature range for the thermal activation (800−830 °C) of heated montmorillonites that is in good agreement with the optimum heating temperature of 800 °C found in this work. The local structure of the C-S-H phase can be described by the average chain length of the aluminosilicate chains (CL ), the average chain length of pure silicate tetrahedra (CL Si), and the average Al(IV)/Si ratio of the chains, following earlier studies of the C-S-H phase in hydrated Portland cements.47,50 These measures are summarized in Table 1S and show only minimal changes in the values for CL and CL Si of the C-S-H phases formed in the reactivity mixtures of the clay materials heated at different temperatures. However, a clear increase and a subsequent decrease in the Al/Si ratio of the C-S-H phases are observed in the temperature range 700− 900 °C (Figure 7b), which matches well with the variation in degree of reaction for the same samples (Figure 7a). Thus, the highest incorporation of Al in the C-S-H phase is observed for the heated clay with optimum reactivity (i.e., 800 °C). The increase in average Al/Si ratio for the C-S-H with heat treatment temperatures from 600 to 800 °C may reflect an increasing concentration of Al(OH)4− ions in the reactivity test solutions and thereby that the high reactivity is associated with an intermediate structure from which the Al3+ species are easily dissolved. If reactivity is viewed in the light of dissolution capability of silicon and aluminum species from the structure, a combination 11469



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from Al(IV) and Al(V) sites become dominating at temperatures above 850 °C, reflecting that an inert material containing highly condensed tetrahedral sites is formed at these high temperatures. Thus, the 27Al MAS NMR spectra in Figure 8 fully support the conclusions on heated montmorillonite reactivity derived from the 29Si MAS NMR analysis (Figures 7 and 8). The optimum reactivity for montmorillonite exposed to heat treatment at 800 °C is in good agreement with earlier studies of Portland cement−heated montmorillonite blends, where optimum mechanical properties were obtained in the vicinity of this temperature.4,26 The high reactivity at temperatures around 800 °C reflects the complete dehydroxylation of the clay mineral in this temperature range and thereby resembles the pozzolanic reactivity for kaolinite which is expected to be proportional to the degree of dehydroxylation.55 However, the abrupt decrease in reactivity at 850 °C for montmorillonite is difficult to account for, considering complete dehydroxylation and the observation of a broad 29Si NMR resonance at this temperature (Figure 1), indicating a nearly amorphous phase. To clarify these observations, inversion−recovery 29Si NMR experiments have been performed in an attempt to gain resolution. Spin−Lattice Relaxation of Heated Montmorillonite. Inversion−recovery (IR) 29Si MAS NMR has been previously used with success to distinguish silicon environments in mixedlayer 2:1 clays56 and in the alite and belite phases of anhydrous Portland cements.57,58 These studies have utilized the fact that the 29Si spin−lattice relaxation for these systems is governed by dipole−dipole couplings between the nuclear spins and the electron spins of the unpaired electron in paramagnetic centers (i.e., Fe3+) present in the materials as impurities. For this case, and in the absence of spin diffusion, the spin−lattice relaxation behavior for 29Si can be described by the stretched exponential relationship:59−61

of factors may lead to the increased dissolution of these elements in the reactivity tests. These factors include (i) the presence of weak Si−O−Si and/or Si−O−Al bonds in the disordered, dehydroxylated phase at 750−800 °C, (ii) the absence of condensed, highly polymerized and stable Q4 units that are formed at higher temperatures, and (iii) the presence of charge-balancing interlayer cations. The Na+ and Ca2+ cations in the dehydroxylated structure may undergo fast hydrolysis when exposed to an alkaline solution and later influence the dissolution kinetics of the major elements. The dissolution of montmorillonite has been studied in solutions with at pH 1.0− 13.5 from which it was reported that dissolution takes place inward from crystal edges in high and low pH solutions.51 This suggests that the heat treatment in the present work may have increased the reactive/edge surface area in the samples. The 27Al MAS NMR spectra of the samples from the reactivity tests (Figure 8) all include a resonance at δcg(27Al) =

Mz(t ) = M 0[1 − (1 + α) exp( − t /T1′ ]

(1)

where M0 is the equilibrium longitudinal magnetization, α ≤ 1 is a constant accounting for pulse imperfections (i.e., α = 1 for an ideal pulse), and T′1 is the stretched exponential spin−lattice relaxation time. IR 29Si MAS NMR spectra of the as-received montmorillonite are shown in Figure 9a and clearly reveal that at least two different 29Si environments are present in this sample. The spectra can be analyzed in terms of a narrow component at −93.7 ppm with a relatively long relaxation time and a broad component with an average chemical shift δ̅ = −92.5 ppm with a short relaxation time. Zero-crossings for the broad and narrow peaks, denoted components A and B, are observed at recovery times of 0.0075 and 0.022 s, respectively, giving spectra where only resonances from components B and A are observed (insets of Figure 9a). These spectra have been deconvolved using one to four partly overlapping peaks, giving subspectra for components A and B that subsequently are used in a full analysis of all IR 29Si MAS NMR spectra. The intensities for components A and B resulting from this analysis are shown by (1 − Mz(t)/M0) as a function of the recovery time, t and √t, in Figure 9b and Figure 9c, respectively. The absence of a linear relationship between ln(1 − Mz(t)/M0) and the time (t) demonstrates that the recovery of magnetization cannot be described by a single-exponential relationship. On the other hand, the observed linear relationship between (1 − Mz(t)/M0)

Figure 8. 27Al MAS NMR spectra (14.09 T, vR = 13.0 kHz) illustrating the central-transition region for the reactivity test mixtures of calcium hydroxide and montmorillonite heated at the listed temperatures. Asterisks indicate spinning side bands.

9.5 ppm, reflecting that a calcium aluminate hydrate phase of the type Ca4[Al(OH)6]2(OH)2·xH2O (x = 4−12) 52 has formed in addition to the C-S-H phase. The peak exhibits its highest intensity for the sample heated at 800 °C with an intensity variation that closely follows the degree of reaction shown in Figure 7a. At 800 °C, three distinct center band resonances are also clearly apparent, which can be assigned to tetrahedral Al in the silicate chains of the C-S-H phase (73 ppm), 5-fold AlO5 units present in the interlayer of the C-S-H structure (33 ppm), and octahedrally coordinated aluminum from a nanostructured aluminate phase formed most likely on the surface of the C-S-H (4.5 ppm).53,54 The resonances are also apparent at 750 °C, in agreement with the second highest degree of reactivity at this temperature. Broad center bands 11470



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dipole−dipole interaction. Since kaolinites62,63 and 2:1 clay minerals64 are known to have segregation of iron in the octahedral layer, the difference in relaxation rate for components A and B may potentially reflect a heterogeneous distribution of paramagnetic centers. Moreover, component A of the as-received material (Figure 9, right-hand inset) appears to be further composed of two distinct resonances (−91.7 and −95.7 ppm) that have not been previously observed for montmorillonites. The high-frequency resonance (−91.7 ppm) may be associated with the small degree of Al3+ incorporation in the silicate sheets, thereby originating from a Q3(1Al) site. The low-frequency part at −95.7 ppm and component B (−93.7 ppm) may potentially reflect different Q3(0Al) silicon sites having slightly different Si−O bond lengths and Si−T distances, potentially arising from Si sites close to the edges and in the bulk part of the silicate sheets, respectively. IR 29Si MAS NMR spectra have also been acquired for four temperatures (Figure 5S) in the vicinity of the heat treatment temperature corresponding to optimum reactivity. In analogy to the IR spectra of the as-received montmorillonite, these spectra can be analyzed in terms of two different components (A and B) exhibiting different relaxation times. The IR spectra corresponding to the zero-crossings for these two components are illustrated in Figure 10 and are deconvolved using the same approach as for the unheated montmorillonite. This results in the average chemical shifts, the stretched exponential relaxation times, and relative fractions for the two components which are summarized in Table 1. Both components exhibit a shift to lower frequency on going from heat treatment at 750 to 1000 °C, reflecting a transformation of Q3 sites into the more polymerized Q4 type of environments. The spin−lattice relaxation times are at least a factor of 5 longer for the narrow component (B) compared to the broad component (A). These observations indicate that component B reflects SiO4 with a high degree of order, whereas component A may be associated with a fast-relaxing, glassy phase. A significant change in relaxation rate is observed for both components when the heat treatment temperature is increased from 850 to 1000 °C. This is most pronounced for component B where the significantly longer relaxation time (T′1 = 11.1 s), compared to component A (T1′ = 0.203 s), suggests a complete phase separation and preferential clustering of paramagnetic impurities in component A. Furthermore, at 1000 °C, there is a macroscopically visible change in the color of the heated particles from brown-white at room temperature to red-orange at 1000 °C (Figure 6S), which also indicates that iron preferentially clusters in the form of hematite domains on the surface of the glassy particles

Figure 9. (a) Inversion−recovery 29Si MAS NMR spectra (4.7 T, νR = 7.0 kHz) of the as-received montmorillonite obtained with the recovery times (t) indicated in seconds. The right- and left-hand insets show the spectra corresponding to the zero-crossings for components A (fast relaxing phase) and B (slow relaxing phase), respectively. Plots of (1 − Mz(t)/M0) as a function of (b) the recovery time (t) and (c) √t for the intensities of components A (open circles) and B (filled circles) derived from the deconvolutions of the individual spectra in (a). The straight lines in (c) and curves in (b) correspond to leastsquares fits of the data to the expression for a stretched exponential relaxation process (eq 1) and the T′1 values in Table 1.

and √t (Figure 9c) is in accord with a stretched exponential relaxation process where linear regression of the data gives the 29 Si spin−lattice relaxation times for a stretched exponential (T′1) listed in Table 1. The stretched exponential relaxation reflects a distribution in internuclear distances (r) between the 29Si spins and the paramagnetic centers which exhibits the r−6 dependence of the

Table 1. Time Constants for the 29Si Spin−Lattice Relaxation (T′1) for Components A and B in Selected Heated Montmorillonite Samples component A temp (°C) 25 750 800 850 1000

component B

T1′ (s)

δ̅ (ppm)

FA (%)

fwhm (ppm)

T1′ (s)

± ± ± ± ±

−92.5 −98.1 −98.2 −96.6 −104.7

57 58 52 67 84

12.6 26.9 27.8 28.3 20.4

0.045 ± 0.009 0.053 ± 0.015 0.053 ± 0.007 0.081 ± 0.014 11.1 ± 2.7

a

0.010 0.011 0.012 0.012 0.203

0.002 0.002 0.001 0.001 0.016

b

c

d

a

δ̅

b

(ppm)

−93.7 −99.5 −101.3 −107.5 −111.7

FB c (%)

fwhmd (ppm)

43 42 48 33 16

4.2 9.8 17.9 18.1 4.8

a

The time constants are determined from inversion−recovery 29Si MAS NMR spectra acquired at B0 = 4.7 T, assuming a stretched exponential relationship (cf., eq 1). bδ̅ represents average chemical shift weighted by the intensities (I) of the individual peaks (1, 2, ...) employed for deconvolution of either component A or B (i.e., δ̅ = [δ1I1 + δ2I2 + ...]/Itot). cFA and FB denote the relative fractions of components A and B derived from the deconvolution of the fully relaxed spectra. dFull width at half-maximum of the deconvolved subspectra for components A and B. 11471



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IR 29Si NMR spectra for this sample (Figure 10) is composed of the slowly relaxing Q4 type component (B, T′1 = 0.081 s) and the Q3 type component with a faster relaxation rate (A, T1′ = 0.012 s). The Q4 type component is not dominantly present at temperatures lower than 850 °C, and thus, the presence of these sites may explain the abrupt change in reactivity at 850 °C, since they most likely originate from an inert, condensed silica phase, in accordance with the average chemical shifts of −107.5 and −111.7 ppm at 850 and 1000 °C (Table 1). A minor fraction of a Q4 type species may also be present at 800 °C; however, it becomes only dominant for the montmorillonite samples heated at or above the 850 °C, where a breakdown of the structure is expected based on thermal analysis.67 Similar 29Si relaxation behavior and enhanced reactivity for Q3 type phases, as opposed to Q4 type species, has also been observed for rice husk ashes used as supplementary cementitious materials (SCMs) in Portland cement blends.68,69 Thus, the pozzolanic reactivity of heated montmorillonite as a function of the temperature (Figure 7) can now be understood in terms of the thermal decomposition sequence for the clay mineral. While the single-pulse 29Si MAS NMR spectra (Figure 1) suffers from significant line broadening, improved resolution is possible from careful analysis of the IR 29Si MAS NMR spectra acquired for recovery times close to the zero-crossings for the individual components (Figure 10) as demonstrated in this study. Deconvolutions of these spectra and determination of the spin−lattice relaxation times for the distinct silicate species may be utilized in qualitative predictions about the pozzolanic reactivity of not only heated clay minerals but also other siliceous pozzolans/SCMs. Iron may largely be considered as an impurity with respect to the commercial applications of heated clay minerals, as colored heated clays are undesirable both in Portland cement blends and in ceramic products. To further examine the effect of iron on the 29Si relaxation behavior for the silicon species, a set of montmorillonite samples have been heated at 1000 °C for different residence times in the furnace. IR 29Si MAS NMR spectra have been obtained for each sample, and the individual

Figure 10. Inversion−recovery 29Si MAS NMR spectra (4.7 T, νR = 7.0 kHz) of the montmorillonite heated at 750, 800, 850, and 1000 °C, obtained with the recovery times (t) indicated in seconds. The spectra in (a) correspond to the zero-crossing for component B and thereby illustrate the resonances from component A, whereas the spectra in (b) are acquired at the zero-crossings for component A, showing peaks for component B.

(component A) that may cover the core bulk Q4-type phase (component B). The presence of such inner crystalline cores housed by glassy surfaces has been reported by TEM observations on fired clays.20 A comparable behavior where clusters of Q3 and Q4 species have strong differential relaxation has also been observed in lithium silicate glasses65 as well as for amorphous silica66 and thus supports the phenomenon reported here for heated clays. Considering the pozzolanic reactivity (Figure 7), the 29Si spin-relaxation behavior for montmorillonite heated at 850 °C sheds light on the puzzling downward trend in reactivity. The

Figure 11. Variation of the 29Si T′1 values obtained at 4.7 T for component A (a) and component B (b) of the montmorillonite heated at 1000 °C as a function of the total heating time. 11472



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Figure 12. 29Si MAS NMR spectra (9.39 T, vR = 6.0 kHz) following the hydration from 1 to 365 days for (a) pure wPc and (b) the wPc−heated (800 °C) montmorillonite blend (70−30 wt %).

i.e., the Q1, Q2(1Al), and Q2 sites of the aluminosilicate chains of the C-S-H as discussed earlier in relation to the reactivity tests (Figures 6 and 7). Moreover, unreacted heated montmorillonite is observed by the broad resonance ranging from approximately −90 to −120 ppm. A visual inspection of the resonances from the C-S-H phases in the two pastes reveals that the presence of the heated clay results in increased intensities for the Q2(1Al) and Q2 resonances after prolonged hydration, demonstrating that longer average aluminosilicate chains are formed in this blend. The individual spectra in Figure 12 have been deconvolved, using the same approach as illustrated in Figure 4S, giving the average aluminosilicate chain lengths and Al/Si ratios for the C-S-H phases and the degree of clay reaction listed in Table 2. The latter shows that the heated

spectra have been deconvolved using two components and the same approach presented above for the sample heated at 1000 °C for 2 h. The stretched exponential relaxation times (T1′ ) determined for components A and B from the deconvolved intensities are shown in Figure 11 as a function of the residence time in the furnace. Generally, the T′1 values for component B are about 30 times longer than the corresponding values for component A, reflecting a higher concentration of Fe3+ ions in the glassy Q3 type phase. For both sites the T1′ values increase with increasing residence time, corresponding roughly to an increase by a factor of 3 on going from a heat treatment time of 1 /2 to 65 h. This variation shows that the distribution/ aggregation of iron is strongly influenced by the total heating time, suggesting that the Fe3+ ions migrate from the silicate phases during prolonged heat treatment forming a separate iron-rich phase on the surface of the silicate particles. Domains of hematite (Fe2O3) have been shown to form and grow in montmorillonites heated at high temperatures in a previous Mössbauer study,70 and the trend observed here suggests that heating time is also an additional factor contributing to the redistribution of iron other than heating temperature and heating rate. Moreover, these spin relaxation studies support the general conclusion of this work that solid-state NMR can be effectively used to follow the thermal decomposition of phyllosilicates. Portland Cement−Heated Montmorillonite Blend. The performance of the SAz-2 montmorillonite, heated at the optimum temperature of 800 °C, as a SCM is investigated by 27 Al and 29Si MAS NMR spectra following the hydration of a white Portland cement (wPc)−heated montmorillonite blend (70/30 by weight) for up to 1 year. 29Si MAS NMR spectra for this blend and a control paste of pure wPc are shown in Figure 12. In agreement with earlier studies of white Portland cement hydration, the spectra include resonances from the clinker phases, alite (Ca3SiO5, −65 to −75 ppm) and belite (βCa2SiO4, −71.3 ppm), as well as the C-S-H hydration products,

Table 2. Al/Si Ratios, Average Chain Lengths of the Aluminosilicate Chains (CL ) and Pure Silicate Chain Lengths (CL Si) for the C-S-H Phase Formed in the Hydrated White Portland Cement−Heated Montmorillonite Blend Determined from 29Si MAS NMRa wPc−heated montmorillonite (800 °C)

pure wPc hydration time (day)

CL

CL Si

Al/Si

CL

CL Si

Al/Si

degree of clay reaction (%)

1 7 28 90 180 365

2.97 3.09 3.21 3.55 3.90 3.97

2.46 2.52 2.60 2.86 3.04 3.09

0.053 0.056 0.055 0.054 0.058 0.057

3.80 3.20 4.80 5.45 5.87 6.04

2.76 2.48 2.87 3.05 3.11 3.21

0.078 0.069 0.115 0.122 0.129 0.125

4.4 22.4 25.7 27.5 31.9 36.8

a

The blend includes 70 wt % wPc and 30 wt % heated clay. The estimated error limits are ±0.25 for CL and CL Si and ±0.015 for the Al/Si ratios. 11473



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Figure 13. 27Al MAS NMR spectra (14.09 T, vR = 13.0 kHz) of (a) pure wPc and (b) the wPc−heated (800 °C) montmorillonite blend (70−30 wt %) hydrated for up to 180 days. The asterisks indicate spinning side bands, while the open circles show the center band from Al(IV) incorporated in the silicate chains of the C-S-H phase. The resonances in the region for octahedrally coordinated Al denoted by E, M, and T are the center bands from the AFt phase (ettringite), the AFm phase (monosulfate), and the nanostructured aluminate phase formed at the surface of the C-S-H, respectively.

exposed to the reactivity tests (Figure 8), i.e., an Al(IV) resonance from Al in aluminosilicate chains, a peak from 5-fold coordinated Al for AlO5 units in the interlayer of the C-S-H, and an Al(VI) resonance from a nanostructured aluminate phase formed on the surface of the C-S-H.53,54 In addition, center bands from octahedrally coordinated Al in the AFt (ettringite) and AFm (monosulfate) phases are observed at 13.2 and 9.7 ppm, respectively. The principal difference between the series of spectra for the two pastes is the relative fraction of the AFt and AFm phases, since the wPc−heated montmorillonite blend contains a larger fraction of the AFm phase, whereas AFt dominates in the hydrated samples of pure wPc. This may reflect that a larger fraction of aluminum is available in the wPc−heated montmorillonite blend, which will promote the transformation from the sulfate-rich ettringite phase (Ca 6 [Al(OH) 6 ] 2 (SO 4 ) 3 ·26H 2 O) to monosulfate (Ca4[Al(OH)6]2SO4·6H2O) when sulfate ions from the Portland cement are a limiting factor for the formation of these calcium aluminate hydrates. Thus, the 27Al MAS NMR spectra are also in accord with the other observations that have shown that a substantial amount of aluminum is released during hydration of the montmorillonite heated at its optimum temperature.

montmorillonite is partly consumed during hydration, reaching a degree of reaction that is similar to the degree of reaction for fly ashes and slags used as SCMs in cement blends. For the pure wPc, the CL , CL Si, and Al/Si parameters are very similar to those reported earlier for hydrated white Portland cements.49 For the wPc−montmorillonite blend it is apparent that the heated clay contributes in a pozzolanic reaction to the formation of the C-S-H phase and leads to longer average aluminosilicate chains and a higher Al/Si ratio. The average chain lengths of pure silicate chains (CL Si) are very similar for the pure wPc and the wPc−clay blend, demonstrating that the increase in CL for the blended system is primarily caused by the incorporation of AlO4 units in the silicate chains; i.e., the Al(OH)4− ions enter the bridging sites of the dreierketten structure of silicate tetrahedra, thereby linking together units of silicate dimers, pentamers, etc. This observation is in full agreement with the results from the reactivity tests for the heated montmorillonites where a significant increase in the Al/Si ratio was observed for optimum reactivity (Figure 7b). A similar increase in CL and the Al/Si ratio for the C-S-H has earlier been reported for Portland cement−metakaolin blends71 and most recently in a systematic study of wPc−metakaolin blends with wPc replacement levels ranging from 0 to 30 wt %.72 The longer average aluminosilicate chains correspond to the formation of a C-S-H phase with a lower Ca/Si ratio, compared to Ca/Si ratio for the C-S-H resulting from hydration of a pure wPc. This lower ratio reflects that a significant part of the Ca2+ ions is consumed by the pozzolanic reaction of the heated montmorillonite, resulting in a larger amount of C-S-H formed in the blended system. However, the degree of reaction of optimally heated montmorillonite is relatively lower than that of optimally heated kaolinite in a similar cement blend.72 The 27Al MAS NMR spectra following the hydration of the wPc and the wPc−heated montmorillonite blend from 1 to 180 days (Figure 13) exhibit the same type of resonances from the C-S-H phase as observed for the montmorillonite samples

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CONCLUSIONS Structural aspects relating to the pozzolanic behavior of a pure montmorillonite, heated at a series of temperatures, have been successfully studied by solid-state NMR. The clay heated at 800 °C is the most reactive among the clays heated at different temperatures, reflecting a high degree of dehydroxylation and the absence of any inert, condensed Q4 type phases. The resolution of distinct 29Si sites for the dehydroxylated montmorillonite has been further improved in the present work compared to an earlier 29Si MAS NMR NMR study where a combination of Q3 and Q4 units was reported but not clearly resolved. The clear identification of inert phases by 29Si MAS NMR may have major implications on the determination of optimum clay heat treatment temperatures for industrial 11474



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Research and General Conclusions. Cem. Concr. Res. 1985, 15, 261− 268. (5) Brigatti, M. F.; Galan, E.; Theng, B. K. G. Structures and Mineralogy of Clay Minerals. In Handbook of Clay Science; Bergaya, F., Theng, B. K. G., Lagaly, G., Eds.; Elsevier: Amsterdam, 2011; Vol. 1, pp 19−87. (6) Brown, I. W. M.; MacKenzie, K. J. D.; Meinhold, R. H. The Thermal Reactions of Montmorillonite Studied by High-Resolution Solid-State 29Si and 27Al NMR. J. Mater. Sci. 1987, 22, 3265−3275. (7) Rocha, J.; Klinowski, J. Solid-State NMR Studies of the Structure and Reactivity of Metakaolinite. Angew. Chem., Int. Ed. 1990, 29, 553− 554. (8) Rocha, J.; Klinowski, J. 29Si and 27Al Magic-Angle Spinning NMR Studies of the Thermal Transformation of Kaolinite. Phys. Chem. Miner. 1990, 179−186. (9) Cadars, S.; Guégan, R.; Garaga, M. N.; Bourrat, X.; Le Forestier, L.; Fayon, F.; Huynh, T. V.; Allier, T.; Nour, Z.; Massiot, D. New Insights into the Molecular Structures, Compositions, and Cation Distributions in Synthetic and Natural Montmorillonite Clays. Chem. Mater. 2012, 24, 4376−4389. (10) White, C. E.; Provis, J. L.; Proffen, T.; Riley, D. P.; Van Deventer, J. S. J. Density Functional Modeling of the Local Structure of Kaolinite Subjected to Thermal Dehydroxylation. J. Phys. Chem. A 2010, 114, 4988−4996. (11) Sperinck, S.; Raiteri, P.; Marks, N.; Wright, K. Density Functional Modeling of the Local Structure of Kaolinite Subjected to Thermal Dehydroxylation. J. Mater. Chem. 2011, 21, 2118−2125. (12) Muñoz-Santiburcio, D.; Kosa, M.; Hernández-Laguna, A.; SainzDíaz, C. I.; Parrinello, M. Ab Initio Molecular Dynamics Study of the Dehydroxylation Reaction in a Smectite Model. J. Phys. Chem. C 2012, 116, 12203−12211. (13) Drits, V.; Besson, G.; Muller, F. An Improved Model for Structural Transformation of Heat-Treated Aluminous Dioctahedral 2:1 Layer Silicates. Clays Clay Miner. 1995, 43, 718−731. (14) Drachman, S. R.; Roch, G. E.; Smith, M. E. Solid-State NMR Characterisation of the Thermal Transformation of Fuller’s Earth. Solid State Nucl. Magn. Reson. 1997, 9, 257−267. (15) Carroll, D. L.; Kemp, T. F.; Bastow, T. J.; Smith, M. E. SolidState NMR Characterisation of the Thermal Transformation of a Hungarian White Illite. Solid State Nucl. Magn. Reson. 2005, 28, 31−43. (16) Fitzgerald, J. J.; Hamza, A. I.; Dec, S. F.; Bronnimann, C. E. Solid-State 27Al and 29Si NMR and 1H CRAMPS Studies of the Thermal Transformations of the 2:1 Phyllosilicate Pyrophyllite. J. Phys. Chem. 1996, 100, 17351−17360. (17) Frost, R. L.; Barron, P. F. Solid-State Silicon-29 and Aluminum27 Nuclear Magnetic Resonance Investigation of the Dehydroxylation of Pyrophyllite. J. Phys. Chem. 1984, 88, 6206−6209. (18) Fajnor, V.; Jesenák, K. Differential Thermal Analysis of Montmorillonite. J. Therm. Anal. Calorim. 1996, 46, 489−493. (19) Presciutti, F.; Capitani, D.; Sgamellotti, A.; Brunetti, B. G.; Costantino, F.; Viel, S.; Segre, A. 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 2005, 109, 22147−22158. (20) McConville, C. J.; Lee, W. E. Microstructural Development on Firing Illite and Smectite Clays Compared with That in Kaolinite. J. Am. Ceram. Soc. 2005, 88, 2267−2276. (21) Rashad, A. M. Metakaolin as Cementitious Material: History, Scours, Production and CompositionA Comprehensive Overview. Constr. Build. Mater. 2013, 41, 303−318. (22) Fernandez, R.; Martirena, F.; Scrivener, K. L. The Origin of the Pozzolanic Activity of Calcined Clay Minerals: A Comparison between Kaolinite, Illite and Montmorillonite. Cem. Concr. Res. 2011, 41, 113− 122. (23) He, C.; Osbaeck, B.; Makovicky, E. Pozzolanic Reactions of Six Principal Clay Minerals: Activation, Reactivity Assessments and Technological Effects. Cem. Concr. Res. 1995, 25, 1691−1702.

supplementary cementitious materials (SCMs). The detection of amorphous phases is a unique capability of solid-state NMR that may be utilized in much more detail in studies of not only heated clay minerals but also other SCMs. It is also shown that the spin−lattice relaxation behavior for 29 Si nuclei can be effectively utilized to improve the resolution of otherwise broadened resonances. Specifically, inversion− recovery experiments have clearly shown that the 29Si spin− lattice relaxation time is highly sensitive to changes in both the heating temperature and time, a unique observation that results from the redistribution and/or aggregation of paramagnetic impurities (mostly iron) in the clay. Since the formation of hematite upon heat treatment imparts color to a clay, the results presented here are also significant for the synthesis of various industrial products like blended white Portland cements, ceramics, chinaware, etc. Finally, evidence of a pozzolanic reaction for heated montmorillonite has been validated in a Portland cement blend, where it is found to be less reactive than heated kaolinite (metakaolin). A better understanding of the dehydroxylated, disordered structures of heated clay minerals, and their influence on clay’s reactivity/dissolution is needed for designing low CO2 embodied cement blends. Further studies on the locally ordered structure of heated clays using advanced solidstate NMR techniques will be an object of future research.

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ASSOCIATED CONTENT

S Supporting Information *

Tabular results from deconvolutions of 29Si MAS NMR spectra, powder XRD patterns, additional 27Al and 29Si MAS NMR spectra, inversion−recovery 29Si MAS NMR spectra, and macroscopic images of heated clays. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: (+45) 8715 5946. Fax: (+45) 8619 6199. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS 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 Carlsbergfondet, is acknowledged. We thank the Danish National Advanced Technology Foundation for financial support of the SCM project. Staff personnel at FLSmidth A/S, Research Dania, and at Aalborg Portland A/S, Cementir Holding SpA, are acknowledged for useful discussions related to the present work.

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REFERENCES

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