Thermal dehydration of tobermorite and jennite

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1 Concrete Science and Engineering, Vol. 1, September 1999, pp Thermal dehydration of tobermorite and jennite P. Yu 1 and R. J. Kirkpatrick 2 (1) Department of Materials Science and Engineering, and Center for Advanced Cement Based Materials, University of Illinois, Urbana, IL (2) Department of Geology and Center for Advanced Cement Based Materials, University of Illinois, Urbana, IL RESEARCH PAPERS ABSTRACT Tobermorite and jennite are important model compounds for the calcium silicate hydrate (C-S-H) phase of hydrated Portland cement. Understanding their behavior during thermal dehydration is important to understanding both the structures and properties of the individual phases and the mechanical and thermal properties of cement paste at elevated temperatures. We present here NMR, DSC and TGA, and powder XRD data for heated 1.4 nm tobermorite and jennite. Both specimens undergo a series of phase transformations during heating and above 800 C form wollastonite as the main final product. Prior to formation of wollastonite the tobermorite loses interlayer water and SiOH groups and the silicate chains cross-link, but the fundamental tobermorite layer structure remains present to above 600 C. In contrast jennite loses interlayer water and SiOH and CaOH groups, but the silicate chains depolymerize during heating, and a depolymerized amorphous phase is present in the C range. Jennite is less stable than tobermorite at elevated temperatures due to the presence of non-silicate oxygens, which allow for the depolymerization. 1. INTRODUCTION Tobermorite and jennite are important model compounds for the calcium silicate hydrate (C-S-H) phase of Portland cement [1-5], form in autoclaved building and thermal insulation materials, and are rare natural minerals [6-8]. Understanding their behavior during thermal dehydration is important to understanding both the structures and properties of the individual phases and the mechanical and thermal properties of cement paste at elevated temperatures. Their thermal dehydration and phase transformations during heating have been studied for many years but are not fully understood [1, 6, 7, 9-17]. The structures of 1.4 nm tobermorite and jennite have not been solved, but based on powder X-ray diffraction (XRD) and spectroscopic data and comparison to the known structure of 1.1 nm tobermorite, they are believed to be layer structures [1, 4, 8, 18-21]. In tobermorite, the layer consists of a central Ca-O polyhedral sheet sandwiched between the single Si-O tetrahedral chains. The silicate chains have a dreierketten structure with two tetrahedra (called the pairing tetrahedra, PT) pointing to the Ca-O sheet and one tetrahedron (called bridging tetrahedra, BT) pointing to the interlayer space. In the ideal structure, each BT has one SiOH bond, and all the oxygen atoms in the Casilicate layer are coordinated to Si. The silicate chains are parallel to the b-axis, with a dreierketten repeat distance of about 0.73 nm [4]. The structural formula for the principal Ca-silicate layer is (Ca 4 Si 6 O 18 H 2 ) 2-, with a negative layer charge of -0.5/Ca-atom. There are Ca 2+ and H 2 O molecules in the interlayer, and the layer spacing varies depending on the water content. 1.4 nm tobermorite has 8 H 2 O molecules per structural unit in the interlayer space, (Ca 4 Si 6 O 18 H 2 )Ca 8H 2 O [1, 4, 6]. Hamid reported that 1.1 nm tobermorite has 2 H 2 O molecules per structural unit in the interlayer space [18], but Taylor reported 4 H 2 O molecules in it [4]. When all the interlayer water molecules are absent, the layer spacing decreases to 0.96 nm (riversideite) [4, 17]. The interlayer water content and the layer spacing decrease gradually as tobermorite is heated at increasing temperatures up to 650 C [6]. The jennite structure is thought to be broadly similar to that of tobermorite but with alternating silicate chains omitted and replaced by rows of OH groups. There must also be associated changes in the Ca polyhedral sheet [1, 4, 5, 8]. The oxygens in the CaOH rows are not bonded to Si [1, 5, 20-21], and are called non-silicate oxygens here. The structural formula of the principal layer of jennite is [Ca 8 Si 6 O 18 H 2 (OH) 8 ] 2-, with a negative layer charge of 0.25/Ca-atom, half that of tobermorite. As in tobermorite, there are Ca 2+ and H 2 O molecules in the interlayer space, ISSN en cours/99 RILEM Publications S.A.R.L. 185

2 Concrete Science and Engineering, Vol. 1, September 1999 and the generally accepted structural formula is [Ca 8 Si 6 O 18 H 2 (OH) 8 ]Ca 6H 2 O [8]. The layer spacing of jennite is 1.05 nm. When jennite loses 4 H 2 O molecules, it becomes metajennite and the layer spacing decreases to 0.9 nm [8]. We present in this paper a systematic investigation of the thermal dehydration of 1.4 nm tobermorite and jennite including XRD, DSC/TG, and NMR data. Based on these data, we establish structural models for the thermal behavior of these phase and improve understanding of the dehydration mechanisms of hydrous Ca-silicates. 2. EXPERIMENTAL The 1.4 nm tobermorite and jennite samples were hydrothermally synthesized by putting the raw materials (CaO and quartz for tobermorite, and CaO and acid silica for jennite) in Parr Acid Digestion Bombs at 80 C for days [5]. The jennite sample contains a small amount of 1.4 nm tobermorite as impurity. To study dehydration, 300 mg of each phase were heated in air in a platinum crucible in a muffle furnace at the temperatures from 110 to 1000 C for 1 hour and quenched in air, then stored in glass bottles and analyzed by XRD, DSC/TG, and NMR at room temperature. The samples were exposed to air during handling and measurement. Powder X-ray diffraction patterns (XRD) were obtained with a RIGAKU D/Max-B X-ray diffractometer, using Cu radiation at 2θ angles from 5 to 65 degree, using a step size of 0.04 and a scanning speed of 5 /min. Simultaneous differential scanning calorimetry and thermogravimetry (DSC/TG) data were obtained with a Simultaneous Thermal Analyzer STA 409 (NETZSCH) with Al 2 O 3 as the reference material, and using 8-9 mg of D- dried sample with a heating rate of 25 C/min in dry nitrogen at a constant flow rate of 75 cm 3 /min. 1 H NMR spectra were recorded in an H o field of 11.74T at 500 MHz using a solid state Varian Unity/INOVA 500 spectrometer running VNMR 5.3B software, and using a Doty Scientific 7 mm supersonic CP/MAS probe with magic-angle-spinning at 8.6 KHz, and a 7 mm Si 3 N 4 rotor and Kel-F caps (proton-free). The π/2 pulse width was 2.85 µs, the recycle time was 5 seconds, and four scans were collected. A spin-echo sequence was used to recover the broad signals due to short T 2 relaxation time. 5% H 2 O in D 2 O was used as the external reference (4.8 ppm). The 29 Si MAS NMR spectra were recorded at an H o field of 8.45T at a resonance frequency of 71 MHz with a home-build spectrometer and magic-angle-spinning at 3.5 KHz in a 7 mm Al 2 O 3 rotor using quadrature single-pulse excitation. The π/2 pulse width was 10 ms, a 30 pulse and a recycle time of 60 seconds were used. Spectra obtained with a longer delay time suggest that some saturation of the Q 4 signal occurred for the tobermorite heated at 1000 C. TMS (Si(CH 3 ) 4 ) was used as the external reference (0 ppm). 3. RESULTS AND INTERPRETATION XRD The powder XRD patterns for tobermorite clearly show the phase transformations due to dehydration and recrystallization (Fig. 1). With one exception these are similar to previously published work [6, 15, 17]. At 23 C (room temperature) the basal spacing is 1.42 nm, and the observed peaks are comparable to those described by Hara et al. and Roberts et al. [22, 23]. At 110 C the basal spacing changes to 1.18 nm, a value intermediate between the known 1.4 and 1.1 nm phases. The (002) peak at θ is quite sharp and is not the result of overlap of peaks for the 1.4 and 1.1 nm phases. We interpret this observation to indicate the existence of a phase with a basal spacing of about 1.2 nm. To our knowledge, this phase has not been previously identified. At higher temperatures the dehydration sequence is the same as that previously described by other workers [6, 17]. 1.1 Fig. 1 Powder XRD patterns of 1.4 nm tobermorite before and after heat treatment at specified temperatures: 23 C, 1.4 nm tobermorite; 110 C, 1.2 nm tobermorite; 250 C, 1.1 nm tobermorite; C, 0.96 nm tobermorite; 600 C, intermediate phase; 800 C, wollastonite-like phase; 1000 C, wollastonite. 186

3 Yu, Kirkpatrick Fig. 2 Powder XRD patterns of jennite before and after heat treatment at specified temperatures: 23 C, jennite and trace amount of tobermorite (T); C, metajennite and tobermorite (T); C, disordered phase; C, wollastonite and larnite (L). nm tobermorite is present at 250 C, 0.96 nm tobermorite is present at C, a transition phase (poorly crystallized wollastonite) is present at 800 C, and well crystallized wollastonite is present at 1000 C. The XRD data for heated jennite show a series of quite different phase transformations (Fig. 2). Metajennite is present between 110 to 250 C due to loss of part of the interlayer water and the resulting decreased layer spacing from 1.06 nm to 0.86 nm, as previously described [8]. The broad and weak peak at 1.4 nm at room temperature, and the peak at about 1.1 nm for the 110 and 250 C samples are due to the impurity of tobermorite. The loss of most of the Bragg diffraction peaks and the presence of a broad envelope centered at about 30 2θ for the C samples indicate that jennite takes on a highly disordered structure in this temperature range. Diffraction data are not very useful for probing the structure of amorphous phases of this type, but the spectroscopic data described below provide important insight into its structure and formation. This phase recrystallizes into wollastonite and larnite (β-ca 2 SiO 4 ) by 800 C. DSC/TG The DSC/TG curves for 1.4 nm tobermorite (Fig. 3 A) indicate a series of detectable dehydration steps with substantial heat absorption and weight loss events at about 90, 200, 260, and 660 C. The sharp endothermic peak and large weight loss at 90 C are due to the phase Fig. 3 DSC and TG data of 1.4 nm tobermorite (A) and jennite (B). transformation to the 1.18 nm phase observed by XRD and confirm the occurrence of this phase. The features at 200 and 260 C are due to the transformations to the 1.1 nm and 0.96 nm phases, respectively. The endothermic peak and weight loss at 670 C can be attributed to decarbonation [15]. The exotherm at 830 C is due to the transformation from dehydrated tobermorite to wollastonite as observed by powder XRD data. The DSC/TG profiles of jennite (Fig. 3 B) show a series of events that is consistent with the XRD data. There is an endotherm and weight loss at about 130 C due to dehydration of jennite to metajennite. The continuous endothermic feature and weight loss between 160 and 360 C can be assigned to release of remnant interlayer water in the metajennite and the small amount of tobermorite present. The broad endotherm and weight loss from 360 to 460 C correspond to the transformation to the disordered phase observed by XRD. The endothermic peak at 680 C is due to decarbonation [15]. The exotherms at 830 and 885 C are due to the formation of wollastonite and larnite based on the XRD data. (1) 1 H MAS NMR The 1 H MAS NMR spectra for 1.4 nm tobermorite before and after heating (Fig. 4 A) are consistent with the XRD and DSC/TG data and provide important new insight 187

4 Concrete Science and Engineering, Vol. 1, September 1999 by XRD and DSC/TG and likewise reveal important information about the role of H. At room temperature, the 1 H NMR spectrum of jennite contains signals at 4.8, 1 and 8 ppm, due to molecular water, CaOH and SiOH, respectively [24-27]. The strong signal at 1 ppm clearly shows the presence of a significant amount of CaOH and supports the structural model proposed by Gard et al. [8]. The CaOH signal is dramatically reduced for the 350 C sample, suggesting an intimate connection between dehydroxylation of CaOH groups and formation of the highly disordered structure observed by XRD. The peaks due to SiOH and CaOH become negligible above 450 C, and the intensity of the signal due to molecular water decreases with increasing temperature, in agreement with DSC/TG data. Fig. 4 1 H MAS NMR spectra of 1.4 nm tobermorite (A) and jennite (B) before and after heat treatment at specified temperatures. into the role of H during dehydration. At 23 C there is a strong signal at 4.8 ppm due to molecular water, a weaker peak at about 1 ppm due to CaOH, and a weak shoulder at about 8 ppm due to SiOH [24-27]. There is no CaOH in ideal 1.4 nm tobermorite, the peak at 1 ppm is probably due to layer-edge sites and defects. The water peak is broad due at least in part to residual 1 H- 1 H dipolar interaction. There are also likely to be multiple water sites and variable degrees of hydrogen bonding. The molecular water could be interlayer water, water molecules adsorbed on the surface, and water in nano-scale intraparticle voids. The spectra of the heated samples have reduced intensity, as expected from the water loss, and narrower peaks. At 110 C the signal at 4.8 ppm due to molecular water becomes narrower, indicating reduced 1 H- 1 H dipolar coupling due to reduced water abundance. At 250 C the spectrum exhibits a broad and strong signal centered at 2.4 ppm and a broad signal with peak maximum at 3.7 ppm. With increasing temperature from 350 to 800 C, the 1 H signals due to SiOH and CaOH disappear and the peak due to water molecules becomes narrower due to reduced 1 H- 1 H dipolar coupling caused by lower water content, consistent with DSC/TG data. The very narrow peak present for the high temperature (800 and 1000 C) samples is probably due to water adsorbed onto the surface from the air. The very weak signal at 1 ppm at high temperature is due to probe background. The 1 H spectra of the heated jennite samples (Fig. 4 B) are also consistent with the dehydration sequence observed (2) 29 Si MAS NMR The 29 Si MAS NMR spectra of tobermorite (Fig. 5 A) show that its chain structure is conserved during dehydration. The spectrum of 1.4 nm tobermorite contains two peaks, corresponding to the predominate Q 2 sites (-85.3 ppm) in the single silicate chains, and minor Q 1 sites (-79.4 ppm) caused by the absence of some bridging tetrahedra [5]. The Q 2 peak progressively broadens up to 600 C, demonstrating disordering of the structure. The disordering is probably caused by varying mean Si-O-Si bond angles and changing structural environments due to removal of interlayer water and SiOH and CaOH groups. The 250 C sample also yields a peak for Q 3 tetrahedra (-95.9 ppm), indicating some cross-linking of the silicate chains in this sample. It is Fig Si MAS NMR spectra of 1.4 nm tobermorite (A) and jennite (B) before and after heat treatment at specified temperatures. 188

5 Yu, Kirkpatrick unclear if signal for Q 3 sites is present for the 350, 450 and 600 C samples, because the peaks are relatively broad. At 800 C the Q 2 peak becomes narrower and shifts toward high field (-90.1 ppm), suggesting increased local structural order and increased mean Si-O-Si bond angles for the poorly crystallized wollastonite-like transition phase observed by XRD. The mean Si-O-Si angles per tetrahedron are 129.1, 137.2, and for 1.1 nm tobermorite [18], whereas they are 136.0, 143.6, and for the three Sisites of wollastonite [28]. The two Q 2 peaks at and ppm at 1000 C demonstrate the formation of wellcrystallized wollastonite [29], consistent with our XRD observation. A spectrum collected with a longer recycle time shows a weak Q 4 signal near -110 ppm for the 1000 C sample. Silica phases often have very long T 1 values and signal for them can be lost due to an insufficient delay time, as in Fig. 5. The Ca/Si ratio of the 1.4 nm tobermorite is 0.9, whereas that of wollastonite is 1. Thus, formation of amorphous SiO 2 probably accommodates the extra Si from the tobermorite. The formation of wollastonite from tobermorite probably does not occur by local (unit cell scale) structural reorganization which retains the individual silicate chains. The arrangement of the Ca atoms and the Ca/Si ratios are different in wollastonite and tobermorite, and wollastonite and amorphous silica probably nucleate and grow separately. The changes in the 29 Si NMR spectra of the heated jennite samples are quite different than for tobermorite but are in good agreement with the XRD and DSC/TG data. The Q 2 peak (-85.4 ppm) broadens with increasing temperature to 250 C, paralleling the dehydration behavior of tobermorite but without the occurrence of Q 3 signal. From 350 to 600 C the peak width increases drastically, and the mean value becomes significantly less negative, indicating progressive depolymerization of the silicate chains [30]. The chemical shift range of this peak for the 600 C sample is from about -70 to -95 ppm. This very broad peak covers the known range of peak positions of Q 0, Q 1 and Q 2 silicate tetrahedra [30] and demonstrates a highly disordered structural state as suggested by the XRD data. At 800 C, wollastonite (2M-CaSiO 3 ) which has three Q 2 sites, and larnite (β-ca 2 SiO 4 ), which has a single Q 0 site, are clearly indicated by the observed peaks. 4. DISCUSSION Dehydration of tobermorite 1.4 nm tobermorite undergoes a series of phase transformations during thermal dehydration and recrystallization. Upon heating it loses interlayer water and the layer spacing decreases from 1.4 to 1.2, 1.1, and to 0.96 nm (Fig. 1). Each step corresponds to an endothermic process and weight loss, indicating independent phases (Fig. 3). Taylor reported that 1.4 nm tobermorite loses almost 50% of its interlayer water on heating to 100 C and considered the phase formed at this temperature to be 1.1 nm tobermorite [1]. Wieker et al. heated synthetic 1.4 nm tobermorite at 100 C for 6 hours and also found 1.1 nm tobermorite [30]. However, our XRD data demonstrate the presence of a 1.2 nm phase at this temperature, which transforms to the 1.1 nm phase by 250 C. Because we heated our samples for only 1 hour, it is likely that the 1.2 nm phase is metastable with respect to the 1.1 nm phase at 110 C. Farmer et al. reported that the natural Crestmore 1.4 nm tobermorite transforms to 1.1 nm tobermorite from 55 to 200 C and to 0.96 nm tobermorite at 200 to 450 C [6]. The same transformations for our synthetic tobermorite occur at higher temperatures, perhaps because of the shorter heating time we used. Analysis of our TG data for tobermorite provides a clear picture of the interlayer compositions of the various phases. The total weight loss is about 18% at 700 C, and the weight loss between 600 to 700 C due to decarbonation is about 1%. Thus the weight loss due to dehydration is about 17%, which is lower than the maximum mass loss of 20% for ideal 1.4 nm tobermorite due to the presence of residual hydrogen at high temperatures (Fig. 4). The weight loss upon heating to 90 C is 8%, corresponding to a loss of 4/9 of the total 9 H 2 O for ideal 1.4 nm tobermorite (C 5 S 6 H 9 ) [4]. Thus the structural formula of the 1.2 nm tobermorite is (Ca 4 Si 6 O 18 H 2 )Ca 4H 2 O. The weight loss upon heating to 200 C is 12.5%, corresponding to 6/9 of the total H 2 O. This suggests the structural formula for 1.1 nm tobermorite to be (Ca 4 Si 6 O 18 H 2 )Ca 2H 2 O, in agreement with Hamid s result for a sample with a Ca/Si ratio of 0.83 [18]. The weight loss at C is about 17%, corresponding to 8/9 of the total H 2 O, indicating a structural formula of (Ca 4 Si 6 O 18 H 2 )Ca for the 0.96 nm phase. The weight loss at different temperatures and the dehydration products are listed in Table 1. Table 1 Tobermorite weight loss and dehydration products Temperature Weight Loss Number of Structural Formula ( C) (Wt.%) H 2 O Lost per Formula Unit (Ca 4 Si 6 O 18 H 2 )Ca 8H 2 O (Ca 4 Si 6 O 18 H 2 )Ca 4H 2 O (Ca 4 Si 6 O 18 H 2 )Ca 2H 2 O (Ca 4 Si 6 O 18 H 2 )Ca 189

6 Concrete Science and Engineering, Vol. 1, September 1999 The occurrence of Q 3 sites in the 29 Si NMR spectra of 1.1 nm tobermorite shows that dehydration of 1.4 nm tobermorite causes cross-linking of some silicate chains. This cross-linking is probably caused by local charge imbalances resulting from loss of H from Si-OH groups via the net reaction 2Si-OH Si-O-Si + H 2 O. The 1 H NMR spectra of Fig. 4 and our recently published near infrared spectra of tobermorites clearly demonstrate the presence of Si-OH linkages [31]. Q 3 sites are well known from many tobermorite samples [5] and link tetrahedra across the interlayer. Such linkages could form cages in the interlayer which would significantly modify the structural environments and the dynamical behavior of the interlayer water. The broad, more shielded 1 H NMR signal observed for water molecules in the 250 C sample is consistent with less mobile molecular water [32] (a greater residual 1 H- 1 H dipolar broadening) that is on average more strongly H-bonded. The broadening of the Q 2 signal of the tobermorite in the temperature range C indicates disordering of the silicate tetrahedral chains, probably dominantly by varying Si-O-Si bond angles, stacking disorder, and rotation of the silicate tetrahedra. However, the majority of the tetrahedra remain in chains, and the fundamental layered tobermorite structure is retained to at least 600 C. This interpretation is consistent with previous work [6]. Dehydration behavior of jennite Jennite experiences a fundamentally different sequence of phase transitions during thermal dehydration and recrystallization than tobermorite. As previously known [1, 8], it loses interlayer water and becomes metajennite at 100 and 250 C. Our XRD, DSC/TG, and spectroscopic data, however, show that by 350 C it loses almost all of its CaOH groups and that the silicate tetrahedra become substantially depolymerized. The result is a depolymerized X-ray amorphous phase that has lost its fundamental layer structure and is structurally disordered both locally and over long distances. The sample undergoes continued depolymerization and structural reconstruction between 350 to 600 C but remains disordered. The formation of wollastonite and larnite by 800 C must occur by nucleation and growth. The lower thermal stability can be attributed to the presence of the non-silicate oxygen atoms in the CaOH rows. Loss of water by dehydration of these sites via the net reaction 2Ca-OH Ca-O-Ca + H 2 O leaves highly reactive O-atoms with a large excess negative charge. These react with the silicate chains by a reaction of the type Ca-O-Ca + Si-O-Si 2Si-O-Ca. This reaction eliminates non-silicate oxygens and bridging oxygens to create nonbridging oxygens and thus depolymerizes the silicate tetrahedra. In contrast, all the oxygen atoms in the Ca-silicate layer of ideal 1.4 nm tobermorite are coordinated to Si. The absence of Ca-OH linkages in its structure prevents depolymerization and allows the layer structure to remain to higher temperatures until the reaction rates become high enough to allow formation of wollastonite. 5. CONCLUSIONS 1. Tobermorite and jennite undergo a series of phase transformations during heating and eventually form wollastonite as the main final product. Jennite also produces larnite (β-c 2 S). 2. The layer structure of jennite has lower thermal stability than that of tobermorite due to the presence of non-silicate oxygens. Jennite loses its basic layer structure upon heating to 350 C, whereas the tobermorite maintains its layer structure to at least 600 C. 3. Tobermorite dehydrates by loss of interlayer water and Si-OH groups, and the silicate chains become crosslinked. 4. Jennite loses interlayer water, Si-OH groups and Ca-OH groups. The non-silicate oxygens coordinate to silicon and form non-bridging oxygens. The silicate chains depolymerize. And the fraction of bridging oxygen decreases during heating. ACKNOWLEDGMENT The authors would like to thank the Center for Cement Composite Materials for financial support. We also thank Mr. John Bukowski for conducting the DSC/TG measurements, and Mr. Paul Moliter for help collecting 1 H NMR spectra. REFERENCES 1. Taylor, H. F. W., Proposed structure for calcium silicate hydrate gel, J. Am. Ceram. Soc. 69 (6) (1986) Taylor, H. F. W., Nanostructure of C-S-H: current status, Adv. Cem. Based Mater. 1 (1993) Cong, X. and Kirkpatrick, R. J., Effects of the temperature and relative humidity on the structure of C-S-H gel, Cem. Con. Res. 25 (1995) Taylor, H. F. W., Tobermorite, jennite, and cement gel, Z. Kristallogr. 202 (1992) Cong, X. and Kirkpatrick, R. J., 29 Si and 17 O NMR investigation of the structure of some crystalline calcium silicate hydrates, Adv. 190

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