J. CHEM. SOC. FARADAY TRANS., 1994, 90(14), 2095-2098 Direct Observation of Aluminium Guest Ions in the Silicate Phases of Cement Minerals by 27AI MAS NMR Spectroscopy Jsrgen Skibsted and Hans J. Jakobsen Department ofChemistry, University ofAarhus , DK-8000 Aarhus C, Denmark Christopher Hall Schlumberger Cambridge Research, PO Box 153,Cambridge, UK CB3 OEL The principal mineral phases in Portland cements are the impure forms of the calcium silicates, Ca,SiO, and Ca,SiO,, known as alite and belite, in which the silicates are modified in composition and crystal structure by incorporation of guest ions such as Mg2+, A13+ and Fe3+. This work reports the first direct evidence for guest- ion substitution in alite and belite by the observation of Al substitution employing 27AI magic-angle spinning (MAS) NMR.For both minerals A13+ is observed to substitute for Si4+ and it is shown that 27AI MAS NMR can be used to quantify the amounts of Al substitution in the silicate phases of ordinary Portland and oilwell cements at levels well below 1 wt.%. The 27AI quadrupole coupling parameters and isotropic chemical shift (6) have been determined for the unique At guest-ion site in belite, which exhibits the most deshielded chemical shift (6 = 96.1) yet reported for a tetracoordinated Al bonded to four oxygens. Portland cements used in the construction industry usually contain 50-70 wt.% alite (Ca,SiO,), 5-25 wt.% belite (Ca,SiO,), 0-15 wt.% tricalcium aluminate (Ca3A1,0,) and 5-15 wt.% ferrite [Ca,(Al,Fe,-,),O, ;0 < x 5 0.71.The sili- cate phases, alite and belite, are the main hydraulic com- ponents responsible for strength development during hydration. Although both silicates can exist in several crys- talline modifications, metal-ion impurities incorporated into the lattice during manufacture normally stabilize the mono- clinic forms M,, MI, and MI,, of alite and strongly stabilize the monoclinic P-form of belite.' However, the presence of such ions can greatly affect the physical and chemical properties of cements. In Portland cements MgO, A1203 and Fe203 are the main oxide impurities of both silicates. Typical concentrations of Al,O, are 1 wt.% in alite and 2 wt.% in belite.' Experimental methods for characterising the nature and quantity of such impurities are essential for understand- ing the material properties of Portland cements, especially the variation in the hydrational reactivity which controls strength development.High-speed '7Al MAS NMR spectroscopy3 has proven a powerful tool in studies of a range of different materials since the 27Al isotropic chemical shift (6) distinguishes clearly between A1 in tetrahedral (AIIv) and in octahedral (AlvI) coordination., Furthermore, the 27Al quadrupole coupling constant (C,) and asymmetry parameter (q)reflect the electric field gradients (EFGs) at the nuclear site and therefore provide additional information about the elec- tronic environments at the A1 nucleus, as recently shown for some aluminate phases of cement materials., This work describes the nature of the incorporation of A1 as an impurity ion in laboratory samples of alite and belite and in an ordi- nary Portland and an oilwell cement as studied by 27Al MAS NMR.Experimental The laboratory samples of alite and belite were provided by Aalborg Portland A/S, Denmark. Powder X-ray diffraction (XRD) of the two samples showed that the alite has a mono- clinic form (MI and/or M,,,) and the belite the monoclinic /?-form. X-Ray fluorescence (XRF) analysis revealed an A1203 content of 1.1 f0.1 wt.% for the alite and 0.7 f0.1 wt.% for the belite sample. Furthermore, XRF showed the presence of minor quantities of MgO, Fe203, Mn,03, P20, and K20 in both samples. The phase compositions for the commercial ordinary Portland and oilwell cement were estimated from a modified Taylor-Bogue using the data from bulk chemical analysis.Solid-state 27Al MAS NMR spectra were obtained on Varian XL-300 (7.1 T) and UNITY 400 (9.4 T) spectrometers equipped with home-made 'A1 background-free' cross-polarisation (CP)/MAS probes for 4 and 7 mm 0.d. Si3N, rotors7 and employing spinning speeds up to 16 kHz. Single-pulse excitation using an rf field strength yB1/2n = 60 kHz, a 0.7 ps pulse (l5O flip angle), and a relaxation delay of 1 s ensured reliable quantitative result^.^.^ The Al,O, contents of the samples were deter- mined from the intensities of the central transition using weighed samples and the "A1 MAS NMR spectrum of syn- thetic Ca,A120, as standard.The results are compared with the data from XRF and X-ray microprobe analysis. "A1 Chemical shifts (6) are reported in ppm relative to an external sample of 1.0 mol dmV3 AlC13.6H20. Simulations of the solid-state 27Al MAS NMR spectra were performed on the SUN Sparc 10 work stations of the UNITY 400 spectrometer using a general software package developed in the Aarhus laboratory for simulation of MAS/variable-angle spinning (VAS) NMR spectra of quadrupolar nuclei." Results and Discussion The high-speed "Al MAS NMR (9.4 T) spectrum of the belite sample is shown in Fig. l(a) and reveals two central transitions for A1 incorporated into the belite. In the Al,, chemical shift range (ca. 100-50 ppm) the edge at 91 ppm, and the singularities at 77 and 56 ppm, characterize the quad- rupolar lineshape for a single A1 site.The Al,, shift range (ca. 20 to -10 ppm) displays an asymmetric resonance with a centre of gravity at -0.6 ppm. The lineshapes of the simu- lated spectrum in Fig. l(b) are in excellent agreement with the experimental spectrum. The simulation employs quadrupole coupling parameters corresponding to a single Al,, site with the optimized parameters C, = 7.1 & 0.1 MHz, q = 0.33 f0.05 and 6 = 96.1 0.5 and to a gaussian dis- tribution of EFG tensor elements for the Al,, resonance. The optimized simulation of the asymmetric resonance for the Al,, site is obtained by summation of a set of simulated quadrupolar lineshapes corresponding to a gaussian distribu- tion of the V,, and V,, EFG tensor elements (I V,, I >, I V I >, V,, 1 ) as recently performed for an amorphous glass.' lyyThis procedure gives 6 = 10.1 and the mean quadrupole coupling J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 120 100 80 60 40 20 0 -20 -40 -60 -80 6 Fig. 1 (a) 27Al MAS NMR spectrum of A1 guest ions in belite recorded at 9.4 T using a spinning speed v, = 15.5 kHz (4 mm rotor) and 101 680 scans. (b)Simulation of the partly overlapping lineshapes of the Al,, and Al,, resonances in (a) using an Al,, : Al,, intensity ratio of 58 :42. For the single Al,, site the simulation employed the optimized 27Al quadrupole coupling parameters and isotropic chemi- cal shift in Table 1. For the asymmetric resonance from the Al,, site (centre of gravity at -0.6 ppm) a set of simulated quadrupolar line- shapes corresponding to a gaussian distribution of the V,, and V,, EFG tensor elements is employed (see text).parameters c, = 4.5 MHz and ij = 0.4 for the Al,, site employing a gaussian distribution function with a halfwidth of 0.75vzz(= 0.75CQh/eQ)for the V,, and V,, tensor elements. Quantitative 27Al MAS NMR,**' using the combined inten- sity for the central transitions from Al,, and Al,,, and a syn- thetic sample of Ca,Al,O, as standard, gives a bulk Al,O, content of 0.7 wt.% for the belite sample, which is in excellent agreement with the result from XRF analysis (see Table 1). The observation of a single Al,, resonance for belite demon- strates that A1 substitutes for Si,, because the crystal struc- ture for /3-Ca,Si04 shows that all Si atoms are equivalent.', We note that the value 6 = 96.1 determined for the Al,, site, although close to the shifts observed for calcium alum in ate^,^ represents the most deshielded chemical shift yet reported for tetracoordinated A1 bonded to four oxygen^.'^ This high- frequency chemical shift further supports substitution of A1 for Si into the crystal structure for /3-Ca,Si04 (Si-0 bond lengths of 1.61-1.65 A)', since we would expect the A1-0 bonds for the A1 guest ion in belite to be shortened relative to the Al,,-O bond lengths in calcium aluminates (1.73-1.81A).'. A less ionic character is therefore predicted for the A1-0 guest-ion bonds in belite which should lead to a shift to higher frequency consistent with earlier observations.' The absence of octahedral sites or vacancies in the crystal structure for /3-Ca,Si04 suggests that the Al,, resonance b..k= 500 400 300 200 100 0 -100 -200 -300 6 Fig.2 27Al MAS NMR spectra of monoclinic alite illustrating that A1 substitution for Si,, is the dominant mode for the incorporation of A1 in alite. (a) Experimental spectrum at 9.4 T (v, = 15.6 kHz, 4 mm rotor, 61440 scans) with (b)expansion of the central transition for the Al,, resonance. (c) Expansion of the central transition in the experimental spectrum at 7.1 T (v, = 8.0 kHz, 7 mm rotor, 8192 scans). The asterisk in (a)indicates the Al,, resonance observed in the alite sample; this corresponds to 3% of the total aluminium intensity.originates from a separate aluminate phase. Furthermore, the distribution of quadrupole coupling parameters observed for the Al,, resonance [Fig. l(b)] strongly indicates that the addi- tional phase is amorphous or contains slightly different Al,, sites. This observation is consistent with a recent electron mi- croscopy study of Al,O,-doped belites, where an amorphous Al-rich grain-boundary phase was observed between the belite grains.16 On this basis we propose that A1 for Si,, sub-stitution is accompanied by charge-balancing oxygen vacancies as earlier proposed by Regourd et al.' 27Al MAS NMR spectra (central transition) of the alite sample (Fig. 2) show an Al,, quadrupolar broadened line- shape [from ca.88 to 30 ppm at 9.4 T, Fig. 2(a) and (b)] which further broadens at 7.1 T [ca. 90-20 ppm, Fig. 2(c)]. The centre of gravity is about 75 ppm at both field strengths. The lineshape bears no resemblance to the well known C,, q-dependent quadrupolar lineshape for a single A1 site. However, it can be ascribed to the superposition of several lineshapes originating from A1 substituting for different Si tetrahedral positions in alite. Simulations of the lineshapes at 7.1 and 9.4 T in Fig. 2, using a set of quadrupolar lineshapes, indicate that the quadrupole coupling parameters and the isotropic shifts are in the range 3.5 5 CQ/MHz 5 7.0, 0.1 5 q 5 0.7 and 80 5 6 5 91 for the A1 guest-ion sites in alite. The observation of several Al,, sites is fully consistent with the broadening observed in the 29Si MAS NMR spectrum" and the crystal structure" of monoclinic alite which contains 18 different Si,, sites in the asymmetric unit for the MI,, form.Table 1 Optimized 27Al MAS NMR data (C,,q and 6) for A1 guest ions in belite and alite and A1,0, contents for belite, alite and for these phases and Ca,Al,O, of an oilwell and ordinary Portland cement wt.% A1,0, sample CQ/MHz v 6IPPm 27Al MAS NMR" XRF or X-ray microanalysis belite Al,, Alm 7.1 _+ 0.2 4.5' 0.33 f0.05 0.4' 96.:01z0.5 ] 0.7 & 0.1 0.7 fO.lb alite oil well cement ca. 3.5-ca. 7' 0.1 5 q 5 0.7' 80 5 6 5 91' 0.9 f0.1 1.5 f0.2 1.1 fO.lb 1.5 f0.1 (alite)' ordinary Portland cement silicate phases 1.1 f0.2 1.7 0.1 (belite)' I Ca,Al,O, ~ phase 1.9 f0.2 I a From the intensity of the central transitions using a 27Al MAS NMR spectrum of synthetic Ca,Al,O, as standard.From XRF analysis. 'Parameters corresponding to a gaussian distribution of the V,, and V,, EFG tensor elements (see text). Estimated range for the C,, q and 6 values from simulation of the central transition at 7.1 and 9.4 T using a set of quadrupolar lineshapes. From X-ray microanalysis of the alite and belite phases in unground clinkers from the same source. Not determined. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 XRD investigations of doped Ca,SiO, have suggested that besides replacing Si,, , A1 can substitute for octahedrally coordinated Ca or vacancies.,' Although a low-intensity Al,, resonance [Fig.2(a)], observed at 4 ppm and constituting 3% of the total intensity, may arise from this type of substitution, the present study shows that A1 substitutes almost exclusively for tetrahedral Si sites in alite. Furthermore, an examination of the Ca-0 bond lengths in the monoclinic MI,, form of alite demonstrates that substitution of A1 for Ca would entail excessively long A1-0 bonds. Thus, again charge-balancing is most likely achieved by creation of one oxygen vacancy for each pair of substituted A1 ions. We note that the A1,0, content of 0.9 wt.%, determined for the alite sample from the intensities of the central transitions in Fig. 2(4, is in excellent agreement with an XRF analysis (Table 1).Furthermore, this value is a typical A1203 impurity level for alites in normal Portland cements.' Direct observations of A1 substitution in the silicate phases of production cements are illustrated by the NMR spectra of an oilwell and an ordinary Portland cement in Fig. 3. Oilwell cements are Portland cements with characteristically low bulk Al,O, : Fe203 ratios (typically < 1.0) and are used in well construction as slurries which are pumped between the wellbore wall and steel casing.'.6*21 For these cements the tricalcium aluminate phase (Ca,Al,O,) is usually absent and A1occurs mainly in the ferrite phase [Ca,(Al,Fe, -J205; 0 < x5 0.71 which is extremely difficult to detect by 27Al MAS NMR because of its high Fe3+ content.The strong similarity of the NMR spectra for the oilwell cement [Fig. 3(a)] and the synthetic alite [Fig. 3(b)] demonstrates the great potential of 27Al MAS NMR to provide detailed information on A1 substitution in the silicate phases of cements. Consider- ing the phase composition of the oilwell cement (70 wt.% alite, 7 wt.% belite and 20 wt.% ferrite from a modified Taylor-Bogue we expect that its NMR spec- trum [Fig. 3(a)] also includes a minor A1 contribution from the belite phase. Quantitative 27Al MAS NMR shows that a total A1 content corresponding to 1.5 0.2 wt.% Al,O, is incorporated into the alite and belite phases of the oilwell cement. By direct X-ray microanalysis of unground clinker from the same source, we obtain comparable Al,03 contents for the two phases (see Table 1).For the ordinary Portland cement (74 wt.% alite, 7 wt.% belite, 10 wt.% tricalcium aluminate and 6 wt.% ferrite2V6) the MAS spectrum [Fig. 3(c)] shows an asymmetric peak at 79 ppm superimposed on a broad resonance extending from CQ. 100 to -40 ppm. These resonances are assigned to A1 in alite-belite and to the tricalcium aluminate phase, respec- tively. Contrary to the well defined quadrupolar lineshapes observed for the two non-equivalent Al,, sites in synthetic Ca,Al,O, ,3*5 the tricalcium aluminate resonance in Fig. 3(c) is featureless. This is ascribed to the incorporation of impu- rity ions in the Ca3Al,0, lattice in Portland cements,' resulting in distortions in the local environments of the AlO, tetrahedra.Fig. 30 illustrates that the lineshape of the broad tricalcium aluminate resonance can be simulated using a gaussian distribution of the elements for the two EFG tensors for Ca3Al,06 ., Deconvolution [Fig. 3(d)] of the spectrum of the ordinary Portland cement, employing the simulated 27Al spectra of alite [Fig. 3(e)] and tricalcium aluminate [Fig. 301, leads to a quantification of Al,O, of 1.1 f0.2 wt.% and 1.9 +_ 0.2 wt.% for the alite-belite and tricalcium alumin- ate phases, respectively. The latter quantity corresponds to 4.9 f0.5 wt.% Ca3A1,0, which we consider an improved value for the calcium aluminate content compared with the somewhat uncertain estimate (10 wt.%) obtained from the Taylor-Bogue We emphasize that high-speed spinning is a prerequisite for the observation (and thereby ll! 1 ~60""'40-4 IA \ i\ /AT------I'..'I.'~'I....I'..'I''"~l'.I I.'..250 200 150 100 50 0 -50 -100 -150 6 Fig. 3 27A1 MAS NMR spectra (9.4 T, v, = 15.8 kHz,4 mm rotor) of an oilwell and an ordinary Portland cement. (a) Experimental spectrum of the oilwell cement (26 750 scans). The close similarity of the expanded Al,, lineshape in (a) and that observed for the alite sample (b) from Fig. 2 demonstrates that A1 is incorporated exclu- sively into the silicate phases (mainly alite) of the oilwell cement. (c) Experimental spectrum of the ordinary Portland cement (20736 scans) showing overlap of resonances from (i) A1 guest ions in alite (and belite) and (ii) the tricalcium aluminate phase.(d) Optimized simulated spectrum for the Portland cement obtained by summation of the simulated lineshapes for Al in (e) the alite phase and cf)the tricalcium aluminate phase. The simulated alite spectrum (e) is obtained by addition of a set of quadrupolar lineshapes with C,, q and 6 values in the range 3.5 < Co/MHz < 7.0, 0.1 < q < 0.7 and 80 < 6 < 91. The simulated tricalcium aluminate spectrum cf) employs addition of a set of quadrupolar lineshapes for each of the two All, sites in Ca,A120, using a gaussian distribution of the V,, and V,, EFG tensor elements similar to the simulation in Fig. l(b). The C,, q and 6 data reported for the two All, sites in synthetic Ca,AI,O, 3*5were used as mean values for these parameters and a gaussian distribution function with a halfwidth of O.lV,, for the V,., and V,, tensor elements were employed for both A1 sites.quantification) of undistorted central transitions for the broad resonances from the Ca,Al,O, phase in Portland cements. In conclusion, our results demonstrate that A1 prefer- entially substitutes for tetrahedrally coordinated Si in the sili- cate phases, alite and belite, of these cements. MAS NMR represents an elegant and very sensitive tool for 2098 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 detailed characterization and quantification of the discr sites for this guest ion in cement and similar minerals. A1 9 10 D. Massiot, C. Bessada, J. P. Coutures and F.Taulelle, J. Magn. Reson., 1990,90,231. J. Skibsted, N. C. Nielsen, H. Bildsse and H. J. Jakobsen, J. 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