J. Am. Ceram. Soc., 87 [12] 2294 –2296 (2004)

journalChemical Speciation of Trace Zinc in Ordinary Portland Cement Using
X-ray Absorption Fine Structure Analysis
Isao Tsuyumoto† and Hiroshi Uchikawa**
Division of Environmental Engineering, Kanazawa Institute of Technology, 7-1 Ohgigaoka, Nonoichi,
Ishikawa 921-8501, Japan
Chemical change of trace zinc in ordinary portland cement(205.1 ppm) was investigated in hydration process using X-ray
absorption near-edge structure (XANES) and extended X-ray
absorption fine structure (EXAFS). Intensities of the peaks
appearing at the same energy of ZnO in XANES spectra weredecreased with cement hydration. The interatomic distances
and the coordination numbers of the first and the second shells
calculated from EXAFS spectra indicated that ZnO hydrolyzed to zincate ion [Zn(OH)4]2ⴚ with cement hydration
keeping their fundamental structure of ZnO4 tetrahedra.
I.

unlike the case with the cement-stabilized wastes with regard to
the concentration and the chemical state. In recent years, techniques to manufacture cements from the incineration ash of urban
refuse and sewage sludge have been developed.11 Much attention
should be paid in using such waste materials because trace heavy
metals incorporated in cement may bring about a hazardous risk.
Thus, it is essential to evaluate the behavior of trace heavy metals
in cement by analyzing their local structures during hydration toassess a potential environmental risk.
We have already reported that trace hexavalent chromium(⬃6.4 ppm) in cement was gradually reduced to trivalent state
during the hydration process in a specified condition using X-ray
absorption near-edge structure (XANES) analysis.12,13 In using theincineration ash of urban refuse and sewage sludge as the raw
materials of cement, contamination with low-volatile heavy metals
such as zinc and copper is supposed. In this study we focused on
the XAFS analysis of the behavior of trace zinc in ordinary
portland cement during the hydration process.

Introduction

S

PECIATION of toxic heavy metals in the environment using new
analytical methodologies has attracted much attention in recent
years, because comprehension of chemical forms of the toxic
heavy metals in the environment is essential for assessing potential
hazards.1 The mobility, bioavailability, and treatability of theheavy metals are affected by the solubilities of the solid-phase andaqueous-phase metal species, and the toxicity is strongly dependent on their chemical forms.2 Many researchers have reported onzinc speciation in environmental samples such as natural water,sediments, humic substances in soils using extended X-ray absorption fine structure (EXAFS),3–7 and the reactions between zinc andcalcium silicate hydrate (C–S–H) have been studied intensively.8,9Cement has been widely used to stabilize and solidify wastematerials containing the heavy metals, and the long-term leachability of the cement-stabilized wastes is the important factor
associated with landfilling. Ziegler et al. used X-ray absorption
fine structure (XAFS) spectroscopy to elucidate the binding
mechanism of Zn(II) to C–S–H and confirmed the presence ofSi–O–Zn bonds.10 Most of the early reports on cement, however,focused on the stabilization of the waste materials with cement andinvestigated the speciation of the heavy metals at rather highconcentrations (ca. 0.2–3 wt%).
On the other hand, the existence of the heavy metals in cement
has become an environmental problem. For example, elution ofhexavalent chromium from cementitious soil stabilizer tends toexceed the environmental criteria of soil occasionally. Chromium
is probably derived from raw materials such as iron slag and therefractory brick. Other toxic heavy metals are also derived fromraw materials and assumed to be incorporated in cement clinker
minerals as solid solution, because they are burned over 1400°C, atwhich chemical reactions through molten phases occur. This isII.

Experimental Procedure(1) Samples
A standard sample of ordinary portland cement was supplied by
the Cement Association of Japan. Hydrated cement samples wereprepared by mixing with water in a sample bottle, and the cement
paste had a water-to-cement ratio of 0.5. The sample bottles weresealed and cured in an incubator at 20°C for 7, 35, 76, and 294
days. Reference samples containing 20 mg of Zn were preparedusing Zn, ZnO, ZnCl2, ZnSO4, and 2ZnCO3䡠Zn(OH)2, respectively. Each reference sample was mixed with 100 mg of BN(boron nitride) and was pressed into pellet of 13-mm ␾ at 80 kN
for 10 min.
The concentration of zinc in the unhydrated cement powder wasdetermined to be 205.1 ppm (0.02051 wt% Zn) by conventional
quantitative analysis using atomic absorption spectrometry. Thiszinc is intrinsic to cement materials and assumed to be derived
from the natural raw materials.

(2) XAFS Spectroscopy
XAFS spectra were recorded at beamline BL38B01 of SPring-8(Japan Synchrotron Radiation Research Institute, JASRI; WestHarima, Hyogo). An Si-(111) double-crystal monochromator wasused, and higher harmonics were eliminated with Rh-coatedmirrors. Beam energy was calibrated by assigning the first inflection on the L3 absorption edge of a Pb foil to an energy of 13.035
keV. The spectra of unhydrated and hydrated cement samples werecollected in fluorescence mode using a Ge 19-element SSD(GL0110S, Canberra Industries, Inc.). The spectra of the reference
samples were collected in transmission mode using two ion
chambers for I0 and I, both of which were filled with N2. The scanstep was 0.4 eV. All scans were collected in triplicate and averaged
to improve the signal-to-noise ratio. The samples were oriented atL. Klein—contributing editor

Manuscript No. 10747. Received December 18, 2003; approved June 2, 2004.
**Fellow, American Ceramic Society.
†To whom all correspondence should be addressed. e-mail: tsuyu@neptune.
kanazawa-it.ac.jp.

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Fig. 3.

Fig. 1. Zn K-edge XANES spectra for ordinary portland cement at theage of 0, 7, 35, 76, and 294 days, along with ZnSO4, ZnO, Zn, ZnCl2, andZnCO3 for reference.

45° to the incident beam, and the measurements were conducted atroom temperature to avoid transformation of cement materials.
Data reduction was conducted using WinXAS 2.2 according tostandard procedures.14 The background offset was removed bysubtracting a line fit to the preedge data, and the step heights werenormalized to unity. The intensities of particular peaks appearing
in XANES spectra were estimated by fitting with a sum of a
Gaussian function and a Lorentzian function. The ␹(k) functions
obtained from EXAFS spectra were weighed by k2 and Fouriertransformed to attain radial structure functions (RSFs). The Fourier
filtering range was between 1.7 and 13.5 Å⫺1, and a Bessel
window with a parameter of 4 was used to suppress artifacts.
Theoretical scattering paths for the fit were calculated with FEFF
8 using the wurtzite structure of ZnO.15
III.

Results and Discussion

The XANES spectra for the Zn K-edge of the cement at eachage are shown in Fig. 1 along with the reference compounds. TheFig. 2. Peak intensities of the XANES spectra for cement which appear
at the same energy of the ZnO spectrum.

2295

EXAFS k2␹(k) function of zinc in cement.

two peaks which appeared in the spectrum of ZnO (depicted byarrows) were also observed in the spectra of cement, and these
intensities were decreased apparently with hydration of cement.
Figure 2 shows their intensities calculated by fitting, and it proves
the decrease of the two peaks. This result indicates that the tracezinc in unhydrated cement has the same local structure as thewurtzite ZnO, in which ZnO4 tetrahedra were stacked. As hydration proceeds, the stacked structure is broken, and the wurtzite
ZnO transforms into the other chemical form.
The background-subtracted, normalized, and k2-weighted
EXAFS spectra for Zn in cement at each age are shown in Fig. 3.
The spectra suggest some change in the local structure around Zn
with age. Figure 4 shows RSFs obtained by the Fourier transformations of the spectra in Fig. 3. The spectra are uncorrected forphase shift. All spectra showed a main peak around 1.5 Å, whichcorresponded to the first coordination shell of Zn. The second peak
appeared around 2.6 Å, which corresponded to the second coordination shell of Zn. It is important to note that the intensity of thesecond peak was decreased with age.
The structural parameters calculated from EXAFS analysis are
summarized in Table I. In the first coordination shell, the coordination number and the interatomic distance remained unchanged
during hydration. Throughout the hydration process, Zn atom wassurrounded by four O atoms, and the Zn–O distance was in therange of 1.97–2.05 Å. The obtained Zn–O distance correspondswith that of the typical ZnO4 tetrahedron in the wurtzite structure(1.80 –2.04 Å). The results indicate the ZnO4 tetrahedra in cement
keep their fundamental structure throughout the hydration process.
In the second coordination shell, the coordination number wasdecreased drastically from N ⫽ 15.4 of the unhydrated cement as
hydration proceeded, whereas the interatomic distance was almostunchanged. This indicates the decrease of the Zn atoms locatedoutside the vertices of the ZnO4 tetrahedra and, consequently, thetransformation from the solid solution to an individual chemicalFig. 4. RSF obtained by Fourier transformation of the EXAFS spectra inFig. 3.

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Table I. Structural Parameters of Zinc in Cement Paste at
Each Age Derived from EXAFS Analyses, Where R Is theInteratomic Distance, N Is the Coordination Number, and⌬␴2 Is the Debye–Waller Factor
Age (days)

R (Å)

Zn–O shellN⌬␴2 (Å)

R (Å)

07
3576
294

2.01
1.98
2.03
1.97
1.98

4.71
4.51
4.09
4.46
4.48

3.16
3.19
3.14
3.21
3.25

0.01510.00970.00310.00740.0057

Zn–Zn shellN⌬␴2 (Å)

15.4
13.4
10.8
8.0
7.5

0.00860.00320.0072
0.00490.0042

species based on the ZnO4 tetrahedron. The individual chemical
species meeting this condition is tetrahydroxozincate(II) ion[Zn(OH)4]2⫺.
Zinc speciation in ordinary portland cement during hydration
was elucidated as follows. In unhydrated cement, zinc exists as asolid solution in cement matrix, and its local structure is the sameas the wurtzite ZnO. This trace zinc hydrolyzes to tetrahydroxozincate(II) ion [Zn(OH)4]2⫺ synchronously with the hydration ofcement. The most stable phase of zinc at pH 12–13, Eh ⫽ 0 V is
tetrahydroxozincate(II) ion [Zn(OH)4]2⫺ from the Eh–pH diagram.16 This corresponds with our results, and the zinc speciationis the same that has been expected thermodynamically. Ziegler etal. have investigated the sorption of Zn(II) to calcium silicate
hydrate (C–S–H(I)) in aqueous suspensions by electron probe
microanalysis (EPMA).17 They have reported that zinc was observed to incorporate into the C–S–H(I) particles but did notappear to substitute for Ca or Si. These early reports are consistent
with our results in this regard that zinc does not form solid solution
after hydration of cement, though the origin of zinc is different. It
is natural to assume that the [Zn(OH)4]2⫺ which was yielded byhydrolysis of solid solution of zinc is adsorbed on the surface orinterlayer of C–S–H particles.
IV.

SummaryXAFS analysis provides helpful information on the local structure of trace elements in cement matrix. In unhydrated cement, theintrinsic trace zinc exists at the same local structure as the wurtzite
ZnO, in which Zn atoms are surrounded by four O atoms in thefirst-coordination shell. As the hydration of cement proceeds, only
the coordination number of the second shell decreases, whereasthat of the first shell and the interatomic distances of both shells
remain constant. Our results elucidate that zinc exists as a solid
solution in unhydrated cement, having the same local structure asVol. 87, No. 12

the wurtzite ZnO, and that this trace zinc hydrolyzes to tetrahydroxozincate(II) ion [Zn(OH)4]2⫺ with the hydration of cement
keeping their fundamental structure of ZnO4 tetrahedra.
Acknowledgments
The authors thank Drs. Hajime Tanida and Tomoya Uruga, Japan Synchrotron
Radiation Research Institute, SPring-8, in performing the X-ray absorption finestructure experiment. The standard sample of ordinary portland cement was suppliedby the Cement Association of Japan.

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