Investigation of damaged interior walls using synchrotron-based XPS and XANES

Evidence of internal sulfate attack in field exposure was demonstrated by the damaged interior wall of a three-year-old house situated in Nakhon Ratchasima Province, Thailand. Partial distension of the mortar was clearly observed together with an expansion of a black substance. Removal of the black substance revealed a dense black layer. This layer was only found in the vicinity of the damaged area, suggesting that this black material is possibly involved in the wall cracking. By employing synchrotron-based X-ray photoelectron spectroscopy (XPS) and X-ray absorption near-edge structure (XANES) techniques, the unknown sample was chemically identified. The S 2p and O 1s XPS results mutually indicated the existence of sulfate species in the materials collected from the damaged area. The XANES results indicated the presence of ferrous (II) sulfate, confirming sulfate-induced expansion and cracking. The sulfate attack in the present case appeared to physically affect the structure whereas the chemical integrity at the molecular level of the calcium silicate hydrate phase was retained since there was a lack of spectroscopic evidence for calcium sulfate. It was speculated that internal sulfate probably originated from the contaminated aggregates used during the construction. The current findings would be beneficial for understanding the sulfate-attack mechanism as well as for future prevention against sulfate attack during construction.

1. Introduction

Sulfate attack is one of the most important phenomena affecting the durability of cement mortars and concrete. It is generally categorized as external or internal sulfate attack, depending on the source of sulfate ions (Ouyang et al., 1988). External sulfate attack typically involves penetration of sulfate ions from solutions such as sea water and ground water to the concrete. Sulfate chemically reacts with hydrated cement and, as a consequence, ettringite and gypsum are formed as primary products (Chen & Jiang, 2009; Santhanam et al., 2002, 2003; Tian & Cohen, 2000). This causes volume expansion and extensive cracking leading to the loss in strength and disruption of the structure. It has been reported that thaumasite can also be formed as a reaction product of sulfate attack at low temperature and causes the degradation of the hydrated cement structure. On the other hand, the internal sulfate attack involves the incorporation of sulfur- and/or sulfate-containing materials during the preparation period. Pyrite (FeS₂), naturally found in the earth's crust, is known to be one of the sulfur-rich minerals. This mineral can be readily oxidized in the presence of oxygen and moisture to produce sulfuric acid and iron sulfates (Lowson, 1982). The products contained in the aggregate materials can cause expansion, affecting the concrete durability. The reaction extent is dependent on the original grain size of the sulfide (Casanova et al., 1996, 1997). Shayan reported the deterioration of the concrete surface of a ten-year-old hospital floor slab due to the oxidation of pyrite in the pyritic aggregate. It was observed that a considerable amount of pyrite was found in the aggregate used in the concrete in the blistering area (Shayan, 1988).

2. Experimental

The samples for measurements were prepared in solid form. The unknown black substance was collected from the damaged interior cement mortar wall. The standard compounds FeS₂ (99.9%, Alfa Aesar), FeSO₄.7H₂O (99.999%, Alfa Aesar), Fe₂(SO₄)₃ (99.999%, Alfa Aesar), Ca(OH)₂ (≥96%, Sigma-Aldrich), CaCO₃ (99.999%, Aldrich), CaO (99.9%, Aldrich) and CaSO₄ (99.0%, Sigma-Aldrich) were used without further purification. The hydrated cement was prepared by mixing the cement with deionized water at a water:cement ratio of 20:1. The mixture was kept at room temperature for 24 h. The hydrated cement was filtered and dried at 313 K and kept in the desiccator prior to the measurement.

X-ray photoelectron spectroscopy (XPS) experiments were carried out at BL3.2a of the SLRI using the CLAM2 energy analyzer. The beamline employs a varied-line-spacing plane grating to monochromatize synchrotron radiation. The excitation energy was 600 eV. The working pressure was controlled within 10⁻⁸ mbar at ambient temperature.

X-ray absorption near-edge structure (XANES) measurements were performed at the time-resolved XAS (Bonn-SUT-SLRI) beamline of the Synchrotron Light Research Institute, Thailand (Poo-arporn et al., 2012). The beamline employs an energy-dispersive monochromator equipped with a Si(111) single crystal and position-sensitive detector to obtain the XANES spectra. The spectra were recorded at an integration time of 1000 ms with an average of ten readouts.

3. Results and discussion

The studied cement mortars were collected from the damaged wall of a house situated in Nakhon Ratchasima Province, Thailand (Fig. 1a). The damage was found on a structural brick wall, covered with painted cement mortar at around 36 months after the construction. Partial distension of the mortar was clearly observed together with an expansion of a black substance. Removal of the black substance revealed a dense black layer under the pale brown cement mortar (Fig. 1b). The typical size of the crack area was approximately 20 mm × 20 mm and 3 mm in depth. The black layer expanded gradually as clearly seen in Fig. 1(c), which was taken seven months after the previous removal of the black substance. Drilling a 6 mm hole through the black layer revealed that the thickness of the black layer was about 2 mm. It was interesting that the black layer was only found in the vicinity of the damaged area, suggesting that this black material is possibly involved in the wall cracking. Further investigation on chemical properties of the black substance would provide significant information for the mechanism of the damages.

Figure 1 Photographs of the damaged area on the interior wall: (a) the crack was initially observed, (b) the same area after partial removal of the black substance, (c) seven months later, and (d) the same area after the expanded mortar.

The black substance was sampled from the damaged wall and characterized using XPS. This technique is usually employed to identify the type and oxidation state of an element in a material of interest. The principle of XPS is based on the photoelectric effect. The interaction between X-rays with sufficient energy and the atoms produces photoelectrons which travel with kinetic energy (KE) given by

where hυ is the energy of the photon, BE is the binding energy of the electron and φ_s is the spectrometer work function. Using an excitation energy of 600 eV, the black substance gave three intense XPS peaks consistent with O 1s, C 1s and S 2p (Fig. 2a). Additionally, the Auger electron peaks corresponding to O KLL and C KLL appeared in the range 60–160 eV and 305–390 eV, respectively. The results indicated that the black substance was composed of O and S. The observed signals from C were attributed to the carbon reference added during the sample preparation. It should be noted that the subtraction of the C 1s signal was impossible due to technical limitation. Since the black substance was collected from the damaged cement mortar, the contribution from Ca and Si was probably expected. The photoelectron peaks corresponding to Si 2p and Ca 2p were previously reported at 101 and 347 eV, respectively (Black et al., 2003). It was clear that the photoelectron energy of Si 2p significantly overlapped the Auger electron peaks of O KLL and that of Ca 2p lay in the energy range of C KLL. To justify the contribution from Ca and Si, the photoelectron peaks were technically shifted from the Auger region by changing the excitation energy. The XPS spectra collected by tuning the excitation energy to 500 eV (Fig. 2b) and 400 eV (Fig. 2c) clearly showed no significant XPS peak corresponding to Ca 2p and Si 2p. Therefore, XPS results suggested that the black substance was a sulfur-rich compound.

Figure 2 XPS spectra of the black substance collected from the cracked area. The survey scans were measured at a photon energy of (a) 600 eV, (b) 500 eV and (c) 400 eV.

The fine-scanned S 2p XPS spectra of the black substance with curve fitting showed only a single type of sulfur at the binding energy of 169.5 eV, consistent with sulfur of the sulfate (Fig. 3a) (Paneva et al., 2007). The fine scan for O 1s displayed a broad and asymmetric feature due to contributions from different oxygen species (Fig. 3b). Peak fitting analysis demonstrated two binding energies at 530.9 and 532.0 eV, which could be ascribed to the oxygen bound to sulfur and hydroxyl group, respectively (Huang et al., 2008). The binding energy of oxygen–sulfur significantly indicated the presence of sulfate in the black substance. The presence of hydroxyl groups in the cement mortar is predictable since Ca(OH)₂ is typically produced during the hydration of cement (Kirby & Biernacki, 2012).

Figure 3 Fine-scan (a) S 2p and (b) O 1s XPS spectra of the black substance with the curve fitting.

The sulfur in the black substance potentially originated from the aggregates used during the construction. Generally, sulfur-containing compounds can be naturally found in both oxidized and reduced forms. Examples of the oxidized forms of sulfur are sulfoxide, sulfone, sulfonate and sulfate whereas the reduced forms include thiophene and sulfide (Prietzel et al., 2007). Pyrite (FeS₂) is one of the most common sulfur-containing compounds found on the earth's crust. Mechano­chemical treatment destabilized the pyrite and resulted in the formation of Fe₂SO₄.H₂O (Godočíková et al., 2002). In addition, the presence of oxygen and moisture could also oxidize the pyrite to produce ferrous sulfate through the reaction (Sammut et al., 1996)

The oxidation of iron pyrite has been previously studied (Singer & Stumm, 1970). The oxygenation reaction of Fe²⁺ occurs in the presence of molecular oxygen. However, the half-time of the reaction in acidic pH is approximately 1000 days, which is relatively slow when compared with the oxidation of pyrite. The atmospheric oxidation reactivity of the surface of pyrite is dependent on species and abundance of such species on a freshly fractured surface (Chandra & Gerson, 2010). Further investigation by the XPS measurement of Fe was required to determine the involved iron species. However, it should be noted here that the signals of Fe 3s and 3p were very weak and that the measurement of Fe 2p XPS spectra was not reported due to the insufficient photon flux at excitation energy higher than 600 eV.

The other spectroscopic technique, i.e. XANES, was utilized in order to verify the involvement of iron in the black substance. XANES is an extremely useful characterization technique as it provides element-specific data as well as local-symmetry information as a consequence of multiple scatterings of the photoelectron (Bianconi, 1988). In this study, Fe K-edge XANES measurements of the unknown sample were carried out and compared with the spectra of the known standard compounds. Fig. 4 illustrates the normalized Fe K-edge XANES spectra of the black substance together with the standard FeS₂, FeSO₄ and Fe₂(SO₄)₃. Typical absorptions of the Fe K-edge were in the energy range 7125–7128 eV. The presence of the Fe K-edge XANES signal in the spectrum collected from the black substance clearly indicated the existence of iron in the sample. Significantly, the white line of Fe K-edge XANES together with the oscillation feature measured from the sample exactly resembled that of the standard FeSO₄, suggesting their chemical similarity. The contrast XANES results between the sample and either FeS₂ or Fe₂(SO₄)₃ ruled out the existence of both species in the black substance. One might also comment on the smaller intensity of the white line of the sample than that of the standard FeSO₄. In fact, the difference in intensity of the white line was generally observed (Lytle et al., 1979; Sham, 1985). The energy of the white line of Fe K-edge XANES is due to the electronic transition from 1s → p states. This absorption cross section is strongly influenced by 3p–3d hybridization. In such a system where the p–d hybridization is weak, the p-like density of states is strongly reduced and only weak structures are observed in this region. Therefore, its intensity is proportional to the density of states of the unoccupied electronic level, related to the Pauling ionicity of the metal–ligand bonds of the compounds (Short et al., 1983; Sinfelt et al., 1981). In this case, the smaller white line peak of the sample suggested the smaller Pauling ionicity between Fe²⁺ and the sulfate ligand.

Figure 4 Fe K-edge XANES spectra of the black substance and the standard compounds.

In order to confirm the above findings, the speciation of sulfate was further examined. The S K-edge XANES spectra of the compounds containing sulfur with oxidation state of +6 were measured (Fig. 5). Although the white-line energy of the S K-edge XANES of the black substance at 2481.5 eV was quite close to those of the standard FeSO₄, Fe₂(SO₄)₃ and CaSO₄ at 2481.6, 2481.8 and 2481.6 eV, respectively, the presence of Fe₂(SO₄)₃ and CaSO₄ was ruled out. The difference in the appearance of S K-edge XANES between these standard sulfate-containing compounds is due to the sensitivity on multiple scattering of the excited electron to multi-atom correlations. This electron confers sensitivity to the spatial arrangement of the neighbouring atom including radial distance, orientation and bond angle. In addition, a small shift of the white line between the black substance and standard FeSO₄ suggests the appearance of an alternate donating atom around S.

Figure 5 S K-edge XANES spectra of the black substance and the standard compounds.

It is commonly known that the formation of gypsum (CaSO₄.2H₂O) occurs especially at the condition of high sulfate concentration. However, the appearance of the characteristic small post-edge peak at 2485 eV in the spectrum of CaSO₄ but not in that of the sample suggested that the presence of CaSO₄ in the sample was improbable. In addition, the spectrum of Fe₂(SO₄)₃ also showed different pre-edge and post-edge features compared with that of the sample, implying that the contribution from Fe₂(SO₄)₃ was irrelevant. Therefore, the above results implied that the black substance is comprised of iron and sulfur in the form of FeSO₄.

The effect of sulfate on the cement mortar was investigated in order to gain information and understand the mechanism of the sulfate attack. Typically, tricalcium silicate (C₃S) is the main phase of the clinker in Portland cement. The hydration products usually consisted of calcium silicate hydrate of variable stoichiometry, denoted as C-S-H (Kirby & Biernacki, 2012). The sulfate attack on the cement mortar usually produces a mineral phase containing calcium and sulfate, namely ettringite and gypsum. Fig. 6 shows the normalized Ca K-edge XANES spectra of the black substance and the standards, Ca(OH)₂, CaCO₃, CaO and CaSO₄. The fully hydrated cement was also used for comparison. The white-line peaks of the black substance, the hydrated cement, Ca(OH)₂, CaCO₃ and CaO were observed at 4044.7 eV, 4044.7 eV, 4045.7 eV, 4049.1 eV and 4051.0 eV, respectively. Clearly, the white-line peak of the C-S-H phase in the hydrated cement appeared at lower energy than that of the standard CaO, suggesting the formation of CaO:SiO₂ solid solution. In addition, Ca(OH)₂ commonly precipitates as portlandite during the cement hydration. Therefore, the Ca K-edge XANES of the hydrated cement was probably due to the contribution from both CaO and Ca(OH)₂. Significantly, the Ca K-edge XANES spectrum of the black substance was precisely identical to that of the hydrated cement, indicating the similarity of their local structures. As the Ca K-edge XANES spectra of CaCO₃ and CaSO₄ were clearly distinguishable from that of the black substance, the presence of both compounds in the cement mortar was excluded. The overall results indicated that the local structure of the C-S-H phase in the cement mortar was insignificantly disturbed by the sulfate. Therefore, the observable expansions and cracks on the interior wall made of cement mortar were presumably the consequence of the precipitation of FeSO₄ phase rather than the attack of sulfate on the molecular structure of the C-S-H phase. It was noticed in this case that only the physical appearance of the interior wall was damaged but the chemical identity of the cement mortar remained unchanged.

Figure 6 Ca K-edge XANES spectra of the black substance and the standard compounds.

4. Conclusions

Expansion and cracking of the cement mortar were shown from the damaged interior wall of a three-year-old house. By employing the powerful combination of X-ray photoelectron spectroscopy and X-ray absorption spectroscopy, identification of the oxidation state and local structure of the unknown compound was accomplished. In the present investigation the unknown black substance formed within the cracked cement mortar was mainly comprised of iron and sulfur in the form of the FeSO₄ phase as indicated by the S 2p and O 1s XPS results. Further investigation by the Fe K-edge and S K-edge XANES measurements confirmed the presence of the FeSO₄ structure. The precipitation of this phase within the mortar caused local expansion and cracking. The formation of the FeSO₄ phase appeared to affect only the physical strength of the mortar whereas the structure at the molecular level of the C-S-H phase was essentially unperturbed. The lack of evidence for the CaSO₄ phase implied that the chemical stability of the C-S-H phase against the sulfate attack as the extraction of calcium ions from the structure was impossible. The sulfate attack in this case is believed to occur internally as the interior wall was free from external sulfate species. It is speculated that the internal sulfate probably originated from the contaminated aggregates used during the construction. Further investigation on the chemical constituent of the aggregates would provide valuable information for the origin of the internal sulfate species.