CEMENT and CONCRETE RESEARCH. Vol. 23, pp. 773-784, 1993. Printed in the USA. 0008-8846/93. $6.00+00. Copyright © 1993 Pergamon Press Ltd.

AN FTIR AND XPS INVESTIGATIONS OF THE EFFECTS OF CARBONATION ON THE SOLIDIFICATION/STABILIZATION OF

CEMENT BASED SYSTEMS-PORTLAND TYPE V WITH ZINC

M. Yousuf A. Mollah ÷, Thomas R. Hess, Yung-Nien Tsai and David L. Cocke

Environmental Chemistry Laboratory Department of Chemistry

Lamar University Beaumont, TX 77710

(Refereed) (Received Dec. 4, 1991; in f'mal form March 29, 1993)

A B S T R A C T The effects of carbonation on the solidification/stabilization (S/S) of zinc using Portland cement Type V (OPC) has been investigated by X-ray photoelectron spectroscopy(XPS) and Fourier transform infrared spectroscopy (FTIR). The FTIR results indicate that carbon dioxide reacts with the m e t a l - b o n d e d and h y d r o g e n - b o n d e d h y d r o x i d e s [CaZn2(OH)6.2H20] generally present in zinc-doped Portland cement (Zn- OPC) samples. XPS examinations of the carbonated products of the undoped and doped (Zn-OPC) samples indicate that the resulting silica phases have chemical states different from standard silica gel. Both FTIR and XPS results suggest that carbonation promotes polymerization of the SiO4uni ts present in OPC matrix. The chemical states of the surface components obtained by XPS analysis are discussed in the light of polymerization of the silicates.

Introduction The design of a model solidification/stabilization (S/S) system

requires the knowledge of the physical and chemical properties of the system and especially an understanding of the way hazardous substances are bound in the S/S medium. In particular, this information can be useful for understanding the leaching mechanisms which will ultimately provide reliable projections and design parameters that will ensure long term immobilization of the wastes in such S/S systems. It is, therefore, absolutely necessary to understand the chemistry of such cement based

+ Visiting Professor, Department of Chemistry, University of Dhaka, Bangladesh.

773

774 M.Y.A. Mollah et al. Vol. 23, No. 4

S]S processes and the factors that govern these processes. In recent publications [1-12] from our laboratory we have reported the mutual interact ions of the priority metal pollutants and cement based S/S systems. Some results have also been reported [7,9,12] on the leaching behaviors of these S/S systems.

However, although the hydration of Portland cement (OPC) is a complex area of research it is generally accepted that poorly- crystalline calcium hydrosi l icate phases (C-S-H) of variable stoichiometries and morphologies and calcium hydroxide (CH) are the two most important chemicals formed from tricalcium silicate (C3S)and [3-dicalcium silicate ([3-C2 S) hydraulic phases. The reaction involved is:

6Ca2+(aq) + 5HSiO3-(aq) + 7OH'caq)--->6CaO.5SiO2.6H20 (C-S-H) (1 )

It is well known that the hydration of OPC is also affected by atmospheric carbon dioxide. The carbonation of cementi t ious materials has been reported in the literature [13-15]. These authors have also discussed the two main reactions of carbonation,

C-S-H + CO2 ---> CaCO3 + silica + H20 (2 ) CH + CO2 ---> CaCO3 + H20 (3 )

The carbonat ion has been found to be a complex process and is dependent on the nature of the cement: type, porosity, permeabili ty, water to cement ratio [13,16], humidity [14,17], carbon dioxide partial pressure [18,19] and nature of the dopant [20-23]. The first reaction converts the C-S-H to calcium carbonate, water and a highly polymerized silica which is acid stable while maintaining the same morphology as the original hydrate [24-26]. The second reaction is expected to be accompanied by an increase in volume of the solid. Each mole of calcium hydroxide (sp. gr. 2.24 g/ml, tool vol 33.0 ml) is converted to the carbonate (sp.gr. 2.71 g/ml, mol vol 36.9 ml) such that there is an 11.8% increase in solid volume [27]. This volume increase will lead to structural problems [27-29], if it occurs in the bulk rather than on the surface. Carbonation of hydrated cement may also lead to a reduction in the pH of the alkal ine cement paste [27]. There is, however , a considerable disagreement as to which of the reactions, 2 or 3, dominate [13,14,30]. However, whichever reaction dominates, the S/S of Lewis acid type metal pollutants by cementitous materials is expected to be dependent on the types and quantities of Lewis bases, the redox status (pe) and the acidity- alkalinity (pH) of the particular environments [31].

Today , spec t roscopic techniques like, IR [30,32-35], FTIR [1,5,11,36], Raman [35,37-39], high resolution NMR [40-45], M6ssbauer [33, 46,47] are being increasingly used to investigate the hydration of OPC. The applications of XPS [48,49] and MAS-NMR [50] for similar type problems are also being investigated. A number of authors have reported

Vol. 23, No. 4 FTIR, XPS, CARBONATION, ZINC, SOLIDIFICATION 775

on the carbonation of cement during hydration [14,17,18,30,51-56]. The bulk of these studies were carried out under low carbon dioxide pressures and are concerned mainly with the effect of carbonation on the rate of strength deve lopment due to the setting of OPC paste. However , the influence of carbon dioxide on the hydration process with respect to the S/S of metal pollutants has not yet received any attention. These effects must be delineated and incorporated into any detailed models of the S/S systems. To this end we present some FTIR and XPS results on the carbonation of OPC and Zn-OPC systems.

Exper imen ta l Sulfate resist ing Portland cement Type V used in this investigation

was received from Texas Industries Inc. The metal, as nitrate, was added at 10% by weight based on the cation. Deionized water was used to prepare the solution and a water to cement ratio (w/w) of 0.35 was maintained to prepare the samples. The wet cement samples were then placed in polys tyrene sample vials and al lowed to set under normal laboratory conditions. Some of the samples were also cured in an empty dessicator which was maintained at a slight positive pressure of carbon dioxide. The samples were analyzed after curing for twenty-eight days.

FTIR: The FTIR spectra were recorded using a Nicolet, Model FTIR 500 (150 scans), and a IBM FTIR, Model 20 instruments (32 scans). The potass ium bromide pellets were prepared by a constant sample to KBr ratio (1.0 mg sample/100 mg of KBr) and total weight (50.0 mg). The FTIR spectra recorded in the range 4000 to 400 cm -1 were obtained at 2 c m -1 resolution.

XPS: The XPS analysis of the air-cured and CO2-cured OPC samples, the air-cured and CO2-cured Zn-OPC samples and the dry OPC clinker was carried out using a Kratos XSAM 800 photoelectron spectrometer fitted to a custom-bui l t vacuum chamber. A portion of the bulk sample was ground into a fine powder and pressed into a stainless steel sample holder. The analysis chamber was maintained at 5X10 -8 Torr or better dur ing the analysis . Mg Ka radiat ion (1253.6 eV) was e m p l o y e d th roughout the exper iment . The spectrometer , control led by a DSS00 operating system, was operated at low magnification, med ium resolution and fixed analyzer transmission (FAT) mode. To compensate for the shift in energy due to surface charging, the observed binding energies were all adjusted by assigning the value for the C( ls ) line f rom advent i t ious carbon to a BE=285.0 eV. The C( ls ) region showed three partial ly overlapping peaks that in some cases required decomposi t ion and fitting to assign the adventitious carbon peak at 285.0 eV. A low binding energy peak was obta ined f rom the metal sample holder and gave direct informat ion on electrical connect ion between the nonconduct ive sample and the spect rometer . An addit ional surface charging that un i fo rmly affected all the other observed peaks was accounted for using the middle carbon peak. The high binding energy carbon peak cor responds to

776 M.Y,A. Mollah et 01. Vol. 23, No, 4

carbonate. Samples were run several times until consis tent binding energies were obtained for all peaks as compared to our laboratory standards and to the literature.

Results and Discussion FTIR: Typical FTIR spectra of the dry clinkers and hydrated Portland

cement (OPC) Type V treated under different experimental conditions are shown in Figure 1.

The corresponding spectra for the zinc-doped OPC (Zn-OPC) are presented in Figure 2. Examination of the FTIR spectra of hydrated cement, (Figure lb and c), in comparison with the spectrum for the dry clinkers, (Figure l a), reveal much of the effects of CO2 on the hydration of OPC. The prominent features that appear upon hydrat ion and subsequent setting of the cement is the appearance of a strong bands at 1480 cm-1 and 875 cm-1 due to the CO3 2" and a broad H20 stretching band that appears at 3400 cm -1. In Figure l b, there is a small sharp peak at 3650 cm -1 due to the hydroxyl (O-H-) stretching band. This band is not very strong because most of the calcium hydroxide species (CH) have reacted with the CO2 in the air according to reactions 3 to form CaCO3.

v, OH ~ . /

VI+V 2 ,H20

V~ H20

cot

a

4000 3060 2soo 2ooo lsoo looo soo Wavenumbers (cm"~

Figure 1: FTIR spectra of, (a) OPC dry clinkers,(b) hydrated OPC, air cured, (c) hydrated OPC, carbon dioxide cured.

Vol. 23, No. 4 FTIR, XPS, CARBONATION, ZINC, SOLIDIFICATION 777

3635 ' ~ 3615

3540 3415

b

a

144~

1050

5

6O3

820

876

'1335

1350

4000 3000 2000 1000 400

Wavenumbers (cm "1) Figure 2. FTIR spectra of, (a) Zn-OPC, cured in air(b) Zn-OPC, cured in

carbon dioxide.

This band does not appear in the CO2-cured sample indicating all of the calcium hydroxide species have reacted. Closer examination of the FTIR spectra shows a marked decrease in the intensity of the Si-O asymmetric s t re tching band due to tr icalcium silicate ( C 3 S ) a n d / o r 1 3 - d i c a l c i u m silicate ( f I - C 2 S ) c e n t e r e d at 925 cm -1 and a new band appears at a higher frequency. In the air-cured sample (Figure lb), the new band is centered at 960 cm-1 while in the CO2-cured sample (Figure lc) the band appears at 1085 cm -1 with a shoulder at 1160 cm -1. This new band is actually the Si- O asymmetr ic stretching band shifted to higher f requency due to the polymer iza t ion of the orthosilicates that occurs upon format ion of the silica phase upon setting of the OPC [30]. The sulfate bands (SO42-) at

1160-1100 cm -1 in the OPC dry clinker spectra are obscured due to polymer iza t ion of the orthosil icate units. Compar ison of the relative intensities of the v4 and v2si l icate bending bands between Figures la, lb and lc also indicate the degree of polymerization. The decrease in the intensity of the out-of-plane Si-O bending vibrations (v4) appearing at 525 c m -1 and the cor responding increase in the in-plane Si-O bending v ibra t ion (v2) at 455 cm -1 also reflects a decrease in the f reedom of motion that occurs upon polymerization of the orthosilicate units.

Since the out-of-plane deformation (v4) is being restricted with the progress of hydration and increased linkage between Si-O-Si units, the v3

778 M.Y.A. Mollah et al. Vol. 23, No. 4

of SiO4 4- vibrat ions in hydrated samples has a reduced intensi ty and appears at a higher energy with respect to the dry clinker. Together, the shifting of the v3 s i l i ca t e band to a much higher frequency region, the change in the relative intensities of v 2 a n d v4 silicate bands and the disappearance of hydroxyl band clearly demonstrate that CO2 is reacting with both CH and C-S-H according to reactions 2 and 3.

Examination of the FTIR spectra of zinc-doped Portland cement (Zn- OPC) (Figure 2a) cured under normal atmospheric conditions with that of the undoped sample reveals much of the effect of zinc doping. The appearances of a number of sharp bands between 3650-3400 cm -1 is due to the f o rma t ion of specif ic m e t a l - b o n d e d and h y d r o g e n - b o n d e d hydroxides which have been delineated and reported by Cocke and co- workers [11].

It has been established that these sharp bands are due to the formation of C a Z n 2 ( O H ) 6 . 2 H 2 0 according to the following reactions:

2 Zn(OH)3- + Ca 2+ + 2 H 2 0 - - - > CaZn2(OH)6.2H20 (4 )

2 Zn(OH)2 + Ca 2+ + 2 OH- + 2 H 2 0 - - - > CaZnz(OH)6"2H20 (5 )

The band due to the silica phase now appears as a poorly defined shallow peak centered at c__a.a. 1000 cm -1. The CO32- ions in calcite have three infrared active vibrational modes occurring at 874, 1429-1492 and 706 c m -1 [25]. The broad and strong bands (Figure 2a & 2b) between 1445- 1420 cm -1 is attributed to v3 of CO32- and the sharp bands at 875 and 715 c m -1 are due to v2 and v4vibrat ions. The unresolved bands at 1445-1420 cm -1 may be attributed to both CaCO3 and ZnCO3. There is no evidence of the formation of the aragonite form of carbonate where the v2 CO32 band appears at 857 cm -1 [30]. The bands at 1390 and 1350 cm -1 are due to nitrates from the zinc nitrate solution. In order to investigate the effect of carbonation on the Zn-OPC system a Zn-doped sample was cured in a CO2- rich atmosphere and the corresponding FTIR spectra is presented in Figure 2b. It is interesting to note that the sharp hydroxyl bands, 3650-3400 cm- 1, in Figure 2a are virtually absent. The absence of these bands in the FTIR spect rum clearly demonstra tes that C a Z n z ( O H ) 6 . 2 H 2 0 is not present in CO2-cured sample. This indicates that the C a Z n 2 ( O H ) 6 . 2 H 2 0 has either reacted with the CO2 or never formed under the CO2-rich condit ions. If, however , under the chang ing pH cond i t ions of the sy s t em the C a Z n 2 ( O H ) 6 o 2 H 2 0 is formed it is likely that subsequent to its formation the following reaction with CO2 takes place:

CaZnz(OH)6.2H20 +3 CO2 . . . . . . . > CaCO3 + 2ZnCO3 +5H20 (6 )

Close examination of the silicate regions in the FTIR spectra of the air-cured and CO2-cured samples, reveals that the chemical environments of silicates have undergone definite structural changes due to exposure to

Vol. 23, No. 4 FTIR, XPS, CARBONATION, ZINC, SOLIDIFICATION 779

carbon dioxide atmosphere. The v3 of SiO44- regions in the CO2- cured sample appears at 1035-1085 cm -1, while the same region in the air- cured sample occurs at 925-1049 cm -1. The relative intensities of v2 and v4 vibrations of SiO44- are also different. The intensity of the v4 vibrations of S i O 4 4- decreases due to po lymer iza t ion , s ince the ou t -o f -p lane deformat ion (v4) is being restricted. The v3 band of SiO4 4" in the CO2- cured sample appears at a higher energy and has a reduced intensity compared with that of the air-cured sample. It thus appears that carbon dioxide has also attacked the C-S-H species. Slegers [30] has reported the following sequence of reactions in the carbonation of hydrated cement,

hydration carbonation C3S, 13-C2S . . . . . . . . . . . . > C-S-H . . . . . . . . . . . . > silica gel

The siliceous product obtained after reaction of C-S-H with CO2 has been character ized as a silica gel type of compound having a degree of polymerization higher than the original C-S-H. The appearance of silica phase at around 1085 cm -1 (Figures lc and 2b) after carbonation is in agreement with literature data [25,30]. The chemical nature of silica and other surface components in the carbonated product has been further examined by XPS and is discussed in the following sections.

X P S : X-ray photoe lec t ron spect roscopy provides valuable information concerning the elements in the near surface region (sampling depth ~ 5 0 /~). Examination of the binding energies of the core-level

e lec t ronic states of the elements in the surface region provides qualitative, semi-quantitative and chemical state information. XPS spectra of the concrete samples showed the near surface region to be composed of O>C>Ca>Zn (in the Zn-doped samples only)>Si>Al>>S (trace amount only). Examination of the Cls region shows two different types of carbon are present in the near surface region. Most of the observed carbon is present as adventitious carbon (BE: 285.0 eV) while the remaining carbon is in the form of carbonate (CO3 2- BE: 289.2 eV). Typically, the carbonate signal accounts for between 10 and 14 percent of the surface carbon signal. The overall amount of carbonate in the near surface region does not appear to be dependent on the nature of the curing atmosphere. This is not unexpected because atmospheric CO2 would react with the near surface calcium hydroxide species (C-S-H and CH) first, so that most of the surface Ca is in the form of CaCO3. The Ols XPS spectra, presented in Figure 3, shows little or no change in the nature of the oxygen in the near surface region. The binding energies for the Ols, (Table I), range between 531.5 eV and 531.7 eV which is not unusual for oxygen in hydroxides, carbonates and metal substituted silicates (-Si-O-M-) (57,58). The Zn- doped samples and the OPC CO2-cured sample show a low binding energy shoulder which is as low as 527 eV. This shoulder is probably due to some differential charging between the sample and the sample holder. The C ls regions for these samples shows a similar broadening at lower binding

780 M.Y.A. Mollah et al. Vol. 23, No. 4

F

E

o

c

B _=

' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' ' I ' ' ' "

540 535 530 525 Binding Energy (eV)

Figure 3: XPS O(1S) spectra of: (A) OPC Dry Clinker, (B) Zn-OPC, air cured, (C) Zn-OPC, carbon dioxide cured, (D) OPC, air cured, (E) OPC,

carbon dioxide cured and (F) Silica gel.

energies. The binding energy for Ols in the silica gel sample falls at 533.9 eV which is characteristics of silica gel [57,58].

The other major elements in the near surface region, zinc and aluminum, do not show any major differences between the air-cured and the CO2-cured samples. The Zn2p3/2 peak occurs at BE: 1021.6 eV for both samples and indicates that the zinc is in the Zn2+ chemical state and is probably due to either ZnO, Zn(OH)2, and/or ZnCO3. However, the lack of hydroxide bands in the FTIR spectra (Figure 2b) all but eliminates the possibility of Zn(OH)2. The A12p binding energy falls between 73.0 eV and 73.5 eV for the air-cured and CO2-cured samples, respectively. These binding energies are well within range expected for A1203.

Table 1: Binding En~ rt~ies for Si, O and Ca

Sample I OPC Dry Clinker

Silica Gel OPC/Air

OPC/CO2 Zn-OPC/Air

Zn-OPC/CO2

Si2p (eV) 100.8 104.1 102.2 102.2 101.1 101.5

Ols (eV) 531.5 533.9 531.6 531.7 531.6 531.7

I Ca2p3/2 /e V) 347.1

347.0 347.1 347.0 347.1

The Ca2p region (Figure 4) exhibits very little change going from cured in air to cured in C02 sample. The Ca2p3/2 peak is observed at a binding energy (Table I) of 347.0 eV and 347.1 eV for the air-cured and

Vol. 23, No, 4 FTIR, XPS, CARBONATION, ZINC, SOLIDIFICATION 781

Ca 2P3

E

c

B

A

3 40 Binding Energy (eV)

Figure 4: XPS Ca(2p) spectra of: (A) OPC dry clinker, (B) Zn-OPC, air cured, (C) Zn-OPC, carbon dioxide cured, (D) OPC, air cured and (E) OPC

carbon dioxide, cured

CO2-cured samples respectively. The Ca2p3/2 binding energy for calcium in a OPC dry clinker sample was observed to be about 347.1 eV, so there is very little change in the Ca2p region upon hydration. For CaO, Ca(OH)2 and CaCO3, the Ca2p3/2 peak falls right around 347.0 eV. Therefore upon reaction of the C-S-H with CO2 to form CaCO3, one would expect very little or no change in this region.

The Si2p XPS spectra for the OPC (air & CO2 cured), OPC dry, Zn-OPC (air & CO2 cured) and silica gel are shown in Figure 5. The binding energies of the Si2p peak are given in Table 1. The spectra for the OPC dry silica linked at only one or two corners of the O-Si-O tetrahedron. A chemical shift (0.4 eV) in the binding energy of Si2p peak due to exposure to CO2 reflects a small change in the chemical environment of the Si as the C-S-H complex is disrupted by the CO2. The Si2p binding energy for the Zn-OPC air-cured sample is about 1 eV lower than the OPC clinker shows the lowest binding energy at 100.8 eV, in agreement with our previous result [5]. This is 3.3 eV lower than the Si2p binding energy for a silica gel sample. From Figure 5, silica gel has a Si2p binding energy of 104.1 eV which is in good agreement with literature values [58].

In the case of silica gel, each of the four corners of the SiO4 tetrahedron is linked by -Si-O-Si- bonds, which is indicative of higher degree of polymerization. The silicate formed by the reaction of C-S-H with the CO2 has the same binding energy (BE: 102.2 eV) for both OPC cured in CO2 and the OPC cured in air samples (Figure 5). It thus appears that the chemical states of Si in C-S-H and as well as silica phase obtained after carbonation of C-S-H are different from that in the standard silica

782 M.Y.A. Mollah et al. Vol. 23, No. 4

si 2P

F

D

c

c B

A • I • ' " " l " " • • I " " " • I

110 105 100 95 Binding Energy (eV)

Figure 5: XPS Si(2p) spectra of: (A) OPC Dry Clinker, (B) Zn-OPC, air cured, (C) Zn-OPC, carbon dioxide cured, (D) OPC, air cured, (E) OPC,

carbon dioxide cured and (F) Silica gel.

gel. The C-S-H silica has, therefore, not been converted into a silica gel type SiO2 by the reaction of C-S-H with CO2. It has been reported [30] that in C-S-H the SiO4 tetrahedrons are bound to each other through two of the four tops to produce an unbranched polymeric structure. It seems, therefore, quite likely that carbonation promotes polymerization with SiO4tetrahedron being reorganised in a different mode as opposed to tridimensional arrangements in silica gel [30]. The Si2p peak at 101.1 eV in Zn-OPC air-cured sample is indicative of small units of polymerized samples. This shift indicates a lower degree of polymerization of the silicates in the Zn-doped samples than in the undoped OPC samples. This observation is in compatible with the changes in the silicate bands in the FTIR spectrum presented earlier.

C.on¢losions The FTIR and XPS results of this study confirm earlier results that

carbon dioxide has significant effects on the hydration of OPC. The FTIR results indicate that the metal-bonded [CaZn2(OH)6.2H20] and hydrogen- bonded hydroxide bands disappear due to carbonation which leads to increased degree of polymerization of the SiO4 4- units in OPC. The Si2p binding energy values obtained from XPS analysis of the hydrated OPC and Zn-OPC demonstrate that small units of polymerized silica linked at only one or two corners of the O-Si-O tetrahedron rather than a highly polymer ized cross- l inked structure is produced as result of polymerization of the orthosilicate units present in OPC.

Vol. 23, No. 4 FTIR, XPS, CARBONATION, ZINC, SOLIDIFICATION 783

Acknowledgments We wish to thank the Gulf Coast Hazardous Substance Research

Center, Lamar University Beaumont for primary support. Partial support for works related to the amorphous materials involved comes from the Welch Foundation, Houston, Texas.

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33.

34. 35. 36.

37. 38. 39. 40.

41

42.

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45.

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