Cement and Concrete Research 53 (2013) 91–103

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Cement and Concrete Research

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A Raman spectroscopic study of the evolution of sulfates andhydroxides in cement–fly ash pastes

Nishant Garg a, Kejin Wang a,⁎, Steve W. Martin b

a Department of Civil, Construction, and Environmental Engineering, Iowa State University, Ames, IA 50011, United Statesb Department of Materials Science & Engineering, Iowa State University, Ames, IA 50011, United States

⁎ Corresponding author.E-mail address: kejinw@iastate.edu (K. Wang).

0008-8846/$ – see front matter © 2013 Elsevier Ltd. Allhttp://dx.doi.org/10.1016/j.cemconres.2013.06.009

a b s t r a c t

a r t i c l e i n f o

Article history:Received 4 October 2012Accepted 13 June 2013Available online xxxx

Keywords:Cement (D)Fly ash (D)Ca(OH)2 (D)Sulfo-aluminate (D)Raman spectroscopy (nominated)

Raman spectroscopy has been employed to study the evolution of sulfo-aluminate and hydroxyl phases in pastesmadewith ordinary Portland cement (OPC) andfly ash (FA). Threefly asheswith different CaO contentswere usedas a cement replacement at the level of 0 and 50% by weight. The pastes were analyzed at 0, 0.2, 2, 4, 8, 12, 16, 20,24, 48, 72 h, and 7, 14, 21, 28, and 56 days after mixing. The wavenumber ranges used for Raman spectroscopicanalysis are 950–1050 cm−1 for evolution of sulfates and 3600–3700 cm−1 for evolution of hydroxides. Gradualdisappearances of gypsum in parallel with the formation of ettringite (AFt) are clearly observed in most pastemixes. Evolution of hydroxides showed the gradual spatial growth of portlandite. In addition to the potential ben-efits, the limitations of using Raman spectroscopy in study of cement-based materials are also noted in this paper.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Fly ashes are commonly used as a supplementary cementitiousmaterial (SCM) in modern concrete due to their economic and environ-mental benefits. Depending upon their physical and chemical properties,fly ashes also enhance the properties of concrete, such as workability,ultimate strength, and durability. Accurate characterization of fly ashesand determination of their chemical reactivity are essential to their fulland optimized use in concrete. Once a fly ash is properly characterized,its replacement level for Portland cement in concrete can be rationallydecided.

Many techniques have been developed to characterize cement-based materials, most of which measure the chemical and physicalproperties of raw materials, such as chemical/phase compositionand particle size distribution, while limited tools are available tofollow the in-situ hydration process. Calorimetric techniques canmon-itor the heat generated from cement hydration, but it has limited use inidentifying the chemical changes and/or in identifying the contributionsof each component of commercial cement during the hydration process.Recent research has indicated that Raman spectroscopy has a potentialto fill these gaps [1].

Raman spectroscopy is a relatively new technique in the field ofcement study. Its application has been primarily limited to pure,synthetic phases commonly found in cement clinker, rather thancommercial cements or SCMs. The present study is aimed at evaluat-ing the capability of this technique in cement science in terms of itsapplication on commercial cementitious materials.

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2. Background

Spectroscopy is a study of the light–matter interaction.When amole-cule is bombarded with a coherent laser in the visible range, most pho-tons are elastically scattered, i.e. without undergoing any change intheir frequency or wavelength. At the same time, a small fraction oflight (approximately 1 in 107 photons) is in-elastically scattered at opticalfrequencies different from the frequency of the incident photons. This in-elastic scattering, a weak phenomenon, results in the Raman shift in theenergy of the photon (shift usually reported in wavenumbers cm−1)and corresponds to the Raman active modes of the virtually excited mol-ecule (or its active functional groups).

Raman spectroscopy can detect certain vibrational and rotationalmodes of the elemental chemical bonds inside a material. This tech-nique was first used in the field of cement chemistry by Bensted whoapplied it to cementitious materials in 1976 [2]. Four years later,Raman microprobe was further successfully applied to cementitiousmaterials by him and his peers [3,4]. Following Bensted, Ghosh andHandoo reviewed the effectiveness of vibrational spectroscopies in thestudy of cement and concrete [5]. However, the potential of Ramanspectroscopy in the field of cement chemistry wasn't recognized untilrecently, as modern instruments and technologies (CCD detectors)were developed to address the inherently weak Raman signal comingfrom most materials, including cementitious materials [6]. Lately,Raman spectroscopy has served as a complimentary technique tomore widely established techniques such as NMR and XRD [1,7].

As the demand for research in the field of cement hydration andmicrostructure analysis increases [8], Raman spectroscopy has beenapplied to multiple aspects of this research area, ranging from studyof hardened concrete surfaces to the understanding of the ever elusive

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structure of the C–S–H gel [9,10]. One of the most significant features ofthis technique is its capability for a real-time in-situ analysis. Using thistechnique, while some researchers have characterized individual clinkercomponents [11], others have focused on studying the progress of timedhydration on clinker phases like C3S and C2S [12,13], C3A [14], C4AF [15].It has been possible to identify various hydration products [1] like AFt,AFm [16] and C–S–H gel [17,18] with good accuracy in syntheticsystems.

Efforts have been alsomade to study other cement relatedmaterialslike fly ash [19–21], metakaolin [22], white Portland cement [23], andcalcium sulfoaluminate cements [24]. Raman has been used not onlyfor characterization but also for studying various environmental attackslike carbonation [25] and thaumasite sulfate attack [26,27]. ThermalRaman spectroscopy has been employed on ettringite [28] and gypsum[29] with promising results. A detailed summary of past work done canbe found in recent literature [30,31].

However, applying Raman spectroscopy to the study of commercialcement-based materials is not an easy task due to high levels of fluores-cence accompanying the Raman scattering spectra [32]. Using long rangewavelengths can eliminate fluorescence, but the photoluminescencebands due to trace level impurities in the mineral lattice become aproblem for effective analysis [33,34]. Very fine powdery materials alsoaffect the weak Raman signal due to the parallel Tyndall scattering [2].Also, addition of SCMs to Portland cement, results in a more complexsystem for Raman microscopic analysis due to the overlapping ofprincipal bands [35]. Thus, studies of OPC and its hydration with orwithout SCMs have so far remained elusive [1].

In the present study, the benefits and limitations of using Ramanspectroscopy for investigating hydration process of commercial cemen-titious materials are re-evaluated. The temporal and spatial evolution ofsulfates and hydroxides formed in the hydration process are examined.

3. Material and methods

3.1. Materials and their general properties

ASTM Type I/II ordinary Portland cement (OPC) and three fly ashes(FA) with varying calcium contents and fineness were used, andTable 1 shows the material properties.

The chemical compositions of the materials as shown in Table 1were identified by a PANalytical PW2404 X-ray fluorescence (XRF)spectrometer. The fineness of the materials, expressed as the per-centage of the materials retained on #325 sieves (45 μm), was mea-sured according to ASTM C430. It is observed in the table that FA1 hasa high calcium content followed by FA2 and FA3. FA1 also has thehighest fineness value among all the three fly ashes studied. Both thehigh calcium content and high fineness are expected to increase the ef-ficiency/reactivity of this fly ash. It is also to be noted that FA1 has thehighest amount of sulfates, followed by FA2 and FA3.

Table 1Chemical and physical properties of cementitious materials.

Compound (%) OPC FA1a FA2 FA3

SiO2 20.2 29.2 46.0 53.5Al2O3 4.7 17.7 17.8 25.2Fe2O3 3.3 5.48 18.2 7.2SO3 3.3 3.41 2.59 0.32CaO 62.9 30.1 8.40 1.46MgO 2.7 7.65 0.95 1.43Na2O N/Ab 2.16 0.59 0.49K2O N/A 0.31 2.16 3.51Eq. Na2O 0.54 2.36 2.01 2.80Others 2.36 3.95 1.3 4.09LOI 1.1 0.40 1.49 4.87% retained on #325 sieve 6.55 11.80 13.25 20.15

a Class C fly ash; the rest are Class F ashes.b Data are not available.

Table 2 shows the mortar compressive strength test results doneas described in ASTM C109. The mortar samples were cast accordingto ASTM C305 with a water-to-binder ratio (w/b) of 0.45 andsand-to-binder ratio (s/b) of 2.75. All the binders consisted of50%OPC + 50%FA (by weight). Three mortar cubes for each agewere cured at 23 °C and ≥95% RH for 1, 3, 7, 28, 56 and 90 daysbefore being tested for their compressive strength. As expected, thefly ash reactivity trend, as expressed by the strength of the FA–OPCmortars after the age of 28 days, was found to be FA1 > FA2 > FA3.

Table 2 also shows the calcium ion (Ca2+) concentration of thebinders measured using atomic absorption spectrometry (AAS). TheAAS analysis was performed on filtrates obtained by using 0.2 μmpolyethylene Whatman membrane filters on binder–water slurries(1.5 g of OPC + 1.5 g of FA were added to 50 ml of deionizedwater, stirred for 1 h, and then stored in a sealed polypropylenevessel) and taking the measurement after 1 day. Consistent with theresults from XRF tests, the AAS test results showed that the Class Cfly ash (FA1) displayed the highest calcium ion content in thesolution, followed by FA2 and FA3. This means that the free calciumfrom the FA1 dissolved rapidly in paste system may promote thecement hydration at an early age.

It is noted that the FA3 paste had very low calcium ion content(144 mg/L), lower than 50% of the calcium ion content in purePortland cement paste (530 mg/L). This indicates a possible a chemi-cal reaction in the FA3 system, which has caused reduction in calciumion content. Another possibility is that FA3 has a relative high loss ofignition (LOI) value, or high content of carbon, which may adsorbsome calcium ions, thus reducing the calcium content.

Fig. 1 shows the rate of heat generation of all of the four corre-sponding mortar samples measured by isothermal calorimetry. It isobserved that the sample made with FA1, a Class C fly ash, displayeda secondary peak of heat generation at 20 h. This may be attributeddue to the renewed formation of ettringite [36]. It is interesting tosee if Raman spectroscopy is able to identify this feature.

Table 3 lists the major phases detected by XRD in the three flyashes studied. The results have also indicated that the amount ofcalcium-based mineral phases decreases from FA1 to FA3, whichcorrelates well with the calcium oxide percentage as presented inTable 1. As indicated by the difference in the centroid of glasshump, FA1 has a calcium aluminate type glass, which is the mostreactive type of glass in fly ashes [37] and might play a significantrole in evolution of sulfo-aluminates, while FA2 and FA3 have anormal siliceous glass.

In summary, XRF, XRD, AAS, strength and heat of hydration testresults all indicate that FA1 might have the highest reactivity followedby FA2 and then FA3. Having a calcium aluminate type glass, FA1might play a significant role in evolution of sulfo-aluminates. Asshown later, these test results help explain the results obtained fromRaman spectroscopy.

3.2. Raman spectroscopy experimental configuration

Raman spectroscopy tests were performed for samples in a dryform (as raw materials) and a wet form (as a paste). The pastesstudied were made with either 100% OPC or 50%OPC − 50%FA (byweight), and they had a water-to-binder ratio (w/b) of 0.45.

To prepare paste samples for Raman spectroscopy tests, 100 g ofbinder and 45 g of water were first mixed by hand for 1 min in a glassflask. The paste was then placed in fifteen separate sealed plastic vialsand stored at 23 °C and ≥95% RH. Each sample was then uncoveredand then analyzed under the Raman microscope at the ages of 0.2, 2, 4,8, 12, 16, 20, 24, 48, 72 h, 7, 14, 21, 28, and 56 days, respectively. Usinga different sample at each different ages was used to reduce/eliminateeffect of carbonation. Raman spectroscopy tests performed for rawmaterials are defined as at the age of 0 (in a dry form).

Table 2Measured properties of fly ash–cement mixes.

Compressive strength development (MPa)

Age (days) 100%OPC 50%FA1 + 50%OPC 50%FA2 + 50%OPC 50%FA3 + 50%OPC

1 14.2 ± 0.4 4.2 ± 0.3 3.2 ± 0.0 4.8 ± 0.23 35.7 ± 0.7 16.5 ± 0.4 8.8 ± 0.2 11.2 ± 0.37 44.7 ± 2.1 22.3 ± 0.8 16.7 ± 0.3 17.3 ± 1.028 54.4 ± 4.6 33.1 ± 0.9 29.7 ± 1.1 22.4 ± 4.156 63.5 ± 0.7 39.1 ± 2.5 31.4 ± 1.4 29.4 ± 0.590 64.8 ± 1.5 44.3 ± 1.4 38.3 ± 1.9 33.7 ± 0.8

Other propertiesCa2+ ion concentration at 1-day (mg/L) 530 364 236 144Cumulative heat (kJ/g) of hydration at 1-day 0.33 0.27 0.23 0.20

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Raman spectra were collected using a Renishaw inVia spectrome-ter with a 488 nm line of an Ar+ laser and a maximum of 25 mW ofpower. A circular spot size of a diameter of about 100 μm wasobtained at the low optical magnification of 5×. A lower magnifica-tion was chosen to obtain a larger more representative area underthe laser spot size in order to overcome the inherent heterogeneityof the materials under study. Selection of the wavenumber rangeschosen for study was 950–1050 cm−1 for evolution of sulfates and3600–3700 cm−1 for evolution of hydroxides, and the exposuretime was kept to the minimum of 10 s. Spectra of the starting, bindermaterials in the dry form were taken at the 0–1200 cm−1 range with30 seconds exposure. All the instrumental configuration was system-atically chosen by targeting for a high S/N ratio and statistical repre-sentativeness whose details can be found in a separate publication[38].

The primary analysis of hydrating mixes included mapping anarea of 3 × 3 mm2 having a total of 49 points spread in a grid matrixon the sample surface. Only one measurement was taken at eachpoint so as to reduce the analysis time and cover as many points aspossible before carbonation occurs. The final spectra shown in thefigures below are the average of 49 spectra taken at 49 spatiallydistributed locations for a given sample at a given age. Fig. 2 showsthe mapping grid employed.

3.3. Data analysis and representation

The collected data were then processed using Wire 3.1 the accompa-nying data collection and handling software with the Renishaw inViaRaman spectrometer and Origin 8.6, a commercial data plotting software.The qualitative analysis of the evolution of sulfates was performed on thedata normalized by the highest peak of the Raman spectrum, while thequantitative analysis of evolution of hydroxides was done without thenormalization. Normalization was found necessary for qualitative and

Fig. 1. Calorimetric data of mortars.

meaningful comparison between the spectra at different ages. Baselinecorrection and a Savitzky–Golay smoothening function were also appliedin the data analysis. Figs. 3 and 4 show the effect of data manipulationprocedures employed.

Fig. 5 shows the process of mapping the intensity distribution ofhydroxides detected in the higher wavenumber region (3619 cm−1). Itis to be noted that 2-d image gives an overall distribution of the concen-tration of the analyte under study, and the singular bright spots do notsuggest overall higher concentration as they may be just outliers. As theimages get filled with brighter spots, one can infer that the given areaunder study is being populated by calcium hydroxide (CH/portlandite).

4. Test results and discussions

4.1. Raman spectra of raw cementitious materials

4.1.1. Ordinary Portland cementRaman study of individual clinker phases has been documented

[38], and therefore it is not repeated in the present study. Fig. 6 showsthe average of 25 spectra taken at 25 different locations on the cementdry powder samples. A square grid of five by five points was drawn onthe sample, making the dimensions of the representative square areastudied to be 2000 × 2000 μm with each point spaced 500 μm aparthorizontally and vertically.

It can be seen from Fig. 6 that all major phases of a commercialclinker, alite, belite, aluminate and ferrite can be detected. However,there is significant overlap between the phases and possibly somefluorescence between 600 and 1000 cm−1 region as suggested byprevious researchers [1,32].

Fig. 7 is also marked by the principal vibrational modes for themolecules responsible for detection of various phases. The mediumintensity broad hump peak at 842 cm−1 (v1[SiO4]4−) is assigned tothe overlap of peaks of both alite and belite as both of them havetheir sharpest peaks in that region [2,4]. The stretching bandsof aluminum–oxygen tetrahedral at ~500 (v1[AlO4]5−) and ~700

Table 3Major phases detected by XRD in the three fly ashes studied.

Mineral phase FA1 FA2 FA3

Quartz x x xLime x x –

Periclase x – –

Anhydrite x x –

Merwinite x – –

Melilite x – –

Yeelimite x – –

Calcium aluminum oxide x – –

Magnetite x – xPortlandite – x –

Mullite – x xHematite – x xCentroid of glass hump 31.0° 23.0° 23.5°

Fig. 2. Mapping setup for analyzing an area on the sample surface.

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(v3[AlO4]5−) cm−1 are assigned to the calcium aluminate phasein the clinker. The assignment of the Ferrite phase at 264 cm−1

(v2[(Fe,Al)O4]5−) is tentative as the small hump in ~250 cm−1 regionmight be just a random spectral feature because the quantity of Ferritepresent in OPC is much less and hence its detection is not straightfor-ward. Ferrite phase also results in a broad hump around 750 cm−1 [4]which maybe overlapping with other phases in this case. Such intersti-tial phases can be better probed by employing combined imaging-Raman systems as recently done by Black and Brooker [36].

4.1.2. Fly ashesFig. 7 shows the spectra of all three fly ashes studied after averag-

ing the spectra from a grid of 25 locations. While the assignment ofquartz at ~460 cm−1 (v2[SiO4]4−) is straightforward due to bending

Fig. 3. Effect of smoot

vibrations [35], the assignment of the glassy phase is a bit tentative[19]. Since there is a cluster of very small peaks at the broad weakpeak between 600 and 800 cm−1 it is difficult to definitely assign aphase in that region. Anhydrite and calcite are also easy to assignbased on the sulfate ~1000 cm−1 (v1[SO4]2−) [2,29] and carbonate~1085 cm−1 (v1[CO3]2−) [9,12] vibrations based on previous litera-ture. It is noted that calcite was not clearly shown in the XRD testresults (Table 3). A possible reason could be that FA1 paste hadsome slight carbonation of the calcium based phases. The carbonategroup is a strong Raman scatterer. Even with insignificant amount,it could be clearly visible in the Raman spectrum. Therefore, there isa challenge to directly relate the phases detected by XRD to thosedetected by Raman, as that would require a focused study oncharacterization of dry ashes.

It is interesting to note when the average spectra of the three flyashes are compared together on the same scale, there are significantdifferences between the calcium based phases. While the quartzphase more or less appears consistently in the fly ashes, the anhydriteand calcite phases are most dominant in FA1, followed by FA2 and theleast dominant in FA3. A similar trend for the glassy phases is alsoobserved. This trend of the content of calcium or calcium basedmineralogical phases illustrated by the Raman agrees well with thatshown by XRD (Table 3) and XRF (Table 1) test results. However,these observations made from a set of averaged, normalized Ramanspectra should not be considered quantitative, as they merely displaya trend of existence of certain phases. Compared with XRD, Ramanspectroscopy is simple and quick, and requires little or no samplepreparation and no more than 10 min for each fly ash analysis.

4.2. Evolution of sulfates in hydrated cementitious materials

4.2.1. Pure gypsum and ettringiteFig. 8 shows the Raman spectra for pure gypsum and ettringite

measured to verify the peak positions assigned in the commercialcementitious systems. The v1[SO4]2− symmetric stretching mode isthe characteristic mode of the species analyzed in this wavenumberregion. The basic reactions that show the formation of ettringite and

hening function.

Fig. 4. Effect of normalization and baseline subtraction.

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its subsequent conversion to monosulfate are also noted for refer-ence. It shall be noted that both the peak location and intensity arequite uniform in such synthetic phases, which may not be necessarilythe case in commercial cement systems discussed later.

Fig. 5. Presentation of evolution of

4.2.2. OPC pasteFig. 9 shows the evolution of sulfates in a 100% OPC paste. The ini-

tial peak at 1007 cm−1 is assigned to gypsum due to v1[SO4]2− sym-metric stretching mode. The second peak at 982 cm−1 is tentatively

hydroxides in a typical paste.

Fig. 6. Raman spectrum for ordinary Portland cement (average of 25 spectra).

Fig. 7. Comparison of the three dry fly ashes based on their spectra. (Q = Quartz,G = Calcium-alumino-silicate glassy phase, A = Anhydrite, C = Calcite).

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assigned to ettringite or AFt type phase based on a literature (althoughpure ettringite is usually around 989 cm−1 and AFm type phase around982 or 983 [16]). Anhydrite at 1018 cm−1 is expected to disappearwithtime due to its hydration and subsequent reaction with aluminates.Fig. 8 also shows that the pure ettringite has a principal band at991 cm−1.

Due to symmetric stretching of sulfate molecule, this ettringitepeak appears to be changing from 982 cm−1 to 987 cm−1 as theage of the samples increase from 24 h to 48 h. It can be inferredthat after 48 h, the structure of ettringite undergoes a change due tothe depletion of sulfate ions in the system. However, prior to 48 hthe band at 982 cm−1 arises from some form of AFt phase andpossibly not AFm (as usually assigned in literature). As it is known,AFt usually converts to AFm not vice versa [39].

Renaudin et al. conducted an extensive Raman study on sulfates[16]. The authors reported that pure forms of ettringite andmonosulfoaluminate can be easily distinguished from each other byRaman spectroscopy. At low range wavenumbers, according to thestudy of Renaudin et al., the ettringite principle band occurs between989 and 991 cm−1 (which matches with the pure ettringite measuredin this study) and for AFm the principal peak occurs at 982 cm−1. Inter-estingly, there is a shoulder peak arising from AFm at 992 cm−1, whichmay overlap the peak arising from the AFt phase. Also,whilemost of theprevious works may agree with this, Frias and Ramirez [22] assignedbands at 986 and 991 cm−1 to AFm instead of AFt which suggestssome difficulty in distinguishing both these phases due to a narrow

Fig. 8. Crystalline and pure sulfates (E = Ettringite, Gy = Gypsum, n = 9).

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difference between their strongest peaks. However, the hydroxyl peakof AFm near 3675 cm−1 is found by both researchers [16,22], which isa characteristic mode of AFm and can greatly help in distinguishing itfrom AFt.

In the present study, AFm is not assigned to any of the peaksobserved in the 950–1050 cm−1 region of the 100% OPC mix, as nocorresponding characteristic hydroxyl peaks for AFm were found inthe higher wavenumber region.

4.2.3. OPC–FA pastesFigs. 10 and 11 show the evolution of Raman peaks arising from

sulfates in the pastes made with OPC and three different fly ashes.Fig. 10 presents the average results of spectra collected from 49different test locations, while Fig. 11 represents the variance in peakposition and intensities of the peaks at all of the 49 locations.

For the OPC–FA1 system, in the early stages of reaction both gypsumand anhydrite are detected, suggesting the strongpresence of sulfates inthe system. This agrees well with the presence of the anhydrite phasedetected strongly in the dry data of FA1 (see Fig. 8). As a result, theAFt phase, with peaks nearing 989 cm−1, is detected at the age of 2 to8 h. During the period of reaction of 12–20 h, there is a sudden shiftof the position of these peaks to 991 cm−1, which may result fromeither recrystallization of ettringite or the initial formation of AFm[16]. This agrees with the calorimetric test results of the OPC–FA1

system. As shown in Fig. 2, the secondary peak appears at the age of ap-proximately 20 h, which is often related to recrystallization or intensereaction of the aluminate phase in a hydrating fly ash–cement system[39]. Beginning from 48 h to 56 days, these peaks fall back to the regionnear 989 cm−1. This peak is assigned to an AFm type phase based onthe hydroxyl peaks observed at higherwavenumber region as discussedin Section 4.3.3. It should also be noted that averaging 49 spectra mayhave induced an unexpected artifact, resulting in loss of trace peaksthat might have existed on selected locations of the mapped surface.

In the OPC–FA2 system, the ettringite formation appears to beslower than that in the OPC–FA1 system. During the first 8 h, noclear peaks of sulfo-aluminate phase are seen. The gypsum and anhy-drite phases are readily available as they exist in the raw materials.During 12–48 h, there is a slight evidence of the parallel existenceof AFt and AFm due to some appearance and disappearance of thepeaks 991 and 980 cm−1 respectively. But, after 7 days, consistentpeaks near the 989 cm−1 region are seen, which can be either AFtor AFm. Since no secondary hydroxyl peak is associated with thepeaks at 989 cm−1, this peak at 989 cm−1 is assumed to arise fromAFt. It is expected that the oscillation between AFt and AFm phasesas seen by the dual peaks at several ages may be related to the highamount of iron present in the fly ash, which may increase the com-plexity of the Raman spectra interpretation due to the iron ion inclu-sion in the phases. There is a possibility of conversion of AFt to AFm

Fig. 9. Evolution of sulfates in 100%OPC paste. AFt = Ettringite; Gy = Gypsum; A = Anhydrite.

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by the inclusion of iron in the lattice, as seen by Black et al. in adetailed study on sulfo-aluminate hydration [15].

For the OPC–FA3 system, the spectra obtained do not provide muchinformation about the ongoing hydration process in this paste. For somereason, there were no peaks obtained at any age (even after repeatedtesting on different samples). In the 950–1050 cm−1 region, there isslight evidence of the formation of the Aft-type phase at 20 h. Otherthan that, there are no spectral peaks other than noise. Since there is50% OPC present in the system, the sulfo-aluminate hydration processshould proceed, and some hydration products should be detected.

Fig. 10. Evolution of sulfates in 50%FA + 50%OPC pastes. (M = Mono

This maybe primarily because of the fact that the total sulfate contenthas been reduced by half, but other potential reasons might be con-tributing behind this phenomenon as well: (a) the products formedin this mix might be highly amorphous or not crystalline enough tobe detected by Raman, (b) FA3 might be merely acting as a filler inthe mix and has no contribution to the pore solution chemistry,(c) abundant portlandite was detected on the surface of the sample,and that might inhibit detection of any other phases. During the ex-periment, slight bleeding was also detected in this OPC–FA3 mix,which might have influenced the hydration; reaction products may

sulfoaluminate; E = Ettringite; Gy = Gypsum; A = Anhydrite).

Fig. 11. Images representing the evolution of sulfates in 50%FA + 50%OPC pastes.

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have crystallized at the surface of the sample and resulted in promo-tion of growth of excessive Portlandite (refer Section 4.3.3).

4.3. Evolution of hydroxides in hydrated cementitious materials

4.3.1. Pure calcium hydroxideFig. 12 shows the calibration curve constructed for a pure CH

(calcium hydroxide) powder with its increasing replacement ofdistilled water (DW) by mass. The clear peaks of CH are locatedat 3619 or 3620 cm−1, and the peak intensity varies somewhatexponentially with increasing mass % in water. This suggests thata big difference in intensity may result from a small difference in solidconcentration of a given species. Although this is a major limitationtowards estimating accurate concentration of a given analyte throughthis technique, quantitative analysis by Raman spectroscopy still dem-onstrates some potential to be applied to the study of ongoing, livereactions [40].

4.3.2. OPC pastesFig. 13 shows the evolution of CH in the 100% OPC paste. The most

intense peak of CH crystal was established at 3620 ± 5 by Padanayi[41] based on his theoretical calculations. The consistent CH peak at3619 cm−1 (v1[OH]−) observed in this work, employing commercialclinkers and fly ashes, agreed well with the peaks observed by severalother researchers [9,14,22]. There were slight changes in the peakposition but not more than ±1 cm−1 as can be seen in the followingfigures. Since the spectra from the evolution of hydroxideswas not nor-malized, the intensities of these peaks at 3619 cm−1 were used to mapthe distribution of concentration of CH on a given surface of 3 × 3 mm2.

A consistent trend of growing CH was obtained in the OPC pastewhich shows the progress of reaction of the calcium silicates resultingin the production of CH as the by product—which is the long established,basic mechanism of cement hydration [42]. Fig. 14 shows the mappeddistribution of CH in the OPC paste. As expected, the CH crystals spreadspatially as a function of time, and their temporal evolution is inagreement with the calorimetric data shown earlier.

Fig. 12. Calibration of intensity with CH concentration.

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4.3.3. OPC–FA pastesFig. 15 shows the evolution of hydroxides in pastes modified by

three different types of fly ashes and a comparison between the CHoverall peak intensities. Fig. 16 shows the spatial distribution of CH inthe OPC–FA mixes.

In the OPC–FA1 paste, CH begins to be detected as early as 4 h, whichis possibly due to the high Ca2+ ion concentration in the mix (Tables 1and 2). However, as seen in the image plots, the intensity of the CH

CH

Fig. 13. Evolution of hydroxides in 100%OPC paste. (CH = calcium hydroxide).

peak at 3619 cm−1 is not significant at early ages but growsmuch stron-ger later. The CH intensity and spatial distribution continue to grow upuntil 72 h, but a decline appeared in the 7-daymeasurement, suggestinga start of the pozzolanic reaction between CH and the reactive silicate/aluminate phases present in thisfly ash. As a result, a secondary hydroxylphase at 3675 cm−1 appears to originate, and it is an AFm type phasebased on the assignments by Renaudin et al. [16], Black et al. [14], andFrias and Ramirez [22]. This matches with the earlier tentative assign-ment of conversion of AFt to AFm (in Section 4.2.2). The rate and amountof CH evolution and spatial distribution in the OPC–FA1 paste imply thatFA1 is the most reactive fly ash, which agrees with that concluded fromXRF, XRD, AAS, strength and heat of hydration test results (as describedin Section 3.1).

For theOPC–FA2 system, a normal trend of growingCH similar to thatin the pureOPCpaste is observed. The CH intensities continue to growupuntil about 7 days, after which the growth has more or less stabilized.Moreover, the spatial distribution has also reached equilibrium whereit is no more increasing in the given area studied. It can be inferredthat FA2 has some pozzolanic effect but it is not as pronounced as FA1.

Much differently, the CH concentration of the OPC–FA3 system in thetest area shows an upward trend once the crystallization begins from 8to 12 h, which matches with the setting and hardening process in thepeak obtained in the calorimetric curve (Fig. 1). At 28 days the concen-tration of CH is well spread on the surface, indicating a high amount ofCH present in this system. As discussed previously, the silicate andaluminate phases in this coarse fly ash might not be reactive and hadlittle contribution to the pozzolanic reaction. This fly ash may be merelyserving as filler in the system. In addition, bleeding water was observedin the OPC–FA3 paste, which may have promoted CH crystal growth onthe surface.

It is noted from the Raman spectroscopy test results of the drymaterials that FA1 has a comparatively dominant broad spectral feature

Fig. 14. Mapping the distribution of CH in 100% OPC paste.

101N. Garg et al. / Cement and Concrete Research 53 (2013) 91–103

near the 600 cm−1 assigned to a reactive glassy phase. This suggeststhat FA1 has a reactive, amorphous glass phase which may also beenhancing its pozzolanic reactivity by consuming the CH to formcalcium aluminate silicate hydrates or more C–S–H. This highreactivity was also predicted by the XRD scans which suggested areactive calcium aluminate type glass (Table 3). The features ofthese fly ashes support the trend of the CH evolution in the threeOPC–FA systems as discussed above.

(a) FA 1 (b) FA 2

Fig. 15. Evolution of hydroxides

5. Conclusions and recommendations

The following conclusions can be drawn from the present study:

(1) Raman spectroscopy can effectively characterize raw cemen-titious materials. The reactivity of fly ashes identified byRaman spectroscopy agrees well with those from XRD/XRFtests and other supplementary tests.

(c) FA 3

in 50%FA + 50%OPC pastes.

Fig. 16. Mapping the distribution of CH in 50%FA + 50%OPC pastes.

102 N. Garg et al. / Cement and Concrete Research 53 (2013) 91–103

(2) Raman spectroscopy can be used tomonitor the hydration processof complex and extremely heterogeneous systems containing

commercial ordinary Portland cement and fly ashes with varyingreactivity. In particular, the focused studies on evolution of sulfates

103N. Garg et al. / Cement and Concrete Research 53 (2013) 91–103

and hydroxides in OPC and OPC–FA pastes have provided a betterunderstanding of the hydration kinetics of the cement-based ma-terials from a Raman spectroscopic perspective.

(3) The disappearance of gypsum and the formation of ettringite(AFt) as a function of time in young pastes can be clearly identi-fied by Raman spectroscopy. The first 24 h of hydration processfollowed by Ramanmicroprobematchedwell with the calorimet-ric data obtained separately. However, the conversion of AFt toAFm type phasewas not very obvious and needs further research.

(4) In case of evolution of hydroxides, the unique ability of Ramanspectroscopy to map the concentration of CH in a representativesurface area is powerful in recording its evolution and distribu-tion as a function of time. The reactivity of fly ashes correlateswith the amount of CH formed in the OPC–FA system.

(5) In comparison with other techniques, Raman was found to becompletely non-destructive and it requires virtually no samplepreparation. Moreover, the analysis times were relatively shorterand an area of 9 mm2 with 49 different locations can be mappedwithin 20 min.

It shall be noted that selection of instrument parameters (such aswavelength, power voltage, and testing time) is very important forobtaining proper Raman spectroscopy test results. To advance applica-tion of Raman spectroscopy in the field of cement-based systems, a setof standard Raman spectra need to be established for quantitativephase analysis.

Acknowledgment

The authors appreciate the sponsorship of the Oak Ridge AssociatedUniversities (ORAU) - Tennessee Valley Authority (TVA) (Grant No.7-22976). Special thanks are given to Dr. Randilynn Christensen forher support and inputs in the Raman spectroscopy tests, and to Dr.Scott Schlorholtz for providing guidance and insightful comments onother experiments. Authors are grateful to an anonymous reviewer forgreatly helping in improving the manuscript.

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