Characterization of alkali silica reaction gels using Raman spectroscopy

Cement and Concrete Research 92 (2017) 66–74

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Characterization of alkali silica reaction gels using Raman spectroscopy C. Balachandran a,⁎, J.F. Muñoz a, T. Arnold b a b

SES Group & Associates LLC, 614 Biddle St., Chesapeake City, MD 21915, USA Turner-Fairbank Highway Research Center, 6300 Georgetown Pike, McLean, VA 22101, USA

a r t i c l e

i n f o

Article history: Received 25 July 2016 Received in revised form 26 October 2016 Accepted 22 November 2016 Available online xxxx Keywords: Raman spectroscopy Alkali silica reaction gels Silica polymerization X-ray diffraction

a b s t r a c t The ability of Raman spectroscopy to characterize amorphous materials makes this technique ideal to study alkali silica reaction (ASR) gels. The structure of several synthetic ASR gels was thoroughly characterized using Raman Spectroscopy. The results were validated with additional techniques such as Fourier transmission infrared spectroscopy, X-ray powder diffraction and thermogravimetric analysis. The Raman spectra were found to have two broad bands in the 800 to 1200 cm−1 range and the 400 to 700 cm−1 range indicating the amorphous nature of the gel. Important information regarding the silicate polymerization was deduced from both of these spectral regions. An increase in alkali content of the gels caused a depolymerization in the silicate framework which manifested in the Raman spectra as a gradual shift of predominant peaks in both regions. The trends in silicate depolymerization were in agreement with results from a NMR spectroscopy study on similar synthetic ASR gels. © 2016 Elsevier Ltd. All rights reserved.

1. Introduction Alkali silica reaction (ASR) is a process of concrete deterioration in which certain siliceous phases present in some aggregates and the hydroxyl ions in the pore solution of the concrete react resulting in the formation of an alkali silica gel reaction product. This reaction product can absorb moisture and expand resulting in cracking of the concrete. ASR damage is one of the many factors affecting the serviceability of concrete infrastructure. Visual manifestations of ASR damage include cracking, typically in a map cracking pattern in unrestrained surfaces, deformation and displacement of structural elements due to expansion, pop outs and gel exudation [1]. The diagnosis of ASR based on visual symptoms alone is often inadequate and needs to be corroborated with laboratory investigations of cores removed from the field. The identification of the presence of ASR gel in the affected concrete offers positive indication of ASR distress [2]. This is mostly achieved by microscopic investigations of extracted cores using petrography or scanning electron microscopy. Other than these laboratory based techniques, the only diagnostic field techniques currently available for detection of ASR gels are staining methods using either uranyl acetate or sodium cobalt and Rhodamine B [3]. The uranyl acetate staining test method is not used extensively owing to it being both hazardous and unreliable. While the sodium cobalt/Rhodamine B test is safer, there is not much information available regarding its effectiveness. Thus, the development of an efficient technique for diagnosing ASR gels will be beneficial for field applications.

⁎ Corresponding author. E-mail address: [email protected] (C. Balachandran).

http://dx.doi.org/10.1016/j.cemconres.2016.11.018 0008-8846/© 2016 Elsevier Ltd. All rights reserved.

Raman spectroscopy is a spectroscopic technique that is sensitive to both amorphous as well as crystalline materials thus making it ideal for the analysis of concrete or cementitious materials. This characteristic of Raman spectroscopy makes it an ideal candidate for potential use as a diagnostic tool for ASR. Ever since pioneering work by Bensted in 1976 [4], this technique has been used to characterize various anhydrous cement phases, hydration products as well as some secondary cementitious materials. Extensive reviews of the important literature and advances in the subject area with Raman shifts of major cementitious phases have been published by several authors [5–11]. An important point to be noted is that most of the research efforts focus on characterization of white cement, synthetic calcium silicate hydrate (C-S-H), and hydration of pure cement phases due to the fluorescence associated with the analysis of grey cements [5,8,11]. Raman spectroscopy has also been used successfully to investigate other hydration products such as sulfate phases and hydroxides [5,12–15] and to study carbonation [8,16] and secondary deposits of sulfate attacked concrete [12,14]. Another potential application of Raman spectroscopy that has hitherto not received much attention is in the area of ASR research. Raman spectroscopy, with its ability to characterize poorly-ordered materials, could potentially be a useful technique to characterize ASR gels. It has been used extensively to study similar amorphous materials such as silicate melts and glasses [17–19]. The availability of portable instrumental configurations of Raman spectrometers opens doors to the possibility of developing a diagnostic tool for ASR gels that can be deployed in the field. A similar approach to investigate the feasibility of applying a portable configuration of Raman spectroscopy to monitor sulfate attack has been recently published [14]. The fiber optic probe of a portable Raman spectrometer can focus the laser beam into cracks of ASR-damaged concrete and potentially collect the Raman signal of gel deposits. The recent

C. Balachandran et al. / Cement and Concrete Research 92 (2017) 66–74

advent of Surface Enhanced Raman Spectroscopy (SERS)-active fiber optic probes [20,21] presents a promising avenue to abate current inherent fluorescence associated with the cementitious matrix surrounding the area of interest [5,8,11], thus allowing for an early detection of the ASR gel. Further, as a research tool, Raman spectroscopy can provide valuable insight into the structure of the gels which in turn will aid in better understanding of the mechanism of the reaction and improve repair strategies for damaged structures [22,23]. Of particular interest is the technique's capability of easily providing information regarding the silicate polymerization of the gels, similar to that obtained using NMR spectroscopy. Published literature based on 29Si MAS NMR investigations indicate that the structure of ASR gels is dominated by silicate tetrahedral Q3 (sheet-like) polymerization [22,24–26]. A detailed study by X. Hou et al., which references more work probing the structure of the ASR gel, revealed that silicate polymerization of a series of synthetic ASR gels as indicated by 29Si MAS NMR varied systematically with the chemical composition [22]. Further, the authors concluded that the structure of the synthetically produced ASR gels was similar to that of gels obtained from field concrete thus implying that is was justifiable to use these synthetic gels in fundamental research [22]. The aim of the study presented in this paper was to evaluate the feasibility of using Raman spectroscopy as a research tool to probe the structure of ASR gels. The current work is part of a larger study, the broad objective of which is to explore the possibility of using Raman spectroscopy as a diagnostic tool for ASR damage. A series of synthetically prepared ASR gels of varying alkali to silica molar ratio and varying calcium to silica molar ratio was thoroughly characterized using Raman spectroscopy, Fourier transmission infrared spectroscopy (FTIR), Inductively Coupled Plasma spectroscopy (ICP), X-ray powder diffraction (XRD) and thermogravimetric analysis (TGA). 2. Materials and experiments 2.1. Materials Three series of synthetic ASR gels, sodium silica gels (Na gels), potassium silica gels (K gels) and sodium calcium silica gels (Na-Ca gels), of controlled composition were produced following a procedure described by Struble et al. [27]. Reagent grade chemicals were used for all the syntheses. In short, the method involved passing a 0.8 M sodium metasilicate solution through an ion exchange column packed with H+ cation exchange resin (Amberlite™ IR120 H). The resulting silica sol was then doped with controlled amounts of 12.5 N sodium hydroxide or 12.5 N potassium hydroxide and subjected to vacuum evaporation until the mass change on consecutive days was less than 0.1%. All gel samples were stored under vacuum in a desiccator until all the analysis was completed in order to minimize carbonation. The alkali oxide/ silica and calcium oxide/silica molar ratios will be referred to as Na/Si, K/ Si and Ca/Si in the rest of the paper. The theoretical Na/Si, K/Si and Ca/Si ratios as well as the exact ratios as determined by ICP are summarized in Table 1. The slight variations in the ratios when compared to the target values were attributed to varying concentrations of the silica sol obtained after exchange from the column. Further, the carbonation of the gels and the moisture content of the gels at the end of the drying period were also monitored using TGA. These results are also included in Table 1. It is evident that as the alkali/Si ratio of the gels increased, their ability to retain moisture also increased. The scanning electron microscopy (SEM) coupled with energy dispersive X-ray spectroscopy (EDS) was used to confirm full distribution of alkalis in the resulting gels (data not presented in this paper). The EDS maps were collected at five random locations on every sample of gel. 2.2. Experimental program The Raman spectra of the gel samples were obtained using a Jasco NRS-3100 Laser Raman Spectrophotometer equipped with a 532 nm

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Table 1 Chemical composition, carbonation and moisture content of the synthesized ASR gels. Molar ratio Gel type Na gels

Gel code

NS1 NS2 NS3 NS4 K gels KS1 KS2 KS3 KS4 Na-Ca gels NCS1 NCS2 NCS3 NCS4

M⁎/Si, target Ca/Si, target Carbonation⁎⁎ Moisture content (Measured) (Measured) (% by weight) (% by weight) 0.25 (0.21) 0.50 (0.40) 0.75 (0.62) 1.00 (1.17) 0.25 (0.32) 0.50 (0.58) 0.75 (0.80) 1.00 (1.08) 0.5 (0.37) 0.5 (0.35) 0.5 (0.37) 0.5 (0.36)

0 0 0 0 0 0 0 0 0.05 (0.07) 0.10 (0.11) 0.20 (0.25) 0.40 (0.49)

5.89 3.01 2.55 4.21 2.23 1.89 3.82 5.62 3.42 3.66 1.14 2.04

10.28 13.06 23.66 20.87 8.63 11.79 12.52 13.36 20.97 20.24 20.41 19.00

⁎ M = Na or K. ⁎⁎ Carbonation (%) is qualitative and used merely to confirm presence of carbonates.

laser and a CCD detector. The laser power at the sample was 8.8 mW. The Raman data was collected with an Olympus BX20 microscope objective, exposure time of 120 s and 2 accumulations which resulted in a sample beam with diameter of 4 μm. For every gel type, a small piece of gel was used for the Raman analysis. Spectra were collected at five different spots on every piece of gel. The spectra were found to be very reproducible with no significant differences between those collected at different points. The asymmetry of broad peaks in the Raman spectra was assumed to be the result of superimposition of multiple symmetrical peaks [28,29]. Consequently, these unresolved bands in each of the spectral regions of interest were separated into its component bands [16,30–34] by curve fitting the broad bands using the Gaussian function [30–32]. The normalized Raman spectra were background corrected and curve fitted using ACD/Labs UV-IR software. The curve fitting procedure in the ACD/Labs UV-IR software automatically optimizes the parameters of all the peaks in a selected region in order to find the best fit of the calculated spectrum to the experimental data. The optimization procedure is based on the Levenberg-Marquardt algorithm. Fig. 1 shows an example of the background corrected and curvefitted spectra for the high and low frequency ranges of a representative Na gel. One of the principal shortcomings of the curve fitting approach is the lack of a unique solution to obtain a good fit. In addition to ensuring the minimum number of peaks to obtain a good fit, the current approach was further justified by the fact that the silica polymerization information resulting from band assignments based on curve fitting was in agreement with previous results of a Raman spectroscopic investigation of amorphous silica materials [17] and, more specifically, of similar synthetic ASR gels using solid-state Nuclear Magnetic Resonance (NMR) data [22]. The FTIR analysis was carried out using a Digilab Excalibur FTS 3000 Series Fourier transform infrared spectrometer. The spectra of KBr pellets with 0.3% sample concentration were collected at 4 cm−1 resolution and co-adding 32 scans per spectrum. Thermogravimetric Analysis (TGA) was used to determine the amount of carbonation and moisture content of the gels. The TGA was performed with TA Instruments Q5000 under nitrogen purge at 25 ml/min. Approximately 50 mg of powdered gel sample was heated to 950 °C at 10 °C/min. The diffractogram of the gels were collected with a PANalytical Empyrean Series 2 X-ray diffraction system. The X-ray source was a Cu anode operating at 45 kV and 40 mA. Data was collected between 10 to 80o in 2θ with a step of 0.0131° and scan step time of 99 s per step. The chemical composition of the gels was verified using a Shimadzu ICPE-9000 Plasma Atomic Emission Spectrometer. For this purpose, 0.5 g of each of the gels was mixed with 5.5 g of lithium tetraborate and 2 ml of a 0.5 g/l ammonium nitrate solution and fused at 980 °C. A total of three repetitions per gel were fused. The samples were digested using hydrochloric acid and

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a

b

Fig. 1. Example of curve fitting of representative spectra of Na gel: a) High frequency region from NS2 gel and b) Low frequency region from NS3 gel.

Si-O-Si linkages

Si-O SS bands C-O SS bands

a

b

NSH bkg.esp

NS4 (1.17)

NSL bkg.esp

NS4 (1.17)

0

0

NS3 (0.62)

-5 NS2 (0.40)

Arbitrary

Arbitrary

NS3 (0.62)

-5 NS2 (0.40)

-10

-10

NS1 (0.21)

NS1 (0.21)

1120 1040

960 880 800 Wavenumber (cm-1)

720

640

800

c

700

600 500 400 Wavenumber (cm-1)

300

d KSH2 BKG.esp

KSL BKG.esp

KS4 (1.08)

0

KS4 (1.08)

0

-5 KS2 (0.58)

KS3 (0.84)

Arbitrary

KS3 (0.84)

Arbitrary

200

-5 KS2 (0.58)

-10

-10

KS1 (0.31) KS1 (0.31)

1120

1040

960 880 800 Wavenumber (cm-1)

720

e

800

640

600 500 400 Wavenumber (cm-1)

300

NCSL bkg.esp

NCS4 (0.49)

0

0

-5 NCS2 (0.11)

-10

Arbitrary

NCS3 (0.25)

Arbitrary

200

f

NCS4 (0.49)

NCSH bkg.esp

700

NCS3 (0.25)

-5 NCS2 (0.11)

-10 NCS1 (0.07)

NCS1 (0.07)

1120 1040

960 880 800 Wavenumber (cm-1)

720

640

800

700

600 500 400 Wavenumber (cm-1)

300

200

Fig. 2. Raman spectra for synthetic alkali silica gels with gel codes and corresponding Na/Si or K/Si or Ca/Si ratios in parenthesis. All Na-Ca gels have a fixed Na/Si ≈ 0.36. Spectra have been stacked for visual clarity. a) Na gels high frequency region b) Na gels low frequency region c) K gels high frequency region d) K gels low frequency region e) Na-Ca gels high frequency region f) Na-Ca gels low frequency region. Note-all the spectra are background subtracted and normalized.

C. Balachandran et al. / Cement and Concrete Research 92 (2017) 66–74

then further diluted for the ICP analysis. The calibration of ICP was performed using corresponding Na2+, K+, Ca2+ and Si standards. The coefficient of variation of the ICP analysis for each of the gels was found to be less than 11%. 3. Results and discussion 3.1. Raman spectroscopy of synthetic ASR gels The Raman spectra of the Na, K and Na-Ca gels, shown in Fig. 2, were characterized by broad peaks suggesting the amorphous nature of the gels. Each spectrum had broad bands in mainly two regions of interest: the high frequency region of 800–1200 cm−1 and the low frequency region of 400–700 cm−1. The broad envelope in the high frequency region of 800–1200 cm−1 contains peaks attributed to the Si\\O symmetric stretching (SS) bands of Q0 (850 cm− 1), Q1 (900 cm−1), Q2 (950– 1000 cm−1) and Q3 (1050–1100 cm−1) silicate units as well as the symmetric stretching bands of the C\\O bonds in carbonate phases [17,29, 35–37]. Bands in the lower frequency region of 400–700 cm− 1 are linked to the presence of bridging oxygens or Si\\O\\Si linkages and are known to vary systematically with changes in composition which in turn implies changes in the silicate polymerization [17]. More specifically, bands in the 520–560 cm−1 range are linked to Q3 units, 590– 600 cm−1 range to Q2 units, ~700 cm−1 to Q1 units and ~430 cm−1 to Q4 silicate units [17]. Following the curve fitting procedure mentioned in Section 2.2, the spectra were then interpreted following an empirical approach involving comparing the spectra with those in published literature for mainly amorphous materials such as silicate glasses and C-S-H gel as well as carbonate phases [17,29,36–41]. Tables 2 to 4 summarize the major bands that were deduced using curve-fitting of the two broad bands in the lower and higher spectral regions for Na-, K-, and Na-Ca gels respectively. The most intense peaks in both the lower and higher frequency range are shown in bold. All the spectra were found to have peaks in the range of 1065 to 1080 cm−1 due to the presence of one or more carbonate phases. The presence of these carbonate phases was confirmed by XRD analysis and the extent of carbonation in each of the samples was quantified using TGA and shown in Table 1. The specific band assignments and trends in the Raman data will be discussed separately for each of the spectral regions of interest in the following sections. Most intense peaks in both the lower and higher frequency range are shown in bold. 3.1.1. High frequency region (800–1200 cm−1) Based on the assignments in Tables 2–4, the Raman spectra of the Na, K and Na-Ca gels were observed to have peaks related to Si\\O SS

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vibrations of Q0, Q1, Q2 and Q3 silicate units approximately in the range of 855 cm−1, 920 cm−1, 1000 cm−1 and 1040 cm−1, respectively. At this juncture, it is important to point out possible areas of overlaps of the SS bands of the various silicate units with other groups. There is the possibility that the bands of Q2 units may have some contribution from a peak linked to Si\\OH stretching mode of silanol groups at ~970 cm−1 [29,36,39]. An attempt was made to use FTIR to verify the existence of silanol in the synthetic gels by confirming the presence of the 950 cm− 1 band which are linked to the stretching of this group [42]. However, it was found that this band of interest was most probably obscured by the broad band between 800 and 1200 cm−1 attributed, among others, to the out of plane bending of carbonate groups, Si\\O\\Si asymmetric stretching band and vibrations of tetrahedral SiO4 groups [43,44]. The second area of overlap is between the SS bands of Q3 silicate units and the SS bands of C\\O in carbonate groups, which appear in the 1030–1090 cm−1 range for various Na/K/Ca carbonate and bicarbonate phases. In the case of some of the synthetic gels, the SS carbonate band appeared as a sharp peak thus making their assignments more straightforward. However, this was not always the case in some others were they overlapped with the Si\\O SS bands resulting in a poorly resolved broad band. Most of the Na gel spectra were found to have peaks at around 1065 cm−1 and 1080 cm−1 which have been assigned to SS vibrational modes from the carbonate phases trona and natrite respectively (Table 2). The K gels spectra were found to have peaks at around 1065 cm−1 attributed to SS vibrational modes in potassium carbonate (Table 3). While not evident from the Raman data, there is a possibility that there may be a peak at around 1030 cm−1 related to kalicinite. Similarly, the presence of trona, natrite as well as calcite was observed in the Na-Ca gels as suggested by the presence of peaks at approximately 1065 cm−1, 1080 cm−1 and 1086 cm−1 respectively (Table 4). Carbonation in the synthetic gel samples was confirmed using TGA (Table 1) and XRD analysis. Fig. 3 shows a representative XRD diffractograms of each of the three different synthetic gel types with the phases linked to the major peaks. The XRD data was found to support the assignments made for the carbonate phases in the Raman data. Further, the presence of trona in ASR gel samples from the field has been noted elsewhere [24]. A close examination of Tables 2–4 reveal that in the case of the Na and K gels, the most intense peak in the silicate SS region (highlighted in bold in the tables) gradually shifted from ~ 1030 cm−1 to ~ 1000 cm−1 with an increase in the alkali/Si ratio. This suggests that the majority of the silicate units in the gels with alkali/Si ratio less than 0.6 were Q3 sites. In the case of the gels with alkali/Si ratio between 0.6 and 1, Q2 units were found to be more prevalent. These gels with alkali/Si ratio greater than 0.6 are beyond the normal range for ASR gels typically found in the field but were included to verify the fact that Raman spectroscopy can indeed detect changes in the polymerization

Table 2 Positions (cm−1) and assignments of the principal vibrational bands in Raman spectra of Na gels. Gel code (Na/Si) NS1 (0.21)

NS2 (0.40)

NS3 (0.62)

NS4 (1.17)

444.93 475.73 518.24 574.82 797.93

446.08 483.48 538.42 596.82 784.16 861.23 921.92 996.91 1037.16 1067.54

442.1 504.85 557.99 595.16 784.52 860.2 924.75 1002.17 1037.98 1071.42 1084.48

443.18 502.48 559.63 599.97 782 849.17 920.88 1006 1040.01 1067.88 1083.73

919.67 990.26 1035.14 1065.97 1084.88

Assignments

Ref.

Si\ \O\ \Si linkages related to presence of Q4 units Internal deformations of Si\ \O tetrahedra/vibrationally isolated 4-membered rings of SiO4 tetrahedra Si\ \O\ \Si linkages related to presence of Q3 units Si\ \O\ \Si linkages related to presence of Q2 units Motions of Si against tetrahedral O with very little associated oxygen movement Si\ \O SS of Q0 tetrahedra Si\ \O SS of Q1 tetrahedra Si\ \O SS of Q2 tetrahedra/Si-OH Si\ \O SS of Q3 tetrahedra Si\ \O SS of Q3 tetrahedra/C-O SS of CO3 group (Trona) C\ \O SS of CO3 group (Natrite)

[17] [37,38] [17] [17] [17] [17] [17] [17,29,36,39] [17,29,36,37] [17,40] [41]

Most intense peaks in both the lower and higher frequency range are shown in bold.

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Table 3 Positions (cm−1) and assignments of the principal vibrational bands in Raman spectra of K gels. Gel code (K/Si) KS1 (0.30)

KS2 (0.58)

KS3 (0.84)

KS4 (1.08)

455.75 497.35 526.7 579.38 792.61 850.95 928.66 998.02 1028.55 1062.47

451.04 495.69 553.31 595.17 784.77 857.58 917.15 997.58 1043.24 1067.2

434.41 494.19 566.03 595.76 781.87 852.38 916.95 1005.52 1040.08 1062.29

450.16 493.37 562.34 597.23 784.62 847.63 920.64 1000.12 1040.47 1061.23

Assignments

Ref.

Si\ \O\ \Si linkages related to presence of Q4 units Internal deformations of Si\ \O tetrahedra/vibrationally isolated 4-membered rings of SiO4 tetrahedra Si\ \O\ \Si linkages related to presence of Q3 units Si\ \O\ \Si linkages related to presence of Q2 units Motions of Si against tetrahedral O with very little associated oxygen movement Si\ \O SS of Q0 tetrahedra Si\ \O SS of Q1 tetrahedra \OH Si\ \O SS of Q2 tetrahedra/Si\ \O SS of CO3 group (kalicinite) Si\ \O SS of Q3 tetrahedra/C\ \O SS of CO3 group (potassium carbonate) Si\ \O SS of Q3 tetrahedra/C\

[17] [37,38] [17] [17] [17] [17] [17] [17,29,36,39] [17,29,36,37,40] [17,41]

Most intense peaks in both the lower and higher frequency range are shown in bold.

of such systems. The observed shift in the SS band of the silicate units with changing alkali/Si ratio has been previously reported in published literature [17,29] and has also been attributed to decreasing polymerization. The Raman spectra of the Na-Ca gels display a similar shift in the most intense peak in the silicate SS region from 1046 cm− 1 to 1012 cm−1 thus implying a progressive depolymerization with increasing Ca/Si ratio as observed in the Na and K gels. More specifically, the gels with Ca/Si ratio less than 0.1 had more Q3 sites while those with the ratio between 0.1 and 0.4 had more Q2 sites. The trends in the NaCa gels are more complex due to the fact that these gels may contain two phases – a Na gel with calcium incorporated into the framework as well as C-S-H gel [22]. Due to this fact, it may not be appropriate to extrapolate the trends in the Raman bands to overall polymerization of the gels. 3.1.2. Low frequency region (400–700 cm−1) The low frequency region of the Raman spectra of Na and K gels had bands linked to Si\\O\\Si linkages related to Q4, Q3 and Q2 silicate sites at approximately 445 cm−1, 520–560 cm−1 and 595 cm−1, respectively. In the case of the Na-Ca gels, the Raman data analysis revealed bands due to Q4, Q3 and Q1 silicate units at 440 cm− 1, 560 cm− 1 and 675 cm−1, respectively. The assignment of the peaks related to Q2 units was slightly more complex due to the fact that the curve fitting procedure revealed the presence of two peaks, one at ~600 cm−1 and the other at ~ 650 cm− 1, in the spectral range valid for Q2 silicate units. The peak at ~600 cm−1 was assigned to Si\\O\\Si linkages related to presence of Q2 units in the Na silica gel with Ca incorporated into the framework [17] while the peak at ~650 cm−1 was assigned to similar Si\\O\\Si linkages in Q2 units which are in the C-S-H phase [29,36, 37]. The presence of the C-S-H phase in conjunction with Na-Ca silica gel

has been previously reported in the case of a similar system of Na-Ca silica gels with Na/Si atomic ratio 0.5 and Ca/Si atomic ratio greater than 0.23 [22]. The presence of Q2 peaks from both of these phases in the lower spectral region further corroborates this fact. A close scrutiny of Tables 2–4 indicate that the most intense peak in the low frequency region for both Na and K gels shifted from ~520 cm−1 to ~600 cm−1. Fig. 4 illustrates the change in position of the most intense peak in the Raman data of the Na and K gels used in this study with varying alkali/silica ratio along with relevant data for alkali silicate glasses from published literature [17]. Essentially, the phenomenon of decreasing silicate polymerization with increasing alkali content is prevalent both in the synthetic gels in this study as well as alkali silicate glasses. Fig. 4 and Tables 2–4 suggest that gels with alkali/silica ratio less than 0.6 were mostly comprised of Q3 units while Q2 silicate sites were predominant in those with alkali/silica ratios between 0.6 and 0.8. In the case of the Na-Ca gels, peaks related to Q3 units were prominent when Ca/Si ratio is less than 0.1 while Q2 sites were more prevalent at Ca/Si ratio of 0.2 and Q1 units start to dominate the structure when the ratio is 0.4. As mentioned in Section 3.1.1, the trends reported for Na-Ca gels only reflect an overall polymerization of the gel and not necessarily changes in polymerization of the Na-Ca gel phase alone due to possible presence of a C-S-H phase in conjunction with a Na-Ca gel. As is evident from Fig. 4 and previously observed by McMillan et al. [17], the peak linked to Q3 units displays a progressive shift towards higher frequencies with increasing alkali content while the peak attributed to Q2 units remains more or less stationary. The data of the position of the Q3 band only for gels that have an alkali/silica ratio close to what can be expected in the field has been replotted in Fig. 5. The good correlation obtained between the peak position of the Q3 (520–560 cm−1) band in the Raman spectra of ASR gels and the alkali/silica ratio implies that this data could possibly be used to estimate the alkali/Si ratio of the

Table 4 Positions (cm−1) and assignments of the principal vibrational bands in Raman spectra of Na-Ca Gels. Gel code (Ca/Si) NCS1 (0.05)

NCS2 (0.10)

484.3 545.54 593.81 656.8

471.16 557.16 604.7 656.25

794.4

794.4

899.06 985.83 1046.65 1068.17 1081.2

903.49 991.39 1040.52 1071.52 1086.81

NCS3 (0.20)

NCS4 (0.40)

438.96 487.06

445.56 485.17

601.95 650.36 673.87 794.4 856.88 920.95 1001.03 1038.87 1064.02 1081.69 1085.53

614 648.29 674.64 794.4 928.97 1012.01 1043.79 1069.07 1085.02

Assignments

Ref.

Si\ \O\ \Si linkages related to presence of Q4 units Internal deformations of Si\ \O tetrahedra/vibrationally isolated 4-membered rings of SiO4 tetrahedra Si\ \O\ \Si linkages related to presence of Q3 units Si\ \O\ \Si linkages related to presence of Q2 units Si\ \O\ \Si linkages related to presence of Q2 units C-S-H gel Si\ \O\ \Si linkages related to presence of Q1 units C-S-H gel Motions of Si against tetrahedral O with very little associated oxygen movement Si\ \O SS of Q0 tetrahedra Si\ \O SS of Q1 tetrahedra Si\ \O SS of Q2 tetrahedra/Si-OH Si\ \O SS of Q3 tetrahedra \O SS of CO3 group (Trona) Si\ \O SS of Q3 tetrahedra/C\ C\ \O SS of CO3 group (Natrite) C\ \O SS of CO3 group (Calcite)

[17] [37,38] [17] [17] [29,36,37] [29,36,37] [17] [17] [17] [17,29,36,39] [17,29,36,37] [17,40] [41] [40]

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a

b

c

Fig. 3. Principal diffraction peaks of carbonate phases in the XRD diffractograms of synthetic alkali silica gels. a) Na gel (NS3), b) K gel (KS2) and c) Na-Ca gel (NCS2). Legend: (N) Natrite, (T) Trona, (K) Kalicinite, (P) Potassium carbonate and (CC) Calcium carbonate.

gel. Thus, Raman spectroscopy could potentially be used as an easy tool to get an idea of the chemical composition of the ASR gel without having to remove a sample and conduct a more detailed scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEMEDS) investigation. 3.2. Comparison of Raman spectroscopic data with NMR data The polymerization information of the various synthetic gels determined by the analysis of the Raman spectra following curve-fitting and subsequent band assignments was validated by comparing the

results with a previously published 29Si MAS NMR spectroscopy study on a series of synthetic alkali silica gels [22] similar to the ones used in the current work. Hou et al. reported the gels to have predominantly Q3 polymerization along with contributions with Q1, Q2 and Q4 units [22]. The fraction (%) of the various silicate polymerization units present in the Na gels in this study, as determined from 29Si MAS NMR spectral deconvolution, has been reproduced in Fig. 6. In general, the 29Si MAS NMR spectroscopy data revealed a progressive depolymerization in the synthetic alkali silica gels with increasing alkali content. Fig. 7 depicts the trends in the relative intensity of the lower spectral region peaks linked to Q1, Q2, Q3 and Q4 silicate units in the Raman

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a

Fig. 4. Relationship between position of the most prominent peak in the Raman spectra (low region) and alkali/silica ratio for Na and K gels and for K silicate glasses [17]. Bands in 520–560 cm−1 range are linked to Q3 units and 590–600 cm−1 range to Q2 units. *M = Na or K.

b

c

Fig. 5. Relationship between position of peak linked to Q3 silicate units and alkali/silica ratio for Na and K gels. *M = Na or K.

spectra for the Na, K and Na-Ca gels analyzed in the current study. It is important to mention that the fact that the assignments of certain bands in the high frequency region were largely affected by the close proximity and possible overlapping of the SS bands of C\\O in carbonate

Fig. 7. Relationship between normalized intensity of Q3 and Q2 peaks in low frequency region and alkali/silica or Ca/Si ratio. a) Na gels, b) K gels and c) Na-Ca gels.

Fig. 6. Relationship between fraction (%) of Q1, Q2, Q3 and Q4 units determined by 29Si MAS NMR spectral deconvolution and alkali/silica ratio for Na gels [22].

groups as well as the peaks related to silanol groups thus rendering a certain ambiguity to the trends described earlier in Section 3.1.1. Considering this fact, the low frequency region data was chosen to draw more definitive conclusions regarding the polymerization of the gels with change in composition. It is evident from Fig. 7 that in the case of both the Na and K gels, there is an overall decrease in the relative intensity of the Q3 and Q4 peaks with increasing alkali/silica ratio while that of the Q1 and Q2 peaks appears to be increasing. The trends for Na-Ca gels are more complex due to the possible presence of two phases in the gel framework. The peaks related to Q3 are observed only in the Na-Ca gels with Ca/Si ratio less than 0.1. The two types of Q2 units, previously described in

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Section 3.1.2, were found in all of the Na-Ca gels with maximum relative intensity at Ca/Si ratio 0.2. The relative intensity of the peak associated to Q1 silicate units, found in the spectra of gels with Ca/Si ratio greater than 0.2, was found to increase with increasing Ca/Si ratio. The presence of Q2 and Q1 units in the Na-Ca gels with Ca/Si ratio greater than 0.2 further substantiates the possible coexistence of a C-S-H gel phase as well as a Na-Ca silica gel, since these sites are more predominant in C-S-H than Q3 units [25,29,45]. In general, the trends in the fraction (%) of the Q1, Q2, Q3 and Q4 units in the gels,as reported by NMR and shown in Fig. 6, are in agreement with the low frequency spectral region Raman data presented in Fig. 7. Based on the NMR data, Hou et al. reported that all the gels with Na/Si ratio less than 0.495 were comprised of predominantly Q3 sites while the gel with ratio 0.495 had slightly more Q2 units. As evident in Fig. 4, gels with Na/Si ratio less than 0.62 were dominated by Q3 silicate units while the gels with higher Na/Si ratios had more Q2 sites. Further, as emphasized in Section 3.1.2, the presence of a C-S-H phase in conjunction with the Na-Ca gel phase, especially in the gels with Ca/Si ratio greater than 0.2 reflects similar observations by Hou et al. based on XRD and NMR data. In general, the Raman data presented in this paper mirrors the trend of depolymerization with increasing alkali/silica ratio which has been previously reported using 29Si MAS NMR spectroscopy [22].

4. Conclusions The work presented shows that Raman spectroscopy is a powerful tool to investigate the structure of ASR gels. The Raman spectra of synthetic ASR gels were found to have two broad bands in the 800 to 1200 cm− 1 (high frequency) range and the 400 to 700 cm− 1 (low frequency) range indicating the amorphous nature of the gel. The high frequency range in the Raman spectra of all the synthetic gels had Si\\O SS peaks of Q0 (855 cm− 1), Q1 (920 cm− 1), Q2 (1000 cm− 1) and Q3 (1040 cm−1) silicate units. The potential overlap between Si\\O SS vibration of Q2 and Si\\OH stretching mode of silanol groups (970 cm−1) could not be ruled out. Further, the Si\\O SS band of Q3 units was found to overlap with SS bands of C\\O in carbonate groups which appear in the 1030–1090 cm−1 range for various Na/K/Ca carbonate and bicarbonate phases. The carbonate phases detected in the Na gels were trona and natrite while that detected in the K gels was potassium carbonate. Trona, natrite as well as calcite were observed in the Na-Ca gels. The bands assignments of carbonate phases in the Raman spectra were in close agreement with XRD results. The low frequency spectral range of the Raman data was found to have bands due to the Si\\O\\Si linkages related to Q2 (595 cm−1), Q3 (520–560 cm−1) and Q4 (445 cm−1) silicate units in the case of the Na and K gels, and Q1 (675 cm−1), Q3 (560 cm−1) and Q4 (440 cm−1) silicate units for NaCa gels. In addition, the Na-Ca gels were also observed to have two Q2 peaks in the low frequency region, one at 600 cm−1 associated to Q2 units in Na-Ca gel and the other at 650 cm−1 attributed to Q2 sites in a C-S-H phase. Increasing alkali/silica and Ca/Si ratio instigated a progressive depolymerization of the gels which manifested as gradual shifts in the predominant peak in the two regions of interest in the Raman spectra. This shift was towards lower wavenumbers in the high frequency region and towards higher wavenumbers in the low frequency region for Na and K gels with alkali/silica ratio above 0.6. The polymerization trends in the Na-Ca gels were more complex as a result of the coexistence of a C-S-H phase and a Na-Ca gel phase. The specific trends in the silicate polymerization of the gels was found to agree well with published literature on a 29Si MAS NMR spectroscopy on a similar series of synthetic ASR gels. A strong correlation was observed between the peak position of the most intense band in the low frequency region (related to Q3 silicate units) and the alkali/silica ratio for gels with a composition similar to field ASR gels.

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Disclosure statement The authors do not have any conflicts of interest that could inappropriately influence this work. The conclusions are the professional opinion of the authors, and do not represent any official policy of FHWA. Funding This research was performed using the resources of Turner Fairbank Highway Research Centre, Federal Highway Administration, Mclean, Virginia; no funding was provided from outside sources. Acknowledgements The authors would like to acknowledge the contribution of LaKesha Perry in the experimental work and Jack Youtcheff for his helpful input during the performance of the study. Finally, the authors also want to thank Richard Meininger and Igor De la Varga for reviewing this manuscript. References [1] M.D.A. Thomas, B. Fournier, K.J. Folliard, Y.A. Resendez, Alkali-silica Reactivity Field Identification Handbook, No. FHWA-HIF-12-022, 2011. [2] S.W. Forster, D.J. Akers, M.K. Lee, A. Pergalsky, C.D. Arrand, D.W. Lewis, R.L. Boone, State-of-the-art Report on Alkali-aggregate Reactivity, 221, Am. Concr. Inst. ACI, 1998 1–23. [3] T.J. Van Dam, L.L. Sutter, K.D. Smith, M.J. Wade, K.R. Peterson, Guidelines for Detection, Analysis, and Treatment of Materials-related Distress in Concrete Pavements, Volume 1: Final Report, No. FHWA-RD-01-163, 2002. [4] J. Bensted, Uses of Raman spectroscopy in cement chemistry, J. Am. Ceram. Soc. 59 (1976) 140–143. [5] S.S. Potgieter-Vermaak, J.H. Potgieter, R. Van Grieken, The application of Raman spectrometry to investigate and characterize cement, part I: a review, Cem. Concr. Res. 36 (2006) 656–662. [6] J. Skibsted, C. Hall, Characterization of cement minerals, cements and their reaction products at the atomic and nano scale, Cem. Concr. Res. 38 (2008) 205–225. [7] S. Martínez-Ramírez, L. Fernández-Carrasco, Raman spectroscopy: application to cementitious systems, in: S.G. Doyle (Ed.), Construction and Building: Design, Materials, and Techniques, Nova Science Publishers, New York 2011, pp. 233–244. [8] I.G. Richardson, J. Skibsted, L. Black, R.J. Kirkpatrick, Characterisation of cement hydrate phases by TEM, NMR and Raman spectroscopy, Adv. Cem. Res. 22 (2010) 233–248. [9] M. Conjeaud, H. Boyer, Some possibilities of Raman microprobe in cement chemistry, Cem. Concr. Res. 10 (1980) 61–70. [10] C.D. Dyer, P.J. Hendra, W. Forsling, The Raman spectroscopy of cement minerals under 1064 nm excitation, Spectrochim. Acta A: Mol. Spectrosc. 49 (1993) 715–722. [11] S.P. Newman, S.J. Clifford, P.V. Coveney, V. Gupta, J.D. Blanchard, F. Serafin, S. Diamond, Anomalous fluorescence in near-infrared Raman spectroscopy of cementitious materials, Cem. Concr. Res. 35 (2005) 1620–1628. [12] K.N. Jallad, M. Santhanam, M.D. Cohen, D. Ben-Amotz, Chemical mapping of thaumasite formed in sulfate-attacked cement mortar using near-infrared Raman imaging microscopy, Cem. Concr. Res. 31 (2001) 953–958. [13] G. Renaudin, R. Segni, D. Mentel, J.M. Nedelec, F. Leroux, C. Taviot-Gueho, A Raman study of the sulfated cement hydrates: ettringite and monosulfoaluminate, J. Adv. Concr. Technol. 5 (2007) 299–312. [14] Y. Yue, Y. Bai, P.M. Basheer, J.J. Boland, J.J. Wang, Monitoring the cementitious materials subjected to sulfate attack with optical fiber excitation Raman spectroscopy, Opt. Eng. 52 (2013) (104107-1-10). [15] N. Garg, K. Wang, S.W. Martin, A Raman spectroscopic study of the evolution of sulfates and hydroxides in cement–fly ash pastes, Cem. Concr. Res. 53 (2013) 91–103. [16] S. Martinez-Ramirez, S. Sanchez-Cortes, J.V. Garcia-Ramos, C. Domingo, C. Fortes, M.T. Blanco-Varela, et al., Cem. Concr. Res. 33 (2003) 2063–2068. [17] P. McMillan, Structural studies of silicate glasses and melts—applications and limitations of Raman spectroscopy, Am. Mineral. 69 (1984) 622–644. [18] H. Aguiar, J. Serra, P. González, B. León, Structural study of sol–gel silicate glasses by IR and Raman spectroscopies, J. Non-Cryst. Solids 355 (2009) 475–480. [19] W.J. Malfait, V.P. Zakaznova-Herzog, W.E. Halter, Amorphous materials: properties, structure, and durability: quantitative Raman spectroscopy: speciation of Na-silicate glasses and melts, Am. Mineral. 93 (2008) 1505–1518. [20] K.L. Wustholz, C.L. Brosseau, F. Casadio, R.P. Van Duyne, Surface-enhanced Raman spectroscopy of dyes: from single molecules to the artists' canvas, Phys. Chem. Chem. Phys. 11 (2009) 7350–7359. [21] C. Liu, S. Wang, G. Chen, S. Xu, Q. Jia, J. Zhou, W. Xu, A surface-enhanced Raman scattering (SERS)-active optical fiber sensor based on a three-dimensional sensing layer, Sens. Biosensing Res. 1 (2014) 8–14. [22] X. Hou, R.J. Kirkpatrick, L.J. Struble, P.J. Monteiro, Structural investigations of alkali silicate gels, J. Am. Ceram. Soc. 88 (2005) 943–949. [23] C.J. Benmore, P.J. Monteiro, The structure of alkali silicate gel by total scattering methods, Cem. Concr. Res. 40 (2010) 892–897.

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