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This is a post-reviewed copy of the paper published in Construction and Building Materials 204 (2019) 450–457

The use of Raman spectroscopy to monitor phase changes in concrete following high temperature exposure

Marleen Vetter a, Jose Gonzalez-Rodriguez a, Elisa. Nauha a, Tanya Kerr b *

a School of Chemistry, University of Lincoln, Lincoln, UK

b Department of Physics, University of the West Indies, Kingston, Jamaica

ABSTRACT

Concrete is one of the most widely used construction materials, having excellent mechanical properties, but may fail in a catastrophic manner post fire. In this study, we present non-destructive Raman testing of concrete walls post fire to track temperature exposure based on the decomposition of the calcium silicate hydrate (CSH) phase. The use of Raman spectroscopy is contrasted with X-ray diffraction to demonstrate its competence in evaluating thermal damage to concrete. It was found that Raman spectroscopy was more adept at identifying the hydrated phases than XRD. Principal component analysis was applied to the Raman data to demonstrate the ability of Raman spectroscopy to distinguish concrete exposed to different temperatures. The decomposition of Calcium silicate hydrate could be followed by Raman monitoring the shifts at 1081, 709 and 278 cm-1. At the highest temperature in this study (950oC) Raman spectra showed the disappearance of these bands with formation of a new one at 1007 cm-1 attributed to the formation of gehlenite following the matrix decomposition.

Keywords: Thermogravimetric Analysis, X-ray diffraction, Raman spectroscopy, calcium silicate hydrate, concrete temperature profiling.

1. Introduction

Worldwide, concrete is one of the most frequently used building material, it has good mechanical properties and can be cast in any shape or place [1]. Concrete is also very versatile as it can have many mix designs based upon its intended purpose. A concrete mix design is developed based on the ratios of cement, sand, crushed rock and water to yield different strengths [2]. Concrete has the added advantage of being fire resistant and contributes nothing to the fire environment, thus making it a safe building choice in terms of its fire resistance [3]. Though concrete has a low thermal conductivity exposure to elevated temperatures will compromise a structure, as the cement matrix is sensitive to dehydration which may result in phase changes and separation of the cement gel from the aggregates [4,5].

While aggregates play an important role in the strength of concrete, the quality of the cement and its hydration plays a significant role. When cement is produced it contains the phases alite (3CaO.SiO2, C3S), belite (2CaO.SiO2, C2S), tricalcium aluminate (3CaO.Al2O3, C3A) and tricalcium aluminate ferrite (4CaO.Al2O3.Fe2O3, C4AF) [6]. In the preparation of concrete, water is added and alite which makes up about 50% of the mineral phases in the cement and has the highest hydrolysis reaction. Belite is found in smaller quantities but forms a part of the important hydrated phases of concrete. The hydrolysis of alite and belite are responsible for the formation of the calcium-silicate-hydrate (CaO.SiO2.H2O, CSH) phase and portlandite [7]. It has been well documented that the CSH phase is largely responsible for the binding of the aggregates in the concrete and hence is a significant contributor to the strength [8].

Numerous studies have also indicated that the decomposition of the CSH phase in the concrete matrix due to elevated temperature exposure, and the subsequent debonding from aggregates, compromise the stability of structures [3–5,9–12]. It is therefore imperative, that structures post fire be accessed for loss in structural stability before being put back into commission. This need for post fire assessment has not been called into question. The question is what is the most effective, non-destructive and safe means of doing this.

The effects of elevated temperatures on concrete and CSH decomposition on different types of concrete have been studied using petrography, x-ray diffraction (XRD) and scanning electron microscopy/energy dispersive detector (SEM/EDS) and thermogravimetric analysis (TGA) [4,5,9–11]. Of these techniques, XRD and TGA provides chemical analysis of the concrete phases. Petrography has been used to study colour change in concrete when subjected to several high temperatures. Observation of color change resulting from chemical changes in the minerals found in the cement gives a rough estimate of temperature exposure [9]. This is a rough estimate particularly if a spectroscope is not used, which is most often the case. The advantage of this technique though, is its portability.

TGA allows for monitoring the weight loss which occurs in the concrete sample with increasing temperature from which the decomposition can be inferred [4]. XRD can directly identify the phases present at a specific temperature and which have been lost [13]. The ability for XRD to effectively identify and quantify phases in concrete is limited by its porous nature, which makes it X-ray amorphous [14]. CSH thus appears as small broad peaks in the diffractogram, or do not appear at all.

These traditional techniques are classified as destructive testing as they require cores to be removed from the structure, and samples need to be powdered. TGA and XRD are also not portable techniques[15]. Ash et al. [16] also describes the difficulty of analyzing concrete with XRD and states that new techniques need to be developed for in-situ testing of concrete. Tang et al. [17] sought to develop a portable technique for monitoring changes in concrete post fire. They utilized Fourier transform infrared spectroscopy (FTIR) to monitor the concrete and create correlations to compressive strength, however, the spectra produced in this study were difficult to analyze and lacked specificity in phase identification. In this study we also propose a technique that can later be adapted for non-destructive in-field testing for providing temperature history which can be correlated to phase change in concrete which can be later used for compressive strength determination.

We propose the use of Raman spectroscopy for determining the phases of concrete and their subsequent chemical changes due to high temperature exposure. Potgieter-Vermaak et.al [18] demonstrated that Raman spectroscopy could be used to assess cement with very good results. In a later study, they showed that this technique was also very promising in studying concrete developed with fly ash and slag, therefore showing the versatility of the technique [19].

Peskova [12] further validated the use of Raman spectroscopy for assessing damage to post fire structures by doing a study on the particles in cracks of concrete blocks heated to 1200 C with reasonable success. This was done by identifying the concrete phases and the changes to their Raman bands. This is the only study to our knowledge where this technique has been attempted.

The current paper will build on Peskova’s work to fully provide correlation between Raman spectra for concrete at different high temperature exposures when measured from a wall surface to facilitate onsite testing and tracking temperature exposure. The Raman data collected will be contrasted with X-ray diffraction data and principal component analysis (PCA) conducted. to establish the competence of Raman spectroscopy as a superior tool for post fire assessment.

PCA is a multivariate analysis tool used to simplify large datasets with multiple correlated variables into a smaller number of uncorrelated variables called principal components (PC). The first principal account for the largest amount of variability in the data, i.e the largest difference between the groups being analyzed. The succeeding components account for the remaining variability, therefore PCs 1 and 2 contains the most information regarding the differences in data being analyzed [20]. A plot of PC1 vs PC2 would result in the separation of groups of data by their differences. This analysis technique will be used in this study to demonstrate the power of Raman spectroscopy in differentiating concrete exposure temperature.

2. Materials and methods

2.1 Specimen making

The materials used in this study were Portland cement, river sand, 6.35 mm aggregates and water. A 1:2:4 cement, sand and stone respectively mix design was done with a water to cement (w/c) ratio of 0.5, resulting in a slump of 6 cm. The approximate concrete mix was 0.133 cubic meters and cast in 24 (15.24 cm x 30.48 cm) cylindrical mold. The average values of the compressive strength were 17 MPa after curing for 28 days. Compression strengths were tested using a Humbolt 7515LE007 Compression Tester fitted with a RiceLine 720 controller (Test Mark Industries, Ohio, USA), which had been calibrated by the Bureau of Standards of Jamaica. The weights of the cylinders were measured using a Defender 3000 measuring scale (OHAUS, USA). The size of the specimen furnaced and tested were approximately 5 cm in diameter, these were collected after the compressive strength of the cylinder was taken.

2.2 Preparation of test samples

To demonstrate that Raman spectroscopy can be used to track the temperature history of concrete, concrete samples taken were heated to different temperatures in a Carbolite furnace CWF 12/13 (Sheffield, UK). Samples from six cylinders were taken and subjected to a heating programme in the furnace at 10 / min to 550, 750 and 900 , hold time 10 min, then cooled to room temperature in the furnace before testing.

2.3 Characterization techniques

To evaluate the efficiency of Raman spectroscopy as a tool for assessing temperature exposure of concrete, concrete samples were subjected to standard testing to identify the expected changes at different temperatures. TGA was carried out on the unexposed samples using the same heating profiles as was done in the furnace. The samples which were furnaced XRD analysis was carried out to identify the phase changes at each temperature. The Raman spectra collected for concrete at each furnaced temperature was then compared to the standard methods to determine how well it performed in identification of phase changes.

2.3.1 Thermogravimetric Analysis

Concrete pieces from six cylinders, were ball-milled into a powder, and a sample of around 1.2-3.5 mg of each analyzed individually using a NETZSCH TG 209F3 TARUS. The samples were heated from 35 C to 900 C at a constant rate of 10 C/min with a nitrogen purge rate of 50 mL/min.

2.3.2 X-ray Diffraction

Samples from the six cylinders produced, were also prepared by ball milling into a powder and analysed by Powder X-ray diffraction. Measurements were made on a Bruker Discover D8 diffractometer using Cu K alpha radiation 5-60° 2Theta in transmission mode on a 24 well transmission plate. The samples that were furnaced were also subjected to XRD analysis.

2.3.3 Raman Spectroscopy

Raman analyses were carried out on the samples removed from the cylinders which represented the surface. Surface samples were taken from both the unexposed and furnaced cylinders for comparison. The data was collected using Horiba Jobin Yvon Labram Raman with an Olympus BX41 confocal microscope attachment. It was fitted with a green laser from Laser Quantum at 532 nm with a 50-100% laser intensity being used for analysis. The 20x objective was used for all spectroscopy analysis. A 100 nm split, 1600 gr/mm and a 1000 µm whole size were used throughout and exposure time was usually between 10-30 seconds. To minimize the effects of sample inhomogeneity, data was collected from multiple points (9) on each sample and averaged to create a Raman profile for all concrete samples.

2.4 Data Analysis

To perform Raman spectral comparisons between concrete at room temperature those exposed to different elevated temperatures, the spectra for each were preprocessed in preprocessed in Bio-Rad Knowitall software. Preprocessing included, baseline correction, Savitsky–Golay seven-point smoothing and normalization. For PCA studies, the original data were converted to give first-order derivatives, to remove typical background and fluorescence interference. After which Savitsky–Golay 15-point smoothing, mean centering and peak area normalization was done. PCA was applied to the data using Tanagra software package.

3. Results & Discussion

3.1Identification of hydrated phases in concrete samples

CSH contributes significantly to the compressive strength of the concrete due to its binding of the aggregates [21]. This being so, CSH degradation was monitored in this study to track the effects of elevated temperatures on concrete. Owing to the complexity of concrete, Raman spectra were collected from several areas to determine the best site for tracking CSH changes. Figure 1 shows a microscopic image of the surface of a concrete sample. It was composed of dark, brown and white areas.

Figure 1: Surface of concrete sample.

The Raman spectrum of the dark areas in figure 2, showed it to contain portlandite and small amounts of ettringite based on the presence of the 350 cm-1 and 991 cm-1 peaks respectively. The CSH phase was also present with peaks at 1081, 707 and 278 cm-1, which were however, less established than the peaks related to unreacted alite and belite phases from the cement, which are evidenced by the presence of peaks at 812-868 cm-1 and 545 cm-1 [12,18]. Present were also peaks related to calcite and ferrite between 233-310 cm-1 and gypsum at 172 cm-1 [22].

Both the brown and white areas on the surface of the concrete contain a larger portion of the CSH phase with no alite and belite phases as they are completely converted. The spectrum of the brown areas shows the Si-O stretching in the amorphous CSH phases at 1011 cm-1 and weak bands of the Si-O-Si bending at 625-668 cm-1 [14]. Of the three areas, the white area showed the strongest and most defined peaks representing the CSH phase in the sample.

The peaks at 1081, 709 and 278 cm-1 have been assigned to which in several studies have been assigned to the presence of calcite [23]. However, Kirkpatrick et al. [14], studied the idea that CSH structure could be understood by modelling it as a defect structure of tobermorite and as being related to jennite. They synthesized CSH based on this idea with the use of NMR, Raman analysis was then performed on the samples. The spectrum obtained for synthesized CSH, tobermorite and jennite were very similar to that obtained in spectrum of the white areas in this study. In the study, the 1081 cm-1 peak was attributed to the symmetrical stretching (SS) of the Si-O tetrahedra, the 709 cm-1 to the Si-O-Si symmetrical bending and deformation of Ca-O. The peak at 460 cm-1 was assigned to the internal deformations of Si-O tetrahedra in the O-Si-O linkage and the 278 cm-1 to Ca-O bond. Designation of these peaks are supported by work done by Leeman on alkali-silica reactions [24] as well as Raman studies on silicates [25]

Figure 2: Raman spectra from the surface of concrete sample

Garg [26] reviewed multiple studies which also support the above assignments and also indicate that the peak at 351 cm-1 is portlandite and the combined peaks at 504 and 757 cm-1 is attributed to C3A. The peak at 1156 cm-1 was also assigned to gypsum.

The concrete samples were also analyzed using XRD to verify the presence of the hydrated phases and the presence of CSH. Figure 3 shows the diffraction pattern for the concrete samples to be studied.

Figure 3: X-ray diffractogram from the concrete sample

The XRD analysis showed no diffraction peak for portlandite. A very small ettringite peak was seen at 2 = 11 [15], however, strong peaks for calcite and quartz are seen. Peaks for C3S and C2S have also been identified at a 2 value of 36 . Several studies on tobermorite, jennite and synthesized CSH which possess very similar structures, indicate that peaks with 2 values approximately 22 , 29 and 49 can be assigned to the CSH phase [14,21,27,28]. The absence of portlandite in the diffractogram may be due to the sample being from the surface of the cylinder where carbonation effects may obscure identification [26] due to preferred orientation of the calcium hydroxide.

3.2Tracking the effects of elevated temperature on concrete by Raman Spectroscopy

In assessing the effects of elevated temperatures on the CSH phase in concrete the spectrum of the white areas will be used for monitoring. This was selected as it had the strongest and most defined peaks of interest.

The temperatures selected for monitoring CSH decomposition by XRD and Raman spectroscopy were, 550 C, 750 C and 900 C. This selection was based on the TG/DSC analysis of samples removed from the cylinders at the start of the study. Figure 4 below shows the DTG and DSC plots for the concrete when heated from 35 C to 900 C. The first peak is observed at 150 C which corresponds to the dehydration of CSH [13]. The second is observed between 450-500 C, which corresponds to the dehydration of portlandite. The third peak appears at 740 C which is an indicator of CaCO3 decomposition to CaO and CO2, as well as the final step of depletion of the CSH hydrated phase of concrete. XRD and Raman studies were not conducted for concrete exposed to 150 C in this assessment, the tool will be used for post fire testing and post fire temperature exposure is usually more than 500 C.

Figure 4: DTG and DSC Thermal analysis for the surface of concrete samples

3.2.1Furnaced experiments

XRD and Raman studies were carried out on the furnaced samples. XRD studies were used to support the existence of and /or decomposition of phases in the sample due to elevated temperature exposure to validate Raman analysis as a tool for tracking concrete exposure. Figure 5 shows the X-ray diffractograms for the concrete samples following exposure to 550, 750 and 950 C.

A series of physical and chemical changes take place in the concrete during exposure to high temperatures as seen from the TG/DSC and XRD analysis. The chemical changes are strongly linked to the compressive strength of concrete [21] and can be detected by Raman spectroscopy which lends specificity to the types of bonds present [12] and hence to the compounds present making it an ideal tool for tracking chemical changes.

Figure 5: X-ray diffractogram from the surface of concrete sample following high temperature exposure

XRD studies by Handoo et al [4] and Alqassim et al. [13] demonstrate that CSH deterioration can be seen at 150 C and is observed in the DSC analysis done in this study. Alqassim’s work demonstrates a continuous decline of the CSH peak from 150-600 C with a complete disappearance at 900 C. The decline of CSH is accompanied by a corresponding increase in alite (C3S) and belite (C2S) peaks. Several papers confirm that CSH degradation will result in the formation of the alite and belite phases of cement [12,21]. In figure 5, evidence of CSH decline was seen with the 2 = 22 peak, this may be due to the 29 and 49 peaks also being assigned to calcite, which in literature, shows an increase with decomposition of the hydrated phases [13]. The alite and belite peak at 36 and SiO2 peaks increased with an increase in temperature as the CSH structure decomposed as with Alqassim’s work. However, CSH in concrete proved difficult to track. A diffraction reflection at 2=18o, which seems to be predominant at higher temperatures, can be observed and can also be identified as belite and confirmed with reflections at 29o as a result of CSH degradation. Peaks present at 23o can be assigned to calcite as seen in figure 3, which disappear at higher temperatures (950oC) as seen with Raman and TGA/DSC and the same for the case of 22o. Peaks found at 34o can also be associated with the formation of alite at those temperatures. Peaks at 39o and 43o associated with calcite have nearly disappeared at the highest temperature. The peaks appearing at 51o and 54o at higher temperatures can be also associated with alite.

Figure 6: Raman spectra from the surface of concrete sample before and after furnacing at different temperatures

The Raman spectrum obtained for the furnaced samples is shown in figure 6 above. At 550 C there is a decrease in CSH peaks at 1081, 709 and 278 cm-1 with the ratio of the latter two peaks to the 1081 cm-1 peak being the same as in the original sample indicating a uniform CSH loss. A small peak appeared at 808 cm-1 which was attributed to the formation of belite. Also seen was a large increase in Si-O bond vibrations at 450 cm-1 resulting from CSH depletion. Concrete at 750 C showed an increase in the peak at 1081 with a slight shift to 1078 cm -1 and 707 cm-1 relative to the 550 C spectrum, this may be explained by the increase in CaCO3 as observed in the x-ray diffractograms at 600 C. In a study reviewed by Garg [26] the shift to 1078 cm-1 and the broadening at the base of the peak is indicative of the formation of amorphous CaCO3. In this spectrum stronger peaks are seen between 808 and 843 cm-1 corresponding to the reformation of the alite and belite phases of cement indicating further decomposition of CSH. The increase the peak at 504 cm-1 also marks the increase in free SiO linkages liberated from CSH. A small peak also appears at 1007 cm-1, this can be attributed to the formation of gehlenite (Ca2Al2SiO7) resulting from matrix decomposition [29]. At 950 C all the CSH and CaCO3 related peaks are no longer present or have decreased significantly such that they can no longer be detected, as was also indicated by XRD studies. The spectrum of concrete at 950 C is dominated by the presence of gehlenite (1002 cm-1), free SiO2 and the alite and belite phases of cement, peaks appearing at 1103 and 1134 cm-1 are assigned to C3S and -C2S glassy phases [12]. There are significant changes in the Raman spectrum of concrete at these temperatures to indicate this technique is a promising tool for onsite analysis of concrete surfaces.

3.2.2 Data Analysis

To determine the discriminating power of Raman spectroscopy between samples exposed to different elevated temperatures, PCA was applied to the dataset obtained from the exposure tests. To achieve the best results, the first order derivative of each spectrum was taken before the PCA was done to remove any noise or fluorescence interference. PCs 1 and 2 together defined 50 % of the variation in the dataset and the separation of data based on these two is illustrated in figure 7. The PC2 vs PC1 plot shows that the concrete samples exposed to 550, 750 and 950 C are well separated and therefore can be distinguished using Raman spectroscopy. The plot also demonstrates the inhomogeneity of concrete samples. Therefore, in using Raman as a tool for temperature profiling of concrete, multiple spectra must be collected, and an average taken to represent the true exposure.

Figure 7: PC plot discriminating concrete samples at different exposure temperatures

In examining the variables that have significant influence or define PCs 1 and 2 the factor loading of each were assessed. A factor loading vs Raman shifts plot is seen in figure 8. Factor loadings give an indication of the weight of the original variables (Raman shifts) have on the different components, thus indicating the influence of them. PC1 factor loadings in figure 8 show that on the positive side the Raman shifts of significance are those related to the 550 oC concrete spectrum. For this reason, this concrete sample is separated from the others along the positive PC1 axis in figure 7. PC1 was also impacted on the negative side of the loadings by Raman shifts associated with the Si-O bonds. PC2 has factor loadings that are related to the alite, belite and gehlenite phases that dominate the 950 oC concrete spectrum, in figure 7 these concrete samples are separated from the others along the negative PC2 axis. PC2 also has no significant contributions in the 707-760 cm-1 which includes a CSH and CaCO3 peaks. Thus, contributing the separation between the room temperature concrete and concrete at 750 oC seen in figure 7

Figure 8: PC Loadings for concrete samples at different exposure temperatures

4. Conclusion

Raman spectroscopy provided clear identification of concrete hydrated phases which were difficult to determine using XRD. The identification of phases using Raman was done without need of sample preparation make it a very good tool for on sight testing. Some tracking of CSH decline with an increase in temperature exposure using XRD was possible only the 2 . While, 2 values corresponding to C3S and C2S peaks could be used for tracking of CSH decomposition. A series of physical and chemical take place in concrete during exposure to high temperatures as seen from the TG/DSC and XRD analysis. The Raman spectra collected in tracking temperature exposure allowed for clear identification several peaks that could be used for distinguishing concrete that had been exposed to different elevated temperatures, as noted from the PCA. Therefore, Raman spectroscopy could prove to be a valuable tool for tracking the changes in concrete phases and strength post fire as well as for temperature profiling of concrete.

AUTHOR INFORMATION

Corresponding author: Tanya J. Kerr

*E-mail: tanya.innis@uwimona.edu.jm

Author Contribution

The manuscript was written through contribution of all authors. All authors have given approval to the final version of the manuscript.

FUNDING

This work was supported by The University of the West Indies, Principal’s New Initiative Fund [grant number: UWI/NIF 16101P].

ACKNOWLEDGEMENTS

The authors are grateful for the use of the facilities and equipment at both UWI Mona School of Engineering and the School of Chemistry at the University of Lincoln, UK.

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Abbreviations: C3S, alite (3CaO.SiO2); C2S, belite (2CaO.SiO2); C3A, tricalcium aluminate (3CaO.Al2O3); C4AF, tricalcium aluminate ferrite (4CaO.Al2O3.Fe2O3); CSH, calcium-silicate-hydrate (CaO.SiO2.H2O