Characterization of cement microstructure by calculation of phase distribution maps from SEM-EDX mappings

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1 Christiane Rößler, Bernd Möser, Horst-Michael Ludwig Characterization of cement microstructure by calculation of phase distribution maps from SEM-EDX mappings 1. Introduction Characterization of native cement hydrates is one of the main challenges towards a complete understanding of hydration processes. Analytical electron microscopy is one of the promising methods to identify cement hydrates by using imaging, spectroscopic and diffraction information. In the present study results of high resolution Scanning Electron Microscopy (SEM) imaging and Energy Dispersive X-ray (EDX) spectroscopy are combined. EDX elemental distribution maps are combined to calculate phase distribution maps of hydrated cement. Results are shown for 28 d hydrated plain Portland cement and cement containing ground granulated blast furnace slag (GGFBS). Results show that phase maps deliver new insights into distribution and presence or absence of phases. Especially hydration products such as C-S-H, C-A-S-H, C-A-M-S-H, AFm and Aft phases can be differentiated if they are present in clusters exceeding a diameter of approx. 2 µm at the given mapping resolution. For increasing GGBFS content (from plain CEM I to CEM III / B) it is shown that the C- S-H phases surrounding alite and belite clinker phases are replaced by C-A-S-H phases. This indicates that already after 28 d of hydration due to the presence of GGBFS the hydrate composition is significantly changed. Further excess of aluminium is indicated by the presence of AFm phases in CEM III / A and B. 2. Experimental program 2.1. Raw Materials CEM I 42.5 R and lab prepared mixtures of this cement with 36 (CEM III/A) respectively 66 (CEM III/B) wt.-% ground granulated blast furnace slag (GGBFS) were used for hydration investigations. Chemical composition of CEM I 42.5 R and GGBFS are given as follows (wt.-%): CEM I 42.5 R (GGBFS) CaO 63.5, (41.1), SiO2 20.0, (35.4), Al2O3 4.4, (11.8), Fe2O3 2.4, (0.7), MgO 1.5, (7.4), MnO 0.0, (0.3), TiO2 0.2, (1.15), K2O 1.3, (0.5), Na2O 0.3, (0.2), SO3 3.7, (0.6), S (1.3). Specific surface area of CEM I 42.5 R and GGBFS were determined by Blaine method (i.e. 460 and 400 m2/kg). Cement and GGBFS have been homogenized in a TURBULA mixer (WAB AG, Switzerland). For hydration investigation cementitious material and water have been mixed (w/c 0.5) and stored at room temperature in sealed plastic containers. Hydration has been stopped by cutting/fracturing samples and immersing them for 6 hours in 2-propanol to remove residual water. For drying samples were placed in vacuum or at 35 C for approximately 12 h. Table 1 : Composition of binders Sample CEM I 42.5 R GGBFS CH w/s [wt.-%] [wt.-%] [wt.-%] CEM I 42.5 R CEM III/A CEM III/B

2 2.2. Methods In the present investigation SEM-imaging (detection of backscattered and secondary electrons, BSE/SE) with EDX mapping is combined. Following to the traditional EDX elemental mapping phase maps are calculated. It is known that for EDX spectroscopy in dependence of the element analyzed, a certain excitation voltage (i.e. approx. 2-3 times the excitation voltage of the X-ray emission line) is needed for quantitative analysis. Thus for analysis of calcium (main component of Portland cement and hydrates thereof) a minimum excitation energy of kev is needed (Kα energy = kev). As any analytical method the SEM-EDX is limited by resolution of the method with respect to the size of phases being analyzed. As already published by previous studies, the nanoscaled nature of cement hydration products, makes it difficult to analyze single phases by SEM-EDX (Wong & Buenfeld [1], Richardson & Groves [2], Möser [3]). Computed excitation volumes for theoretical composition and density of C-S-H, ettringite and portlandite reveal that for the necessary minimum accelerating voltage of kev the emitted X-rays emerge from a volume of approximately 0.5 µm 3 (calculated by software Flight-E Version 3.1-E, EDAX/Ametek, 0.5 µm radius, Figs. 3A&B). This volume can be even larger, if porosity is increased and thus embedding resin is contained in the analyzed area, lowering the average sample density (compare porosity of three circles in Fig. 3A). Thus using an accelerating voltage of 12 kv is clearly a compromise towards the small size of cement hydrates but introduces a deficiency regarding quantification of heavier elements. For elements up to Kα energy of 6 kev (i.e. Ti, Cr, Mn) the applied 12 kev acceleration voltage is still sufficient to get a quantitative measure. For analysis of iron and zinc (Kα energy exceeding 6 kev) also L-level X-ray emission lines were used for quantification of EDX spectra. Thus it is clear that combination of required acceleration voltage, nanoscaled size and intermixing of cement hydrate phases SEM-EDX resolution on bulk samples is insufficient to reveal composition of for example single C-S-H phases. Because this is a known fact previous studies analyzed many single spots in order to find the approximate true mean value [4]. This approach leads to large scatter of results. A scanning electron microscope of type Nova NanoSEM 230 (FEI) equipped with a field emission gun (Shottky emitter) and various detectors for imaging (secondary electron (SE) detectors, backscattered electron detectors, (BSE)) and analysis (EDX- Silicon Drift Detector, EDAX/Ametek Pegasus XM4 with Genesis 6.35 software) were used for investigation. All SE-imaging on fractured surfaces was done at high vacuum and low accelerating voltage (2 kv) thus no electric coating was necessary. Under high vacuum conditions the through the lens detector (TLD) is used for imaging of secondary electrons (SE). Polished sample surfaces were coated with nanometer layer of carbon and used for backscattered electron (BSE) imaging and EDX mapping. For SEM-EDX mappings samples were embedded in epoxy resin and mechanically polished with diamond slurry up to 1 µm. Electrical coating of a few nanometers of carbon was applied on polished surfaces. The following EDX mapping conditions have been applied: acceleration voltage 12.0 kv, approximately 9000 cps, 512x400 pixel mapping resolution (approx and 0.63 µm/pixel depending on mapped area), and dead time of EDX detector was always below 30 %. The number of 9000 cps is deliberately set low (i.e. up to cps is possible) by choosing a medium beam current (approx. 1 na) for the applied 12 kev accelerating voltage. This was done to limit the sample (especially cement hydrate) deterioration under electron bombardment. To get a high total count number for one mapping 256 frames were recorded. Thus to a total mapping time of 13.7 hours and a total count number of 2

3 approximately 440 million counts per map were obtained. The benefit of using longer mapping period is that the accumulation of charge and interaction with the sample is limited. The standard area mapped was mm 2. Quantification of EDX spectra was carried out without standards. Software Genesis 6.35 (EDAX/Ametek) was used to quantify EDX spectra by peak deconvolution (applying ZAF and background correction). Phase analysis on EDX mappings was carried out with software package Genesis 6.35 (EDAX/Ametek). The principal workflow of phase analysis consists of: 1. Acquiring of EDX elemental mapping of the sample, 2. Quali- and quantitative analyses of EDX spectra, 3. Definition of phases of interest by using an EDX library for pure phases or by defining manually phases of interest, 4. Setting up match parameters (based of Chi square goodness of fit and by setting up pixel size for summed up EDX spectra), 5. Assigning a phase to each pixel, summing up of identified pixels to larger areas (computing phase maps). The current study uses Software package Genesis 6.35 (EDAX/Ametek). The applied setup of matching parameters averaged spectra of an area of 3x4 EDX pixels. This leads at the applied standard magnification to a phase map resolution of 1 µm (i.e. lowest resolution applied in the current study). The allowed variability for phase composition was set to 90% for the applied Chi square fit. Phase maps were obtained by comparing average spectra of the defined EDX pixel size (i.e. sum of 3x4 pixels in present case) with surrounding EDX spectra. If a certain fit (90 % Chi square) of spectra is detected the analysed pixels are combined and coloured in one colour. In a first run an automated phase cluster analysis was carried out to check which phases can be differentiated. The obtained phases were validated by manually checking the composition of the phases found on the samples against the phase composition found in the cluster analysis. In this way, we assured that the mean phase composition was as close as possible to an expected cementitious phase. Determined average phase compositions were saved in a phase library and applied to all investigated samples. Still due to the definition of the ChiSquare fit a certain deviation of phase composition from predefined phase composition is allowed. Because the software Genesis 6.35 offers at maximum only seven different phase colours (see Figs. 1A) porosity is not differentiated yet and also in GGBFS containing samples alite and belite is not differentiated. By comparing phase maps with BSE images it becomes obvious that identification of alite, belite and portlandite as shown by phase maps matches quite well with areas indicated in BSE grey value images. In addition hydration products can be differentiated by phase mapping showing also areas where C-S-H (yellow colour in Fig. 1A), CH (red colour in Fig. 1A) and AFm only (magenta colour in Fig. 1A) occur. As shown in Fig. 3B hydration products such as ettringite, AFm, CH and C-S-H also intergrow on a submicron scale. Because on polished sections these phases cannot be resolved separately they are defined as matrix phase (grey colour in Fig. 1). This matrix phases also contains porosity. Phase maps also provide a better differentiation of aluminate and silicate clinker phases (blue, cyan and green colour in Fig. 1A). 3

Color index: green alite and belite, cyan - ferrite, red -

1A. Fig. 2A: EDX phase map of 28 d hydrated CEM III /B.

4 3. Results and Discussions Fig. 1A: EDX phase map of 28 d hydrated CEM I. Color index: green alite and belite, cyan - ferrite, red - CH, yellow - C-S-H, blue - aluminate, grey - matrix, magenta - AFm. Fig 1B: SEM-BSE image of phase map in Fig 1A. Fig. 2A: EDX phase map of 28 d hydrated CEM III /B. Color index: green alite, cyan GGBFS, red - CH, yellow - C-S-H, blue - C-A-M-S-H, grey - matrix, magenta AFm, black - belite. Fig 2B: SEM-BSE image of phase map in Fig 2A. 4

Fig. 4A: Dense C-S-H rim around alite (CEM

4B: Radial growing fibrous C-S-H in CEM I

5A: GGBS particles surrounded by hydrate

5 Fig. 3A CEM I hydrated for 28 d. Circles indicate analyzed area of EDX analysis at 12 kv accelerating voltage. Fig. 3B: Circle size indicates that EDX analysis always contains contributions from all phases (i.e. C-S-H, AFm/AFt, CH). Fig. 4A: Dense C-S-H rim around alite (CEM I 42.5 R, 28 d hydrated). Fig. 4B: Radial growing fibrous C-S-H in CEM I 42.5 R (28d). Fig. 5A: GGBS particles surrounded by hydrate phases in CEM III/B, 28 d hydrated. Fig 5B: Hydrates of CEM III/B 28d: foil like C-A-S-H phases. 5

Coloured phase images clearly depict the spatial distribution of identified phases (Figs. 1A & 2A).

6 Comparing BSE images and phase maps of reference cement (CEM I 42.5 R) hydrated for 28 d it becomes obvious that phase maps - as shown in Figs. 1A- allow a clear distinction between four hydrate phases (i.e. C-S-H surrounding silicate clinker, CH, AFm and matrix). Coloured phase images clearly depict the spatial distribution of identified phases (Figs. 1A & 2A). Clearly, C-S-H phases merely surround partly or fully hydrated silicate clinkers, CH and AFm are deposited in the pore space between clinkers. Also silicate (alite, belite) and aluminate clinker phases can be differentiated (Figs. 1A, 2A). At proper mapping resolution (i.e µm/pixel) also aluminate and ferrite clinker phases can be discriminated (Fig. 1A). Due to mentioned physical limitations of EDX analysis on nanoscaled hydrate phases and limitations of the software (only seven phases that can be colour coded) the matrix phase cannot be further differentiated and thus contains hydrates together with pore space (grey area in Figs. 1A and 2A). Correlating phase maps with high resolution SE imaging shows that the C-S-H rim around alite particles is merely a single phase (Figs 3A, 4A). In contrast, the CEM I 42.5 R matrix (28 d) consists of nanoscaled C-S-H, ettringite, AFm/AFt and portlandite that is intensely intergrown (Fig. 3B). Comparing the microstructure of CEM I 42.5 R and CEM III/B matrixes (after 28 d of hydration) it is found that due to presence of GGBFS the fibrous habit (indicated by nanoscaled tips growing into capillary pores) of C-S-H is changed to a foil-like habit (Figs. 4B, C& 5B). This is already described in previous studies (Richardson & Grooves [5]). Comparison of results of EDX-quantification on phase maps for inner C-S-H of CEM I 42.5 R and CEM III/B reveal an increased Mg, Al and Si content and a reduction in Ca concentration in the presence of GGBFS. Thus a reduction in Ca/Si value of inner C-S-H in CEM III/B is depicted. This is in accordance with findings from EDX point counting on hydrated slag cements [6]. EDX phase maps in Figs. 6 furthermore reveal that after 28 d only in CEM III/A a significant amount of unhydrated alite is found. CEM III/B at the same stage merely shows remnants of belite and much fewer alite (not shown by Fig. 6B). In the microstructure of CEM III/B after 28 d also the identification of pure C-S-H is difficult. This can be taken as clear indication that by ongoing hydration in the presence of GGBFS the C-S-H formed by alite hydration is transformed into Mg and Al rich foil-like hydrates of C-A-S-H type. Fig. 6A: Phase map of CEM III A, 28d: green alite, belite, cyan GGBFS, blue C-A-M-S-H, yellow C-S-H, red portlandite, magenta AFm, grey matrix. Fig. 6B: Phase map of CEM III/B, 28d: color index see left image except: green only belite. 6

7 4. Conclusions Results of the present investigation show that for a complete SEM microstructural characterization of cementitious materials various SEM imaging and analysis techniques need to be combined. The newly applied phase analysis for EDX elemental maps is shown to be a valuable tool to differentiate phases and their spatial distribution during cement hydration. In that way hydrate phases such as C-S-H, C-A-(M)-S-H, CH, AFm and ettringite can be differentiated and their spatial distribution was revealed. Compared to EDX point counting method the approach in the present study was to record EDX spectra at an increased resolution (i.e. minimum of 0.6 µm point distance). All collected spectra are recorded in combination with BSE images and the following EDX phase analysis resembles phases already identified by BSE imaging plus allows differentiation of phases with similar BSE grey level. By comparing high resolution SEM-SE images with images of EDX phase distribution maps it is revealed that after 28 d of hydration GGBFS particles (in hydrated CEM III and CH activated GGBFS) are surrounded by an approx. 500 nm thick layer of hexagonal-platy crystals followed by 1-2 µm thick layer of foil-like hydrates of C-A-S-H type. EDX analysis on these hexagonal platy crystals reveals that they contain Ca, Si, Mg, Al and S. Previous studies describe them as hydrotalcite-like [7, 8] or as Mg-Al layered double hydroxides (LDH) phase [9]. Since as observed in the investigated CEM III (28 d) the quantity of these phases is very limited it is questionable whether they can be identified by X-ray diffraction analysis. Thus as Richardson et al. [10] already suggested, a structural similarity to AFm phases has to be considered. The hydration rim around slag particles consists of two types of hydrates. One is the hydrate typically described as hydrotalcite, but according to the present study may contain significant amount of calcium. Thus further investigations are needed to characterize these phases. The outer part of the slag hydration rim consists of the described C-A-M-S-H phases. These are also found together with other hydrates (nanoscaled ettringite, calcite, portlandite etc.) in the CEM III matrix. They replace fully C-S-H phases if the clinker content is as low as for the investigated CEM III/B. Acknowledgement The research was supported by the Deutsche Forschungsgemeinschaft (DFG), grant number Ro 2424/4-1. 7

8 References [1] Wong HS, Buenfeld NR. Monte Carlo simulation of electron-solid interactions in cement-based materials. Cem Concr Res 2006; 36: [2] Richardson IG, Groves GW. Microstructure and microanalysis of hardened ordinary Portland cement pastes. J Mat Sci 1993; 28(1): [3] Möser B, Stark J. High resolution imaging of wet building material samples in their natural state using Environmental Scanning Electron Microscope. 11th Intern. Congress on the Chemistry of Cement, 2003; South Africa May, CD-ROM. [4] Gallucci E, Zhang X, Scrivener K.L. Effect of temperature on the microstructure of calcium silicate hydrate (C-S-H). Cem Concr Res 2013; 53: [5] Richardson IG, Groves GW. Microstructure and microanalysis of hardened cement pastes involving ground granulated blast-furnace slag. J Mat Sci 1992; 27 (22): [6] Mark Whittaker, Maciej Zajac, Mohsen Ben Haha, Frank Bullerjahn, Leon Black, The role of the alumina content of slag, plus the presence of additional sulfate on the hydration and microstructure of Portland cement-slag blends, Cem Concr Res, 66, 2014, [7] Wang S-D, Scrivener KL. 29Si and 27Al NMR study of alkali-activated slag. Cem Concr Res 2003; 33: [8] Richardson IG, Groves GW. Microstructure and microanalysis of hardened ordinary Portland cement pastes. J Mat Sci 1993; 28(1): [9] Taylor R, Richardson IG, Brydson RMD. Composition and microstructure of 20-yearold ordinary Portland cement-ground granulated blast-furnace slag blends containing 0 to 100% slag. Cem Concr Res 2010; 40 (7): [10] Richardson IG, Skibsted J, Black L. Kirkpatrick RJ. Characterisation of cement hydrate phases by TEM, NMR and Raman spectroscopy. Adv Cem Res 2010; 22: Corresponding Autor: Dr. rer. nat. Christiane Rößler Bauhaus-Universität Weimar Coudraystr Weimar christiane.roessler@uni-weimar.de 8

HYDRATION AND MICROSTRUCTURE DEVELOPMENT OF PORTLAND CEMENT BLENDED WITH BLAST FURNACE SLAG

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