ESEM-BSE coupled with rapid nano-scratching for micro-physicochemical analysis of marine exposed concrete

15th_{Euroseminar on Microscopy Applied to Building Materials•17-19 June 2015•Delft, The Netherlands}

ESEM-BSE coupled with rapid

nano-scratching for

micro-physicochemical analysis of marine

exposed concrete

Damian Palin

_{; Arjan Thijssen}

_{, Virginie Wiktor}

Henk M. Jonkers

, and Erik Schlangen

a_{Delft University of Technology, Faculty of Civil Engineering & Geosciences,}

Section of Materials and Environment, Stevinweg 1, 2628 CN Delft, The Netherlands.

b_{Bartels Building Solutions, Linie 524, 7325 DZ Apeldoorn, The Netherlands.} ⇤_{d.palin@tudelft.nl}

Abstract

Ordinary Portland cement (OPC) mortar specimens submerged in sea-water were analysed through environmental scanning electron microscopy (ESEM) in back scattered electron (BSE) mode and nano-scratching. Results from both sets of analysis show the presence of distinct phases associated with aragonite, brucite and cement paste. Phases associated with porosity and aggregates were also distinguishable through the BSE analysis and less defined in the nano-scratch data. This study indicates the powerful nature of coupling BSE image analysis with nano-scratching to obtain information on the quality of concrete. Work is underway to improve the method in order to apply it for better understand on the micro-physicochemical properties of marine exposed concrete.

Keywords: marine environment, concrete, ESEM-BSE, nano-scratches, micro-physicochemical.

I. Introduction

Concrete quality must derive from its micro-chemical and-physical properties. Concrete may un-dergo degradation and healing processes which can affect its quality and subsequent durability. Evalua-tion and interpretaEvalua-tion of the relaEvalua-tionship between micro-chemical and -mechanical analysis should then provide key insights for better assessment of concrete durability. Since its use in the early1980s (Scrivener and Pratt, 1983), electron microscopy in backscattered electron (BSE) mode has shown great potential for assessing cementitious materials. BSE analysis produces a grey level image whose grey level values are based on the mean atomic num-ber of the target providing chemical information on the sample being analyzed. Scrivener and Pratt (Scrivener and Pratt, 1983; Scrivener et al. 1986; Scrivener, 2004) in their pioneering work revealed local variations in the microstructure of cementi-tious materials allowing the identification and quan-tification of distinct phases. A major draw back of the technique is, however, its inability to access ma-terials mechanical properties. Recent advances in nanoindentation have allowed greater

understand-ing of nano- and micro-mechanical material prop-erties (Sanchez and Sobolev, 2010). Specimens pre-pared for nanoindentation must be smooth and flat making them ideally suited for BSE analysis. Re-cent combination of nanoindentation with scanning electron microscopy (SEM) in BSE mode has pro-vided new insights into the micro-physicochemical properties of cementitious materials (Chen et al., 2010). Nanoindentation by virtue analyses me-chanical properties at a point meaning that a large number of indents are necessary when analysing heterogeneous materials. An adaptation of nanoin-dentation is the nano-scratch technique whereby the indenter is scratched across a sample surface providing information on its scratch hardness. The continuous nature of the nano-scratch technique provides far greater information over time, and also property gradients and discontinuities across interphases, than its nanoindentation counterpart. This work is part of a larger study looking at the development of autonomous self-healing concrete for application in the marine environment. To the authors knowledge this is the first paper to combine BSE image analysis with nano-scratching for micro-physicochemical material analysis of cementitious

Palin et al.

Table 1: Mix-design for mortar specimens. Amount [kg/m3] Constituent CEM I 42.5 N

Cement 507

Water 253

Water cement ratio 0.5

Sand fraction [mm]:

1-2 608

0.5-1 426

0.25-0.5 167

0.125-0.25 319

materials, illustrating the potential of the combined technique for analyzing marine exposed concrete.

II. Experimental approach

Mortar cubes were cast from OPC (CEM I 42.5 N, ENCI, the Netherlands) in accordance with EN 1015-11 (EN, 1999). The applied mortar mix de-sign is shown in Table 1. 24 hours after casting the mortar specimens were carefully removed from their moulds, tightly sealed in polyethylene plas-tic bags and kept at room temperature for a total curing period of 28 days.

Autogenous healing incubation conditions Plastic buckets were prepared containing 4 l of synthetic sea-water (20±2 C). The synthetic sea-water, to be called sea-water for the remainder of the text, was produced from technical grade chemi-cals (Sigma-Aldrich), the composition of which is shown in Table 2. Three cubes were submerged in each bucket. Water was changed every two weeks to mimic in situ conditions and prevent ion deple-tion. The buckets were kept open to the atmosphere during the experiment to allow for gas diffusion across the water-air interface.

Table 2: Synthetic sea-water composition based after the major constituents of sea-water (Stumm and Morgan, 1996). Compound Amount [g/l] NaHCO3 0.19 CaCl2·2H2O 1.47 MgCl2·6H2O 10.57 Na2SO4·10H2O 9.02 KCl 0.75 NaCl 24.08

Sample preparation and micro-physicochemical analysis

Cubes submerged for 140 days were removed from the sea-water, air-dried, impregnated with epoxy and polished sections prepared. In order to make the thickness of the specimens comparable and level, a thin section machine was used for cutting and grinding the specimens. After grinding, each sample was polished on a lapping table with 6 µm (1 hour), 3 µm (1 hour), 1 µm (1 hour) and 0.25

µm (4 hours) diamond spray (DP-Spray P, Struers,

Copenhagen, Denmark). In between each spray the samples were submerged in ethanol and ultrasoni-cated (5 min) to remove debris. Following polishing samples were kept in a desiccator until testing.

Scratch locations were identified before scratch-ing and digital images made usscratch-ing ESEM (Philips XL 30 ESEM, Eindhoven, Netherlands) in backscat-tered electron (BSE) mode. The instrument was operated at 20 kV accelerating voltage and at a working distance of 10 mm between the final con-denser lens and the specimen. The spot size was 5.0

µm and the magnification of the images was 500⇥. Agilent Nano Indenter G200 (Agilent Technolo-gies, Oak Ridge, Tennessee, USA) with a diamond Berkovich tip was then used to produce the nano-scratches. A quartz standard was indented before and after each test series to ensure accuracy. A series of scratches were made at a wear load of 1 mN and velocity of 25 µm.s 1_{. These scratches}

were 25 µm apart, perpendicular to the specimen surface and ran for a length of 200 µm. BSE images were taken of the scratched specimens. Compari-son between the initial and subsequent BSE images could then be made. MATLAB R2013b (The Math-Works, Inc., Natick, Massachusetts, USA) was used to isolate the scratch line pixels in the unscratched images. Figure 1 (C and D) shows two scratch lines plotted in the form of two histograms. Each plot is the average grey level values of 6 pixels approxi-mating the scratch width.

III. Results and discussion

Figure 1 (A) and (B) show BSE micrographs of selected areas of the mortar specimen after submer-sion in sea-water. Detailed considerations on the effects of submerging concrete in sea-water have been given elsewhere (Palin et al., 2015, Glasser et al., 2008; Conjeaud, 1980). Based on this work it is known that concrete tends to leach ions increasing permeability, making the concrete susceptible to further attack including erosion due to wave action. Reaction of these ions with sea-water constituents can also form an aragonite-brucite mineral layer on its surface, which may reduce permeability, con-tributing specific properties to concrete. Evidence

15th_{Euroseminar on Microscopy Applied to Building Materials•17-19 June 2015•Delft, The Netherlands}

Figure 1: (A) and (B) BSE micrographs of the scratch locations; (C) and (D) the histograms grey level distributions along the scratch lines; and (E) and (F) their corresponding scratch depth profiles.

of these phenomena can be seen in the micrograph with the light grey area on the left hand side rep-resenting aragonite and the darker grey to its right representing brucite. This double layer is formed on top of the mortar, which can be seen as the area on the right hand side of the micrograph. Running horizontally through the micrograph are two white lines, which graphically represent the scratch paths. The histograms below the micrographs (Figure 1 (C and D)) represent the grey scale levels along the scratch lines. Due to compositional and hence grey level variations along the lines various phases can be distinguished. The relatively flat sections towards the left of the histograms represent the ho-mogenous phases of aragonite and brucite. As we move towards the right hand side of the two his-tograms more variance can be seen in the grey level values representing variance in the mortars chemi-cal composition, darker levels representing porosity and lighter levels anhydrous phases (Zhang et al., 2011). Figure 1 (E) and (F) show two nano-scratch profiles. Scratch depth variations are based on the micromechanical properties of the phases. We can see from the nano-scratch test that both aragonite and brucite have similar scratch profiles, indicat-ing similar hardnesses in line with the literature (Santhanam, 2013). This is interesting as the two

phases have quite different grey scale values and hence chemical properties. However like the BSE histograms, the relative flatness of the scratch pro-files represents the homogeneity of the two min-eral phases. Once the nano-indenter leaves the aragonite-brucite double layer and begins scratch-ing the mortar, the scratch profile becomes more varied representing the heterogenic nature of the material. Shallower depths represent harder materi-als such as aggregates and deeper depths porosity. Nano-scratch analysis provides information on the nano- and miro-mechanical properties of cementi-tious materials such as their scratch and mar resis-tance. Interesting properties when trying to better assess the performance of cementitious materials, particularly those in the marine environment.

The objective this research was to see if BSE mi-crographs could be coupled with nano-scratching to assess the micro-physicochemical properties of concrete exposed to the marine environment. Dis-tinct phases can be seen in both sets of analysis. Both techniques showed aragonite and brucite to have relatively flat profiles representative of their homogeneity, while the mortar profiles having more variance representative of its heterogeneity. We also saw correlation between some of the peaks and dips of the two techniques within the mortar

Palin et al.

resenting aggregates and pores. Although good correlation can be seen between the two methods, differences do remain. These differences may be based on fundamental differences between the two techniques, but may also be explained by differ-ences in the micro interaction volumes of the two techniques. Any difference in the interaction vol-umes would mean that the mechanical information provided by the nano-indenter and the chemical information provided by ESEM-BSE would not be directly comparable (Chen et al., 2010). Further investigation is required to assess whether the in-teraction volumes of the two techniques are closely comparable in location and size.

IV. Conclusion

BSE imaging and nano-scratching techniques have been combined for micro-physicochemical infor-mation on cementitious materials. Data generated from the two techniques correlated well, with both able to distinguish between phases associated with aragonite, brucite and cement paste. Though fun-damentally different, improved correlation between the techniques may be achieved by better matching the micro interaction volumes. Work is underway to assess and better match the interaction volumes of the two techniques. On a general level, this study illustrates the great potential in coupling BSE mi-croanalysis and nano-scratching for advancing our understanding on the relationships between the chemistry and mechanics of cementitious materials, particularly those in the marine environment.

V. Acknowledgements

The authors wish to thank Ravi Patel of the Uni-versity Gent for his input regarding the MATLAB analysis, while the research leading to these re-sults has been funded through the European Union Seventh Framework Programme (FP7/2007-2013) under grant agreement no _{290308 – SheMat.}

References

Chen, J.J., Sorelli, L., Vandamme, M., Ulm, F.-J., Chanvillard, G. (2010): “A Coupled Nanoindentation/SEM-EDS Study on Low Water/Cement Ratio Portland Cement Paste: Evidence for CSH/Ca(OH)2 Nanocompos-ites” Journal of American Ceramic Society. Vol. 5, 5, pp. 1484-1493.

Conjeaud, M. (1980): “Mechanism of sea water attack on cement mor-tar”, ACI Special Publication, 65

Glasser, F.P., Marchand, J., Samson, E. (2008): “Durability of concrete — degradation phenomena involving detrimental chemical reactions”,

Cement and Concrete Research. 38 226-246.

Palin, D., Wiktor, V., Jonkers, H.M. (2015): “Autogenous healing of marine exposed concrete: Characterization and quantification through visual crack closure”, Cement and Concrete Research, 73, pp. 17-24.

Sanchez, F., Sobolev, K. (2010): “Nanotechnology in concrete - A re-view”, Construction and Building Materials. 24 pp. 2060-2071.

Santhanam, M., (2013): “Magnesium Attack of Cementitious Materi-als in Marine Environments, in: Performance of Cement-based MateriMateri-als in Aggressive Aqueous Environments”, Springer, pp. 75-90.

Scrivener, K.L., (2004): “Backscattered electron imaging of cementi-tious microstructures: understanding and quantification”, Cement and Concrete Composites. 26 pp. 935-945.

Scrivener, K.L., Patel, H., Pratt, P., Parrott L. (1986): “Analysis of phases in cement paste using backscattered electron images, methanol adsorption and thermogravimetric analysis, in: MRS Proceedings”, Cam-bridge Univ Press, pp. 67.

Scrivener, K.L., Pratt, P. (1983): “Characterisation of Portland cement hydration by electron optical techniques, in: MRS Proceedings”, Cam-bridge Univ Press, pp. 351.

Stumm, W., Morgan, J., (1996): “Aquatic chemistry, chemical equili-bra and rates in natural waters”, Env Sci Technol.

Zhang, X.Y., Gallucci. E., Scrivener, K. (2011): “Prognosis of Al-kali Aggregate Reaction with SEM, Advanced Materials Research.” 194 pp. 1012-1016.