ATOMIC FORCE MICROSCOPY FOR CEMENTITIOUS MATERIALS

Paramita Mondal (1), Surendra P. Shah (1) and Laurence Marks (2)

(1) Center for ACBM, Northwestern University, USA

(2) Northwestern University, USA

Abstract

Atomic force microscopy (AFM) is a technique that is capable of both imaging surface structure and recording local mechanical properties of the sample. AFM was used to image nano- and microstructure of cement paste after a few hours of hydration and after it is completely hardened. A Hysitron TriboIndenter has been used to determine mechanical properties while also providing in-situ SPM images. This technique can be used to investigate the effects of different additives and how properties change at the nanoscalel. Mechanical properties adjacent to fibers and aggregates are also of interest where both AFM and the TriboIndenter can be useful. This introduces new possibilities for characterizing the properties of the interfacial transition zone that has a considerable effect on the overall performance of cement-based materials.

Keywords: Atomic force microscopy, cementitious materials, nanostructure, mechanical properties, nanoindentation

1. INTRODUCTION

It is widely believed that the fundamental properties of concrete such as strength, ductility, early age rheology, creep and shrinkage, fracture behavior, durability, etc are affected by material properties at the nanoscale. However, the nano and microstructure of the most important hydration product in cement paste, calcium silicate hydrates (C-S-H, with C = CaO, S = SiO2, and H = H2O) is not well understood. In order to improve cement and concrete properties, it is necessary to understand the nanoscale properties. The goal of our research is to characterize the properties of cementitious materials using different imaging techniques and evaluation of the mechanical properties of different phases of hydrated cement paste.

We present here results using Atomic Force Microscopy (AFM) on cementitious materials. This is a relatively new technique for investigating the nanoscale properties of cementitious materials ([1], [2], [3], [4], [5]). Experiments were performed on early age and on hardened cement paste to image the nanostructure of the products of hydration. Preliminary results on determination of the mechanical properties of cement paste at the nanoscale are also presented. Our technique shows promise for the nanoscale characterization of cementitious materials and shows potential for the investigation of the effects of different additives.

2. IMAGING OF HARDENED CEMENT PASTE 2.1 Experimental details

Hardened cement paste with a water to cement ratio of 0.45 was used in this study. Specimens were cut and polished using silicon carbide paper down to 6.5 μm, then polished using diamond paste down to 0.1 μm to obtain a very smooth surface. Two different Atomic Force Microscopes (AFMs), a Digital Instrument Nanoscope MultiMode Scanning Probe Microscope and a JEOL JSPM-5200 were used. Both Contact and Tapping Mode were used for imaging, though Tapping Mode was found to be more effective. 2.2 Results

(a) (b) 350 nm

Figure 1 4.7 μm × 4.7 μm AFM image of C-S-H gel, (a) Topography, (b) Change in amplitude

Images of C-S-H gel shows nearly spherical particles of different sizes in different areas,

and the typically size of the spherical particles ranges from 100 nm to 500-700 nm. Figure 1 is a 4.7 μm × 4.7 μm AFM image of C-S-H gel, on the left the topography and on the right the amplitude signal . The topography image shows the coarse height variations of the sample, brighter regions being higher than darker ones. The amplitude image shows local variations in height after removal of the coarse topography variations, in effect a high-pass filtered image. The image clearly shows particles of size 200 nm to 350 nm.

Figure 2 is a 5 μm × 5 μm AFM image of an unhydrated cement particle. Unhydrated particles were very flat, and it was not uncommon to observe polishing scratches indicating that this is a consequence of the sample preparation. Small particles on the surface could be of calcium carbonate that developed due to exposure to atmospheric conditions after polishing.

250 nm

350 nm

200 nm

(a) (b) 1000 nm

Figure 2 5 μm × 5 μm AFM image of unhydrated cement particle, (a) Topography, (b) Change in amplitude

3. IMAGING OF CEMENT PASTE AT EARLY AGE 3.1 Experimental details

Cement paste at an early age is too soft for a polishing approach, so a different method was used. Cement paste was cast in a small sample holder that fits into the AFM and cured for 24 hours. A Scanning Electron Microscope was used to locate well hydrated smooth areas on the surface that were later imaged with the AFM. 3.2 Results

Figure 3 shows one AFM image (topography) of C-S-G gel after 24 hours of casting. The image shows nearly spherical particles of several tens of nanometers in size. More experiments need to be performed at different ages to understand the changes in the nanostructure of cement paste during the hydration process.

Figure 3 2 μm × 2 μm AFM image (topography) after 24 hours of casting 4. DETERMINATION OF THE MECHANICAL PROPERTIES AT NANOSCALE FOR HARDENED CEMENT PASTE 4.1 Experimental details Our goal was to identify different phases in the cement paste and determine local mechanical properties. A Hysitron TriboIndenter was used to provide in-situ SPM imaging to allow pre and post-test observation of the sample, so one can identify different phases and position the indenter probe at specific test locations. Post-test imaging also provides the ability to verify that the test was performed at the anticipated location and also to retroactively measure the indent size. Both a Berkovich tip with a total included angle of 142.3 degrees and a cube corner tip of total included angle of 90 degrees were used; the cube corner tip was found to be more effective since it is sharper. Hardened cement paste sample was prepared using the method mentioned previously. The sample was first imaged to identify different phases and then indentation was performed. Though nanoindentation has been used with cementitious materials ([6], [7]), it is new to image the nanostructure and simultaneously indent to obtain local the mechanical properties. Figure 4 shows one typical force-indentation curve with cyclic loading and unloading. Elastic properties were evaluated using the Oliver and Pharr method [8] from the unloading part of the curve.

Figure 4 Load-indentation curve with cyclic loading 4.2 Results

Several Indentations were performed on different areas of C-S-H gel and on unhydrated cement particles. Figure 5 shows images of different areas after indentation. For unhydrated particles, the calculated modulus and the hardness values were 125 GPa and 7.66 Gpa respectively. For C-S-H gel close to an unhydrated particle, the values measured were 40 GPa and 1.32 GPa. For C-S-H gel further away from an unhydrated particle, the calculated modulus was 23 GPa and the hardness 1.01 GPa. We reproducibly observed differences in of the mechanical properties of the C-S-H gel in different areas.

Indentation depth (nm)

Forc

e (μ

N)

(a) (b) (c)

Figure 5 10 μm × 10 μm image after indentation, (a) Unhydrated particle, (b) C-S-H gel close to unhydrated particle, (c) C-S-H gel further away from unhydrated particle

5. CONCLUSIONS

Characterization of the nanoscale properties of cementitious materials is necessary to understand and finally improve the properties of concrete. AFM is a new technique for imaging the nanostructure of cementitious materials, which is just starting to be exploited. Our technique for determining the nanoscale mechanical properties shows potential to simultaneously identify and image the nanostructure of different phases and determine nanomechanical properties removing ambiguities which might exist with just nanoindentation results. Further experiments are underway to characterize the structure of C-S-G gel at nanoscale that shows differences in mechanical properties. Experiments are also underway to characterize the nanostructure at an early age, the effects of different additives and the properties at the interfacial transition zone.

Unhydrated particle

REFERENCES

[1] Mitchell, L.D., Prica, M., Birchall, J.D., ‘Aspects of Portland cement hydration studied using atomic force microscopy’, Journal of Materials Science, 31 (1996) 4207-4212 [2] Papadakis, V.G., Pedersen, E.J., Lindgreen, H., ‘An AFM-SEM investigation of the effect of silica fume and fly ash on cement paste microstructure’, Journal of Materials Science 34 (1999) 683-690 [3] Yang, T., Keller, B., Magyari, E., Hametner, K., Günther, D., ’Direct observation of the carbonation process on the surface of calcium hydroxide crystal in hardened cement paste using Atomic Force Microscope’, Journal of materials Science 38 (2003) 1909-1916 [4] Yang, T., Keller, B., Magyari, E., ’AFM investigation of cement paste in humid air at different relative humidities’ Journal of Physics D: Applied Physics 35 (2002) L25-L28 [5] Kauppi, A., Andersson, K.M., Bergström, L., ‘Probing the effect of superplasticizer adsorption on the surface forces using the colloidal probe AFM technique’, Cement and Concrete Research 35 (2005) 133-140 [6] Constantinides, G., Ulm, F.J., ‘The effect of two types of C-S-H on the elasticity of cement-based materials: Results from nanoindentation and micromechanical modeling’, Cement and Concrete Research 34 (2004) 67-80 [7] Velez, K., Maximilien, S., Damidot, D., Fantozzi, G., Sorrentino, F., ‘Determination by nanoindentation of elastic modulus and hardness of pure constituents of Portland cement clinker’, Cement and Concrete Research 31 (2001) 555-561 [8] Oliver, W.C., Pharr, G.M., ‘An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments’, Journal of Material Research 7 (1992) 1564-1583