Keywords: nanoindentation, micromechanical properties, cement paste, alkali activated fly-ash, statistical deconvolution.

The paper describes the identification of intrinsic micromechanical properties measured by means of nanoindentation on two types of structural materials - cement paste (CP) and alkali-activated fly ash (AAFA). Nanoindentation is an experimental technique that can directly access mechanical properties at small dimensions starting from several tens of nanometers (depending on the sample and the probe). Nowadays, this technique is commonly used also for characterization of structural materials based on cement [1,2]. Since these materials exhibit a high degree of heterogeneity even at the microscale it is uneasy to identify their intrinsic phase properties by direct (focused) measurements.

An experimental programme described in the paper covers nanoindentation measurements of large grids of indents producing a statistical set of elastic properties [4] at volumes of around 1 µm on both types of samples (CP and AAFA). The data were used to construct histograms from which intrinsic properties were extracted by means of statistical deconvolution method [3]. First, distinct material phases of CP and AAFA samples were detected using samples' chemistry, morphological observations and X-ray analysis in electron microscopy (ESEM). Gaussian distributions of individual material phases were determined based on minimizing of the standard error between the constructed probability density function and the experimental results.

Furthermore, results of the deconvolution were compared with independent results from an image analysis. Good correlation of volume fractions of individual phases was achieved for cement paste. Thus it can be concluded that the grey level in an ESEM image corresponds well with mechanical properties of the CP samples. However, results from the image analysis of alkali-activated fly-ash indicate that mechanical properties are not directly linked to the grey levels in the ESEM images for all phases except for non-activated phases and fly-ash particles that can be easily identified. For the main phases (N-A-S-H gel and partly-activated phases), very different results have been obtained. Our findings document well the exceptional capabilities of nanoindentation as a perfect tool for micromechanical analysis and the separation of individual material phases that can hardly be supplemented by other methods.

1

G. Constantinides, F.J. Ulm, K. Van Vliet, "On the use of nanoindentation for cementitious materials", Materials and Structures, 36, 191-196, 2003. doi:10.1007/BF02479557

2

G. Constantinides, F.J. Ulm, "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, 1, 67-80, 2004. doi:10.1016/S0008-8846(03)00230-8

3

G. Constantinides, K.S.R. Chandran, F.J. Ulm, K.J. Van Vliet, "Grid indentation analysis of composite microstructure and mechanics: Principles and validation", Mat. Sc. Eng. A, 430(1-2), 189-202, 2006. doi:10.1016/j.msea.2006.05.125

4

W.C. Oliver, G.M. Pharr, "An improved technique for determining hardness and elastic modulus using load and displacement sensing indentation experiments", Journal of Material Research, 7, 1564-1583, 1992. doi:10.1557/JMR.1992.1564

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