Cement-based materials characterization at nanoscale: Nanoindentation and ultrasonic Atomic Force Microscopy (AFM)

Characterizing mechanical properties of cement-based materials is a basic task to be performed before using it as a structural material. An array of different mechanical tests has been developed and applied to measure the stiffness or strength of materials. A recent addition to this field concerns nano-mechanical characterization. It is an extension of our interests: how rigid is a certain volume of materials in nanoscale space? Measuring the properties of interfacial transition zone (ITZ) locating between an inert aggregate and bulk paste is a key application example for nanoscale characterization. In order to measure the properties of ITZ, spatial resolution should be enhanced less than the size of ITZ (a few tens of micrometers), while the compressive strength test, a conventional macroscale measurement, usually uses a specimen of 150 mm diameter 300 mm height (6 in by 12 in) cylinder. This paper discusses two nanoscale techniques, that is, nanoindentation and ultrasonic atomic force microscopy (AFM) used at the center for Advanced Cement-Based Materials (ACBM). The principle of both techniques measuring the elastic modulus at the nanoscale is clarified with an application to a cement paste sample having 50% water-binder ratio. The measurements by both techniques are not exactly the same due to their different mechanism. However, both techniques identically find that the peak probability of the measured elastic modulus of the cement paste is distributed between 10 and 20 GPa (1450 and 2900 ksi).