Creep effects in nanoindentation of hydrated phases of cement pastes

This paper focuses on the nanoindentation of cement pastes and their micromechanical response. Since cement paste is a heterogeneous material at microscale, micromechanical properties have to be assessed separately for its individual material phases, i.e. hydrated products and unhydrated phases. Our study addresses important issues concerning experimental loading paths and their effects on the evaluation of elastic properties using nanoindentation. The effect of creep is shown on a series of multicycle experiments. A wide range of maximum loads and corresponding final penetration depths (approximately 200–1200 nm) are covered in this study. Experimental measurements on both hydrated cement samples and on unhydrated clinker minerals have proved that significant creep deformation can be attributed purely to hydrated phases. Further, it has been found that cement paste exhibits a strong size effect on elastic properties measured by nanoindentation and evaluated by the standard Oliver–Pharr procedure for loading–unloading tests with no dwell period at the peak load. Such property was already reported for plastic materials like metals [Elmustafa AA., Stone DS., Indentation size effect in polycrystalline F.C.C. metals, Acta Materialia 2002; 50 (14): 3641–3650., Wei, Y., Wang, X., Zhao, M., Size effect measurement and characterization in nanoindentation test, Journal of Material Research 2004; 19 (1): 208–217.], due to various effects but creep. As shown in the paper, cyclic loading and using long dwell periods at peak load leads to minimizing of this spurious type of size effect. Moreover, comparison with results obtained for the COC/PE polymer (75% cycloolefin copolymer + 25% polyethylene) has been performed. This polymer has been found to exhibit similar time-dependent behavior as hydrated cement paste.