A coupled nanoindentation/SEM-EDS study on low water/cement ratio portland cement paste: Evidence for C-S-H/Ca(OH)₂ nanocomposites

A low water/cement ratio (w/c=0.20) hydrated Portland cement paste was analyzed by grid-indentation coupled with ex situ scanning electron microscope-energy-dispersive X-ray spectra (SEM-EDS) analysis at each indentation point. Because finite element and Monte-Carlo simulations showed that the microvolumes probed by each method are of comparable size (approximately 2 μm), the mechanical information provided by nanoindentation was directly comparable to the chemical information provided by SEM-EDS. This coupled approach provided the opportunity to determine whether the local indentation response was a result of a single- or a multiphase response - the latter being shown predominant in the highly concentrated w/c=0.20 hydrated cement paste. Results indicate that, in the selected microvolumes where C-S-H and nanoscale Ca(OH)₂ (CH) are present, increasing fractions of CH increase the local indentation modulus (and hardness), yielding values above those reported for high-density (HD) C-S-H. Micromechanical analyses show that C-S-H and CH are associated, not merely as a simple biphase mixture, but as an intimate nanocomposite where nanoscale CH reinforces C-S-H by partially filling the latter's gel pores. The paper discusses the mechanism of forming the C-S-H/CH nanocomposite, as well as the impact of nanocomposites on various macroscopic properties of concrete (e.g., shrinkage, expansion). On a general level, this study illustrates how a coupled nanoindentation/X-ray microanalysis/micromechanics approach can provide otherwise inaccessible information on the nanomechanical properties of highly heterogeneous composites with intermixing at length scales smaller than the stress field in a nanoindentation experiment.