Elucidating the nano-mechanical behavior of multi-component binders for ultra-high performance concrete

The nanomechanical signature of highly heterogeneous ultra-high performance (UHP) cement pastes are explored in this paper. The UHP pastes are proportioned using 30% or 50% (by mass) of commonly available cement replacement materials including fly ash, microsilica, and fine limestone. Nanoindentation experiments coupled with a Bayesian information criterion-based statistical approach is used to develop modulus-hardness (M−H) clusters for the UHP pastes. While typical low-density (LD) and high-density (HD) C-S-H phases are present in early-age UHP pastes, it is shown that an ultra-high stiffness (UHS) phase, which is a composite of HD C-S-H and nanoscale CH, is predominant at later ages. Nanoindentation data points to the presence of significantly higher proportions of mixed phases in the UHP pastes, comprising of cement hydrates/pozzolanic reaction products and unreacted phases including fine limestone and microsilica acting as micro-aggregates to enhance the stiffness of the paste. The presence of such mixed phases complicates upscaling of the elastic modulus using multi-scale homogenization models, which is to be carefully accounted for in such highly heterogeneous systems.