Investigation of compressive creep of calcium-silicate-hydrates (C-S-H) in hardened cement paste through micropillar testing

This study presents an experimental method for investigating the compressive creep behavior of C-S-H micropillars at micro-scale. The micropillars with two different diameters (5 μm and 0.5 μm) in hardened cement paste (HCP) are fabricated using focused ion beam (FIB) milling and subjected to compression using an indenter with flat-end tip. Multi-stage loading protocols are used to sequentially obtain the elastic modulus, creep behavior, and compressive strength of the individual C-S-H micropillar sample. The sinking effect of C-S-H micropillars during the compression introduces a part of irrelevant deformation to the displacement measured by the indenter. To address this, a numerical procedure is used to quantify the sinking deformation and to determine the true stress-strain relationship of the C-S-H micropillar from the measured indentation force-displacement relationship. The results show that the measured specific creep compliance of all C-S-H micropillars can be well described by power-law functions with an average R2 > 0.99. In contrast, if the specific creep curves are fitted by logarithmic functions, the average R2 is only 0.59. It is indicated that the micropillar creep tests capture the short-term creep of C-S-H. Moreover, micropillar creep tests measure the creep compliance of C-S-H that is one order of magnitude faster than the most macroscopic creep tests. This experimental study emphasizes the impact of imposed stress status on the creep behavior of C-S-H at microscale.