Influencing factors on micromechanical properties of calcium (alumino) silicate hydrate C-(A-)S-H under nanoindentation experiment

In Portland cement-based concrete, calcium (alumino) silicate hydrate (C-(A-)S-H) is the principal binding phase that governs its physical, mechanical, and durability properties. In this study, micromechanical properties of compacts made of synthetic C-(A-)S-H powder were investigated with nanoindentation, and various influencing factors were identified. It was revealed that a normal distribution and convergence of test data could be attained with an indentation depth > 350 nm. Higher conditioning relative humidity, pressure, and holding time aided consolidation of the powder and led to an improved elastic modulus. No remarkable influence of strain rate (0.025–0.1 s⁻¹) and vibration frequency was noted. While the Al-incorporated C-A-S-H exhibited superior mechanical properties relative to C-S-H, all the C-(A-)S-H compacts yielded lower elastic modulus values as compared to data from fully hydrated C₃S paste, mainly attributable the phase bonding, packing density, chemical composition, and the presence of phases other than C-(A-)S-H in the C₃S sample.