In Situ Soft X-ray Spectromicroscopy of Early Tricalcium Silicate Hydration

The understanding and control of early hydration of tricalcium silicate (C(3)S) is of great importance to cement science and concrete technology. However, traditional characterization methods are incapable of providing morphological and spectroscopic information about in situ hydration at the nanoscale. Using soft X-ray spectromicroscopy, we report the changes in morphology and molecular structure of C(3)S at an early stage of hydration. In situ C(3)S hydration in a wet cell, beginning with induction (~1 h) and acceleration (~4 h) periods of up to ~8 h, was studied and compared with ex situ measurements in the deceleration period after 15 h of curing. Analysis of the near-edge X-ray absorption fine structure showed that the Ca binding energy and energy splitting of C(3)S changed rapidly in the early age of hydration and exhibited values similar to calcium silicate hydrate (C–S–H). The formation of C–S–H nanoseeds in the C(3)S solution and the development of a fibrillar C–S–H morphology on the C(3)S surface were visualized. Following this, silicate polymerization accompanied by C–S–H precipitation produced chemical shifts in the peaks of the main Si K edge and in multiple scattering. However, the silicate polymerization process did not significantly affect the Ca binding energy of C–S–H.