Monitoring Early Hydration of Cement by Ex Situ and In Situ ATR-FTIR - a Comparative Study

Diffuse Reflection Fourier Transform Infrared (DR-FTIR) spectroscopy has previously proven to provide time-resolved insights into early cement hydration spanning similar to 30s to similar to 36h after completing the mixing. Here, a previously validated ex situ freeze-dry procedure to stop hydration at preset times is complemented by an in situ Attenuated Total Reflectance (ATR) infrared spectroscopy method. The qualitative overall agreement between ex situ freeze-drying and in situ monitoring is demonstrated. Moreover, water conversion during hydration comes out clearly in the time-resolved ATR-FTIR spectra. This information is absent in DR-FTIR where buildups of crystal water and hydroxides are observed, while quenching of the hydration process requires removal of free water prior to acquiring the spectra. The ability of the IR technique to monitor the initial rate of hydration as a function of time is validated by comparing to calorimetry. The two approaches are understood to be complementary in that the former monitors alite grain surface hydration, while the latter reflects bulk hydration. IR is complementary to the calorimetry in cases of surface processes in conjunction with low enthalpy changes, that is, initial C-S-H formation and additive related surface chemistry.