  In this study, in situ quasi-elastic neutron scattering (QENS) has been
employed to probe the water dynamics and reaction mechanisms occurring during
the formation of NaOH- and Na2SiO3-activated slags, an important class of
low-CO2 cements, in conjunction with isothermal conduction calorimetry (ICC),
Fourier transform infrared spectroscopy (FTIR) analysis and N2 sorption
measurements. We show that the single ICC reaction peak in the NaOH-activated
slag is accompanied with a transformation of free water to bound water (from
QENS analysis), which directly signals formation of a sodium-containing
aluminum-substituted calcium-silicate-hydrate (C-(N)-A-S-H) gel, as confirmed
by FTIR. In contrast, the Na2SiO3-activated slag sample exhibits two distinct
reaction peaks in the ICC data, where the first reaction peak is associated
with conversion of constrained water to bound and free water, and the second
peak is accompanied with conversion of free water to bound and constrained
water (from QENS analysis). The second conversion is attributed to formation of
the main reaction product (i.e., C-(N)-A-S-H gel) as confirmed by FTIR and N2
sorption data. Analysis of the QENS, FTIR and N2 sorption data together with
thermodynamic information from the literature explicitly shows that the first
reaction peak is associated with the formation of an initial gel (similar to
C-(N)-A-S-H gel) that is governed by the Na+ ions and silicate species in
Na2SiO3 solution and the dissolved Ca/Al species from slag. Hence, this study
exemplifies the power of in situ QENS, when combined with laboratory-based
characterization techniques, in elucidating the water dynamics and associated
chemical mechanisms occurring in complex materials, and has provided important
mechanistic insight on the early-age reactions occurring during formation of
two alkali-activated slags.
