quasi-elastic neutron scattering study on the water dynamics and reaction mechanisms in alkali-activated slags

• Published in: Physical chemistry chemical physics : PCCP Vol. 21; no. 2; pp. 1277 - 1292
• Main Authors: Gong, Kai, Cheng, Yongqiang, Daemen, Luke L, White, Claire E
• Format: Journal Article
• Published: 22.05.2019
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c9cp00889f

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 Na 2 SiO 3 -activated slags, an important class of low-CO 2 cements, in conjunction with isothermal conduction calorimetry (ICC), Fourier transform infrared spectroscopy (FTIR) analysis and N 2 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 Na 2 SiO 3 -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 by 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 N 2 sorption data. Analysis of the QENS, FTIR and N 2 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 Na 2 SiO 3 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. Determination of key amorphous-amorphous phase transformations in alkali-activated slags by combining in situ quasi-elastic neutron scattering and isothermal calorimetry.