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

TL;DR

This study uses in situ quasi-elastic neutron scattering combined with other techniques to elucidate water dynamics and reaction mechanisms during the formation of alkali-activated slags, revealing distinct water transformations and gel formation pathways.

Contribution

It demonstrates the effectiveness of in situ QENS combined with FTIR and N2 sorption in revealing water and reaction mechanisms in alkali-activated slags, providing new mechanistic insights.

Findings

NaOH-activated slag shows transformation of free water to bound water during gel formation.

Na2SiO3-activated slag exhibits two reaction peaks with distinct water conversions.

The formation of C-(N)-A-S-H gel is driven by Na+ ions and silicate species.

Abstract

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.