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

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 Na2SiO3activated 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. IntroductionAlkali-activated materials (AAMs) are a class of sustainable cements synthesized by mixing aluminosilicate precursors with alkaline activating solutions, where the precursor particles dissolve and reprecipitate to form an interconnected gel network. When properly formulated, the resulting AAM binders exhibit favorable mechanical properties similar to hydrated ordinary Portland cement (OPC), 1 that is the main binder material used in concrete production. Due to the massive usage of OPC around the world (~4.1 billion tons in 2017), 2 the cement industry alone is responsible for approximately 5-9% of the global anthropogenic CO2 emissions. [3][4][5] The negative sustainability aspects of OPC have catalyzed the research and development of alternative cementitious binders, with AAMs being one of the most promising candidates. 3 Compared with OPC, AAMs can have a substantially lower CO2 emissions (up to ~80% 4 ) as they are produced using precursor powders from industrial by-products (e.g., blast furnace slag and coal-derived fly ash) and naturally abundant ashes and clays. 5,6 In addition to the sustainability benefits outlined above, certain AAMs exhibit superior thermal properti...

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