ALKALI-ACTIVATED SLAGS: MECHANISMS OF FORMATION AND SULFATE-INDUCED DEGRADATION

Abstract

Alkali-activated materials (AAMs) constitute an important class of sustainable cements with a huge potential to substantially lower the CO2 emissions of the cement industry. However, much remains unknown about this class of materials, especially regarding the complex molecular-level mechanisms occurring during their formation and subsequent degradation in aggressive environments. Understanding these mechanisms is crucial for the (i) optimization of the AAM properties for specific applications, (ii) development of testing standards, and (iii) development of physics-based predictive models for performance evaluation/prediction, all of which are important for the large-scale commercial adoption of this important technology. Hence, this thesis focuses on bringing clarity to (i) the chemistry and atomic structure of a ground granulated blast-furnace slag (GGBS), a commonly used precursor material for synthesis of AAM, (ii) the complex reaction mechanisms occurring during formation of alkali-activated GGBS and (iii) the fundamental chemical degradation mechanisms in alkali-activated GGBSs under sulfate exposure, a major durability issue that has plagued numerous concrete structures around the world. To achieve these goals, a range of experimental and computational techniques have been employed.

The initial part of thesis is devoted to generating realistic structural representations for the reactive amorphous phase (i.e., calcium and magnesium aluminosilicate (CMAS) glass) in GGBS, which is not yet available in the literature. This is achieved by combining molecular dynamics (MD) simulations and density functional theory (DFT) calculations with reverse Monte Carlo refinement against X-ray and neutron pair distribution function (PDF) data. The resulting structural representation not only agrees with experiments (including X-ray/neutron scattering data from this study as well as literature data) but also is thermodynamically favorable (according to DFT calculations). Detailed analysis of the final structure enabled existing discrepancies in the literature to be reconciled and has revealed new structural information on the CMAS glass, specifically, (i) the unambiguous assignment of medium-range atomic ordering, (ii) the preferential role of Ca atoms as charge compensators and Mg atoms as network modifiers, (iii) the proximity of Mg atoms to free oxygen sites, and (iv) clustering of Mg atoms.

The middle part of the thesis centers on furthering understanding of the kinetics and mechanisms of reactions during alkaline activation of GGBS. This is achieved firstly by using in situ X-ray PDF analysis and high-resolution XRD to trace phase formation and evolution of local atomic structure in a NaOH-activated GGBS. By combining GGBS structure modeling with X-ray PDF data, all the phases in the NaOH-activated GGBS (including the amorphous unreacted GGBS) are quantified as a function of reaction time. This phase quantification also enables estimation of the degree of reaction (DOR) with the progress of reaction, which will be useful for studying the reaction kinetics in the NaOH-activated GGBS. Second, in situ quasi-elastic neutron scattering (QENS) and inelastic neutron scattering (INS) are used to study the impact of activator chemistry on the reaction kinetics and mechanisms occurring during the formation of alkali-activated GGBS. By fitting the QENS data with commonly used models, the different water populations present in alkali-activated GGBS have been quantified. By tracing how the different types of water environments evolve with the progress of reaction, and comparing these results with the heat evolution data from isothermal conduction calorimetry and Fourier transform infrared spectroscopy (FTIR) data, important mechanistic insight into the reaction kinetics and mechanisms in the two AAS samples has been obtained.

Following the studies on the formation of alkali-activated GGBS, another important component of the thesis focuses on the final reaction products formed in the alkali-activated GGBS. Specifically, the impact of the inherent chemical variability in GGBS on the phase formation and local atomic structure of the final alkali-activated GGBS pastes are investigated using X-ray total scattering and subsequent PDF analysis. Analysis of the local structural differences in conjunction with published PDF data identifies the important role calcium plays in dictating the atomic structure of the disordered binder gel in cementitious materials. This study serves as a crucial step forward in linking GGBS chemistry with phase formation in alkali-activated GGBS pastes, revealing key information on the local structure of highly disordered cementitious materials.

The final part of the thesis is devoted to elucidating nanoscale degradation mechanisms for alkali-activated GGBS and associated binder gel under sulfate attack. This is achieved by first studying the impact of different types of sulfate-bearing solutions (i.e., Na2SO4, MgSO4 and H2SO4) on the phase formation and local atomic structure of a NaOH-activated GGBS by combining X-ray PDF analysis, synchrotron-based XRD, and Fourier transform infrared (FTIR) spectroscopy. The XRD, FTIR and PDF results show dramatically different behaviors for the different forms of sulfate attack, which appear to be directly correlated with the ability of the ions (i.e., Na+, Mg2+, H+) accompanying SO42− to alter the pH of the pore solution in the samples. By correlating these differences with the pH data from the equilibrated solutions, this study has provided important mechanistic insight into the fundamental sulfate-induced degradation reactions occurring in hydroxide-activated GGBS. Second, time-resolved in situ X-ray PDF analysis and high-resolution XRD are employed to trace the changes occurring to the local atomic structure and phases of a synthetic sodium-containing calcium-alumino-silicate-hydrate (C-(N)-A-S-H) gel during MgSO4 attack. By combining the in situ PDF analysis with simulated PDF data of structural models, the detailed changes occurring to the local atomic structure of the C-(N)-A-S-H gel were elucidated as a function of reaction time. This study highlights the power of combining in situ PDF analysis with simulation of PDF data in providing molecular-level details on chemical processes involving amorphous/disordered-to-amorphous/disordered transformations.