Structural Evolution during Geopolymerization from an Early Age to Consolidated Material

Abstract

Time-resolved rheology, small angle X-ray scattering (SAXS), and electron paramagnetic resonance (EPR) techniques were used to study the polymerization of geopolymers. These polymers are inorganically synthesized by the alkaline activation of an aluminosilicate source (metakaolin) in aqueous solution. The influence of the alkali activator (Na(+), K(+), and Cs(+)) was investigated at room temperature. As observed through the variation of the viscoelastic moduli (G', G"), curing proceeds in steps that are well pronounced when NaOH is used. These steps correspond to a specific dissolution/polycondensation mechanism and are smoothed when the size of the alkali cations increases. This size effect also has an impact on the gelation time (maximum of tan δ). Structural analysis through SAXS experiments allows us to characterize these mechanisms on the nanoscale and to show that the growth of the geopolymer is due to the aggregation of oligomers with a size that is even smaller than the cation is chaotropic. Finally, water behavior during geopolymerization was assessed by using a spin probe. The results show that the spin-probe signal progressively disappears during the first moment of the reaction and reappears when the solid polymeric gel is formed, highlighting the role of water molecules in the different chemical reactions during the process. The EPR signal is in fact increasingly masked as the ion size decreases (because of the strength of the hydration shell). At the end of the reaction, some water molecules were released within the pores, restoring the visibility of the isotropic spin-probe signal.

Establishment of a preconditioning regime for air permeability and sorptivity of alkali-activated slag concrete

“…It is believed that the water does not notably participate in the formation of the main product phases and does not form a strongly hydrated binder phase similar to Portland cement. Therefore, water in AASC is reported to be easily removed from specimens [27,42,43].…”

Characterising the Reaction of Metakaolin in an Alkaline Environment by XPS, and Time- and Spatially-Resolved FTIR Spectroscopy

“…However, the direct experimental validation of this mechanism has to date proven elusive, although significant steps towards defining the nature of the initial gel phases have been taken recently [6,7]. This paper contributes to the understanding of the initial stages of dissolution of metakaolin in alkaline environments, through the analysis of leached metakaolin samples by Fourier transform infrared (FTIR), Auger emission spectroscopy and X-ray photoelectron (XPS) spectroscopy.…”

Performance of Different Grades of Palm Oil Fuel Ash with Ground Slag as Base Materials in the Synthesis of Alkaline Activated Mortar

“…In addition, the reactivity of slag with POFA due to their fineness induced early setting especially in AAG-UPOFA and AAG-TPOFA mortars. These eventually led to the reduction of water available for lubrication of particles thereby increasing the inter-particle abrasion (Steins et al 2012;Zuhua et al 2009). It was clear that carbon content and particle sizes had significant effects on the rheology of the mixtures.…”