Relating chemical composition, structure, and rheology in alkali‐activated aluminosilicate gels

Abstract

Model aluminosilicate (N-AS -H) gel systems give insight to the complex reaction occurring in alkali-activated binders, allowing for more precise control over chemical composition and exhibiting simpler kinetics. Small angle neutron scattering, oscillatory rheology, and solid-state NMR were used to characterize N-AS -H gel samples and relate structure, chemical composition, and rheology. Aluminum concentration was varied in the gel samples at fixed silica and sodium concentration. Past the percolation point, the gel develops in two regimes distinct in structure and rheological behavior. The first gel regime is characterized by increase in modulus and initial formation of fractal structures following a diffusion-limited monomercluster aggregation mechanism. The second gel regime at higher aluminum concentrations is characterized by a modulus plateau and structure development via reaction-limited aggregation of existing clusters. NMR data indicate a saturation of aluminum in the network at the transition between gel regimes, which may be pH driven. Understanding the structure-property relationships critical to formation of the N-AS -H gel can give valuable insight to the reaction mechanism in alkali-activated binders and the influence of chemical composition on resultant structure and mechanical properties.

Time–connectivity superposition and the gel/glass duality of weak colloidal gels

“…More generally, using Muthukumar's relation for 0 ≤ α ≤ 0.5 yields a range of possible fractal dimensions 2.0 ≤ d f ≤ 2.5 consistent with the RLCA mechanism that is characteristic of these weakly attractive gels. Another recent investigation has also concluded that aluminosilicate gels are formed through RLCA for a range of compositions (20). This analysis suggests that our results should hold for all gels that are in the so-called solid-cluster limit, i.e., gels that display a long-time elastic-like behavior (G ≥ G at low frequencies) with α ≤ 0.5 (5).…”

“…S5 and S7), illustrating that, despite slight differences in the chemical balance of the initial reactions, these weak colloidal gels are fractal clusters that are formed through reactionlimited cluster aggregation of smaller building blocks. Recent structural studies on weak colloidal gels with short-range attraction have also reported similarly dense fractal structures with d f 2 (20,27).…”

“…Here we study the gelation of aluminosilicate gels and investigate the evolution in both their microstructural characteristics and their mechanical response at meso and macro scales through time-resolved small-angle X-ray scattering (TR-SAXS) and time-resolved mechanical spectroscopy (TR-MS), respectively. The evolution of aluminosilicate gel systems is of great interest in many applications including the production of zeolites and alkali-activated binders, also known as geopolymers (19,20). The gelation process is initiated by mixing alkali silicate and alkali aluminate solutions together.…”

Experimental and Molecular Dynamics Simulation Study on Sol–Gel Conversion Process of Aluminum Carboxylate System

“…The sol sample exhibited good elastic behavior, where tanδ < 1. When tanδ > 1, the increase in tanδ value corresponded to better fluidity [ 30 , 31 , 32 ]. As shown in Figure 6 b, this tanδ value was increased with the rise in temperature for all sol models, indicating that the fluidity of aluminum sol improved with the increase in temperature.…”

Shear-induced memory effects in boehmite gels

“…This result strikingly contrasts with previous observations on depletion gels, in which preshear at "low" rates yielded weaker gels with reduced elasticity due to a largely heterogeneous microstructure inherited from the that acquired during shear 17 . Moreover, the viscoelastic spectrum involves a storage modulus that is an increasing function of frequency, whereas the loss modulus is independent or a mildly decreasing function of frequency, similar to aging systems including gels 69,70 and soft glasses 71 . The loss factor tan δ decreases with increasing frequency, which is reminiscent of glass-like behavior, where the slowest relaxation modes dominate 58 .…”