Jannie S. J. van Deventer

Geopolymerisation of multiple minerals

Abstract Geopolymerisation can transfer large scale alumino-silicate wastes into value-added geopolymeric products with sound mechanical strength and high acid, fire and bacterial resistance. However, due to the complexity of source materials as well as the interaction between source materials during the geopolymerisation, previous studies have mostly dealt with single or bi-component systems, which could potentially narrow the application of this technology. The present work selects three industrial materials, i.e. fly ash, kaolinite and albite to investigate various combinations. The results show that when appropriate reaction parameters are used, the three component system gives geopolymers possessing the highest compressive strength and the lowest probability of cracking. It is believed that the higher reactivity of the fly ash and albite, the interaction between the source materials and the gel phase, and the reinforcing effect caused by the large unreacted albite particles are responsible for this satisfactory mechanical behaviour.

Geopolymers for immobilization of Cr6+, Cd2+, and Pb2+

Alkali activation of fly ash by sodium silicate solutions, forming geopolymeric binders, provides a potential means of treating wastes containing heavy metals. Here, the effects on geopolymer structure of contamination of geopolymers by Cr(VI), Cd(II) and Pb(II) in the forms of various nitrate and chromate salts are investigated. The addition of soluble salts results in a high extent of dispersal of contaminant ions throughout the geopolymer matrix, however very little change in geopolymer structure is observed when these materials are compared to their uncontaminated counterparts. Successful immobilization of these species will rely on chemical binding either into the geopolymer gel or into other low-solubility (silicate or aluminosilicate) phases. In the case of Pb, the results of this work tentatively support a previous identification of Pb(3)SiO(5) as a potential candidate phase for hosting Pb(II) within the geopolymer structure, although the data are not entirely conclusive. The addition of relatively low levels of heavy metal salts is seen to have little effect on the compressive strength of the geopolymeric material, and in some cases actually gives an increase in strength. Sparingly soluble salts may undergo some chemical conversion due to the highly alkaline conditions prevalent during geopolymerization, and in general are trapped in the geopolymer matrix by a simple physical encapsulation mechanism. Lead is in general very effectively immobilized in geopolymers, as is cadmium in all except the most acidic leaching environments. Hexavalent chromium is problematic, whether added as a highly soluble salt or in sparingly soluble form.

The effect of alkali metals on the formation of geopolymeric gels from alkali-feldspars

Abstract The effect of alkali metal cations in alkaline solutions and alkali metals structured in alkali-feldspars on the formation of gel phase and the formed geopolymers has been investigated by using ICP, PAS-FT-IR, Raman, 29Si MAS NMR and SEM/EDX techniques. In a concentrated alkaline solution, the dissolution of alkali-feldspars is found to be inhibited, which is partly due to the reversible dissolution and precipitation reactions. DSC results indicate that K contained in the gel phase, either derived from KOH solution or from the dissolution of K-feldspar, is observed to increase the extent of disorder of the gel phase as well as the mechanical strength of the formed geopolymers. The reason for this effect is determined by 29Si MAS NMR analysis as being the higher polymerising activity between silicate species and between silicate and aluminosilicate species caused by the presence of K. The elemental analysis of gel phase using SEM/EDX shows that when the molar ratio of Na/K ranges from 3.5 to 85.6, the geopolymers synthesised from alkali-feldspar/kaolinite matrices demonstrate a high compressive strength. This study also reveals that a higher dissolution tendency as well as the involvement of K can increase the compressive strength of the geopolymers synthesised from alkali-feldspar/kaolinite matrices in alkaline solutions.

Microscopy and microanalysis of inorganic polymer cements. 2: the gel binder

By scanning electron microscopy and microanalysis of fly ash-based and mixed fly ash-slag inorganic polymer cement (i.e., “fly ash geopolymer”) binders, a more detailed understanding of the gel structure and its formation mechanism have been developed. The binder is predominantly an aluminosilicate gel charge balanced by alkali metal cations, although it appears that calcium supplied by slag particles becomes relatively well dispersed throughout the gel. The gel itself is comprised of colloidal-sized, globular units closely bonded together at their surfaces. The microstructure of the binder resulting from hydroxide activation of fly ash is much less uniform than that which forms in a corresponding silicate-activated system; this can be rationalized in terms of a newly developed explanation for the differences in reaction mechanisms between these two systems. In hydroxide activation, the newly formed gel phase nucleates and grows outwards from the ash particle surfaces, whereas the high silica concentration in a silicate-activated system enables a more homogeneous gelation process to take place throughout the inter-particle volume.

Microscopy and microanalysis of inorganic polymer cements. 1: remnant fly ash particles

Accurate and precise electron microscopic analysis of the remnant solid precursor (fly ash and blast furnace slag) particles embedded in an inorganic polymer cement (or “fly ash geopolymer”) provides critical information regarding the process of gel binder formation. Differential solubility of phases in the fly ash is seen to be important, with insoluble mullite crystals becoming exposed by the retreat of the surrounding glassy phases. High-iron particles appear to remain largely unreacted, and the use of sectioned and polished specimens provides a view of the inside of these particles, which can show a wide variety of phase separation morphologies and degrees of intermixing of high iron and other phases. Calcium appears to be active in the process of alkali activation of ash/slag blends, although the competitive and/or synergistic effects of ash and slag particles during the reaction process remain to be understood in detail.

Characterization of Aged Slag Concretes

Slag concretes, activated by carbonates or carbonate/hydroxide mixtures and cast between 1964 and 1982, are examined. These concretes have served for prolonged periods under conditions in which portland cements would have deteriorated rapidly, and yet have remained sound and actually increased in strength over their service life. By a combination of microscopic and nuclear magnetic resonance (NMR) analysis, this durability is attributed to the combination of a highly polymerized, relatively low-Ca, amorphous C-S-H outer product, with an inner product that undergoes continuing hydration via a cyclic process involving carbonate anions. The relatively consistent Ca/Si ratio across all phases is believed to contribute to durability, as is the low Al content of the C-S-H phases formed in systems using hydroxide activators. The low permeability of the concretes also appears to have contributed to their durability.

Microstructural characterisation of geopolymers synthesised from kaolinite/stilbite mixtures using XRD, MAS-NMR, SEM/EDX, TEM/EDX, and HREM

When crystalline aluminosilicates partially dissolve in a concentrated alkaline medium, an amorphous geopolymeric gel is formed interspersed with undissolved crystalline particles. Some aluminosilicates dissolve more readily than others to give an equilibrium ratio of aluminium to silicon in the gel. In this case study, kaolinite and stilbite mixtures were used to investigate the relative reactivity of different minerals when present in different ratios. XRD and 29Si and 27Al MAS-NMR were used to determine when a specific mineral was completely transferred into the gel phase. Electron diffraction using transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HREM) were employed to establish the amorphous nature of the gel phase. Scanning electron microscopy (SEM)/energy dispersive X-ray (EDX) and TEM/EDX were then used to determine the composition of the gel. By using simple mass balance assumptions, the quantity of gel and the extent of partial dissolution of an aluminosilicate could then be calculated. It was found that a geopolymer containing a higher weight percentage of CaO in its gel, a lower ratio of (average surface area)/gel, and where the undissolved crystalline particles have a higher hardness had higher mechanical strength. The method developed in this paper is also applicable to other cementitious materials.

Density functional modeling of the local structure of kaolinite subjected to thermal dehydroxylation.

Understanding the atomic-level changes that occur as kaolinite is converted (thermally dehydroxylated) to metakaolin is critical to the optimization of this large-scale industrial process. Metakaolin is X-ray amorphous; therefore, conventional crystallographic techniques do not reveal the changes in local structure during its formation. Local structure-based experimental techniques are useful in understanding the atomic structure but do not provide the thermodynamic information which is necessary to ensure plausibility of refined structures. Here, kaolinite dehydroxylation is modeled using density functional theory, and a stepwise methodology, where several water molecules are removed from the structure, geometry optimization is carried out, and then the process is repeated. Hence, the structure remains in an energetically and thermodynamically feasible state while transitioning from kaolinite to metakaolin. The structures generated during the dehydroxylation process are validated by comparison with X-ray and neutron pair distribution function data. Thus, this study illustrates one possible route by which dehydroxylation of kaolinite can take place, revealing a chemically, energetically, and experimentally plausible structure of metakaolin. This methodology of density functional modeling of the stepwise changes in a material is not limited in application to kaolinite or other aluminosilicates and provides an accurate representation of the local structural changes occurring in materials used in industrially important processes.

What is the structure of kaolinite? Reconciling theory and experiment.

Density functional modeling of the crystalline layered aluminosilicate mineral kaolinite is conducted, first to reconcile discrepancies in the literature regarding the exact geometry of the inner and inner surface hydroxyl groups, and second to investigate the performance of selected exchange-correlation functionals in providing accurate structural information. A detailed evaluation of published experimental and computational structures is given, highlighting disagreements in space groups, hydroxyl bond lengths, and bond angles. A major aim of this paper is to resolve these discrepancies through computations. Computed structures are compared via total energy calculations and validated against experimental structures by comparing computed neutron diffractograms, and a final assessment is performed using vibrational spectra from inelastic neutron scattering. The density functional modeling is carried out at a sufficiently high level of theory to provide accurate structure predictions while keeping computational requirements low enough to enable the use of the structures in large-scale calculations. It is found that the best functional to use for efficient density functional modeling of kaolinite using the DMol3 software package is the BLYP functional. The computed structure for kaolinite at 0 K has C1 symmetry, with the inner hydroxyl group angled slightly above the a,b plane and the inner surface hydroxyls aligned close to perpendicular to that plane.

Reduction of gold(III) chloride to gold(0) on silicate surfaces.

The mechanism of adsorption and reduction of the gold chloride complex on silicate minerals is investigated. Gold chloride, supplied as HAuCl(4) solution, is rapidly adsorbed on the silicate surfaces, the Au(III) is reduced to metallic gold, and gold particles grow on the surface. SEM images show agglomerates of gold unevenly distributed on the surface of the silicates, including in some areas forming agglomerates, especially on quartz and feldspar. Silica gel forms via dissolution of silicates in acidic conditions and also has strong adsorption/reduction potential for gold. A mechanism for the adsorption and reduction is proposed, involving ligand substitution between gold chloride and OH() groups on defect sites in silicate surfaces. Consequently, gold can be reduced by hydrogen or silicon radicals at the defect sites. Adsorption of Au(III) by silicate minerals, followed by reduction, could play an important role in the deposition of gold in natural systems, as well as causing loss of gold from leaching processes during hydrometallurgical gold recovery.