J.S.J. van Deventer

The geopolymerisation of alumino-silicate minerals

Abstract Geopolymers are similar to zeolites in chemical composition, but they reveal an amorphous microstructure. They form by the co-polymerisation of individual alumino and silicate species, which originate from the dissolution of silicon and aluminium containing source materials at a high pH in the presence of soluble alkali metal silicates. It has been shown before that geopolymerisation can transform a wide range of waste alumino-silicate materials into building and mining materials with excellent chemical and physical properties, such as fire and acid resistance. The geopolymerisation of 15 natural Al–Si minerals has been investigated in this paper with the aim to determine the effect of mineral properties on the compressive strength of the synthesised geopolymer. All these Al–Si minerals are to some degree soluble in concentrated alkaline solution, with in general a higher extent of dissolution in NaOH than in KOH medium. Statistical analysis revealed that framework silicates show a higher extent of dissolution in alkaline solution than the chain, sheet and ring structures. In general, minerals with a higher extent of dissolution demonstrate better compressive strength after geopolymerisation. The use of KOH instead of NaOH favours the geopolymerisation in the case of all 15 minerals. Stilbite, when conditioned in KOH solution, gives the geopolymer with the highest compressive strength (i.e., 18 MPa). It is proposed that the mechanism of mineral dissolution as well as the mechanism of geopolymerisation can be explained by ion-pair theory. This study shows that a wide range of natural Al–Si minerals could serve as potential source materials for the synthesis of geopolymers.

THE EFFECT OF COMPOSITION AND TEMPERATURE ON THE PROPERTIES OF FLY ASH- AND KAOLINITE -BASED GEOPOLYMERS

Fundamental research into the geopolymerisation process is increasing rapidly because of the potential commercial application of this technology. Despite this, however, very little work has been undertaken to determine the relationship between composition and temperature on the final chemical and physical properties of geopolymeric products derived from waste materials. The present study shows that the differences in reactivity of source materials, used during the synthesis of waste-based geopolymers, significantly affect the final properties of the geopolymeric material. It is proposed that these observed changes in material properties are due to the incomplete dissolution of the waste material. The water content, the fly ash/kaolinite ratio, as well as the type of metal silicate used have a substantial effect on the final properties of the geopolymer. In particular, the current work shows that the thermal history of the source materials, such as kaolinite, as well as the curing regime for the geopolymer are important factors that must be taken into consideration when designing a geopolymer product for a specific application.

The potential use of geopolymeric materials to immobilise toxic metals: Part I. Theory and applications☆

Abstract During the last decade geopolymerisation has emerged as a possible technological solution for the effective stabilisation and immobilisation of toxic materials. Despite the fact that this technology is based on a very old principle, surprisingly little is known about the nature of these reactions or their products. It is only in the last fifteen years that it has been rediscovered and attention has been drawn to its useful chemical and physical properties. This paper will therefore attempt to briefly discuss the available literature on geopolymerisation in terms of its history, reaction kinetics and structure as well as investigations into the application of geopolymerisation to various waste forms. It is evident from the literature that factors governing the formation of geopolymers are still poorly understood, although the physical and chemical properties suggest that these matrices are well suited for the immobilisation of toxic materials and specifically toxic metals. It is finally concluded that geopolymers offer attractive options towards simple industrial applications where large volumes of waste materials need to be stabilised. It must also be acknowledged that these advantages can only be applied optimally once all relevant interactions regarding the formation of geopolymers from waste materials have659 been quantified scientifically. Hence, further research is required regarding the formation of geopolymers and their application in industry.

The characterisation of source materials in fly ash-based geopolymers

In recent years geopolymers have emerged as novel materials having unique and highly desirable chemical and mechanical properties. The technology of geopolymerisation is gaining commercial interest because it has been demonstrated that, in certain cases, the properties of geopolymeric materials are superior to existing cementitious systems. Source materials used during the synthesis of geopolymers from industrial by-products such as fly ash have an important role in determining the final properties of the geopolymer matrix. It is proposed in the current work that this is caused by the fact that not all of the waste material is dissolved and, therefore, some of the original structures of waste particles remain intact, becoming part of the new geopolymer structure and serving to either weaken or strengthen the newly formed structure. The current work uses XRD and FTIR techniques to characterise fly ash obtained from different sources in order to gain a greater understanding of the effect of phase composition on the dissolution behaviour, reactivity, and final physical and mechanical properties of fly ash-based geopolymeric materials.

The potential use of geopolymeric materials to immobilise toxic metals: Part II. Material and leaching characteristics

The stabilisation and solidification of waste materials by the technology of geopolymerisation is fairly unknown and has not been studied in any depth. This paper presents some experimental evidence as to the physical and chemical characteristics of geopolymers manufactured from fly ash originating from two different regions. It has become apparent that these materials could be used for a wide variety of environmental and other applications such as the immobilisation of heavy metals and the fabrication of structural products. In this study compressive strength testing, specific surface area determinations, Transmission Electron Microscopy (TEM), Nuclear Magnetic Resonance (NMR) and leaching tests were used in characterising a number of geopolymer matrices. It is also shown how the inclusion of heavy metal ions, alkali metal cations and different processing conditions affect the physical and chemical characteristics of the final product.

The effects of inorganic salt contamination on the strength and durability of geopolymers

Two geopolymer systems were prepared by alkali activation of fly ash and kaolin at room temperature with alkaline silicate solutions. System I was synthesised using a less concentrated alkaline silicate solution than System II. Inorganic salts were dissolved or suspended in the mixing water and added to the early pastes before setting to simulate process contamination involving the activating solutions. The strength and durability of the resultant products were examined by comparing the compressive strengths, Fourier-transform transmission infrared spectroscopy (FTIR) spectra, X-ray diffraction (XRD) diffractograms and scanning electron microscopy coupled with energy dispersive spectrometer (SEM–EDS) analysis at different ages. It was found that chloride salts such as KCl, CaCl2 and MgCl2 were detrimental to System I geopolymer. They decreased the product durability by gradually causing precipitation and crystallisation in the aluminosilicate gel—the binding phase of geopolymer. On the other hand, carbonate salts, K2CO3 and CaCO3, were beneficial by lowering the dissolved water contents and preventing hydrolytic attacks on the gels. The greater alkali contents in the System II geopolymers were found to promote greater solid dissolution but also caused aluminosilicate gel precipitation at a very early age. This lowered the product compressive strength and masked the effects of the inorganic salt contamination in System II geopolymer.

Microanalysis of calcium silicate hydrate gel formed within a geopolymeric binder

It is possible to have geopolymeric gel and calcium silicate hydrate (CSH) gel forming simultaneously within a single system. Scanning electron microscopy was employed in studying the morphology and elemental composition of the two phases. The elemental composition within the different phases was consistent. However, CSH gel formed in such system had a significantly lower Ca/Si ratio than the CSH commonly formed from the hydration of ordinary Portland cement. In addition, there were some calcium precipitate along the interface between the CSH and geopolymeric gels. It is suggested that the properties (e.g., size, elemental composition) of the geopolymeric and CSH gels forming simultaneously, and the reactivity of the calcium precipitates along the interfacial region, will hold the key in reformulating a new generation of concrete that matches the durability of ancient concrete. A chemical mechanism on how the presence of slag in the alkali activation of metakaolin would lead to the formation of both geopolymeric and CSH gels has been proposed.

Effect of silicate activator pH on the leaching and material characteristics of waste-based inorganic polymers

Abstract An increasing demand for utilization of large scale industrial wastes as added-value products is projecting geopolymer technology to the forefront of solidification/ stabilization applications over and above its potential as a ‘green’ alternative to concrete. The present work examines the effect of alkali metal silicate precursor solutions on geopolymerization, in terms of its initial pH-control, on the efficiency of heavy metal (Pb and Cu) containment in geopolymers. The effect of the activator and pH on the source materials was investigated through a variety of dissolution experiments. The immobilization efficiency was assessed through TCLP analyses. By means of compressive strength analyses it was possible to correlate the effects of pH, alkali metal silicate and variable calcium sources with the physical and chemical characteristics of the final product and to postulate reaction mechanisms. The greatest compressive strength was attained utilizing KOH/K-Silicate and the most efficient immobilization of Pb was attained utilizing NaOH/Na-Silicate. Both these maxima occurred at high pH (pH=14).

Effect of the silicate activator pH on the microstructural characteristics of waste-based geopolymers

Abstract An understanding of the processing of aluminosilicate mineral wastes is important for the development of large-scale waste-based geopolymerisation, especially for the encapsulation of heavy metals. This paper examines the effect of alkali metal silicate precursor solutions on geopolymerisation. Specifically, it addresses the effect of the degree of polycondensation of silicate solutions on the microstructure of waste-based geopolymers containing heavy metals (Pb and Cu). The microstructural mechanisms of heavy metal immobilisation were assessed through a variety of techniques such as X-ray diffraction (XRD), Infrared (IR) spectroscopy and Scanning Electron Microscopy (SEM) and compared to alternative polysilicate remediation methods. Nuclear Magnetic Resonance (NMR) spectroscopy was used to investigate the effects of pH and alkali metal on the speciation and equilibria of silicate solutions. The role of variable calcium sources on the physical and chemical characteristics of the final product was also investigated. Calcium silica hydrate (CSH), although present at pH 12, was largely unobserved at high pH (pH=14), which is the optimal condition for geopolymerisation.

The effect of metal contaminants on the formation and properties of waste-based geopolymers

Abstract The stabilisation and solidification of waste materials by the technology of geopolymerisation is receiving increasing attention from researchers; immobilisation of metal contaminants in these structures seems to be a viable alternative to present stabilisation techniques. This paper presents some experimental evidence concerning the effect of the inclusion of mainly Cu and Pb on the physical and chemical characteristics of geopolymers manufactured from fly ash. A variety of experimental and analytical techniques were used in this investigation, including compressive strength testing, specific surface area analyses, transmission electron microscopy, nuclear magnetic resonance, X-ray diffraction, and infrared spectroscopy. It was found that contaminants are being immobilised through a combination of chemical bonding and physical encapsulation. The nature of the contaminant seems to have a fairly large effect on both the physical and chemical characteristics of the final product, with subsequent long-term implications as far as durability is concerned. It is therefore concluded that a definite interaction exists between matrix-forming components and the immobilisation of the contaminant, where the amount of contaminant is a critical factor in the analysis.