A brief history and review of geopolymer technology is presented with the aim of introducing the technology and the vast categories of materials that may be synthesized by alkali-activation of aluminosilicates. The fundamental chemical and structural characteristics of geopolymers derived from metakaolin, fly ash and slag are explored in terms of the effects of raw material selection on the properties of geopolymer composites. It is shown that the raw materials and processing conditions are critical in determining the setting behavior, workability and chemical and physical properties of geopolymeric products. The structural and chemical characteristics that are common to all geopolymeric materials are presented, as well as those that are determined by the specific interactions occurring in different systems, providing the ability for tailored design of geopolymers to specific applications in terms of both technical and commercial requirements.

Geopolymer technology: the current state of the art

The geopolymerisation of alumino-silicate minerals

Low bone mass (osteoporosis) is a silent epidemic of the 21st century, which presently in the UK results in over 200,000 fractures annually at a cost of over one billion pounds. Figures are set to increase worldwide. Understanding the factors which affect bone metabolism is thus of primary importance in order to establish preventative measures or treatments for this condition. Nutrition is an important determinant of bone health, but the effects of the individual nutrients and minerals, other than calcium, is little understood. Accumulating evidence over the last 30 years strongly suggest that dietary silicon is beneficial to bone and connective tissue health and we recently reported strong positive associations between dietary Si intake and bone mineral density in US and UK cohorts. The exact biological role(s) of silicon in bone health is still not clear, although a number of possible mechanisms have been suggested, including the synthesis of collagen and/or its stabilization, and matrix mineralization. This review gives an overview of this naturally occurring dietary element, its metabolism and the evidence of its potential role in bone health.

Silicon and bone health.

Alumina mesophases have been synthesized by reacting aluminum alkoxides and carboxylic acids with controlled amounts of water in low-molecular-weight alcoholic solvents. Calcination of these materials yields aluminas that are thermally stable to 1073 K and contain randomly ordered pores. Specific surface areas as high as 710 m2/g and narrow pore size distributions centered at 20 A that do not contain “zeolitic” micropores are exhibited by the calcined solids.

Synthesis of pure alumina mesoporous materials

Initial geopolymeric reaction processes governing dissolution of solid aluminosilicate particles in alkali solutions have been investigated using conventional experimental techniques, and the data analysed by speciation predictions of the partial charge model (PCM). For metakaolin powders activated with 5.0 M NaOH, solid-state nuclear magnetic resonance (NMR) spectra disclose the existence of monomeric [Al(OH)4]− species after two hours of dissolution, consistent with PCM predictions. However, no equivalent monomeric silicate species were observed for 5.0–10.0 M NaOH activator solutions characteristic of systems with nominal Si/Al ≤ 1. The apparent absence of monomeric silicate species suggest rapid condensation of silicate units with [Al(OH)4]− to generate aluminosilicate species, as indicated by the evolution of the shoulder at around −87 ppm in the 29Si NMR spectra. Of the two possible stable silicate species [SiO2(OH)2]2− and [SiO(OH)3]−, the latter appears most likely to condense with [Al(OH)4]− to produce aluminosilicate oligomers, from which larger oligomers subsequently form through further condensation with [Al(OH)4]− leading to a gradual build up of aluminosilicate networks and a lowering of system alkalinity. This dissolution and hydrolysis sequence at the early stages of synthesis suggests a reaction path wholly consistent with predictions of the partial charge model.

Dissolution processes, hydrolysis and condensation reactions during geopolymer synthesis: Part I—Low Si/Al ratio systems

Penta-co-ordinated aluminates are shown to give 27Al nmr chemical shifts of approximately 35 ppm using solid-state nmr spectroscopy

Penta-co-ordinated aluminium: a solid-state 27Al, n.m.r. study

Changes in the pH or concentration of sodium silicate solutions produce changes in the degree of polymerisation of the silicate species, and these have been followed by the technique of trimethylsilylation. Provided that the pH remains relatively high, above ca. 10 or 11 according to concentration, equilibrium is rapidly re-established after such changes. At lower pH, equilibrium is approached more slowly, and changes in distribution with time can be followed.

Silicate species in solution. Part 1. Experimental observations

Structure and Properties of Aluminosilicate Solutions and Gels

The rate of gelation of aluminosilicate solutions depends on the cation present; solutions containing Na+, K+, Cs+, and Me4N+ have been investigated, and some n.m.r. evidence is presented.

The gelation behaviour of aluminosilicate solutions containing Na+, K+, Cs+, and Me4N+

The method of trimethylsilylation was used to study anion distributions in sodium silicate solutions with Na2O: SiO2 ratios of 1:1 and 1:3.41 at various concentrations. The polysiloxanes produced were characterised, mainly by g.l.c., g.p.c. and C and H analyses, to give a picture of the anion distribution in the original solution. The results were compared with those from previous studies. The observations of earlier workers—that the degree of polymerisation increases with increasing concentration and decreasing soda:silica ratio—were confirmed.

Lesley S. Dent Glasser

Papers

Penta-co-ordinated aluminium: a solid-state 27Al, n.m.r. study

Silicate species in solution. Part 1. Experimental observations

Structure and Properties of Aluminosilicate Solutions and Gels

The gelation behaviour of aluminosilicate solutions containing Na+, K+, Cs+, and Me4N+

Studies on sodium silicate solutions by the method of trimethylsilylation