Glass-ceramics are polycrystalline materials of fine microstructure that are produced by the controlled crystallisation (devitrification) of a glass. Numerous silicate based wastes, such as coal combustion ash, slag from steel production, fly ash and filter dusts from waste incinerators, mud from metal hydrometallurgy, different types of sludge as well as glass cullet or mixtures of them have been considered for the production of glass-ceramics. Developments of glassceramics from waste using different processing methods are described comprehensively in this review, covering R&D work carried out worldwide in the last 40 years. Properties and applications of the different glass-ceramics produced are discussed. The review reveals that considerable knowledge and expertise has been accumulated on the process of transformation of silicate waste into useful glass-ceramic products. These glass-ceramics are attractive as building materials for usage as construction and architectural components or for other specialised technical applications requiring a combination of suitable thermo-mechanical properties.

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In the continuing quest for improved performance, which may be specified by various criteria including less weight, more strength and lower cost, currently-used materials frequently reach the limit of their usefulness. Thus materials scientists, engineers and scientists are always striving to produce either improved traditional materials or completely new materials. Compos ites are an example of the latter category.Not that composites are really new. A composite is a material having two or more distinct constituents or phases and thus we can classify bricks made from mud reinforced with straw, which were used in ancient civilizations, as a composite. A versatile and familiar building material which is also a composite is concrete; concrete is a mixture of stones, known as aggregate, held together by cement. In addition to synthetic composites there are naturally occurring composites of which the best known examples are bone, mollusc shells and wood; bone and wood are discussed later in the chapter.Within the last forty years there has been a rapid increase in the production of synthetic composites, those incorporating fine fibres in various plastics (polymers) dominating the market. Predictions suggest that the demand for composites will continue to increase steadily with metal and ceramic based composites making a more significant contribution (Figure 1.1).The spur to this rapid expansion over the last few decades was the development in the UK of carbon fibres and in the USA of boron fibres in the early 1960s. These new fibres, which have high elastic constants, gave a significant increase in the stifTness of composites over the well-established glass fibre containing materials, and hence made possible a wide range of applications for composites. One of the key factors was the very high strength-to-weight and stifTness-to-weight ratios possessed by these new composites (section 1.6). Early employment in aviation has led, in more recent times, to wide use in other areas such as the leisure and sports

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Lightweight glass ceramic foams have been produced from a mixture of silicate wastes, namely 20 wt-% coal pond ash and 80 wt-% bottle glass cullet. A powder sintering route with the incorporation of 2 wt-%SiC as foaming agent was used. The pore morphology achieved under different sintering conditions was investigated in detail using X-ray microtomography. The apparent density of the foams ranged from 0 . 2 to 0 . 4 g cm 23 , and the porosity ranged from 70 to 90%. Other variables, such as pore wall thickness, pore size and roundness, all behaved consistently with sintering temperature. The optimum sintering temperature was found to be in the range 1000-1050uC, at which porosity was about 75% and was the most uniform. Foams produced under this condition exhibit satisfactory compressive strength of about 1 . 5 MPa and show relatively high thermal shock resistance, with compressive strength gradually decreasing as quenching temperature increases.

This paper reports the results of a study of the feasibility of recycling the solid residues from domiciliary waste incineration by producing a glass-ceramic. The major components of the raw material (TIRME F+L), which was from a Spanish domiciliary incinerator, were CaO, SiO 2 and Al 2 O 3 but nucleating agents, such as TiO 2 , P 2 O 5 , and Fe 2 O 3 were also present in reasonable amounts. It was found that a relatively stable glass with suitable viscosity could be obtained by mixing 65 wt% TIRME F+L with 35 wt% glass cullet. The heat treatment required to crystallise the glass produced from this mixture, designated TIR65, was nucleation at 560°C for 35 min followed by crystal growth at 100°C for 120 min. The resulting glass-ceramic contained a number of crystalline phases, the most stable being clinoenstatite (MgSiO 3), or perhaps a pyroxenic phase which incorporates Ca, Mg and Al in its composition, and åkermanite (Ca 2 MgSi 2 O 7). The microstructure contained both fibre-like and dendritic crystals. The mechanical properties were acceptable for applications such as tiles for the building industry.