Other products used within construction include the alkaline earth silicate and borosilicate glasses; these have significantly different chemical compositions giving rise to their particular physical properties. The composition of alkaline earth silicate glass is typically 55-70% silica, 5-14% potassium oxide, 3-12% calcium oxide, 0-15% aluminium oxide, with quantities of zirconium, strontium and barium oxides. Borosilicate glass is typically 70-87% silica, 0-8% sodium oxide, 0-8% potassium oxide, 7-15% boron oxide, 0-8% aluminium oxide, with small quantities of other oxides. A particular characteristic of borosil-icate glass is that it has a coefficient of expansion one-third that of standard soda lime silicate glass, making it significantly more resistant to thermal shock in case of fire.
Glass ceramics are characterised by a zero coefficient of expansion making them highly resistant to thermal shock. The composition of glass ceramics is complex comprising typically 50-80% silica, 15-27% alumina, with small quantities of sodium oxide, potassium oxide, barium oxide, calcium oxide, magnesium oxide, titanium oxide, zirconium oxide, zinc oxide, lithium oxide and other minor constituents. The glass is
Fig. 7.2 Glazed facade - Faculty of Law, University of Cambridge. Architects: Foster and Partners. Photograph: Arthur Lyons initially produced by a standard float or rolling technique, but subsequent heat treatment converts part of the normal glassy phase into a fine-grained crystalline form, giving rise to the particular physical properties.
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