Glass recycling in porcelain stoneware tiles: densification behaviour and microstructure development on firing

Recycling of scrap glass in a porcelain stoneware tile body mix was studied by replacing different amounts of the fluxing component by two types of scrap glass: (i) 5, 10 and 15wt% soda-lime-silica (SLS) container glass and (ii) 2wt% Pb-based cathode ray tube glass. The densification behaviour of the standard stoneware composition and four other mixes containing recycled glass was evaluated in terms of water absorption and linear shrinkage. The microstructural evolution on firing the resulting body mixes was investigated by X-ray diffractometry (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Substituting the fluxing component with 5-10wt% SLS glass yields stoneware with reduced water absorption. The presence of 2wt% Pb-based glass in the body mix made it possible to decrease the sintering temperature by 20°C. The microstructure of the standard stoneware consists of a glassy matrix with 100-1000μm long mullite needles, feldspar relicts and partially dissolved α-quartz crystals. Using 10wt% or more SLS glass in the body mix composition led additionally to the crystallisation of a plagioclase, (Ca,Na)Al2Si2O8, which partially inhibited mullite formation. SEM analysis revealed that Ca-rich areas developed adjacent to quartz particles on firing as a result of interactions between quartz and SLS glass. Rounded wollastonite and elongated sodium silicate crystals have been also identified in SLS glass fluxed stonewares. PbO was not detected in stoneware samples containing Pb-based recycled glass, suggesting that it may evaporate on firing and/or remain as trace amounts of lead evenly distributed in the glassy phase