Microstructural Variation in Porcelain Stoneware as a Function of Flux System

Abstract

Fired microstructures of standard porcelain stoneware tile and tile made from mixes containing waste glass as part of the flux system were studied by XRD, SEM, and TEM. The standard porcelain stoneware microstructure consists of 100–1000 μm long mullite needles, feldspar relics, and partially dissolved α‐quartz embedded in a glassy matrix. The use of soda–lime–silica (SLS) glass in the flux system led to crystallization of plagioclase, wollastonite, and sodium silicates. CaO‐rich areas adjacent to quartz particles, as a result of interactions between SLS glass and silica from the quartz, and eutectic morphologies, revealed that SLS glass accelerated liquid formation and thus sintering and densification. Formation of these additional phases led to lower levels of quartz, mullite, and Na‐feldspar in the microstructure although lower firing temperatures could be used to achieve full density due to generation of more fluid liquid. Use of PbO‐containing waste glasses had little effect on the microstructure compared with standard composition while use of mixed PbO‐containing and SLS glasses led to microstructures containing plagioclase but to lower extent than in tile with higher levels of SLS.

NMR techniques: A non-destructive analysis to follow microstructural changes induced in ceramics

“…During cooling, this liquid phase forms a glassy matrix which surrounds the residual crystal phases and the newly formed ones. 8,9 Because of the high viscosity of the silica-rich liquid phase, bubbles of gas, deriving both from the decomposition of the raw materials and from the different solubility of the gases present, can remain trapped into the bulk of the material. In this way, even if a sintered sample shows very low water absorption and low NMR signal, a nonnegligible amount of non-connected porosity is present in the bulk, as observed for B2 -a (Fig.…”

Mullite formation in clays and clay-derived vitreous ceramics

“…24 A typical porcelain stoneware body formulation contains 40 wt.% illite, 1 wt.% kaolinite, 15 wt.% K-feldspar and 35 wt.% Na-feldspathic sand containing quartz. Replacement of 15 wt.% of the Na-feldspar (which contains ∼18 wt.% Al 2 O 3 ) with SLS glass (which contains 1.5 wt.% Al 2 O 3 ) led to detectably lower contents mullite in the resulting stoneware (Souza et al 23 ) presumably due to the Al 2 O 3 deficiency in the aluminosilicate liquid making it more difficult for mullite to grow. While mullite content was reduced new phases formed with the SLS flux including wollastonite, sodium silicates and plagioclase solid solution between sodium and calcium aluminosilicates, the latter likely competing with mullite for Al/Si ions.…”

“…Porcelain stoneware is the world leader in the tiling market 23 and has been the subject of much recent research especially into use of waste material such as recycled soda-lime-silica (SLS) glass as replacement for feldspar flux in its formulation. The high firing temperatures of porcelain stoneware >1200 • C mean that different types of waste can be incorporated while still producing an as-fired material with a level of desired microstructural heterogeneity comparable to that of its waste-free counterpart.…”

Water1H NMR Technique to Analyse the Porous Structure of Ceramics

“…In this way, even if a sintered sample shows very low water absorption, a non-negligible amount of closed porosity is present in the bulk. [11,12] The cotto tile, Sample B, is characterized by relevant values of water absorption, 28.7 wt%, and connected porosity, 32.2 %. Its MRR analysis is summarized in Figure 5, which shows T 1 and T 2 relaxation time distributions for one of the samples examined after water saturation under vacuum.…”