Glass Powders as Fluxes: Brick Manufacture and Glazes
A flux is an additive to a manufacturing process that decreases the operation temperature, thereby uplifting the efficiency of the overall process whilst simultaneously decreasing costs in energy-intensive processes. In the manufacture of bricks and other building materials, not only does the added flux decrease operation temperature but can also enhance the physical and mechanical properties of the finished clay brick post-firing(1).
Reducing the energy requirement for the production of bricks is a priority, with production in the UK stated at using 706 kWh of electricity per ton of brick produced(2). In the ceramic glaze space, where a post-application firing is commonly used to finish the product, fluxes reduce the temperature required and, in some cases, enhance the finish.
Sources of ground glass can be wide ranging, from glass powder produced as a by-product from plate/container glass manufacture or from glass recycling streams. Eliminating glass from landfill is a priority. Clear glass tends to be preferred for brick addition, but due to the small quantities of colourants in coloured glass, these can also be used.
Glass Powder Fluxes In Bricks
Bricks are one of the most common building materials, essential to the modern global economy. The basic brick consists of clay, which is fired (either in a furnace/kiln or an electric industrial oven) whereupon it dries out and hardens, becoming mechanically and compressively stronger, in addition to more porous and lighter. Common additions of ground glass do not exceed 20% by weight, as much more than this will cause workability problems, and diminishing economic returns(3).
For the manufacture of bricks, glass is required to be in a particle size of less than 1mm so it can effectively pass through pan mills ahead of mixing and firing(4). In general, finely ground glass with a particle size of less than 75 μm lowers firing temperatures(5).
The chemistry that occurs during the firing of a brick is complex, and the addition of ground glass generally decreases the firing temperature required. Notwithstanding the complex chemical reactions taking place, the addition of glass powders impacts the process via two major manifolds:
Clay Body Maturation
During firing, the heat causes a sintering of clay particles together to form a bond from the fusion of silica and alumina (giving the brick its characteristic strength), lower melting point glass powders melt ahead of the sintering process, and thus reduce the overall sintering temperature if they are uniformly distributed throughout the wet clay(6)
Soda lime glass acts as a flux due to its lack of crystallinity and high sodium oxide content. The reduced temperature required to form a glassy bond and the increased glassy phase component in the finished brick have been associated with better overall structure and durability properties. Furthermore, research has shown that increased glass content leads to lower porosity in the final product, thus less water can be absorbed by it - lengthening brick lifetimes - as water absorption rates are inversely proportional to porosity levels(7). Finally, due to recrystallisation processes in the heating/cooling phases, there is a reduction in the amount of amorphous species, leading to an enhancement of mechanical properties with glass content as little as 5%(8).
Shrinkage is a concern during the brick manufacturing process, as a brick that does not conform to size standards will be discarded if it does not fall within acceptable tolerances. Over the years, manufacturers have sought to understand shrinkage and have developed methods to moderate it. One such method is the addition of ground glass to the wet clay. Research has shown that firing shrinkage in glass powder-containing clays is equal to or less than unadulterated clay(9). Vitrification is increased when glass is used in the mixture, leading to higher density and less water absorption, itself leading to less shrinkage during firing(10). This phenomenon is explained by the filling of pores with the aforementioned glassy phase. One study utilising an unusually high 45 weight% ground glass reported almost no linear shrinkage and significantly less cracking(11).
Scanning electron microscopy demonstrates that as the proportion of glass added to the brick, the denser the overall brick matrix became. Overall density increases are associated with heightened values for compressive and mechanical strength. It has been proposed that glass particle sizes akin to sand grains are effective at reducing plasticity, thus reducing shrinkage in the shaping-drying step ahead of firing(12).
Glass Powder Fluxes In Glazes
Ceramic glazes are thin layers of glass created on the surface or ceramics to provide enhanced aesthetic and durability properties, and are typically based on quartz and silica, alongside a flux such as sodium or potassium oxide. Studies have successfully looked into replacing some or all of the flux with ground glass from a variety of sources, in loadings of up to 30% by weight(13). The main advantage to using ground glass as a flux is cost savings by utilising this instead of fritted silica/quartz(14), even though modest firing temperature reductions are observed, due to the lower melting point glass reducing overall firing temperature(15).
Crazing (cracking due to thermal expansion temperature mismatch) is one of the major defects observed in ceramic glazes(16), and additions of glass to the glaze mixture can ensure closer expansion temperatures between the ceramic and the glaze - thereby reducing the potential for crazing. Like in bricks, the addition of ground glass can reduce plasticity in advance of firing.
- Glass powder is an inexpensive material that can be used as fluxes in brick and glaze production, replacing other materials with no detrimental effects
- In bricks, addition of glass powder decreases firing temperature, increases mechanical strength and reduces shrinkage upon firing. Density is increased and water absorbance is decreased. Glass powders can be used in quantities up to 20% by weight.
- In ceramic glazes, ground glass replaces expensive fluxes such as sodium and potassium oxides, and causes a mild decrease in firing temperature. The use of ground glass can reduce crazing, and is used in quantities up to 30% by weight.
1 F. Andreola et al., Ceram. Int., 2008, 34, 1289
2 The Carbon Trust (online), 2015, Industrial Energy Efficiency Accelerator, Guide to the Brick Sector, accessed 10 Feb 2020, https://www.carbontrust.com/resources/industrial-energy-efficiency
3 T. Manfredini et al., Ceram. Int., 2016, 42, 13333
4 M. Dondi et al., Waste Manag., 2009, 29, 1945
5 York Handmade Bricks Co. Ltd., Glass Addition Trials, The Waste and Resources Action Programme, Banbury, United Kingdom, 2005
6 S. Mustafi et al., J. Sci. Res., 2011, 24, 2
7 D. Eliche-Quesada et al., Appl. Clay Sci., 2011, 52, 270
8 C. Zanelli et al., Ceram. Int., 2008, 34, 455
9 N. Phonphuak et al., J. Cleaner Prod., 2016, 112, 3057
10 V. Loryuwnyonga et al., Waste Manag., 2009, 29, 2717
11 Italian patent 1404928, 2013
12 F. Andreola et al., Environ. Manag. J., 2013, 12, 57
13 K. Strcker and H. B. Costa, Mater. Sci. Forum, 2014, 775-776, 635
14 D. V. Van Gordon, Recovered Soda-Lime Glass as a Ceramic Body flux, in: W. J. Smothers, Materials and Equipment - Whitewares - Structural Clay, Wiley, New York, 2009
15 US patent US415808A, 1978
16 D. V. Van Gordon and W. C. Spangenberg, J. Am. Ceram. Soc., 1955, 38, 331