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/Vitrification
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).
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