Abstract
This paper studies the mineralogical and textural changes that take place during the firing in an electric kiln at 800, 950 and 1100 °C of brick samples made with or without additives. Samples were made with a clayey raw material which was mixed with either halite or calcined diatomite sludge and then fired. These samples were then compared with control samples made without additives. Different analytical techniques (X-ray fluorescence, thermogravimetric and differential scanning calorimetric analyses, X-ray diffraction, polarized optical microscopy and scanning electron microscopy) were used to reconstruct the changes that took place inside the bricks from a mineralogical and textural point of view, changes that are similar to those that take place in nature during pyrometamorphism. The carbonates decomposed and reacted with silicates to form gehlenite, diopside and wollastonite; the plagioclase was enriched in calcium and the quartz concentration fell; the clay minerals favoured the melting of the matrix and the appearance of mullite, and K-feldspar changed from microcline to sanidine. The extent of vitrification increased in line with the increase in the firing temperature. When halite was added, new silicates appeared earlier at lower firing temperatures and molysite was formed, while the most important mineralogical difference in the bricks made with added calcined diatomite sludge was the presence of cristobalite, a component of the sludge. It is interesting to observe that the newly-formed phases contain certain chemical elements that are not normally found in their standard chemical composition.
Highlights
Conventional brick is one of the oldest building materials in the world and one of the most frequently used over the course of history due to its great resistance and durability (Fernandes et al, 2010)
This study enabled us to chart the process of formation of metastable mineral phases which developed at high temperature and atmospheric pressure and which replicate, albeit at small scale, pyrometamorphic processes (Grapes, 2006)
Xray diffraction identified phases such as gehlenite, diopside, wollastonite, anorthite and mullite, whose appearance and abundance varied depending on the firing temperature, the change in the polymorphs of Kfeldspar, and the presence of molysite only in bricks made with added halite
Summary
Conventional brick is one of the oldest building materials in the world and one of the most frequently used over the course of history due to its great resistance and durability (Fernandes et al, 2010). Brick production process has hardly changed over the course of time and consists basically in kneading the clayey earth with water so as to work it into the right shape; it is left to dry before being fired. What has changed in recent times is the increasingly strict control of all aspects of the manufacturing process (selection of raw materials, the addition of grogs, melting agents and additives, the process of moulding the unfired pieces, the types of kilns), so transforming the manufacture of bricks from an art into a science, guaranteeing a much more uniform, standardized product (Shakir & Mohammed, 2013). A careful selection of the raw materials and strict control of all phases of the production process are essential for obtaining high-quality bricks and other ceramic products
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