State whether the following is true or false.
More bentonite always makes a ceramic body easier to work with.
Answer: A
Beyond a small addition the drying shrinkage rises steeply and cracking becomes the governing problem. It is a case where the optimum is a narrow window rather than a direction of improvement. Shrinkage and workability move in opposite directions here.
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Halloysite differs from kaolinite chiefly in its
Answer: B
Correct answer: tubular particle morphology
Halloysite has essentially kaolinite’s composition and layer structure but rolls into tubes rather than forming plates, because a water layer between the sheets curls them. The tubular habit changes how it packs, flows and disperses, and it is why halloysite is of interest as a nanotube filler as well as a ceramic raw material.
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Consider the following statements about the named clays.
1. China clay is chiefly kaolinite and fires white.
2. Ball clay is finer and more plastic than china clay.
3. Fireclay is chosen for its whiteness after firing.
Answer: C
The first two are correct. The third names the wrong property, since fireclay is valued for refractoriness and is generally too impure to fire white. Alumina content rather than colour is what a refractory clay is bought for.
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A body contains 25 per cent china clay, 25 per cent ball clay, 25 per cent feldspar and 25 per cent quartz. What proportion is clay?
Answer: D
Adding the two clays gives 50 per cent of the batch. Clay content largely determines whether a body can be formed plastically or must be pressed or cast. Below about twenty per cent clay a body is generally too short to throw or extrude.
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Dehydroxylation of kaolinite occurs at approximately
Answer: A
The hydroxyl groups leave as water vapour and the lattice collapses to metakaolin, an amorphous material. This is the irreversible change that makes a fired piece impossible to return to slip, and it happens well above the boiling point of water.
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Metakaolin formed on heating kaolinite is
Answer: B
Correct answer: an amorphous material with no long-range order
Between roughly 500 and 600 degrees kaolinite loses its hydroxyl groups, and the structure collapses into metakaolin - a dehydroxylated, largely amorphous aluminosilicate. It is neither a melt nor a crystal, and it is highly reactive, which is why metakaolin is used as a pozzolan in cement. On further heating past about 1,000 degrees it recrystallises to mullite.
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Assertion (A): Calcined clay is pozzolanic whereas raw clay is not.
Reason (R): Calcination destroys the crystal lattice and leaves a reactive amorphous material.
Answer: C
Both are true and the reason gives the change: the disordered structure will react with lime and water, which is exactly what a pozzolana must do and what the intact crystal will not. This is the basis of the calcined clay used in modern blended cements.
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Mullite begins to form in a fired clay body at around
Answer: D
Correct answer: 1,000 degrees Celsius and above
Mullite begins to appear around 950 to 1,000 degrees as metakaolin reorganises, and it develops into the needle-like crystals that give a fired body its strength as the temperature rises further. Below 600 degrees the clay is only losing water, and 2,000 degrees is above the melting point of anything in the body.
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State whether the following is true or false.
Mullite crystals contribute significantly to the strength of a fired porcelain.
Answer: A
The interlocking needles reinforce the glassy matrix in much the same way as fibres in a composite. Encouraging their growth is one of the aims of the firing schedule for a fine body. A soak near peak temperature encourages the needles to grow.
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Consider the reactions of clay on heating.
1. Dehydroxylation gives metakaolin at 500 to 600 degrees.
2. Mullite forms above about 1,000 degrees.
3. The changes can be reversed by adding water again.
Answer: B
The first two are correct. The third is exactly what cannot happen, since the lattice has been destroyed and the changes are chemical rather than physical. This is what separates firing from drying in kind rather than degree.
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