Polymorphism in alite means that
Answer: A
Several polymorphs occur depending on temperature and on the minor elements present, and they differ somewhat in reactivity. Stabilising a more reactive form is one route by which minor components influence cement strength.
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State whether the following is true or false.
Minor elements present in small quantities can influence clinker phase formation appreciably.
Answer: B
Magnesium, alkalis, sulphur and fluorine all affect which polymorphs form and how readily the phases crystallise. This is why raw materials are analysed for far more than the four principal oxides. Trace constituents can shift phase formation more than their quantity suggests.
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Mineralisers such as fluoride are sometimes added to a raw mix in order to
Answer: C
Correct answer: allow burning at a lower temperature
A mineraliser such as calcium fluoride lowers the temperature at which the liquid phase forms and at which alite is stable, so the same clinker can be made a good deal cooler. That saves fuel and reduces the thermal load on the refractory, at the cost of a small addition and some care over the effect on setting.
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Assertion (A): A small addition of a mineraliser can reduce the fuel a kiln requires.
Reason (R): It allows alite to form at a lower burning zone temperature.
Answer: D
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Alite normally requires about 1,450 degrees; with a mineraliser present it forms 50 to 100 degrees lower. Since a great deal of fuel is spent reaching the last part of that temperature, the saving is worth having - though the effect on setting time must be checked.
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The burnability of a raw mix describes
Answer: A
Correct answer: how readily it forms clinker at a given temperature
Burnability depends on the fineness of the meal, the mineralogy of the coarse particles, and the chemical moduli, and it is measured by burning a sample and determining the free lime remaining. A poorly burnable mix needs a hotter kiln and more fuel to reach the same free lime.
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Coarse quartz particles in a raw meal are particularly troublesome because they
Answer: B
Silica in the form of large quartz grains dissolves much more slowly than finely divided silica from clay. The lime that should have combined with it is left free, so meal fineness and quartz content are watched together.
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Assertion (A): A raw meal may meet its residue specification and still burn badly.
Reason (R): The residue figure says nothing about which mineral the coarse particles are.
Answer: C
Correct answer: Both A and R are true, and R is the correct explanation of A
Both are true and the reason gives the mechanism. Coarse calcite reacts readily, but coarse quartz does not: it must be reached to its centre by solid-state diffusion, and a large grain cannot be. Two meals with identical residues therefore burn quite differently according to what the residue consists of, which is why the coarse fraction is examined mineralogically.
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State whether the following is true or false.
Two raw meals with the same sieve residue necessarily have the same burnability.
Answer: D
Burnability depends on which minerals make up the coarse fraction as well as how much there is. Quartz of a given size is far more troublesome than limestone of the same size. Coarse quartz is often specified separately for this reason.
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A clinker has a lime saturation factor of 98 and free lime of 2.5 per cent. If a target free lime of 1 per cent is wanted, the most likely first correction is to
Answer: A
A high factor means more lime than the silica can combine with, so the excess remains free. Lowering it brings the two into balance, though better burning or a finer meal could also help. Raw mix, meal fineness and burning are all levers on the same figure.
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The clinkering reaction that forms alite is
Answer: B
Belite forms first at lower temperature and then takes up further lime once the melt appears. This explains why free lime and belite are both left behind when burning is insufficient. The two faults appear together and have a single underlying cause.
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