Cement and Refractories Questions and Answers

Answer: C

Correct answer: carries weight while it is hot

A refractory in a furnace roof or wall carries the load of the structure above it at working temperature, and many materials that resist high temperature unloaded will deform under even modest stress when hot. Refractoriness under load applies a specified pressure while heating and reports the temperature at which deformation begins, which is the figure that matters in service.

Answer: D

Correct answer: softening temperature

The pyrometric cone equivalent compares a cone made of the refractory with a series of standard cones fired alongside it, and reports the number of the standard cone that deforms to the same extent. It measures the temperature at which softening begins under its own weight, and it says nothing about behaviour under load, thermal shock or slag attack.

Answer: A

The trapped air impedes heat flow, which is what an insulating brick wants, but the same voids admit slag and weaken the brick. A working face in contact with slag therefore needs a dense brick with an insulating layer behind it.

Answer: B

Uneven expansion between a hot face and a cooler interior sets up stresses that break pieces away, and rapid temperature cycling is the usual cause. A low coefficient of expansion and a controlled porosity both help resist it.

Answer: C

Correct answer: coefficient of thermal expansion

Thermal spalling happens because the hot face expands more than the cooler interior, setting up stresses that crack the brick. A low expansion coefficient reduces those stresses directly, which is why materials such as fused silica and zircon do so well under rapid cycling. Some porosity also helps by accommodating strain, so a very dense brick is not necessarily the best choice here.

Answer: D

Correct answer: volume change accompanying its crystalline inversions

Silica undergoes crystalline inversions below about 600 °C, and the accompanying volume changes are large and abrupt enough to crack the brick if it is cycled through them. Above that temperature the material is stable and performs excellently, which is why silica linings in coke ovens and glass tanks are held hot and cooled only slowly at shutdown.

Answer: A

It burns away in the presence of oxygen, so it is used in reducing conditions such as a blast furnace hearth or under a protective coating. Its high thermal conductivity and excellent thermal shock resistance are its merits.

Answer: B

Correct answer: 63.5 per cent

Combined lime is the total lime less the free lime: \[ 65 - 1.5 = 63.5\ \text{per cent.} \] The distinction matters because only the combined lime has reacted into the useful clinker compounds. Free lime hydrates slowly after the concrete has hardened and expands, which is the unsoundness the Le Chatelier test is designed to detect.

Answer: C

Correct answer: 74.1 per cent of the mix

The limestone is 78 per cent of the mix and is itself 95 per cent calcium carbonate: \[ 0.78 \times 0.95 = 0.741 = 74.1\ \text{per cent of the mix.} \] The clay contributes essentially none, so this is the whole carbonate content, and it is what determines how much lime will be available after calcination.

Answer: D

Correct answer: 56 kg

The calcination is \( \mathrm{CaCO_3} \rightarrow \mathrm{CaO} + \mathrm{CO_2} \), with molar masses 100, 56 and 44. So 100 kg of pure carbonate yields \[ 100 \times \frac{56}{100} = 56\ \text{kg of lime,} \] the remaining 44 kg leaving as carbon dioxide. That 44 per cent loss is why a cement kiln releases so much carbon dioxide from the raw materials alone, quite apart from the fuel.