Cement and Refractories Questions and Answers

Answer: A

Correct answer: about 15.9 litres

Volume is mass divided by density, and a specific gravity of 3.15 means a density of 3150 kg/m³: \[ V = \frac{50}{3150} = 0.0159\ \mathrm{m^3} = 15.9\ \text{litres.} \] This is the figure behind the familiar statement that a 50 kg bag of cement occupies about 0.035 m³ loose - the difference between the two being the voids between the grains.

Answer: B

Correct answer: 25 per cent

Porosity is the fraction of the volume that is void, given by comparing bulk and true density: \[ P = 1 - \frac{\rho_{bulk}}{\rho_{true}} = 1 - \frac{2.1}{2.8} = 1 - 0.75 = 0.25. \] Porosity matters because it governs how readily slag penetrates the brick and how well it insulates - and those two pull in opposite directions.

Answer: C

Correct answer: 40 N/mm\( ^2 \)

The area of a 50 mm cube face is \( 50 \times 50 = 2500 \) mm², so \[ f = \frac{100 \times 10^3}{2500} = 40\ \mathrm{N/mm^2}. \] The commonest error is to forget the conversion of kilonewtons to newtons, which gives an answer a thousand times too small.

Answer: D

Correct answer: 1030 kg

The gypsum is added as a percentage of the clinker, so 3 per cent of 1000 kg is 30 kg and the ground cement weighs \[ 1000 + 30 = 1030\ \text{kg.} \] Reading it as 3 per cent of the finished cement would give 1031 kg, a difference too small to matter here but worth being deliberate about in a mass balance.

Answer: A

Correct answer: 1-Q, 2-R, 3-P

The three principal compounds divide the work between them. Tricalcium silicate hydrates quickly and gives the early strength. Dicalcium silicate hydrates slowly and supplies the later strength, contributing little in the first week. Tricalcium aluminate gives almost no strength but reacts violently with water, producing a flash set and the greatest heat of any of the compounds.

Answer: B

Correct answer: 1-R, 2-P, 3-Q

The classification follows the chemistry. Magnesite is magnesia-rich and therefore basic. Silica is acidic. Chromite is neutral, which is why it is used as a buffer layer between an acidic and a basic lining where the two would otherwise react. Matching the lining to the slag is the first decision in selecting a refractory.

Answer: C

Moisture goes first, the carbonate decomposes next, the compounds form at the hottest point, and the clinker is quenched last. In a modern plant the first two stages happen in the preheater and precalciner rather than in the kiln itself.

Answer: D

Correct answer: 1 and 3 only are correct

Statements 1 and 3 are correct: gypsum retards the flash set caused by tricalcium aluminate, and cement sets by hydration rather than by drying, which is why it hardens under water. Statement 2 reverses the two silicates - tricalcium silicate gives the early strength and dicalcium silicate the later.

Answer: A

Correct answer: raising the tricalcium aluminate content

Tricalcium aluminate has the highest heat of hydration of all the clinker compounds, so raising it increases the heat rather than reducing it. The other three all reduce it: dicalcium silicate reacts slowly and releases less heat, a pozzolana replaces some of the clinker, and coarser grinding slows the reaction and spreads the heat over a longer time.

Answer: B

Correct answer: high porosity

Porosity is what an insulating refractory needs, not a working-face one. At the hot face, pores admit slag and gas into the body of the brick and accelerate its destruction, so a dense low-porosity brick is wanted there, backed by a porous insulating layer behind it. The other three - spalling resistance, refractoriness under load and slag resistance - are all working-face requirements.