Cement and Refractories Questions and Answers

Answer: A

Correct answer: a calcareous and an argillaceous material

Portland cement needs a source of lime and a source of silica, alumina and iron. The calcareous material - limestone, chalk or marl - supplies the lime, and the argillaceous material - clay or shale - supplies the rest. Everything else, including the gypsum, is a minor addition, and the whole chemistry of the clinker follows from the proportions in which these two are blended.

Answer: B

The sulphate reacts at the aluminate surface and the coating slows further access of water for long enough to place the concrete. The same compound formed later, from sulphate entering hardened concrete, is expansive and destructive.

Answer: C

Decomposition proceeds at around 900 degrees and consumes a large share of the kiln's heat, which is why a precalciner is fitted ahead of the kiln in modern plants. The clinkering reactions occur higher up the temperature scale.

Answer: D

Correct answer: 1450 degrees Celsius

The burning zone reaches about 1450 °C, which is the temperature at which part of the charge melts and the liquid phase allows lime and silica to combine into tricalcium silicate. Calcination of the limestone is complete well below this, at around 900 °C; the extra heat is needed for the clinkering reactions themselves rather than for driving off carbon dioxide.

Answer: A

Correct answer: let the desired compounds decompose

Tricalcium silicate is unstable below about 1250 °C and reverts slowly to dicalcium silicate and free lime if the clinker is allowed to cool through that range at leisure. Rapid cooling freezes it in as a metastable phase. Since tricalcium silicate is what gives the early strength, quenching the clinker is not a convenience but a requirement.

Answer: B

Correct answer: tricalcium silicate

Tricalcium silicate hydrates quickly and is responsible for most of the strength developed in the first week or so. Dicalcium silicate hydrates far more slowly and contributes the later strength, so the two together give a cement that gains strength quickly at first and continues to gain it for months. Adjusting their ratio is how rapid-hardening and low-heat cements are made.

Answer: C

The silicates hydrate at very different rates, and the dicalcium form is still reacting long after the tricalcium has finished in the first week. Its low heat of hydration is also why a low-heat cement is made rich in it.

Answer: D

Correct answer: flash set and the highest heat of hydration

Tricalcium aluminate reacts almost instantly with water, releasing a great deal of heat and stiffening the paste before it can be placed. That is exactly why gypsum is ground in with the clinker: it reacts with the aluminate to form ettringite, which coats the grains and delays the reaction. The compound contributes little to strength.

Answer: A

Correct answer: tricalcium aluminate

Sulphate attack works by sulphate reacting with the hydrated aluminate to form ettringite, which occupies a greater volume and disrupts the hardened paste. Limiting the tricalcium aluminate content, typically to five per cent or less, removes most of what the sulphate has to attack. The silicates are unaffected, so the cement retains its strength.

Answer: B

Correct answer: iron compounds

The grey colour comes from the iron-bearing phase, chiefly tetracalcium aluminoferrite. White Portland cement is made from raw materials chosen to be very low in iron and manganese, which is why it is more expensive: the limestone and clay must be selected, and the fuel and grinding media must not contaminate the mix.