Types of Cement and Composition Questions and Answers

Answer: C

Adding 65, 22, 6 and 3 gives 96 per cent, the remainder being magnesia, alkalis, sulphate and loss on ignition. The four principal oxides account for nearly all of a Portland cement. The balance among them decides which clinker phases form.

Answer: D

A high value gives less liquid phase in the kiln, which makes the mix harder to burn but produces a clinker of different character. It is used alongside the lime saturation factor to control the mix. Together the two moduli largely fix the character of the clinker.

Answer: A

Periclase, the crystalline form of magnesia, reacts long after hardening and causes unsoundness in much the same way as free lime. Standards therefore cap its content. Dolomitic limestone must therefore be used with care in the raw mix.

Answer: B

Each hydrates slowly and expands after the concrete has set, so both are limited by specification. The Le Chatelier and autoclave tests exist to detect exactly this class of fault. Both faults appear only after the concrete has hardened, which is what makes them serious.

Answer: C

Types II, III, IV and V cover moderate sulphate resistance, high early strength, low heat and high sulphate resistance respectively. The numbering is worth learning as a set. Indian standards use different designations for broadly similar products.

Answer: D

Correct answer: 100 kg

\[ 2000\ \mathrm{kg} \times 0.05 = 100\ \mathrm{kg}. \] The 50 kg offered treats the two tonnes as one. Gypsum is added at this few per cent to control setting: without it the tricalcium aluminate would hydrate almost instantly and flash set the cement before it could be placed.

Answer: A

Correct answer: expand slightly on setting to offset shrinkage

Concrete shrinks as it dries, and where it is restrained that shrinkage puts it into tension and cracks it. An expansive cement generates a controlled expansion during setting - usually by forming ettringite deliberately - which, held by the reinforcement, leaves the concrete in mild compression instead. It is used for shrinkage-compensating slabs and for grouts that must fill a space completely.

Answer: B

Expansive cements use a controlled expansion deliberately to offset shrinkage. What is undesirable is uncontrolled expansion of the kind free lime and magnesia cause. Controlled and uncontrolled expansion are quite different things.

Answer: C

It is pumped a great distance down a hot borehole and must not stiffen on the way. Retarders are used to control the thickening time under those conditions. A premature set would leave the casing uncemented and the well unusable.

Answer: D

Plasticisers and air-entraining agents give a mortar that spreads and holds water well. High strength would in fact be a disadvantage in a mortar joint, which should be weaker than the units it bonds. Strength is only one property among several that a mortar must balance.