Cement Interview Questions And Answers

Cement is a material with adhesive and cohesive properties and it is used as a binding material that sets, hardens, and adheres to other materials to bind them together.

It is a major and important construction material that used to construct most of the modern world including buildings bridges, highways, and dams. Presently cement is the most consumed commodity in the world after water and there is not any other material till now which could replace cement which describes the importance of cement on its own.

Flash setting of cement means rapid development of rigidity or premature hardening in freshly mixed cement paste and there is an early loss of work-ability in paste. Cement does not gain prominent strength during flash set. To prevent the cement from flash set, gypsum is added.

Gypsum controls the setting time of cement, so it prevents the flash setting i.e., gypsum slows down the hydration process of cement after adding the water to cement. This ensures that concrete will not set too quickly before it is placed and compacted at required place.

Weight of 1 cement bag = 50 kg and volume of 1 cement bag is 0.035 cu.m.

On the addition of water to cement, a chemical reaction which is exothermic in nature takes place and produces a significant amount of heat is referred as hydration of cement.

Setting of cement means stiffening of the paste form of cement i.e., change of cement paste from a fluid state to rigid state whereas hardening refers to the gain of strength of a set cement paste and it happens after setting state. At the time of setting, cement acquires some strength, however, for practical purpose, the term setting and hardening are separately defined.

Grades of cement represents the compressive strength of cement i.e., the compressive strength of cement cube of size 7.06×7.06×7.06 cm after 28 days of curing in N/mm² (MPa). Higher the grade of cement superior will be the strength of cement used in construction work and there are only 3 grades of cement available i.e., 33 grade, 43 grade and 53 grade cements.

By increasing the C₃S content, decreasing the C₂S content and by fine grinding of the cement clinker, we can get the rapid hardening property.

No, generally we can’t use the rapid hardening cement for mass concreting because in early stage, this cement exerts large amount of heat of hydration due to which the temperature inside the concrete rises leading to the formation of the undesirable cracks on cooling.

By varying the quantity of gypsum, we can control the setting time of cement.

With the addition of water in the cement paste, sometimes a premature stiffening occurs within 1 to 5 minutes. This condition is known as false set. This condition occurs due to the presence of anhydrous gypsum which is formed due to grinding of gypsum with too hot clinker. By continuous mixing, we can break the false set without harming any property of the concrete.

The soundness of cement is an ability of hardened paste after setting to retain its volume. If the volume change is unstable after setting and hardening then the concrete structure will crack.

Flash set means a rapid development of rigidity in freshly cement paste which caused because of inadequate amount of gypsum are added to the cement. Further mixing can’t dispel this rigidity and the heat produce in this process is of significant amount.

Whereas false set also makes the freshly cement paste rigid but this occurs because some of the gypsum dehydrates as a result of contacting too hot clinker or very high temperatures in the grinding mill. In this process the heat generated is of less amount and further mixing can solve the issue of false set.

Major compounds of Portland cement are C₃S (Tricalcium Silicate), C₂S (Dicalcium Silicate), C₃A (Tricalcium aluminate) and C₄AF (Tatra calcium alumino ferrite).

HAC also known as calcium aluminate cement (CAC) or aluminous cement, is composed of calcium aluminates unlike Portland cement which is composed of calcium silicates. This cement contains about 40% alumina and is highly reactive and about 80% of strength of concrete is developed within 24 hours.

This cement is highly resistant to chemical attacks, have very low pH and also high resistant to chemical corrosion. The refractive index of this cement is high and also has high durability in sulphuric acid.

Due the property of rapid hardening and strength, it is widely used in undersea concreting and also in sewer infrastructure. This cement is also used in refractory concretes where more strength is required because of high temperature.

Blended cements are the uniform mixture of OPC and other constituents (blending materials) like fly ash, silica fumes and slag to enhance the properties of cement. By varying the proportions of constituents, we can improve the workability, strength, durability and chemical resistance of concrete.

The minimum w/c ratio for full hydration of cement is 23%.

This cement should not be stored more than 1 month.

Low heat evolution can be achieved by reducing the content of tricalcium silicate and tricalcium aluminate which are the compounds exerts maximum heat of hydration and by increasing dicalcium silicate.

• In PPC, some parts of costly raw materials are replaced by cheaper pozzolanic material.
• Soluble calcium hydroxide is converted into insoluble cementitious products resulting in improvement of permeability.
• Heat of hydration and rate of hydration are low.

The cement which are obtained by grinding Portland cement clinker with water repellent, film-forming substances like oleic acid or stearic acid. The water repellent film reduces rate of deterioration when the cement is stored under unfavourable conditions. This cement is also known as Hydrographic cement.

Generally, the quantity of gypsum added varies from 3 to 5% depending upon C₃A (tri calcium aluminate) content. If the addition of gypsum is more, than the required quantity will be combined with C₃A and the excess quantity of gypsum will remain in free state leading to the expansion and consequent disruption of the set cement paste.

The IRS-T 40 cement grade is a special type of cement grade which is mainly used in railway sleepers only. This special cement is manufactured under the specifications that are approved by the ministry of Indian Railways under IRS-T40 – 1985. This cement contains high volume of C₃S content which are finally grinded in order to develop high early strength.

Surkhi is an artificial pozzolanic material made by powdering bricks or burnt clay balls. Surkhi is used for making waterproof cement mortars and concrete and also make the concrete more resistant to alkalis and salt solutions.

Principle raw materials are

1. Argillaceous compounds (silicates of alumina) in the form of clays and shales and
2. Calcareous compounds (CaCO₃) in the form of limestone, chalk and marble which is a mixture of clay and CaCO₃.

PPC is manufactured by the intergrinding of OPC clinker with 15 to 25% of pozzolanic material. A pozzolanic material is essentially a silicious or aluminous material which while in itself possessing no cementitious properties. But when water is added, the pozzolanic materials shows some cementitious properties.

Following are the field tests:

1. There should not be any visible lumps into the cement bag and the colour of cement should normally be greenish grey.
2. It must be cool feeling on thrusting your hand into the cement. It shows that there is not any moisture into the bag and also no heat of hydration.
3. Take a pinch of cement and the feel between the fingers should be smooth and not a gritty feeling.
4. On throwing a small amount of cement into the bucket full of water, the cement should float for some time before it sinks.

Digits 33, 43 and 53 represents 28 days compressive strength in N/mm2 of standard cube of face area 50 sq.cm made up of cement mortar 1:3.

Rounded shape of aggregates is considered to be the best for concrete making as they can be packed to get minimum voids and have low water/cement (w/c) ratio. But we are not using them because they haven’t the good interlocking capacity.