Cement MCQ Quiz - Objective Question with Answer for Cement - Download Free PDF

Concept:

The water-cement ratio is the ratio of the weight of water to the weight of cement used in a concrete mix. It is a key factor influencing the strength, durability, and workability of concrete.

Calculation:

A standard bag of cement in most countries, weighs 50 kg.

If the water-cement ratio is 0.6, this means for every 1 kg of cement, 0.6 kg of water is required. Therefore, for a 50 kg bag of cement:
Water required = 0.6 × 50 kg = 30 kg of water.

Explanation:

The following table attachment of Vicat's apparatus for different tests

Additional Information

The Vicat Apparatus is used to determine the setting time and consistency of cement and other hydraulic binders.

It helps assess:

• Initial setting time: When the paste begins to stiffen.

• Final setting time: When the paste hardens sufficiently.

• Standard consistency (normal consistency): Water content required to prepare cement paste of a standard plasticity.

Components of Vicat Apparatus

• Frame with movable rod: Holds the plunger or needle.

• Plunger/Needles:

  • Plunger (10 mm dia, 50 mm long): for consistency test.

  • Initial setting needle: for initial setting time.

  • Final setting needle with annular collar: for final setting time.

• Graduated scale: Measures penetration depth.

• Mould (Vicat mould): Conical frustum-shaped mould, typically:

  • Top diameter: 60 mm

  • Bottom diameter: 70 mm

  • Height: 40 mm.

• Glass base plate: on which the mould is placed.

Explanation:

• The presence of small amounts of sugar (typically less than 0.25% by weight of cement) in mixing water retards the setting of cement.

• Sugar molecules interfere with the hydration process by forming a thin coating on cement particles, slowing down the chemical reactions between water and cement.

• Retardation of setting allows more time for placing and compacting concrete, which can be beneficial in hot weather concreting.

• However, if the sugar content is too high, it can excessively delay setting or even prevent proper hardening of the concrete.

Additional Information

Retarders

• Retarders are chemical admixtures added to concrete to delay the setting time of cement.

• They slow down the rate of hydration reactions between cement and water, giving more time for transportation, placing, compacting, and finishing of concrete, especially useful in hot weather or large pours.

• Common retarders include sugar (in very small amounts), lignosulphonates, and certain acids.

Accelerators

• Accelerators are chemical admixtures that speed up the setting time and early strength gain of concrete.

• They promote faster hydration reactions, allowing the concrete to set and gain strength quickly, which is helpful in cold weather concreting or where rapid turnaround is needed.

• Common accelerators include calcium chloride, triethanolamine, and some nitrates.

• The use of retarders helps prevent cold joints and ensures better surface finishes by keeping the concrete workable for longer periods.

• On the other hand, accelerators are used when rapid construction is required or when early strength is needed for formwork removal.

• Both retarders and accelerators must be used in controlled quantities. Overuse of retarders can lead to excessive delays or incomplete setting, while excessive accelerators can cause shrinkage, cracking, or reduced long-term strength.

Explanation:

• Long-term strength:
  PPC develops strength at a slower rate initially but achieves higher long-term strength due to the pozzolanic reaction continuing over time.

• Heat of hydration:
  The presence of pozzolanic materials in PPC leads to a reduction in heat of hydration, making it suitable for mass concreting where thermal cracking is a concern.

• Permeability:
  PPC produces a denser and less porous concrete, leading to lower permeability and improved durability in aggressive environments.

 Additional InformationPortland Pozzolana Cement (PPC) 

• Composition:
  PPC is manufactured by grinding Portland cement clinker with pozzolanic materials (such as fly ash, calcined clay, or silica fumes) and gypsum.

• Durability:
  Due to its low permeability and dense microstructure, PPC offers greater resistance to chemical attacks, especially from sulphates, chlorides, and alkalis.

• Environmental Benefit:
  The use of industrial by-products like fly ash in PPC makes it more eco-friendly by reducing carbon emissions and promoting sustainable construction.

• Workability and Finish:
  PPC provides better workability and surface finish due to the finer particles and longer setting time, which allows better compaction and finishing of concrete surfaces.

Explanation:

• The percentage of voids in cement powder is approximately 40%.

• This means about 40% of the volume of cement is empty space between the particles.

• These voids get filled with water during mixing, which is essential for the hydration reaction.

• The actual value may vary slightly depending on the fineness of cement.

Additional Information

• The voids in cement affect the water demand and ultimately the strength and durability of concrete.

• Lower voids mean denser packing of particles and better strength potential.

• Understanding voids helps in optimizing the water-cement ratio for mix design.

Explanation:

Standard consistency test:

• This test determines the percentage of water required to make workable cement paste.
• Vicat’s apparatus is used to perform this test.
• Temperature during test = 27 ± 2°C
• Relative humidity = 90%
• As per Vicat’s test ‘the percentage of water added to the cement at which the needle can not penetrate 5-7 mm from bottom of the mould is called consistency.
• For OPC consistency is around 30%
• In order to make a cement paste of normal consistency, the percentage of water varies from 25 to 35%.

Important Points

Concept:

Consistency: Vicat’s Apparatus is used to find out the consistency, initial setting time and final setting time of the cement. In the normal consistency test we have to find out the amount of water to be added to the cement to form a cement paste of normal consistency.

Important Point:

Standard consistency is the consistency of the cement paste that permits the Vicat’s plunger of diameter 10 mm and height 50 mm to penetrate in to the mould up to depth of 33 to 35 mm from the top. 

Concept:

Absolute density of different concrete materials are as follows:

Following are parameters and properties of cement:

1. Bulk density of cement = 1440 kg/m3

2. Specific gravity of cement = 3.15

3. Weight of one bag of cement = 50 kg

4. Volume of cement bag = 50/1440 = 0.035 m3

5. Number of cement bags in 1 m3 = 30 approx.

6. Volume of dry mortar is 30% more than volume of wet mortar.

∴ Volume of dry mortar = 1.3 × volume of wet mortar

Consistency: Vicat’s Apparatus is used to find out the consistency, initial setting time and final setting time of the cement. In the normal consistency test we have to find out the amount of water to be added to the cement to form a cement paste of normal consistency.

Important Point:

Standard consistency is the consistency of the cement paste that permits the Vicat’s plunger of diameter 10 mm and height 50 mm to penetrate in to the mould up to depth of 33 to 35 mm from the top. 

Approximately 23% water by weight is required for complete hydration i.e to complete all chemical reactions and 15% water is entrapped in between the voids of cement. The total water required for complete hydration and workability is 38% by weight.

∴ water required to complete all chemical reaction in cement is 23 %

Note:

The water required for completing chemical reaction is 23 %

A total of 38% water by weight of cement is required to complete the chemical reaction and fill the space within the gel pores.

Concept:

The following table shows the composition of cement and its properties.

Crazing: Crazing is development of networks of very fine random cracks or fissures on the surface of concrete due to differences in shrinkage between the surface and the interior. The cracks rarely exceed 12 mm or so in depth, and are therefore not serious, apart from the unsightliness.

Efflorescence: Efflorescence is a whitish coloured powdered deposition of salts (such as sulphates and carbonates of calcium, sodium and potassium) on the concrete surface that is formed due to evaporation of water from the water soluble salts in the concrete.

Sulphate deterioration: Sulphate attack on concrete is a chemical breakdown mechanism where sulphate ions attack components of the cement paste. Due to sulphate attack, ettringite is formed which is very expansive in nature, results in deterioration in concrete in form of cracking and spalling.

Laitance: In case of excess water in concrete mix, the cement along with water slurry comes to the surface by capillary action and formsa thin layer over surface known as laitance. This weakens the bond between the successive lifts of concrete. The excess water may leak through the form work, resulting in honeycombed concrete and on evaporation makes the concrete porous.

Explanation:

If the fineness of the cement is greater, surface area of the particles of the cement is also greater. And greater surface area leads to an increase in rapid hydration reaction (i.e. exothermic reaction between cement particles and water).

And hydration reaction is the backbone of the strength development.

So, increase in fineness of the cement will ultimately lead to increase in development of the strength of the cement.

Explanation:

​Plastering work

• Plaster is a thin layer of mortar applied over the masonry surface and it acts as a damp-proof coat over the brick masonry work.
• Plastering also provides a finished surface over the masonry that is firm and smooth hence it enhances the appearance of the building.
• The primary objectives of plastering are to protect the surface from atmospheric influences, cover the defective workmanship in masonry, conceal porous materials, and provide a suitable surface for painting.

Procedure of Plastering Work

1. Preparation of Surface for Plastering

• Keep all the mortar joints of the wall rough, so as to give a good bonding to hold plaster.
• Roughen the entire wall to be plastered.
• Clean all the joints and surfaces of the wall with a wire brush, there should be no oil or grease, etc. left on the wall surface.

2. Groundwork for Plaster

• In order to get a uniform thickness of plastering throughout the wall surface, first, fix dots on the wall. A dot means a patch of plaster of size 15 cm × 15 cm and having a thickness of about 10 mm.
• Dots are fixed on the wall first horizontally and then vertically at a distance of about 2 meters covering the entire wall surface.
• Check the verticality of dots, one over the other, by means of plumb-bob.
• After fixing dots, the vertical strips of plaster, known as screeds, are formed in between the dots. These screeds serve as the gauges for maintaining the even thickness of plastering being applied.

3. Applying Under Coat or Base Coat

• In the case of brick masonry, the thickness of the first coat of plaster is in general 12 mm, and in the case of concrete masonry, this thickness varies from 9 to 15 mm.
• The ratio of cement and sand for first coat plaster varies from 1:3 to 1:6.
• After leveling, left the first coat to set but not to dry, and then roughen it with a scratching tool to form a key to the second coat of plaster.

4. Applying Finishing Coat

• The thickness of the second coat or finishing coat may vary between 2 to 3 mm.
• The ratio of cement and sand for second coat plaster varies from 1:4 to 1:6.
• Before applying the second coat, damp the first coat evenly.
• Apply the finishing coat with wooden floats to a true even surface and using a steel trowel, give it a finishing touch.