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

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.

These compounds are known as Bogue’s Compounds.

Tricalcium aluminate (C3A): Celite is the quickest one to react when the water is added to the cement. It is responsible for the flash setting. The increase of this content will help in the manufacture of Quick Setting Cement. The heat of hydration is 865 J/Cal.

Its proportion is 11 – 25%

Tricalcium silicate (C3S): This is also called Alite. This is also responsible for the early strength of the concrete. The cement that has more C­­­­3S content is good for cold weather concreting. The heat of hydration is 500 J/Cal.

Its proportion is 25 - 50%

Dicalcium Silicate (C2S): This compound will undergo a reaction slowly. It is responsible for the progressive strength of concrete. This is also called as Belite. The heat of hydration is 260 J/Cal.

Its proportion is 25 - 40%

Tetra calcium Alumino ferrite (C4AF): This is called Felite. The heat of hydration is 420 J/Cal. It has the poorest cementing value but it responsible for long term gain of strength of the cement. 

Its proportion is 9 - 11%

Explanation:

Vicat's Apparatus

Definition: Vicat's apparatus is a device used in civil engineering to determine the physical properties of cement, specifically its consistency and setting times. These properties are critical in ensuring the quality and suitability of cement for various construction applications. The apparatus consists of a plunger, needles, and a graduated scale to measure the penetration of the plunger or needle into the cement paste.

Working Principle: The working of Vicat’s apparatus is based on the principle of measuring the penetration resistance of cement paste. The consistency and setting times are determined by observing the depth of penetration of a plunger or needle into the cement paste under standardized conditions. The paste is prepared by mixing cement with water in specific proportions, and the penetration is measured at different time intervals to assess the paste's behavior as it sets and hardens.

Applications:

• Normal Consistency of Cement: Vicat's apparatus is used to determine the amount of water required to prepare a cement paste of normal consistency. Normal consistency is the water content at which the paste allows a standard plunger to penetrate to a specified depth.
• Initial Setting Time of Cement: The apparatus is used to measure the time it takes for the cement paste to lose its plasticity and resist a standard needle's penetration.
• Final Setting Time of Cement: Vicat's apparatus is also used to measure the time it takes for the cement paste to harden completely and resist penetration entirely.

Advantages:

• Provides accurate and reliable measurements of cement consistency and setting times.
• Helps in ensuring the quality and performance of cement in construction projects.
• Allows for the standardization of cement properties for various applications.

Explanation:

Gypsum Addition in Portland Cement Manufacturing

Definition: Gypsum is a mineral commonly added during the manufacturing of Portland cement to regulate the setting time of the cement. Portland cement is produced by grinding clinker, a product obtained by heating limestone and clay in a rotary kiln, into a fine powder. Gypsum plays a critical role in controlling the hydration of cement, which in turn influences its workability and strength development.

Gypsum is added at the beginning of grinding the clinker.

This option is correct because gypsum is typically introduced into the grinding process at the very start when clinker is being ground into powder. The primary purpose of adding gypsum at this stage is to control the setting time of cement. Without gypsum, the hydration of the cement would occur too rapidly, resulting in "flash setting," where the cement hardens almost immediately upon mixing with water, making it unworkable.

Gypsum reacts with the compounds present in the clinker, particularly tricalcium aluminate (C₃A). The reaction between gypsum and C₃A forms calcium sulfoaluminate, a compound that delays the hydration process and extends the setting time. This controlled setting process ensures that the cement remains workable for a sufficient period, allowing construction workers to place and finish it properly.

Additionally, introducing gypsum at the beginning of the grinding process helps in achieving a uniform distribution of the mineral throughout the cement mixture, ensuring consistent performance in terms of setting time and strength development.

Explanation:

• As per IS 4031 (Part 5): 1988, which outlines the procedure for determining the initial setting time of cement using the Vicat apparatus, the initial setting time for Portland-Pozzolana Cement should not be less than 30 minutes.
• This standard defines the time it takes for the cement paste to lose its plasticity and begin to set, and for Portland-Pozzolana Cement, it is specified as 30 minutes or more

 Additional Information The Vicat Apparatus is an essential tool used for determining the setting times (initial and final) of cement, and it also measures the consistency of the cement paste. It is widely used in accordance with standards like IS 4031 (Part 5): 1988 to ensure the quality of cement used in construction.

Components of Vicat Apparatus:

1. Frame: The apparatus is typically mounted on a metal or wooden frame that holds all the components together.

2. Plunger: The most important part is the Vicat needle or plunger (a metal rod with a pointed end), which is used to test the consistency and setting times of cement.

3. Scale: A vertical scale attached to the apparatus that allows accurate measurement of the needle's penetration depth.

4. Needle Holder: The needle is fixed to a holder, which is part of a movable system that ensures precise downward movement.

5. Base Plate: A flat surface where the cement paste is placed for testing.

6. Initial and Final Setting Time Needle: There are two different needles for testing:

   • Initial Setting Time Needle: A needle with a 1-mm sqaurearea used for the initial setting time.

   • Final Setting Time Needle: A thicker needle, typically with a 5-mm diameter, used for the final setting time.

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.