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WBBSE Madhyamik Vocational Construction study materials
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WBBSE Madhyamik Vocational Construction practice questions

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Question 1Construction Materials and Their Properties

A mason needs a mortar for general brickwork in a load-bearing wall of a two-storey house. Which mix ratio by volume of cement to sand is the standard recommendation for this purpose?

  1. 1:3
  2. 1:1
  3. 1:6 Correct answer.
  4. 1:10

Explanation

For ordinary brickwork in load-bearing walls, a cement-sand mortar of about 1:6 by volume gives adequate strength and workability at reasonable cost. Richer mixes are reserved for structural concrete or watertight work, and leaner mixes lose bond strength.

Question 2Construction Materials and Their Properties

While testing a sample of good quality burnt clay brick, a student strikes two bricks together. Which sound indicates well-burnt brick?

  1. A clear metallic ringing sound Correct answer.
  2. No sound at all
  3. A dull thud
  4. A soft crushing noise

Explanation

Well-burnt bricks are hard and vitrified enough to give a clear ringing sound when struck together. A dull sound suggests under-burning, excess moisture or internal cracks.

Question 3Construction Materials and Their Properties

Which timber is most suitable for door and window frames because it is durable, takes paint well and resists warping after proper seasoning?

  1. Rubber wood
  2. Bamboo
  3. Coconut palm stem
  4. Teak Correct answer.

Explanation

Teak is a dense, naturally oily hardwood that resists moisture, insects and warping, so it is preferred for frames and joinery. Bamboo and palm stems are not true structural timber for frames.

Question 4Construction Materials and Their Properties

Ordinary Portland cement is stored in a godown. Which practice best prevents loss of strength during storage?

  1. Keeping bags on a raised wooden platform in a dry, airtight shed Correct answer.
  2. Storing bags in tall stacks directly on the floor
  3. Storing bags in an open yard exposed to monsoon air
  4. Storing bags beside a water tank to keep them cool

Explanation

Cement absorbs moisture from air and floor, which causes lumping and loss of strength. Bags should be kept on raised dry platforms in a dry, well-sealed shed and used in the order received.

Question 5Construction Materials and Their Properties

A concrete mix is designated M20. What does the number 20 represent?

  1. The characteristic compressive strength in N/mm² at 28 days Correct answer.
  2. The maximum size of coarse aggregate in mm
  3. The water-cement ratio multiplied by 100
  4. The cement content in bags per cubic metre

Explanation

In the M-grade system, the number after M is the characteristic cube compressive strength in N/mm² measured at 28 days. M20 therefore means 20 N/mm² characteristic strength.

Question 6Construction Materials and Their Properties

Which of the following is a naturally occurring binding material that hardens only on exposure to air and is used in traditional lime plaster?

  1. Portland slag cement
  2. Gypsum plaster
  3. Epoxy resin
  4. Fat lime Correct answer.

Explanation

Fat lime, obtained by burning pure limestone, hardens by absorbing carbon dioxide from air and is a classic air-setting binder for traditional plaster. The other materials set by different mechanisms.

Question 7Construction Materials and Their Properties

Which property of a good building stone is tested by the Los Angeles abrasion machine?

  1. Resistance to wear Correct answer.
  2. Porosity
  3. Water absorption
  4. Crushing strength

Explanation

The Los Angeles abrasion test measures how much material wears away under repeated impact and abrasion, indicating resistance to wear. Porosity and absorption are found by water absorption tests, and strength by compression testing.

Question 8Construction Materials and Their Properties

In reinforced cement concrete, why is steel used together with concrete?

  1. Steel has high tensile strength and bonds well with concrete Correct answer.
  2. Steel increases the water absorption of concrete
  3. Steel prevents the concrete from setting
  4. Steel makes the concrete lighter in weight

Explanation

Concrete is strong in compression but weak in tension, while steel is strong in tension and bonds well to concrete. Their thermal expansion coefficients are also nearly equal, so they work together.

Question 9Construction Materials and Their Properties

A plasterer wants a smooth, crack-free finish on internal walls. Which material is commonly added to lime or cement mortar to improve workability and reduce shrinkage cracks?

  1. Sand only
  2. Broken brick bats
  3. Coarse gravel
  4. Fine sand with a small quantity of gypsum Correct answer.

Explanation

Fine sand with a little gypsum improves workability and controls shrinkage, giving a smoother plaster. Gravel and brick bats are too coarse for finishing coats.

Question 10Construction Materials and Their Properties

A sample of sand is tested and found to have a fineness modulus of 3.5. How should this sand be classified?

  1. Medium sand
  2. Fine sand
  3. Very fine sand
  4. Coarse sand Correct answer.

Explanation

Fineness modulus increases with coarseness; values around 3.2 to 3.8 indicate coarse sand. Fine sand has a much lower fineness modulus, typically below 2.5.

Question 11Construction Materials and Their Properties

A concrete cube of 150 mm side fails at a load of 540 kN. What is its compressive strength?

  1. 24 N/mm² Correct answer.
  2. 54 N/mm²
  3. 36 N/mm²
  4. 12 N/mm²

Explanation

The cube face area is 150 × 150 = 22500 mm². Dividing 540000 N by 22500 mm² gives 24 N/mm², which corresponds to an M20 grade mix.

Question 12Construction Materials and Their Properties

Which cement is preferred for mass concrete work such as a dam, where the heat generated during setting must be kept low?

  1. Ordinary Portland cement
  2. Portland pozzolana cement Correct answer.
  3. White Portland cement
  4. Rapid hardening Portland cement

Explanation

Portland pozzolana cement generates less heat of hydration and gives better long-term durability, making it suitable for mass concrete. Rapid hardening cement does the opposite.

Question 13Construction Materials and Their Properties

A brick is soaked in water for 24 hours and its absorption is measured. Which result indicates a first-class brick?

  1. About 25 percent by weight
  2. Not more than about 5 percent by weight Correct answer.
  3. About 20 percent by weight
  4. About 15 percent by weight

Explanation

First-class bricks are dense and should absorb not more than about one-fifth of their weight, commonly quoted as around 5 percent in standard tests, after 24-hour immersion. High absorption indicates porous, weak bricks.

Question 14Construction Materials and Their Properties

Which test is used to determine the consistency of a cement paste and thereby fix the water content for setting-time tests?

  1. Slump test
  2. Le Chatelier test
  3. Compaction factor test
  4. Vicat apparatus test Correct answer.

Explanation

The Vicat apparatus is used with a standard plunger to find the water needed for standard consistency. Setting times are then measured on a paste of that consistency, and soundness uses the Le Chatelier apparatus.

Question 15Construction Materials and Their Properties

A wall is built with bricks that were not soaked before laying in cement mortar. What is the most likely consequence?

  1. The mortar will set faster and gain extra strength
  2. The wall will become waterproof
  3. The bricks will absorb water from the mortar, weakening the bond Correct answer.
  4. The bricks will expand and crack the wall

Explanation

Dry bricks suck water out of the mortar, leaving too little water for cement hydration at the interface. This weakens the bond between brick and mortar, so bricks are soaked before laying.

Question 16Construction Materials and Their Properties

Which of these is the main reason for adding a small quantity of gypsum to cement clinker during manufacture?

  1. To give the cement a white colour
  2. To increase the heat of hydration
  3. To reduce the cost of grinding
  4. To retard the setting time of cement Correct answer.

Explanation

Gypsum is added as a retarder so that cement does not flash-set when water is added, giving time for mixing, transport and placing. It does not primarily change colour or reduce grinding cost.

Question 17Construction Materials and Their Properties

A concrete mix has a slump of 25 mm. For which type of work is this mix most appropriate?

  1. Free-flowing floor topping
  2. Thin sections with congested reinforcement
  3. Mass concrete in foundations with light reinforcement Correct answer.
  4. Pumped concrete for tall columns

Explanation

A low slump of about 25 mm indicates a stiff mix suitable for mass concrete with light reinforcement. Congested or pumped work needs higher workability.

Question 18Construction Materials and Their Properties

Which of the following correctly describes the seasoning of timber?

  1. Coating timber with paint to make it waterproof
  2. Treating timber with preservative chemicals against insects
  3. Controlled drying of timber to reduce moisture content Correct answer.
  4. Cutting logs into planks with a saw

Explanation

Seasoning is the controlled removal of moisture from timber so that it becomes stable, strong and less liable to warp or crack. Preservation and sawing are separate operations.

Question 19Construction Materials and Their Properties

A cement bag shows an ISI mark under IS 269. Which type of cement does this standard primarily cover?

  1. Portland slag cement
  2. Ordinary Portland cement, 33 grade Correct answer.
  3. Sulphate resisting cement
  4. Portland pozzolana cement

Explanation

IS 269 covers ordinary Portland cement of 33 grade, while pozzolana and slag cements have their own standards. The ISI mark indicates conformity with the relevant Indian Standard.

Question 20Construction Materials and Their Properties

Two mortar mixes are prepared, one with river sand and one with crushed stone sand of the same grading. Which property is likely to differ most noticeably?

  1. The colour of the cement used
  2. Bleeding and water retention of the fresh mortar Correct answer.
  3. The setting time of the cement
  4. Compressive strength of the hardened mortar

Explanation

Crushed stone sand has rougher, more angular particles than rounded river sand, which changes water demand and reduces bleeding. Strength differences are usually small if grading is the same, and the cement itself is unchanged.

Question 21Construction Materials and Their Properties

A site engineer finds that a consignment of cement has formed hard lumps that do not crumble between the fingers. What is the correct action?

  1. Use it after sieving out the lumps
  2. Use it for non-structural plaster only
  3. Reject the consignment as unsuitable for structural work Correct answer.
  4. Mix it with fresh cement to dilute the lumps

Explanation

Hard lumps that do not break down show that the cement has partly hydrated and lost strength. Such cement should be rejected for structural work rather than sieved or blended into fresh cement.

Question 22Construction Materials and Their Properties

A concrete mix is designed with a water-cement ratio of 0.60. If the cement content is 320 kg per cubic metre, what is the water content?

  1. 320 kg per cubic metre
  2. 256 kg per cubic metre
  3. 192 kg per cubic metre Correct answer.
  4. 160 kg per cubic metre

Explanation

Water content equals the water-cement ratio multiplied by cement content, so 0.60 × 320 = 192 kg per cubic metre. This is the free water added per cubic metre of concrete.

Question 23Construction Materials and Their Properties

A stone sample is found to have high porosity. Which conclusion is most reasonable about its use in an external wall?

  1. It will be immune to chemical attack
  2. It will have very high crushing strength
  3. It will absorb water and may suffer frost damage Correct answer.
  4. It will be excellent for damp-proofing

Explanation

High porosity means the stone can absorb water, which lowers strength and durability and can cause frost damage in cold weather. Dense, low-porosity stones are preferred for exposed external work.

Question 24Construction Materials and Their Properties

A beam is designed with reinforcement placed near the tension face. If the steel is instead placed near the compression face, what is the likely structural effect?

  1. The concrete becomes more workable
  2. The beam becomes stronger in tension
  3. The beam becomes lighter
  4. The tensile capacity of the section is largely lost Correct answer.

Explanation

Steel must be placed where tension occurs, which is near the tension face of a simply supported beam. Placing it near the compression face wastes its tensile strength and leaves the section vulnerable to cracking and failure.

Question 25Construction Materials and Their Properties

A plaster coat is applied on a very smooth, dry concrete surface without any preparation. Which defect is most likely to appear?

  1. Shrinkage cracking of the base
  2. Crazing due to excess cement
  3. Blistering and debonding Correct answer.
  4. Efflorescence

Explanation

A smooth, dry surface gives poor mechanical key and sucks water from the plaster, so the coat may blister and debond. The surface should be roughened and wetted before plastering.

Question 26Construction Materials and Their Properties

Which of the following is a naturally occurring binding material that hardens only on exposure to air and is used in traditional construction?

  1. Bitumen
  2. Cement
  3. Lime Correct answer.
  4. Gypsum

Explanation

Lime is a traditional binding material produced by burning limestone; it hardens by absorbing carbon dioxide from the air (carbonation) and is used in mortars and plasters. Cement and gypsum harden through chemical hydration, and bitumen is a hydrocarbon binder that sets by cooling or evaporation, not air hardening.

Question 27Construction Materials and Their Properties

A plasterer wants a smooth, crack-free finish on internal walls. Which material is commonly added to lime or cement mortar to improve workability and reduce cracking?

  1. Gypsum Correct answer.
  2. Lime
  3. Water
  4. Sand

Explanation

Gypsum is added to lime or cement mortar to improve workability, retard setting, and reduce shrinkage cracks, giving a smooth finish. Sand is a filler, lime improves plasticity but may increase shrinkage, and water is needed for mixing but does not control cracking.

Question 28Construction Materials and Their Properties

A mason needs a binding material for general brick masonry that sets and hardens by chemical reaction with water, both in air and under water. Which material meets this requirement?

  1. Gypsum plaster
  2. Non-hydraulic fat lime
  3. Ordinary Portland cement Correct answer.
  4. Hydraulic lime

Explanation

Cement is a hydraulic binder: it reacts chemically with water and continues to gain strength even when submerged, which is why it is the standard binder for masonry and concrete exposed to damp conditions. Lime and gypsum set mainly by drying, carbonation or re-crystallisation and cannot be relied on under water.

Question 29Construction Materials and Their Properties

A timber beam for a roof is to be sawn from a freshly felled log. Which practice best reduces subsequent warping and shrinkage cracks in the beam?

  1. Convert the log to sections and stack them with spacers for air-seasoning before use Correct answer.
  2. Soak the sawn sections in water for a week and then install them
  3. Paint the green sections with cement slurry and use them at once
  4. Saw the log immediately and fix the beam while it is still green

Explanation

Air-seasoning stacks converted timber with spacers so that moisture is lost slowly and evenly from all faces; this relieves drying stresses gradually and limits warping, twisting and splitting. Using green timber or sealing it while wet traps uneven moisture gradients, so defects develop after the member is fixed in place.

Question 30Construction Materials and Their Properties

A concrete cube is tested and the failure surface passes cleanly through the coarse aggregate particles rather than around them. What does this indicate about the mix?

  1. The aggregate is weaker than the hardened cement paste, so the aggregate is the limiting component
  2. The cube was tested before it had set, so the result is invalid
  3. The cement paste is weaker than the aggregate, so the bond and matrix govern the strength Correct answer.
  4. The water-cement ratio was too low, making the paste excessively strong

Explanation

When a crack travels through the paste or along the aggregate-paste interface rather than splitting the stones, the matrix or the bond is the weaker link, so strength is governed by the paste and its adhesion. Failure through the aggregate itself shows the opposite: the stone is the weaker element.

Question 31Building Components and Construction Methods

In a traditional brick masonry wall, what is the standard thickness of a mortar joint between two bricks laid in a stretcher course?

  1. About 10 mm Correct answer.
  2. About 60 mm
  3. About 40 mm
  4. About 25 mm

Explanation

Brick masonry relies on a thin, uniform mortar bed so that load is transferred evenly through the wall and shrinkage cracking is limited. A joint of roughly 10 mm (about 3/8 inch) is the accepted practice for both bed and head joints. Thicker joints waste mortar and reduce the compressive strength of the masonry.

Question 32Building Components and Construction Methods

Which component of a building is designed primarily to transfer the load of the superstructure safely to the soil below?

  1. The foundation Correct answer.
  2. The lintel
  3. The plinth beam
  4. The parapet

Explanation

The foundation is the lowest structural element and its function is to spread the building load over a sufficient soil area so that bearing pressure stays within safe limits. Other elements such as lintels or parapets carry only local loads. The plinth beam ties the footings but does not by itself perform the load-spreading role.

Question 33Building Components and Construction Methods

A mortar made by mixing cement, sand and water only, with no lime, is best described as which type?

  1. Mud mortar
  2. Surkhi mortar
  3. Lime mortar
  4. Cement mortar Correct answer.

Explanation

Cement mortar is the mix of cement, fine aggregate and water, and it is known for high strength and quick setting. Lime mortar uses lime as the binding material, mud mortar uses clay soil, and surkhi mortar uses burnt brick powder. The composition stated in the stem corresponds only to cement mortar.

Question 34Building Components and Construction Methods

What is the main purpose of providing a damp-proof course (DPC) at plinth level in a load-bearing wall?

  1. To increase the compressive strength of the wall
  2. To bond the wall to the plinth beam
  3. To stop rising moisture from the ground entering the wall Correct answer.
  4. To allow thermal expansion of the masonry

Explanation

A damp-proof course is a continuous moisture-resistant layer placed near plinth level so that groundwater cannot rise by capillary action into the wall. Materials such as bitumen, polythene sheet or rich cement mortar are commonly used. It does not add strength or accommodate movement, so the other options confuse its purpose.

Question 35Building Components and Construction Methods

In a framed structure, the horizontal member that spans between columns and supports the slab or wall above is called a:

  1. A footing
  2. A beam Correct answer.
  3. A column
  4. A staircase

Explanation

A beam is the horizontal structural member that carries transverse loads and transfers them to the supporting columns or walls. Columns are vertical compression members, footings are foundation elements, and a staircase is a circulation component. The description in the stem matches a beam only.

Question 36Building Components and Construction Methods

Which of the following is the usual minimum bearing of a lintel on each side of an opening in a brick wall?

  1. About 50 mm
  2. About 100 mm
  3. About 230 mm Correct answer.
  4. About 460 mm

Explanation

A lintel must sit on sound masonry at each end so that the load is transferred safely into the wall. Practice generally requires a bearing of about 230 mm, roughly one brick length, on either side. Shorter bearings risk crushing the masonry under the end reaction.

Question 37Building Components and Construction Methods

Which plastering defect appears as a pattern of fine hairline cracks on a newly plastered cement surface?

  1. Blistering
  2. Peeling
  3. Efflorescence
  4. Crazing Correct answer.

Explanation

Crazing is the network of fine, shallow cracks that forms when the plaster surface dries and shrinks faster than the material beneath. It is usually caused by rich mixes, rapid drying or poor curing. Blistering, efflorescence and peeling are different defects with different causes.

Question 38Building Components and Construction Methods

The vertical distance between the top of one stair tread and the top of the next tread is known as the:

  1. Riser Correct answer.
  2. Tread
  3. Landing
  4. Nosing

Explanation

In a staircase, the riser is the vertical rise between successive treads, while the tread is the horizontal surface where the foot rests. The nosing is the projecting edge of a tread, and a landing is a flat platform between flights. The stem defines the riser.

Question 39Building Components and Construction Methods

A brick is described as being laid in 'header' orientation when:

  1. Its longest face lies flat and parallel to the wall face
  2. It is laid on edge with the frog upward
  3. Its short end face is exposed on the wall face Correct answer.
  4. It is cut diagonally to form a corner

Explanation

A header is a brick laid so that its short end face, roughly 90 mm wide, shows on the face of the wall. This orientation ties the two leaves of the wall together. Stretchers, by contrast, show the long face. The other descriptions do not correspond to standard brick positions.

Question 40Building Components and Construction Methods

In English bond brickwork, alternate courses consist of:

  1. Only stretcher courses throughout
  2. A course of stretchers with a header every fourth brick
  3. A full stretcher course followed by a full header course Correct answer.
  4. Only header courses throughout

Explanation

English bond is formed by alternating a complete course of stretchers with a complete course of headers. This arrangement gives great strength and avoids continuous vertical joints. Flemish bond, by contrast, alternates stretchers and headers within the same course.

Question 41Building Components and Construction Methods

A concrete mix designated M20 in Indian practice indicates that the concrete has:

  1. A characteristic compressive strength of 20 N/mm² at 28 days Correct answer.
  2. A cement content of 20 kg per cubic metre
  3. A slump of 20 mm
  4. A maximum aggregate size of 20 mm

Explanation

The letter M stands for mix, and the number denotes the characteristic cube compressive strength in N/mm² measured at 28 days. Thus M20 concrete is expected to reach 20 N/mm² in standard cube tests. The designation does not refer to cement content, workability or aggregate grading.

Question 42Building Components and Construction Methods

Why is a frog provided on one face of a burnt clay brick?

  1. To indicate the direction of the kiln during firing
  2. To form a key for the mortar so the brick grips the bed joint Correct answer.
  3. To reduce the weight of the brick for easier handling
  4. To allow the brick to be cut more easily on site

Explanation

The frog is the shallow depression moulded into one bed face of the brick. When the brick is laid frog-up, mortar fills the depression and forms a mechanical key that improves the bond with the course above. It is not a weight-saving or cutting feature.

Question 43Building Components and Construction Methods

A retaining wall 3 m high is to be built in a garden where slight movement is acceptable and appearance matters. Which material is most appropriate for the facing?

  1. Timber boarding fixed to posts
  2. Random rubble masonry in cement mortar Correct answer.
  3. Stone cladding fixed to a brick backing
  4. Reinforced cement concrete with formwork finish

Explanation

Random rubble masonry in cement mortar is a common, economical choice for low retaining walls, and its irregular stone face is acceptable where appearance is a secondary concern. The wall relies on mass and batter for stability. RCC would be stronger but is costlier for a small garden wall, while cladding and timber are not suitable as the primary retaining structure.

Question 44Building Components and Construction Methods

During plastering of an internal brick wall, the mortar dries out rapidly in hot weather. What is the most appropriate corrective action?

  1. Cure the plaster by keeping it moist for several days Correct answer.
  2. Apply a thicker single coat to retain moisture
  3. Increase the sand proportion to make the mix leaner
  4. Add more cement to the mix to speed setting

Explanation

Rapid drying prevents cement from completing hydration, leading to shrinkage cracks and weak plaster. Keeping the surface moist for several days allows proper curing and strength gain. Adding cement or changing the mix ratio does not replace the moisture lost to evaporation.

Question 45Building Components and Construction Methods

A door frame is to be fixed in a 230 mm brick wall opening. Which method gives the most rigid and durable fixing?

  1. Building the frame into the wall with holdfasts or lugs embedded in the masonry Correct answer.
  2. Fixing the frame with adhesive only
  3. Holding the frame with wooden cleats driven into the mortar joints
  4. Nailing the frame directly into the brick face

Explanation

Frames are best secured by holdfasts or lugs that are built into the masonry as the wall rises, giving a rigid connection that resists slamming and settlement. Nailing into brick or relying on mortar-joint cleats gives a weak, temporary hold. Adhesive alone cannot carry the repeated impact loads on a door frame.

Question 46Building Components and Construction Methods

In a load-bearing brick wall, why are successive vertical joints staggered rather than aligned?

  1. To make the wall lighter in weight
  2. To avoid a continuous vertical plane of weakness Correct answer.
  3. To reduce the number of bricks required
  4. To allow rainwater to drain through the wall

Explanation

If vertical joints line up from course to course, they form a continuous plane that splits the wall into independent leaves and allows easy separation. Staggering the joints, as in proper bonding, distributes load and ties the masonry together. Economy, weight and drainage are not the reasons for staggering.

Question 47Building Components and Construction Methods

A concrete slab shows a crack running parallel to its supporting beams near mid-span. Which cause is most consistent with this pattern?

  1. Use of rounded river sand instead of crushed sand
  2. Insufficient curing water on the day of casting
  3. Poor quality of the formwork oil
  4. Inadequate reinforcement or excessive loading causing flexural tension Correct answer.

Explanation

A crack parallel to the supports near mid-span is characteristic of flexural cracking, where tensile stress exceeds the concrete's capacity because reinforcement is insufficient or loads exceed design. Curing-related cracks are usually random or map-cracked on the surface. Formwork oil and aggregate shape affect finish and workability, not this crack pattern.

Question 48Building Components and Construction Methods

Which of the following is the correct order of operations when constructing a load-bearing brick wall from the ground up?

  1. DPC, then foundation, then superstructure
  2. Superstructure, then foundation, then DPC
  3. Foundation, then superstructure, then DPC
  4. Foundation, then DPC, then superstructure Correct answer.

Explanation

Construction proceeds upward: the foundation is built first and brought to plinth level, the damp-proof course is laid on the level plinth, and only then is the superstructure raised. This sequence keeps rising moisture out of the wall from the outset. Placing the DPC after the wall is built defeats its purpose.

Question 49Building Components and Construction Methods

In a dog-legged staircase, the two flights are separated by a landing at mid-height. What is the main functional reason for this landing?

  1. To reduce the number of risers in the staircase
  2. To carry the load of the floor slab above
  3. To increase the total floor height served
  4. To provide a resting place and allow the direction of travel to reverse Correct answer.

Explanation

A mid-landing breaks a long climb into manageable flights, giving users a place to rest and pause, and it is where the direction of travel turns through 180 degrees in a dog-legged stair. It does not change the floor-to-floor height or the total riser count. Slab support is a structural role of beams, not the landing's purpose.

Question 50Building Components and Construction Methods

A brick wall built with a rich cement mortar shows extensive shrinkage cracks within a week of construction. Which factor most likely contributed?

  1. Thorough wetting of bricks before laying
  2. Use of well-graded sharp sand
  3. Excessive cement content relative to sand in the mortar Correct answer.
  4. Slow drying of the wall in humid weather

Explanation

Rich mixes contain more cement paste than the sand can restrain, so the mortar shrinks significantly as it dries and sets, producing cracks. Well-graded sand, pre-wetted bricks and slow drying all reduce shrinkage risk. The high cement-to-sand ratio is therefore the most probable cause.