Mortar

Mortar is a paste of binding material (cement, lime or mud), fine aggregate (sand) and water, used to join building units. Its functions are:

1. Binds bricks, stones or blocks into a solid mass.
2. Distributes load evenly over the bedding surface of masonry units, avoiding point loads.
3. Bedding: gives an even, level bed so the units sit properly and courses stay horizontal.
4. Fills joints and the gaps and voids between units, making the wall weather-tight and preventing entry of rain, wind, dust and insects.
5. Bonds reinforcement/anchors such as fixing door-window frames, hold-fasts and steel bars in masonry.
6. Plastering and pointing: used for finishing walls, floors and giving a decorative surface.
7. Resists weathering and increases durability of the structure.
8. Absorbs small movements and shrinkage (flexible lime/mud mortar), and is used for flooring, damp-proofing and as a base for tiles.

Cement mortar is prepared by hand-mixing for small jobs or by a mixer for large work.

Steps

1. Select the proportion (by volume), for example 1:4 cement:sand for brick masonry and 1:6 for plaster; select the sand (clean, sharp, free from clay and silt).
2. Measure materials by volume using a gauge box (such as a measuring box 1 ft x 1 ft x 1 ft).
3. Mix dry: spread sand on a clean, watertight platform (wood or concrete), spread cement on top and turn the heap with shovels, 3-4 times, until the colour is uniform.
4. Add water gradually (about 20-25% by weight of the dry mix for a workable mix), while mixing again, forming a crater and turning the dry mix into it. Do not over-wet.
5. Mix thoroughly until a uniform, workable paste of the right consistency is obtained. In machine mixing, load the sand and cement, and add water, running for 1-2 minutes.
6. Use within 30 minutes (before initial setting). Do not re-temper or use mortar that has begun to set; keep the platform clean.

Mortar is a workable paste obtained by mixing a binding material (cement, lime or mud) with fine aggregate (sand, surkhi or cinder) and water. It is used to join masonry units such as bricks and stones, and for plastering and pointing. After setting it hardens into a solid mass.

How it is prepared

1. Measure the binder and sand in the required ratio by volume (for example, cement:sand = 1:4).
2. Mix them dry on a clean, watertight platform until the colour is uniform.
3. Add clean water little by little, turning the mass until a uniform paste of workable consistency is obtained.
4. Use the mortar before it starts to set (within about 30 minutes for cement mortar).

Lime mortar is made by grinding lime and sand in a mortar mill or tank mill with water; mud mortar from clay soil and water.

Precautions before and during the application of cement mortar:

1. Materials: use fresh cement of the right grade (not lumpy), clean sharp sand free from silt, clay and organic matter, and potable water.
2. Proportions: measure the proportion by volume correctly, for example 1:3 to 1:6 as required by the job.
3. Mixing: mix dry first, then wet; mix thoroughly for uniform colour on a clean, watertight platform. Do not use too much water.
4. Time: use the mortar within about 30 minutes of adding water; do not re-temper mortar that has started to set.
5. Surface preparation: clean and wet the masonry surface (or bricks should be soaked in water) so it does not suck water from the mortar; rake out joints for plaster adhesion.
6. Joint thickness: keep the joints uniform and thin (about 10 mm), full of mortar with no hollows.
7. Alignment: check levels, plumb and lines while laying.
8. Curing: keep the work wet for at least 7 days (10 days for plaster); protect it from sun, wind, rain and frost during the early days.
9. Weather: avoid work in hot, dry or freezing weather without proper care.

A good cement mortar should have the following properties:

1. Workability: it should be easy to mix, spread and use, and plastic enough to fill the joints completely.
2. Adhesion (bond strength): it should stick firmly to bricks and stones.
3. Strength: it must have enough compressive strength (for example 5-10 MPa for 1:4 mix, as required) to carry loads from the masonry.
4. Durability: it should resist weathering, frost and chemical attack.
5. Water retentivity: it should retain water long enough to prevent the masonry units from sucking it out.
6. Setting: it should set neither too fast nor too slow; it should not shrink or crack.
7. Impermeability: it should resist moisture penetration.
8. Economy and appearance: it should be cheap and give a good finish; it should be free from efflorescence-causing salts.
9. Mobility: it should flow under the trowel and fill all spaces.

The type of mortar is chosen according to the requirements of the work, the load, exposure, cost and availability.

Factors for selection

1. Type and importance of work: load-bearing walls, foundations and structures below ground level need strong cement mortar (1:3 to 1:4); ordinary walls use 1:6; plastering uses 1:4 to 1:6; non-load-bearing partitions and temporary works can use lime or mud mortar.
2. Strength required: cement or gauged mortar for heavy loads; lime mortar for light loads.
3. Exposure: use cement mortar in wet, damp or exposed places (basements, parapets, water retaining structures); use lime mortar for dry interior places.
4. Setting time needed: cement mortar for quick setting; lime mortar for slow.
5. Durability and weather: in frost/rainy areas, cement or gauged mortar.
6. Cost and availability: mud mortar is the cheapest and used in rural areas; surkhi/lime mortar where lime is available.
7. Special purposes: fire-resistant (fire-clay mortar), sound-proof, X-ray shielding (barium), packing mortar for grouting.

Mortars are classified in the following ways.

1. According to binding material

• Mud mortar: mud (clay soil) and water; used in rural areas.
• Lime mortar: lime, sand/surkhi and water (fat lime or hydraulic lime).
• Surkhi mortar: lime and surkhi (powdered burnt brick).
• Cement mortar: cement, sand and water.
• Gauged mortar (composite mortar): cement, lime and sand together.
• Gypsum mortar: plaster of Paris/gypsum, used for plastering and decorative work.

2. According to use

• Bricklaying mortar: for masonry joints.
• Finishing mortar: for plaster and pointing.

3. According to special characteristics

• Light-weight mortar: with sawdust, wood powder or perlite.
• Fire-resistant mortar: with aluminous cement and fire-clay.
• Packing mortar: for oil wells, grouting and tunnels.
• Sound-absorbing, X-ray shielding and chemical resistant mortars.

Surkhi mortar is a mortar made by mixing hydraulic lime or fat lime with surkhi (finely ground burnt brick or clay) in a ratio of about 1:2 and water. The surkhi acts as a pozzolana and gives the mortar hydraulic properties. It is used for ordinary buildings, foundations, and in wet places where cement is costly; it is cheaper and resists dampness but is weaker than cement mortar.

Gauged mortar (composite mortar) is made by mixing cement, lime and sand, in proportions like 1:1:6 or 1:2:9. Lime gives workability and water retention, and cement adds strength and early setting. It is economical, workable, less shrinkage-prone, and used for superstructure brickwork and plastering.

For brick masonry or plaster, cement mortar is applied as follows:

1. Prepare the surface: clean the bricks/wall surface of dust and loose parts, and wet bricks (soak for at least 2 hours) so they do not draw water from the mortar. For plaster, rake joints 10-12 mm deep and wet the wall.
2. Prepare the mortar in the required ratio (1:4 or 1:6) and use within 30 minutes.
3. Spread a mortar bed about 10-12 mm thick with a trowel over the lower course.
4. Place the brick with a firm pressure, tap it to level, and fill the vertical joint with mortar (frog up/filled).
5. Check line, level and plumb for each course; remove excess mortar from joints.
6. Finish joints: point or strike the joints neatly.
7. Plaster: apply a first coat (12 mm), then finishing coat (6-8 mm) and level with straight edge and float.
8. Cure with water for 7-10 days.

Mortar is a paste of binder, fine aggregate and water used to join masonry units and for plastering and pointing.

Cement mortar

• Composition: cement, sand and water, 1:3 to 1:6.
• Properties: strong, durable, sets fast (initial set in 30 minutes or more), good bond, impervious and resistant to moisture and weather.
• Uses: load-bearing brick and stone masonry, foundations, RCC structures, plaster, flooring, damp places, and for pointing.

Mud mortar

• Composition: clay soil, water, sometimes cow dung/straw or husk added to prevent cracks.
• Properties: cheap, workable and easily made, but weak, easily eroded by rain, and not durable.
• Uses: low-cost rural houses, temporary walls, garden walls and for plastering with protection from water.

Mortar is a plastic mixture of a binding material (cement, lime or mud), sand and water used for joining masonry units and for plastering and pointing.

Engineering characteristics

• Good workability and plasticity so it fills joints.
• Adequate bond with masonry units and sufficient compressive strength.
• Good water retentivity and impermeability.
• Appropriate setting time; no shrinkage cracks.
• Durable against weather and economical.

Cement mortar versus lime mortar

Merits

1. It is economical, with good workability and plasticity, and easy to apply.
2. It has less shrinkage and cracking, and is flexible, accommodating small movements; it has good bond with bricks.

Demerits

1. Its strength is low and it gains strength slowly because setting is slow (days to weeks).
2. Fat lime mortar does not set in damp places or under water, and needs a long curing period.

Mortar is used:

1. To bind bricks, stones and concrete blocks into masonry and distribute loads evenly.
2. To fill joints between masonry units and make walls weather-tight.
3. For plastering walls and ceilings and for decorative finishes.
4. For pointing of masonry joints.
5. For flooring, bedding of tiles and stone slabs, and damp-proof courses.
6. For fixing doors and window frames, hold-fasts and fixtures.
7. For grouting, repair of cracks and as a base for concrete (screed).