What is Mortar? | 13 Types of Mortar | Preparation of Mortar | Tests for Mortar | Functions & Properties

  1. Introduction  

Mortar is a mechanical mixture in varying proportions of a  binding material such as  cement or lime and an inert material or fine aggregate like sand, surkhi, etc.

The binding material and fine aggregate are sometimes referred to as matrix and adulterant respectively.

Mortars are normally described according to the binding material used in their preparation e.g. Cement-mortar, Lime mortar, etc.

Mortar is used as a binding material in stone or bricks masonry, or concrete, as a covering product to walls in the form of plaster to give, a smooth, hard, and decorative surface.

On account of its plastic nature, when green, the mortar forms an excellent, even bed for the irregular surface of the stone and this results in equal distribution of pressure over it.

  2. Functions of Mortar  

1. It binds together stones or bricks properly (so as to provide strength to the structure).

2. In any concrete, it holds coarse aggregates together (so as to form a solid mass).

3. It provides durable weather resisting layer between different courses of masonry in the structure.

4. In stone masonry and brick masonry, it fills up empty joints; a thin liquid mortar is used for such purposes is termed as grout.

5. It forms a homogeneous mass of structure so that it may resist all the loads coming over it and transfers the same uniformly to its foundation.

6. It does pointing or plastering to the structures.

  3. Properties of Good building Material  

Following properties are required of a good building mortar:

1. It should be easily workable.

2. It should develop adequate strength in tension, compression, and bond for the work for which it is used.

3. It should be durable and should not affect the durability of other materials.

4. It should be set quickly so that the speed of construction is ensured.

5. It should cohere the bricks or stones to provide a tight joint through which water cannot pass.

6. It must be cheap.

7. It must be able of developing the calculated stresses.

8. The joints formed by mortar should not develop cracks and they should be able to maintain their appearance for quite a long period.

  4. Uses of Mortar  

Followings are the uses of the mortars:

1. It is utilized to fill up the spaces between bricks or stones for preparing walls weather-tight.

2. It is used to bind together the bricks or stones in brick or stone masonry.

3. It is used to provide a soft even bed between the various layers of brick or stone masonry for the same distribution of pressure over the bed.

4. It is used as a matrix.

5. It is used in pointing and plastering to protect the joints and surfaces of brick and stone masonry.

6. It is employed for molding purposes.

7. It is used to form joints of pipes.

8. It is used to increase the normal appearance of the structure.

9. It is used to hide open joints of brickwork and stonework.

  5. Ingredients of Mortars  

The ingredients of various mortars used for different engineering purposes are enumerated and described below.

a. Binding or cementing materials

b. Fine aggregates

c. Water

 a. Binding or Cementing Materials:  

(i). Cement : 

It is used as a binding material for the preparation of mortars required for different engineering applications where strength and durability are the primary requirements.

For this purpose, ordinary portland cement (also called normal setting cement) is mostly used; however, for special works rapid hardening cement, low heat cement, or any other type of cement may also be suitably used.

Functions:

In cement mortar, cement performs the following functions:

(i). It makes the mortar impermeable by filling up the voids existing in the fine aggregate.

(ii). It imparts strength to the mortar on setting and hardening.

(iii). It binds the aggregates into a solid mass by virtue of its setting and hardening properties when mixed with water.

(ii) Lime:

It is also used as a binding material in preparing the mortars for various purposes but its strength is less than that of cement.

The different types of lime, eminently hydraulic, semi-hydraulic, and fat limes are used for preparing the lime mortars for different purposes.

For preparing the mortar, the lime should be slaked before mixing it with sand, surkhi, etc.

Functions: 

In a lime mortar, lime performs the following functions.

(i) It imparts strength to the mortar on setting and hardening.

(ii) It binds the fine aggregate into a solid mass by virtue of its setting and hardening properties when mixed with water.

 b. Fine aggregate  

(i) Sand:

It is commonly used as fine aggregate in preparing the cement as well as lime mortars.

Sand contains tiny angular or rounded grains of Silica.

It is made by the decomposition of sandstone by the effect of weathering agencies.

It may be either natural sand (e.g river sand, Nala sand, pit sand, and sea sand ) or artificial sand ( prepared by crushing stones and gravels) mortars that are to be exposed to view should not be prepared with sea sand.

The sea sand should be washed with freshwater ( to remove a maximum of its salt contents) before use.

Classifications of sand

The sand is classified as fine, coarse, and gravelly, according to the size of the grains.

Fine sand should pass through a screen with clear openings of 1.5875 mm and is mostly utilized for plastering.

Coarse sand will pass through a screen with a clear opening of 3.175 mm and is generally used for masonry work.

Gravelly sand will pass through a screen with a clear opening of 7.62 mm and is generally used for concrete work.

Requirements / Characteristics or properties of Good sand:

Good sand should possess the  following characteristics:

(i) Its grains should be strong and durable.

(ii) It should be chemically inert.

(iii) It should not contain salts that attract moisture from the atmosphere.

(iv) Its grains should be coarse, angular, sharp, and hard.

(v) It must not contain any organic or vegetable matter.

(vi) It should be well graded. The fineness modulus of sand should be within the middle of 2 and 3.

(vii) It must be clean and free from coatings of clay and slit.

Functions of  Sand in Mortar

Followings are the functions of  sand in mortar.

(i) It doesn’t increase the strength of mortar but is used as an adulterant for the economy.

(ii) It increases the resistance of mortar against crushing.

(iii) It reduces the shrinkage of the binding material and hence cracking of mortars due to setting is avoided.

(iv) It helps in setting/hardening of flat lime by making it porous is absorbed through the voids of sand which causes a flat lime to set/harden effectively.

(v) It subdivides the paste of the binding material into a thin film and then more surface is offered for its spreading and adhering.

(vi) It helps in making mortars of any desired strength by varying their proportion with the binding material.

(ii) Surkhi:

Surkhi is also used as a fine aggregate in the preparation of lime mortar.

It is finely powdered burnt clay and generally made from slightly underburnt or pilla bricks.

It should be free from admixture of foreign substances.

It plays the same functions as those of sand but in addition, it gives strength and improves the hydraulic property of the mortar.

Surkhi if used wholly in place of sand particularly in exposed situations, it causes the mortar to disintegrate.

(iii) Ashes or cinders: 

These are in the form of small nodules ( obtained from furnaces or locomotives) which are ground and used as fine aggregate in lime mortar.

They impart strength and hydraulic properties to the lime mortar.

These act as adulterants, hence the bulk or volume of mortar is increased which results in the reduction of cost.

 c. Water  

Water plays a significant role in mixing, transporting, and proper laying of the mortar.

The water to be used for preparing the mortar should meet the following requirements.

– Should be clean and fresh

– Should be free from organic impurities, hygroscopic, greasy, and oily substances.

– Should preferably be drinkable.

Functions:

Below following functions are done by water in the preparation of mortar.

(i) It wets the surfaces of aggregate.

(ii) It makes the mortar workable (by acting as a lubricant for the aggregate).

(iii) It facilities the spreading of cement/lime over the fine aggregate.

(iv) It causes hydration of cement and eminently hydraulic lime due to which setting and hardening of cement and lime take place.

  6. Types of Mortars  

Mortars are divided in several ways, i.e, depend on the binding agent, features of binding agent and aggregate, etc.

In normal, mortars are classified as follows:

1. Cement mortars

2. Lime mortars

3. Lime-cement mortars(or composite or gauge mortar)

4. Special mortars

(i) Mud mortars

(ii) Cement-clay mortar

(iii) Light and heavy mortars

(iv) Decorative Mortar

(v) Air-entrained (plasticized) mortar

(vi) Gypsum mortar

(vii) Fire-resistant mortar

(viii) Packing mortar

(ix) Sound-absorbing mortar

(x) X-ray shielding mortar

  6.1. Cement Mortar  

Cement mortar consists of cement as binding material, sand in different proportions, and water.

It should be noted that surkhi and cinder are not chemically inert substances and hence they can not be used as adulterants with the matrix as cement. Thus the sand only can be utilized to prepare cement mortar.

It is the strongest type of mortar and is therefore preferred for use in a different construction of structures subjected to heavy loading.

The ratio of cement to sand should normally be 1 : 3 to 1: 6 by weight for the mortar to be workable and strong.

The use of mortars of lower cement content is not satisfactory since any notable reduction in cement contents leads to a reduction in workability and less cohesion and will produce porous joints with a tendency for low frost resistance.

1: 8 cement mortar is nearly twice as strong as 1: 3 lime mortar.

Cement mortar when used for protective plaster provides a waterproof layer and protects the elements covered from weathering effects.

  Preparation of Cement Mortar:  

Cement mortars are prepared by the followings methods:

(i) Hand mixing                       (ii) Machine mixing

(i) Hand mixing: 

This method is usually taken when a small quantity of mortar is needed.

In this method first, clean dry sand is spread in a uniform layer on a pucca platform. The needed quantity of cement is uniformly spread.

The whole mass is then mixed dry by working with spades till the whole mass becomes uniform in color.

Depression is then formed in the center of the mortar mix and the required quantity of water is added to this dry mix.

Dry material from the sides is placed on the edge of the depression containing water. It is first done normally till the water is fully absorbed by the dry mass. Care is done not to let the water collapse the banks and flow out.

The wet mass of mortar is then worked with spades to get a mortar of unvarying consistency.

Nearly 28 liters of water per bag of cement is required for making the mortar suitable consistency.

Cement mortar should be prepared in small quantities that can be used before the initial setting time of the cement.

(ii) Machine mixing:

When a large quantity of mortar is required continuously at a fast rate, it is prepared by mixing the ingredients in mechanical mixers.

Cement and sand in specified proportion are put into the drum of the mixture and then the required quantity of water is immediately added before revolving the drum.

The drum is then revolved for a sufficient period to form a uniform mixture of the required consistency.

The mixed mortar is then flowed out for utilization.

  Uses of Cement Mortar:  

(i) Cement mortar is used where high strength is required and in structures that are subjected to wet conditions such as piers, dams, deep foundations, etc.

(ii) It is also used for external plastering and pointing work.

(iii) It is utilized for the damp proof courses below the ground level, exposed work like exterior free-standing walls and parapets, engineering construction with bricks of high strength.

(iv) In concrete, cement mortar binds the particles of coarse aggregate into one solid mass.

  Precautions in Cement Mortar:

Following precautions should be taken while using cement mortar.

1. It should be prepared by mixing it uniformly to a workable consistency.

2. It should be used immediately after preparing and should be consumed within half an hour after adding water.

3. Before laying the bricks or stones in the structure, they should be fully saturated with water, otherwise, they may absorb the greater part of the water from the mortar, thus making the mortar joint weaker due to the lack of water required for hydration.

4. The masonry or plastered surface should be continuously kept moist by sprinkling water for at least seven days.

  6.2 Lime Mortar  

These are further classified as:

(i) Non-hydraulic lime mortars

(ii) Hydraulic Lime mortars

(iii) Black mortars

(i) Non-hydraulic lime mortars

These mortars are prepared by mixing fat lime (well slaked before use) with sand. The usual proportions of lime and sand are 1: 2 or 1 : 3 by volume.

They have a light color.

They do not cause efflorescence.

They are not good for damp situations, foundations, thick walls, as their setting action depends on the presence of CO2.

They are best only for thin joints in brickwork.

When used in moderately thick sections, their quality can be improved by the addition of surkhi or cement.

(ii) Hydraulic lime mortars

These mortars are made from class A and class B limes.

Mortars prepared from class A lime is next to cement mortar in strength and so is used in all heavy and important engineering works, in foundations, and for external plastering and pointing works.

Mortar made with 1 part of the fat lime and 2 parts of surkhi or 1 part of the lime plus  1 part of surkhi and 1 part of sand is mostly used in the foundations and superstructures of ordinary buildings.

In works where more strength or hydraulicity is needed pure hydraulic lime without sand is preferred.

(ii) Black mortars:

These mortars are so-called because of their color.

In these mortars, the usual proportions are 1 part lime to 3 parts of ash or cinder (1 : 3 ).

They become hard on the setting.

Due to their less pleasing color, they are used in internal walls.

  Properties of Lime mortars  

1. They are quite plastics and workable when wet.

2. They have good working qualities if made from high-calcium limes.

3. They develop strength very slowly but gain continuous strength over long periods.

4. They do not “set” but stiffen as water is lost by absorption by the units or blocks in contact, and by evaporation. Further, the gain in strength is acquired by a very slow reaction of lime with carbon dioxide from the air.

5. They provide a fairly strong surface when used for plastering.

6. They provide enough bonds between masonry blocks or bricks when used for masonry joints.

  Uses of Lime Mortars   

(i) Lime mortar is utilized in masonry to tie stones, bricks, or concrete blocks together.

(ii) It is suitable for masonry and plastering in cheap and light load-bearing wall construction.

(iii) It is also used for internal work with very thin mortar joints, or external walls in sheltered conditions where the mortar is produced by a frost-resistant pointing.

  Precautions in Lime Mortars   

While using lime mortar, the following precautions should be taken:

1. When hydraulic lime is used as the main building material, the mortar should be used within 4 hours after grinding.

2. Lime mortar made with surkhi or other pozzolana is hydraulic in nature and should be used within 24 hours after grinding.

3. It is not advisable to allow the mortar surface to dry suddenly; it is essential to sprinkle water mildly on masonry or plastered surface.

4. After grinding, all lime mortars must be kept wet and should be never be allowed to dry during maturing and storage( to do so, make a small hollow in the middle of the heap and keep it filled with water all time).

5. The workers who are to handle lime mortar should protect their skin by using oil, rubber gloves, etc.

  6.3 Lime Cement mortars  

Lime cement mortars are also known as “Compo” mortar or gauged mortar.

It is made by mixing cement with lime mortar in the best proportions.

The addition of cement increases the hydraulicity and strength.

The process of adding cement is known as gauging. It makes the lime mortar economical, strong, and dense.

Non-hydraulic, semi-hydraulic, or hydrated lime are generally used for preparing these mortars.

The normal proportions of cement, lime, and sand are:

1:1:6, 1:2:9 and 1:3:12

These mortars should be used within two hours after the cement is added.

   Uses of Lime Cement Mortars  

(i) It is used for bedding and for thick walls.

(ii) It can also be used for cavity walls, in the masonry to bind stones, bricks, or concrete blocks, etc.

(iii) It is also used for plastering wall surfaces and outside weatherproof pointing.

(iv) It is used efficiently in masonry providing thin joints thus leading the economy.

  6.4. Special Mortars  

  6.4.1 Mud Mortar  

Sticky clay is puddled or plugged with water until it comes to the required consistency. It is called “gara“.

Sometimes certain fibrous material (or gobber) is also added which prevents the shrinkage cracks; the mortar ingredients are mixed thoroughly.

Mud mortar is very cheap and the ingredients are locally available.

The life of mud mortar surface varies from 5 to 15 years depending on its waterproofing treatment and weather conditions.

Uses:

These mortars are also used for the brickwork of ordinary buildings.

It is used for surfacing floors and plastering the internal and also external surfaces.

In order to improve the weather resistance of mud mortar on the outside walls which are exposed to rain, the surface is sprayed or painted with bituminous material ( Sometimes foe providing weatherproof properties, the bituminous material is fixed during the preparation stage).

  6.4.2 Cement Clay Mortar  

Here clay is introduced as an effective finely ground additive in quantities ensuring a cement clay proportion of not over 1:1.

The addition of clay improves the grain composition, the water-retaining ability, and the workability of mortar and also increases the density of mortar.

This type of mortar has better covering power and can be used in thin layers.

Uses:

It is used for masonry joints and plastering.

  6.4.3 Lightweight and heavy mortars  

Lightweight mortar

These are prepared from light porous sands from pumice and another fine aggregate. They are also prepared by mixing wood powder, wood shaving, or sawdust with cement mortar or lime mortar.

In such mortars, the fiber of jute coir and hair, cut into pieces of suitable size, or asbestos fibers can also be used.

These mortars have a bulk density of less than 15 KN/m3.

Uses:

They are used in cases where a reduction in the conductivity of structure is desirable and also structure is to be kept lightly loaded.

Heavy Mortar

These are prepared from heavy quartz or other sands.

They have a bulk density of 15KN/m3.

Uses:

They are used in load-bearing constructions.

  6.4.4 Decorative Mortars  

These mortars are obtained by using:

Color cement or pigments, and the fine aggregate of appropriate color, texture, and surface.

Uses

They are used to impart a pleasant outer appearance to the surfaces of structures.

  6.4.5 Air-entrained (Plasticized) mortars  

The workability of lean cement-sand mortar can be changed by entraining air in it (air serves as a plasticizer producing minute air bubbles which help inflow characteristics and workability).

The air bubbles rise the volume of the binder paste and support to fill the voids in the sand.

The air-entraining also makes the mortar lightweight and better heat and sound insulator.

  6.4.6 Gypsum Mortars  

These mortars are prepared from gypsum binding materials such as building gypsum and anhydrite binding material.

  6.4.7 Fire-resistant mortar  

It is prepared by adding luminous cement to a finely crushed powder of fire brick (Usual proportion being one part of luminous cement to two parts of powder of fire bricks).

Uses:

This mortar, being fire-resistant, is used with fire bricks for lining furnaces, fireplaces, ovens, etc.

  6.4.8 Packing Mortar  

For packing oils, special, mortars are required which should possess the following properties.

(i) Water resistance

(ii) High homogeneity

(iii) Predetermined setting time

(iv) Resistance to subsoil water pressure

(v) Ability to form solid water-proof plugs in cracks and voids of rocks.

The composition of this type of mortar depends on hydrogeologic conditions, type of timbering, and packing methods.

The varieties of packing mortars are:

Cement-sand, cement-loam, and cement-sand-loam packing mortars.

  6.4.9 Sound Absorbing Mortar  

These mortars may have binding materials such as cement, lime, gypsum, slag, etc.

The bulk density of such a mortar is 6 to 12 KN/m3.

Noise levels can be reduced by using sound-absorbing plaster formed with the help of sound-absorbing mortar.

  6.4.10 X-ray Shielding Mortar  

The aggregates for this type of mortar are obtained from heavy rock. In order to enhance the protective property of such a mortar, bed suitable admixtures are added.

It has a bulk density of 22 KN/m3.

Uses:

It is used for providing the plastering coat to walls and ceiling of X-ray cabinets.

  7. Precautions in the Use of Mortars  

1. All mortars prepared for masonry works should be used as soon as possible with the maximum Limits given below.

2. Only that much quantity of cement mortar should be prepared at a time which is likely to be used before the initial setting of time of cement used in mortars.

3. All lime mortars after grinding, should be kept damp by covering them with wet sacks. They shall never be allowed to go dry.

4. Partly set and dried mortars should not be retempered and used.

5. The bricks or stones should be thoroughly soaked in water till all the air bubbles cease. Otherwise, they might absorb moisture from mortar and prevent it from setting.

6. The mortar should be used as stiff as possible consistent with workability since excess water weakens the mortar. Grout (a thin liquid mortar) should only be used in joints that cannot be filled by the mortar of proper consistency.

7. In the case of masonry work with lime mortars, work after every height of 1.5m or less should be allowed to be set for at least 2 days before starting further construction over it.

8. After the work has been laid, it should be kept wet for a week or so to prevent the rapid drying of the mortar particularly in hot weather.

  8. Test For Mortars  

In order to test the quantity of mortar, the following tests are done:

a. Adhesiveness to building units test

b. Crushing strength test

c. Tensile strength test

d. Setting time test

  a. Adhesiveness to building units test   

This test is conducted as follows:

i. Two bricks of size 190mm x 90mm x 90mm are jointed together and cemented with the mortar under test. This gives a common central sectional horizontal joint surface area of 90mm x 90mm forming a cross with 50 mm free projections on either side of both the bricks.

ii. The upper brick is tilted from an overhead support and a board is hung from the finish of the lower brick.

iii. The board is loaded till the joint between the brick fails.

The ultimate adhesive strength of mortar is obtained by dividing the maximum load by area of contact.

  b. Crushing Strength Test  

The following steps are utilized to run the test:

i. The brickwork is carried out with the mortar to be tested.

ii. A sample of this brickwork is taken and it is gradually loaded in a compression testing machine till the failure occurs due to crushing.

iii. The ultimate load causing failure divided by the cross-sectional area gives the ultimate crushing strength of the mortar per sq mm.

  c. Tensile Strength Test  

This test is conducted as follows:

i. A briquette made with water is tested in a testing machine.

ii. The total applied tensile load at which the briquette at the place of its breaking, gives the ultimate tensile strength of the mortar per sq mm.

  d. Setting Time Test  

This test is preferred as follows:

Mortar is tested in Vicat apparatus for the set time.

Depending on the type of lime mortar, the test may be conducted after one day, three days, or one week.

In the case of cement mortar, this test is conducted within 3 to 5 minutes of adding water.

   9. Selection  of Mortars for Different Engineering Works  

The various engineering works along with a recommended mix of different mortars are given in Table.