Mortar

The term mortar is used to indicate a paste prepared by adding required quantity of water to a mixture of binding material like cement or lime and fine aggregate like sand.

The binding material used may also be referred as matrix and the aggregate used may also be referred as adulterant. Hence, mortar is a mixture of matrix & adulterant. The matrix binds the particles of the adulterant and as such, the durability, quality and strength of mortar will mainly depend on the quantity and quality of the matrix. The combined effect of the two components of mortar is that the mass is able to bind the bricks or stones firmly.

Following are the properties of a good mortar:

1.      To carry out pointing and plasterwork on exposed surfaces of masonry.

2.      To form an even and soft bedding layer for building units.\

3.      It should be capable of resisting penetration of rain water.

4.      To form joints of pipes.

5.      To improve the general appearance of a structure.

6.      It should be cheap, durable.

7.      It should be easily workable.

8.      It should not affect the durability of materials with which it comes into contact.

9.      To serve as a matrix or cavity to hold the coarse aggregates, etc.

10.  It should set quickly so that speed in construction may be achieved.

11.  The joints formed by mortar should not develop cracks and they should be able to

 

According to the kind of binding material, the mortars are classified into the following five categories:

(i) Lime mortar

(ii) Surkhi mortar

(iii) Cement mortar

(iv) Gauged mortar

(v) Gypsum mortar.

 

(i) Lime Mortar:

In this type of mortar, the lime is used as binding material. The lime may be fat lime or hydraulic lime.

The fat lime shrinks to a great extent and hence it requires about 2 to 3 times its volume of sand. The lime should be slaked before use. This mortar is unsuitable for water-logged areas or in damp situations. For hydraulic lime, the proportion of lime to sand by volume is about 1:2 or so. This mortar should be consumed within one hour after mixing.

It possesses more strength and can be used in damp situations. The lime mortar has a high plasticity and it can be placed easily. It possesses good cohesiveness with other surfaces and shrinks very little. It is sufficiently durable, but it hardens slowly. It is generally used for lightly loaded above-ground parts of buildings.

(ii) Surkhi Mortar:

This type of mortar is prepared by using fully surkhi instead of sand or by replacing half of sand in case of fat lime mortar. The powder of surkhi should be fine enough to pass BIS No. 9 sieve and the residue should not be more than 10% by weight.

The surkhi mortar is used for ordinary masonry work of all kinds in foundation and superstructure. But it cannot be used for plastering or pointing since surkhi is likely to disintegrate after some time.

(iii) Cement Mortar:

In this type of mortar, the cement is used as binding material. Depending upon the strength required and importance of work, the proportion of cement to sand by volume varies from 1:2 to 1:6 or more. It should be noted that surkhi and cinder are not chemically inert substances and hence they cannot be used as adulterants with matrix as cement.

Thus the sand only can be used to form cement mortar. The proportion of cement with respect to sand should be determined with due regard to the specified durability and working conditions. The cement mortar is used where a mortar of high strength and water-resisting properties is required such as underground constructions, water saturated soils, etc.

(iv) Gauged Mortar:

To improve the quality of lime mortar and to achieve early strength, the cement is sometimes added to it. This process is known as the ganging. It makes lime mortar economical, strong and dense.

The usual proportion of cement to lime by volume is about 1:6 to 1:8. It is also known as the composite mortar or lime-cement mortar and it can also be formed by the combination of cement and clay. This mortar may be used for bedding and for thick brick walls.

(v) Gypsum Mortar:

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