What Is Mortar? | Functions Of Mortar | Types Of Mortars | LCETED

WHAT IS MORTAR?

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

 

FUNCTIONS OF MORTAR IN BUILDING WORKS

The following are the major functions of mortar:

1. To bind building materials such as bricks and stones into a solid mass.

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

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

4. To form joints of pipes.

5. To improve the general appearance of a structure.

6. To prepare moulds for coping, corbels, cornice, etc.

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

8. To distribute uniformly the super-incumbent weight from the upper layer to the lower layer of bricks or stones.

9. To hide the open joints of brickwork and stonework.

10. To fill up the cracks detected in the structure during the maintenance process, etc.

 

TYPES OF MORTARS AND THEIR PREPARATION

The following major types of mortars are recognized based on the type of the cementing material used in its preparation: lime mortars, cement mortars and gauged mortars are the most common types.

 

LIME MORTARS

Lime mortars are defined as mixes of lime with fine sand and /or pozzolanic materials like surkhi, pumice, ash and cinder in water.

Preparation

The following are the main stages in the preparation of lime mortars: selection of the raw materials, proportioning of the raw materials and mixing of the raw materials.

 

Selection of the raw materials

Lime, sand, surkhi, pumice, ash and cinder form the major raw materials commonly used in the preparation of lime mortars.

Lime: All types of lime can be used for making mortars, although all lime mortars cannot be used for different situations. The main types of lime, namely the fat lime and hydraulic lime, yield mortars of differ- ent qualities, the former being suitable only for limited purposes like jointing and light loaded masonry work whereas the latter being more useful in other situations as well. It is essential that the lime to be used be free from impurities like silica, iron oxide and, especially, gypsum.

Sand: The sand for making a strong and durable mortar must be of good quality, i.e., it must be clean and sharp grained. The recommended fineness modulus of sand for mortar is 2.00–3.00.

Sand is added to the lime for at least two purposes:

i. To avoid shrinkage and cracking of mortar on hardening because lime used alone shrinks and cracks on hardening.

ii. To increase the bulk of the mortar; this is essential to make it more economical.

When used inappropriately proportions, sand facilitates the hardening of the lime paste by yielding a porous structure and allowing the atmospheric carbon dioxide access to the inner lime.

Pozzolanic materials: These include a variety of lightweight materials like surkhi, cinder, pumice and ash, which have been found to increase the strength of mortars when used in place of sand or even along with sand.

Water: In general, water suitable for drinking purposes can be used in the preparation of mortar. The water must be essentially free from alkalies, acids and organic residues.

Sometimes, cement is also added in small quantities to lime mortars to enhance the strength and setting properties of mortar. Depending on the type of aggregate used, lime mortars are further distinguished into lime–sand mortars, lime–surkhi mortars, lime–sand–surkhi mortars and so on.

Proportioning of the raw materials

The main aim of proportioning is to fix such ratios of the raw materials that will result in a mortar of desired quality, i.e., required strength, durability and finish. Since the mortars used in different situations in construction are not subjected to the same forces and conditions, no single rule for proportioning a mortar can be framed. The widely recommended and commonly adopted proportions for different conditions are summarized in Table.

Mixing of the raw materials

It is of paramount importance that the ingredients of mortar be mixed as thoroughly as possible. Two common methods adopted are manual mixing and mortar mill mixing.

Manual mixing: This is done on a watertight platform made of masonry or in a tank of suitable dimensions. A spade is the main tool required. Measured volumes of slaked lime and the aggregate are placed on the platform or on the tank. These are first mixed in dry state by giving turns with the help of the spade. Water is added gradually and mixing continued simultaneously until a mortar of uniform consistency is obtained.

Mortar mill mixing: A mortar mill is a mechanical device for grinding the mortar ingredients in the presence of water and is run either by animals or by power. In the traditional method, animal-driven mortar mills are used.

Table: Various Lime Mortar Compositions for Different Construction Works 

Situation	Mortar composition recommended	Remarks
(A) Foundations
1. Foundation concrete in dry subgrade with water level below 2.4 m of the foundation level	Any one of the following mixes: 1 lime, 2 sand 1 lime, 1 sand, 1 surkhi 1 lime, 2 surkhi 1 cement, 3 lime, 12 sand	For moist subgrade where water table is within 2.4 m of the foundation level, only cement–sand mortar of 1:3 mix should be used for all foundation work
2. Foundation masonry with loading less than 4 tons/ ft2	1 lime, 2 sand 1 lime, 1 sand, 1 surkhi 1 lime, 2 surkhi 1 cement, 3 lime, 12 sand
3. Foundation masonry in heavy and medium loading in dry subgrade	1 cement, 1 lime, 6 sand
(B) Superstructures
4. Load-bearing walls with brick masonry.	1 cement, 1 lime
i) Light loading	1 lime, 1 sand, 1 surkhi 1 lime, 1 sand, 1 cinder	Loading less than 4 tons/ft2
ii) Medium loading	1 cement, 3 lime, 12 sand	Loading between 4 and 6 tons/ft2
iii) Medium loading	1 cement, 2 lime, 9 sand to 1 cement, 1 lime, 6 sand	Loading more than 8 tons/ft2
5. Non-load bearing partition wall with concrete slabs or hollow blocks	1 lime, 3 sand
(C) Plasters
6. i) External plasters below damp proof course	1 cement, 1 lime, 6 sand
ii) External plasters on all walls	1 cement, 3 lime, 9 sand
iii) Internal plasters on all walls	All mixes are suitable
General-purpose mortar	1 cement, 1 lime, 6 sand	Suitable for most of masonry work and plasterwork

Hardening of lime mortars

Mortars made of fat lime or hydrated lime hardens by a the simple process of crystallization preceded by a loss of water due to evaporation. The lime takes up the carbon dioxide from the atmosphere and forms a crystal mass of calcium carbonate that is quite hard and responsible for the strength of the mortar.

 

Cement mortar

The most common type of mortars of the present-day generally consists of mixtures of cement with sand in the presence of a suitable quantity of water. In all situations where a strong, durable and resistant mortar is desired, the cement mortar becomes indispensable.

 

Table: Proportions Recommended for Cement–Sand Mortar

Type and situation of the work	Recommended proportions
1. For ordinary masonry work with bricks or stones as structural units.	1 cement, 3 sand to 1 cement, 6 sand
2. For reinforced brickwork and for all work in moist situations.	1 cement, 2 sand to 1 cement, 3 sand

       

 

Preparation

The preparation of cement mortar involves the same stages as lime mortar, namely selection of raw materials, their proportioning and thorough mixing. Among the materials, cement, sand and water are the essential ingredients. The cement used for preparing mortar must be in perfectly undamaged and undeteriorated condition. Sand in addition to possessing its usual properties must be free from impurities like oxides of iron, clay and mica and must be thoroughly cleaned before being used for good quality mortar. Generally, no other aggregate can replace sand completely in a cement mortar, although nowadays several other alternatives are being used. The functions of sand in the mortar include increasing the bulk of the mortar, providing resistance against shrinkage and cracking on the setting of cement and also making the mortar more strong.

 

Proportioning

Only very carefully proportioned volumes of the ingredients are to be used in the preparation of cement mortars. The proportioning is usually by volume. Different mix ratios are specified for different works, the most commonly used are shown in Table

 

Mixing of the ingredients

For small jobs, the manual mixing of mortar is commonly practised and for large-scale construction where larger batches of the mortar are required the mortar mill mixing is indispensable.

 

Gauged mortars

These mortars contain both cement and lime as the essential ingredients besides the fine aggregate, which is generally sand. The addition of cement to lime mortar improves considerably the quality of mortar, especially in respect of its workability, time of the setting, hardening and ultimate strength.

Since the gauged mortars consist of cement as one of the essential ingredients, it is important that such mortars are consumed within 2 hours of their preparation to avoid any deterioration in quality.

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