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.
14.2.1 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.
14.2.1.1 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 different 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:
- To avoid shrinkage and cracking of mortar on hardening, because lime used alone shrinks and cracks on hardening.
- To increase the bulk of the mortar; this is essential to make it more economical.
When used in appropriate 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 the 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 14.1.
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 14.1 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: | 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 |
| 1 lime, 2 sand 1 lime, 1 sand, 1 surkhi 1 lime, 2 surkhi 1 cement, 3 lime, 12 sand | ||
| 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) Heavy 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, 2 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 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.
14.2.2 Cement mortar
The most common type of mortars of the present day generally consists of mixtures of cement with sand in the presence of suitable quantity of water. In all situations where a strong, durable and resistant mortar is desired, the cement mortar becomes indispensable.
Table 14.2 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 |
14.2.2.1 Preparation
The preparation of cement mortar involves the same stages as of lime mortar, namely selection of raw materials, their proportioning and thorough mixing. Among the materials, cement, sand and water are the essential ingredients. 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.
14.2.2.2 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 14.2.
14.2.2.3 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.
14.2.3 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 setting, hardening and ultimate strength.
Since the gauged mortars consist of cement as one of the essential ingredients, it is important that such mortars be consumed within 2 hours of their preparation to avoid any deterioration in quality.