7

MORTAR

7.1 INTRODUCTION

The cementing materials discussed in the previous chapters can not be used directly as a construction material unless they are combined with some other inert material like aggregates. Thus, a combined material formed by mixing intimately a binding material (cement or lime) with a fine aggregate (sand or surkhi) in a certain proportion and with an adequate quantity of water is called mortar. Here lime or cement enables a proper bond to be developed among the particles, and sand adds mechanical strength to the mortar.

The properties of binding materials, lime and cement, have been discussed at length in the previous chapters. Hence, it is necessary to know more about the fine aggregate and water to be used to get the best mortar.

Properties of mortar and their kinds along with the quality of sand and water to be added are discussed in the following sections.

7.2 QUALITIES OF SAND FOR MORTAR

Sand particles are small soil grains of silica. As per the Indian Soil Classification, particles of diameter greater than 0.075 mm and less that 4.75 mm are categorized as sand.

Sand is naturally available and may be classified as (i) pit sand, (ii) river sand and (iii) sea sand. Sea sand is not suitable for mortar.

For the purposes of mortar, sand is classified as fine, coarse and gravelly sand. Fine sand is one that passes through a screen with clear openings of 1.59 mm, coarse sand passes through a screen with clear openings of 3.18 mm and gravelly sand passes through a screen with clear openings of 7.62 mm. Fine sand is used for plastering, and coarse sand is used for masonry work.

The properties of sand required for mortar are as follows:

1. It should be free from any organic matter.
2. It should be chemically inert.
3. It should be well graded.
4. The fineness modulus of sand should be between 2 and 3.
5. It should be sharp, angular, coarse and clean.

7.3 QUALITIES OF WATER FOR MORTAR

Water is used for construction purposes of Civil Engineering Works such as the construction of earth structures, highway construction, mortar and concrete works, etc. Although water is an important constituent of construction materials, it does not receive the attention due to it particularly in the construction of earth structures and highway construction.

However, due importance is given in case of preparation of mortar and concrete. The general belief is that if it is suitable for drinking, then it is considered suitable for construction including mortar and concrete. The quality of water in connection with the preparation of mortar and concrete and for curing is further discussed.

As water helps to form cement gel, which is the governing factor for the strength of mortar or concrete, the quantity and quality of water are to be carefully decided.

The quality of mixing water may be determined based on any one of the following approaches:

1. If the water is obtained from an unknown source, the strength of the mortar or concrete made with questionable water is compared with similar mortar or concrete prepared with pure water.
2. If the pH value of water lies between 6 and 8, the water is assumed to be free from organic matter and declared as suitable.

The presence of the following impurities may affect the mortar if excess quantity is allowed:

1. Presence of suspended particles ≯ 0.02%
2. Presence of salts: chlorides ≯ 10,000 ppm

   sulphates ≯ 3,000 ppm

3. Presence of acids and alkalis: harmful
4. Presence of algae: harmful

The guidelines to assess the quality of water for concrete discussed in Section 8.4.3 and Table 8.1 are also applicable for mortar.

7.4 FUNCTIONS OF INGREDIENTS OF MORTAR

As discussed earlier, the ingredients of mortar are:

1. Binding or cementing materials, viz., cement or lime
2. Fine aggregates, viz., sand, surkhi, cinder, etc.
3. Water

7.4.1 Functions of Cementing Materials

The primary requirement of cementing materials is to provide strength and durability to the structure.

For cement mortar mostly Ordinary Portland Cement is used. However, for special types of works other cements may be used appropriate to the work. Cement performs the following functions when used in cement mortar:

1. Primarily, it fills the voids formed by fine aggregates and thereby makes the mortar impermeable.
2. It binds the fine aggregates into a solid mass with time on adding water to the mix.
3. It imparts strength after setting and hardening.

For lime mortar, different types of lime are used, viz., eminently hydraulic, semi-hydraulic and fat limes. For preparing lime mortar, the lime should be slaked before mixing it with the fine aggregate. However, the strength of lime mortar is less than that of cement mortar. Lime performs the following functions when used in lime mortar:

1. It binds the fine aggregates into a solid mass. However, it takes more time to set.
2. It imparts adequate strength to the lime mortar on setting and hardening.

7.4.2 Functions of Fine Aggregates

The fine aggregate mainly used is sand. The required properties of good sand were discussed earlier. Sand plays the following functions in mortar:

1. It reduces the shrinkage of the binding material and thereby prevents the formation of cracks.
2. It provides high resistance to the mortar to withstand compressive forces.
3. It divides the paste of the binding material and spreads to provide more surface contact area.
4. It makes the fat lime porous to assist in setting and hardening.
5. It provides any proportion of the mortar.

Surkhi is another form of fine aggregate, which is usually prepared by powdering slightly unburnt bricks. It should be free from admixtures of any foreign materials. Surkhi is generally used with sand. If used on its own, it causes the mortar to disintegrate. When used in combination with sand, it performs all the functions as that of sand. In addition, it provides strength and improves hydraulic properties of the mortar.

Sometimes ash or cinder is provided as fine aggregate in lime mortar. It provides strength and improves hydraulic properties. It acts as an adulterant and hence increases the volume.

7.4.3 Functions of Water

The quality of water required for mortar has been discussed earlier. Water plays a significant role in the formation of mortar. It helps in mixing, transporting and in proper laying of the mortar. As a matter of fact the quality and quantity of water contribute to the strength of the mortar. Water plays the following roles in mortar:

1. It wets the surface of the aggregates to provide better bonding.
2. It helps in the spreading of the cementing material over the surface of the fine aggregate.
3. By acting as a lubricant for the aggregate, the mortar is made workable.
4. It causes hydration of cement and lime because of which setting and hardening of cement mortar and lime mortar take place.

7.5 FUNCTIONS OF MORTAR

Quality mortar should satisfy the following functions:

1. It forms a binding or cementing layer between two courses of masonry in the structure.
2. Because of this binding property, it provides strength and stability to the structure.
3. In brick or stone masonry, it forms a uniform layer by filling open joints and holes as a grout in a thin liquid form.
4. It makes the masonry structure a homogeneous mass of structure, which can withstand both dead and live loads and loads due to natural reasons.
5. When used for pointing purposes, it provides high rigidity and protects the structure.
6. Provides a uniform smooth surface when used as a plaster.
7. It also forms a good surface for taking finishing materials like white/colour washing or painting.
8. It holds the aggregates together in concrete to form a homogeneous solid mass.

7.6 PROPERTIES OF MORTAR

In general, the properties required for a good mortar are as follows:

1. The workability of mortar should be easy.
2. Setting should be quick, particularly in constructions that have a faster turnaround time.
3. Should be capable of developing good adhesion with building units such as bricks, stones, etc.
4. Should be in a position to develop the designed stresses.
5. Should not react with the material on contact and affect its durability.
6. Cracks should not be formed on the plastered surface.
7. Joints should be intact and not peel off or crack, particularly in pointing.
8. Should have high durability.
9. Should be comparatively cheap.

7.7 TYPES OF MORTARS AND THEIR USES

Mortars using different binding materials have been used for various purposes. These are discussed below.

7.7.1 Lime Mortar

It is the mortar wherein fat lime or hydraulic lime is used as the binding material. The lime should be slaked before use. If fat lime is used, 2–3 times the volume of sand has to be used as it shrinks to a great extent. This lime mortar is not suitable for water-logged areas or in damp situations.

It is easy to handle and is highly plastic in nature. It has high cohesiveness and adheres to the surface and undergoes less shrinking. Although it is sufficiently durable, it hardens slowly. It is suitable for light-loaded structures and is used for construction purposes above the ground.

7.7.2 Surkhi Mortar

It is prepared using lime and surkhi instead of sand. In case of fat lime mortar only 50% needs to be replaced by surkhi. Surkhi should be powdered well.

Surkhi mortar is used for ordinary masonry work. It can not be used for pointing works and for plastering as the surkhi is likely to disintegrate.

7.7.3 Cement Mortar

It is obtained by using cement as the binding material. Cement mortars are assigned certain grades based on their minimum compressive strength at 28 days curing and mix proportion by volume. Some of the grades that are frequently used are given in Table 7.1.

The recommended cement mortar mix for different purposes are as follows:

1. MM 2 – For external works with protection and foundation masonry in very wet soil.
2. > MM 2 – For external walls without protection.
3. MM 3 – For foundation masonry in soil with saturated water and for masonry subjected to vibrations.
4. MM 5 to MM 7.5 – For plastering.
5. MM 7.5 – For floor finishes and damp-proof courses.

Cement mortar is used for the following purposes:

1. In brick and stone masonry.
2. For plastering of surfaces of structures like walls, beams, columns, etc.
3. For floor finishes.
4. As binding material for decorative and aesthetic works.
5. For pointing of stone or brick masonry works.
6. For irregular surfaces of stone masonry to distribute the load uniformly on the bed.
7. For underground construction purposes, water-saturated soils, etc.

7.7.4 Lime–Cement Mortar

This type of mortar is also called Gauged Mortar or Composite Mortar. It is the one wherein the quality of lime mortar is improved by adding cement to get early strength. This makes lime mortar economical, strong and dense. The general proportion of cement to lime by volume is about 1:6–1.8. This mortar is useful for purposes like bedding and for thick brick walls.

7.7.5 Mud Mortar

Cohesive clay is puddled or pugged with water till the required consistency is attained. In order to prevent shrinkage and cracks, certain fibrous materials are also added. All the ingredients are mixed thoroughly. The life of mud mortar varies from 5–15 years depending on its water-proofing treatment and weather conditions.

Mud mortar has been in use in ancient days and mostly in villages. This can be used for brickwork of ordinary buildings. It is also used for surface floors and plastering internal walls and to some extent external walls also. In order to protect the exposed wall to rain, the mud plaster surface may be sprayed or painted with bituminous material. Instead the bituminous material is mixed in the preparation stage itself so as to add weather-proof properties to the mud mortar.

7.7.6 Fire-resistant Mortar

It is obtained by adding aluminuous cement to the finely ground powder of fire bricks. This is fire-resistant mortar, which can be used with fire bricks for furnace lining, fire places, etc.

7.7.7 Light-weight Mortar

It is obtained by mixing materials like saw dust, wood powder, asbestos fibres, jute fibres, etc., to the lime mortar or cement mortar. This type of mortar is used for sound-proof and heat-proof construction.

7.7.8 Packing Mortar

It is a special type of mortar possessing properties of high homogeneity, water resistance, predetermined setting time and the ability to form solid and water-proof plugs. This property of the mortar is useful for plugging cracks or voids of rocks, resistance to subsoil water pressure, etc., in oil wells. Different kinds of packing mortars are cement–sand, cement–loam and cement–sand–loam.

7.7.9 Sound-absorbing Mortar

This type of mortar is formed by compositioning Portland Cement, lime, gypsum, slag, etc. The crushed aggregates are selected from light-weight porous materials such as pumice, cinders, etc.

7.7.10 X-Ray Shielding Mortar

It is a heavy type of mortar, which is obtained from crushed heavy rocks, and suitable admixtures are added to get the required property. A plastering coat of this mortar is applied to the walls and ceiling of X-ray cabinets.

7.8 PREPARATION OF MORTARS

Preparation of lime mortar and cement mortar have different procedures, which are discussed in subsequent paragraphs.

7.8.1 Preparation of Lime Mortar

Ordinary lime mortar and plaster are made by grinding lime with sand. There are three methods of preparing lime mortar.

In the first method, a dry mixture of lime and sand are placed in a trough or on a platform in the specified proportion. The required quantity of water is sprinkled over the mixture in stages and mixed each time after the water is sprinkled. Then the mixture is ground to a plastic mixture by pounding with heavy wooden hammers.

The second method is adopted when a large quantity of mortar is needed. A circular annular trough is made and the mixture is placed into it. The mixture is ground by rotating a wheel through the trough with the help of bullocks. This type of mortar production is called a bullock-driven mortar mill. The mixture is ground for about 200 revolutions to get a suitable mortar.

The third method is the modern method, which is used for preparing large quantities of lime mortar. Here, a machine-driven pug or mortar mill is used. For brickwork, the grinding is done for three minutes and longer for plaster for plasterwork, pointing and terrace works. While the construction work is in progress, the mortar is raked up continuously.

Ordinary lime mortar can be stored for a maximum period of 3 days after mixing. Hydraulic lime should be used within 2–4 hours after grinding. The lime mortar should be kept moist by covering it with a wet sack cloth and frequently moistened.

7.8.2 Preparation of Cement Mortar

Cement mortar is mixed by hand for small construction purposes, while large quantities of cement mortar may be mixed by a machine.

1. Hand Mixing

Sand is measured in boxes, and the specified quantity of sand is spread first. Cement is measured by the weight of bags or by considering the volume of 0.035 m³. Cement is spread over the sand. Then they are dry mixed by turning over and over and backward and forward using a shovel or a spade till the mixture attains a uniform colour.

The quantity of mix that has to be used within 30 minutes is separated and mixed again and formed into a heap. After making a small depression at the top, water is added to the centre of the depression and mixed well. Adequate water must be added, as excess water may draw the cement out. The water required is roughly about 70% of the weight of cement for 1:3 mortar. The whole mass is mixed thoroughly for 10–15 minutes by hand mixing.

2. Machine Mixing

Machine mixing of cement mortar is done in a concrete mixer. The sand and then the cement are put into the mixer. Initially, it is dry mixed and then water is gradually added. Wet mixing is then continued for more than a minute till the mixture is brought to a plastic consistency.

Cement mortar has to be used before it begins to set. The entire cement mortar has to be used within a maximum time of 2 hours. While using the cement mortar, water may be added to the mortar when the mortar gets stiffened. This process may be continued to restore the necessary consistency. Such retempering may be allowed only up to a period of 2 hours, beyond which the mortar should not be used.

7.8.3 Preparation of Lime–Cement Mortar

In this case, it should be ensured that the lime is slaked well. Then the lime is mixed with sand and ground well to form an intimate mixture. Water is then added and mixed well to form a regular lime mortar. The lime mortar is transferred to a mechanical mixer to which the required cement is added and mixed with additional water. Mixing is continued for 3–5 minutes to get the required consistency. This mortar can be used similarly as that of cement mortar within 30 minutes.

7.9 SELECTION OF MORTAR FOR DIFFERENT PURPOSES

Selection of mortar for particular civil engineering purposes are tabulated in Table 7.2.

Table 7.2 Selection of mortar for different purposes

1. Mortar is a combined material formed by intimately mixing a binding material (cement or lime) with a fine aggregate (sand or surkhi).
2. The primary requirement of cementing materials is to provide strength and durability to the structure.
3. Sand reduces the shrinkage of the binding material, provides high resistance, divides and spreads the paste of binding material over a larger surface area.
4. Water helps in mixing, transporting and in proper laying of the mortar.
5. If lime mortar, fat lime or hydraulic lime is used as the binding material. The fine aggregate used is sand. If surkhhi is used, it is called surkhi mortar.
6. Cement mortar is obtained by using the binding materials, cement and sand. There are four grades of cement mortar (CM), viz., MM 1.5, MM 3.0, MM 5.0 and MM 7.5.
7. Lime–cement mortar is one where the quality of lime mortar is improved by adding cement to obtain early strength. This is also called gauged mortar or composite mortar.
8. Fire-resistant mortar is obtained by adding aluminous cement to the finely ground powder of fire-bricks.
9. Light-weight mortar is obtained by mixing materials like saw dust, wood powder, asbestos fibres, jute fibres, etc., to lime mortar or cement mortar.
10. Packing mortar is a special type of mortar possessing properties of high homogeneity, water resistance, predetermined setting time and the ability to form solid and water-proof plugs.
11. Sound-absorbing mortar is formed by compositioning Portland Cement, lime, gypsum, slag, etc. The crushed aggregates are selected from light-weight porous material such as pumice, cinders, etc.
12. X-ray shielding mortar is a heavy type of mortar, which is obtained from crushed heavy rocks, and suitable admixtures are added to get the required property.

1. What is mortar? What are the functions of its constituents?
2. What are the properties of sand that are used in mortar?
3. Explain the importance of the effect of the quality of water on the strength of mortar.
4. Discuss the function of mortar.
5. Explain the properties of good mortar.
6. What are the properties of an ideal mortar? Explain.
7. How are mortars classified?
8. How is lime mortar prepared?
9. What are the mixing methods of cement mortar?
10. Discuss different types of mortar selected for a variety of civil engineering works.
11. What type of cement mortar would you recommend to use for external and internal walls of brick? Why?
12. What would be your recommendation of mortar proportion for the following works.

    (i) Masonry work in super structure

    (ii) Pointing works of stone masonry

    (iii) Plastering surface for paint works.