Mortar - Civil Engineering SSC JE (Technical) - Civil Engineering (CE)

Mortar is the paste obtained by adding water to a mixture of a binding material (such as cement or lime) and an aggregate (such as sand) in a definite proportion. The binding material is often referred to as the matrix and the aggregate as the adulterant. Hence, mortar is a mixture of matrix and adulterant.

Constituents of Mortar

• Matrix (Binding material) - cement, lime, gypsum or a combination of these, which provides cohesion and strength after setting.
• Adulterant (Aggregate) - usually sand, which provides bulk, dimensional stability and affects workability and shrinkage.
• Water - activates the binder and determines workability; the amount must be controlled.
• Admixtures (optional) - plasticisers, retarders, water-proofing agents, pigments or air entrainers used to modify properties for special requirements.

Qualities of Good Sand for Mortar

• It should be free from organic and vegetative matter, which can affect setting and durability.
• It should be free from harmful chemical salts (for example excessive chlorides or sulfates) that can damage metal reinforcement or reduce durability.
• It should be chemically inert with respect to the chosen binder (should not react adversely with cement, lime, etc.).
• It should be well graded so that particles of different sizes fill voids effectively, producing a compact mortar with good strength and workability.
• Particle shape and surface texture should be suitable: angular particles generally give better bond but require more water; rounded particles give better workability but less mechanical interlock.
• Excessive silt or clay in sand should be avoided because it reduces bond and strength; the sand should be clean.

Function of Sand in Mortar

• To provide bulk - sand increases the volume of mortar and reduces cost because the aggregate is less expensive than the binder.
• To influence strength - the proportion of sand relative to binder affects the mortar strength; increasing sand content generally reduces strength and vice versa.
• To increase surface area and bond - sand divides the binder paste into many layers and increases the contact surface available for adhesion between masonry units and plaster surface.
• To reduce shrinkage - sand restrains excessive shrinkage of the binder paste during setting and drying, thereby reducing cracks.

Qualities of Good Mortar

• The mortar should not adversely affect the durability of the construction unit with which it is in contact.
• It should be capable of developing the required design stresses and transferring stresses between masonry units.
• It should resist penetration of water to the extent required for its use (weather resistance, damp-resistance for DPC, etc.).
• It should be economical and durable.
• It should possess adequate workability - easily spread and finished without sticky or harsh behaviour; it should not set too early during handling.
• It should have compatible thermal and moisture movement characteristics with the masonry units to minimise cracking and debonding.

Common Types of Mortar and Their Uses

Mortars are classified by the binder used or by intended application. Common binders are cement, lime, gypsum or a combination of cement and lime. Typical types are:

• Cement mortar - high early strength, suitable for pointing, bed joints, DPC, and areas requiring durability.
• Lime mortar - more workable and flexible, used in restoration, historic masonry and where greater plasticity is required.
• Cement-lime mortar - combines strength of cement with improved workability and water retention of lime.
• Gauged mortar - lime or gypsum mortar mixed with small quantity of cement for improved strength.
• Special mortars - including waterproofing, fire-resistant, or decorative mortars made with admixtures and special binders.

Recommended Mortar Types for Typical Construction Activities

Proportioning and Mixing of Mortar

• Proportions of binder to sand are usually given by volume (for example, 1:3 means one part binder to three parts sand).
• Well-defined proportions ensure repeatable strength and workability; field practice commonly measures by bucket or box volume.
• Mixing must ensure uniform distribution of binder and aggregate; dry mixing followed by gradual addition of water gives better control of consistency.
• For lime mortars, adequate slaking of quicklime and proper gauging time are important for consistent properties.

Preparation, Laying and Curing

• Prepare mortar shortly before use to avoid loss of workability; avoid re-tempering (adding water to stiffened mortar) unless specifically allowed.
• Apply mortar on clean, damp surfaces for good bond; remove loose debris and dust from masonry units.
• Use appropriate joint thickness as per specification; too thin or too thick joints affect performance.
• Curing is necessary for cement-based mortars to obtain full strength and durability. Keep the mortar moist for the initial curing period as specified.

Quality Control and Tests

• Check sand for cleanliness, grading and absence of deleterious materials.
• Control mix proportions and record batches.
• Workability can be judged by consistency and ease of application; adjust water carefully.
• Where required, carry out laboratory tests on hardened mortar specimens to measure compressive strength, water absorption and bond strength.

Applications of Mortar

• Binding masonry units (brick, block, stone) in walls and partitions.
• Plastering and rendering of walls and ceilings.
• Pointing of joints to protect masonry from weather and to provide a finished appearance.
• Forming bedding for paving, tiles and flooring in certain cases.

Precautions and Best Practices

• Use clean water and clean sand to avoid contamination.
• Store cement and lime in dry conditions to prevent premature hydration.
• Do not add excessive water to improve workability; use plasticisers where necessary.
• Follow specified mix proportions and workmanship for durable construction.
• Protect new mortar from rapid drying, frost and rain until adequately cured.

Summary

Mortar is a carefully proportioned mixture of a binder (matrix), aggregate (adulterant) and water used to bind masonry units and for finishes. The choice of sand, correct proportions, proper mixing, application and curing determine the performance and durability of the mortar. Common mortars include cement, lime and cement-lime mortars; their selection depends on the required strength, workability and exposure conditions.