Introduction
Mortar is a vital construction material, primarily used to join bricks, stones, and blocks in masonry work. It can be defined as a paste formed by adding water to a mixture of fine aggregates, such as sand, and a binding material like clay, gypsum, lime, cement, or their combinations. This paste not only binds structural units together but also provides strength and durability.
Historically, mortar has played a critical role in architectural marvels. The pyramids of Egypt, for instance, were constructed using clay-gypsum, gypsum-lime, and lime mortars. In India, lime mortar was extensively used in iconic structures such as the Taj Mahal and ancient forts. Over time, advancements in mortar technology led to the development of hydraulic lime, known for its superior water-resistant properties, by burning limestone mixed with clay. This innovation eventually gave rise to Roman cement. By 1824, Portland cement, which is now regarded as the strongest binding material for mortars, revolutionized construction practices.
Mortar compositions are typically determined either by the volume or weight of the materials used in 1 m³ of mortar or by the ratio of the ingredients. For simple mortars, consisting of a single type of binding material like cement, the proportion is denoted as 1:4, signifying one part binding material to four parts sand by weight or volume. For combined mortars with multiple binding materials or mineral admixtures, the proportions are expressed with three figures, e.g., 1:0.4:5 (cement: clay: sand).
Mortar serves various essential purposes, including:
- Brick and Stone Masonry – It is applied in vertical joints and layered as a bed for binding successive masonry units together.
- Plastering and Pointing – Used to coat walls and seal joints, it enhances durability by protecting against weathering while also improving aesthetics.
- Concrete Matrix – Mortar acts as a binding medium within concrete to ensure structural integrity.
Function of Mortar
A quality mortar must fulfill the following essential functions:
Binding Layer
Mortar acts as a binding agent, cementing two courses of masonry to ensure structural integrity.Strength and Stability
By providing strong adhesion between masonry units, it enhances the overall strength and stability of the structure.Joint Filling
In brick or stone masonry, mortar fills gaps, joints, and holes, often in the form of a thin liquid grout, creating a uniform layer.Structural Homogeneity
Mortar integrates individual masonry units into a cohesive mass, enabling the structure to withstand dead loads, live loads, and forces caused by natural phenomena like earthquakes and winds.Pointing and Rigidity
When used for pointing, mortar ensures rigidity and offers protection against environmental wear, extending the durability of the structure.Plastering
Mortar provides a smooth and uniform surface when applied as plaster, enhancing both the functionality and aesthetics of the structure.Base for Finishing
It creates an ideal surface for finishing materials such as whitewashing, color washing, or painting.Concrete Formation
In concrete, mortar binds the aggregates together, forming a solid, homogeneous mass capable of bearing significant loads.
Properties of Good Mortar
A good mortar possesses a combination of physical and chemical properties that ensure its effectiveness in construction and its compatibility with building materials. These properties are critical in both its fresh (green) and hardened states.
Key Properties of Fresh Mortar
Workability
Fresh mortar should be easy to mix, spread, and apply uniformly to surfaces or masonry joints with minimal effort and cost.Consistency (Mobility and Placeability)
The mortar must have appropriate mobility to flow and spread easily, ensuring thin, uniform application. Placeability refers to the ease of applying mortar without segregation or excessive effort.Water Retention
Mortar should retain sufficient water during application and hardening to avoid premature drying, which can weaken bonds and reduce strength. Low water retention can lead to defects such as cracking or incomplete hardening.Resistance to Stratification
The mix should remain homogeneous during transportation and handling, without separating into layers.Setting Time
Mortar should set quickly enough for construction projects requiring a fast turnaround but not so rapidly that it affects proper application.
Key Properties of Hardened Mortar
Strength
The mortar must achieve adequate compressive and tensile strength, depending on its application. This depends on factors such as the water-cement ratio, quality of binding material, and sand properties. For instance:- Using fine sand reduces mortar strength and density compared to coarse sand.
- Increasing the water content beyond what is needed for a workable mix reduces strength.
Bonding Ability
Mortar should develop a strong adhesive bond with building units like bricks, stones, or blocks, ensuring structural stability and uniformity.Durability
It should resist weathering, erosion, and chemical reactions with surrounding materials, maintaining its integrity over time.Resistance to Cracking
A good mortar minimizes the occurrence of shrinkage cracks, ensuring the long-term stability of the structure. Excessively strong cement mortars can lead to cracking, while excessively weak mortars reduce durability.Compatibility with Building Units
Mortar should not react adversely with building materials in contact, such as bricks or stones, ensuring their durability is not compromised.Frost Resistance
Mortar should form non-porous joints to resist frost damage, especially in colder climates.Plasticity and Flexibility
Mortars made with additives, such as lime or plasticizers, exhibit better flexibility, which aids in autogenous healing of small cracks.Economy
Mortar should be cost-effective, balancing strength, workability, and durability with affordability.
Special Considerations for Mortar Composition
- Cement-Sand Ratio: For stronger bonds, mortars like 1:4 (cement) are used, while very weak mixes (e.g., 1:10) may lead to porous joints and reduced performance.
- Cement-Lime Mixes: Adding hydrated lime to cement mortar enhances flexibility, workability, and self-healing properties of cracks.
- Impurities: Even small amounts of mica or similar materials can significantly reduce tensile and compressive strength.
Selection of Mortar for Different Purposes
Selection of mortar for particular civil engineering purposes is tabulated below:
| Sl. No. | Nature of Civil Engineering Work | Cement Mortar | Lime Mortar |
|---|---|---|---|
| 1 | General brickwork (using stock bricks) | 1:6 | – |
| 2 | Brickwork below ground level | 1:3 to 1:4 | – |
| 3 | Brickwork with thin joints | – | 1:3 (fat lime) |
| 4 | Reinforced brickwork | 1:3 | – |
| 5 | Damp-proof course | 1:2 | – |
| 6 | Internal walls and surfaces of less importance | – | 1:3 (ashes or cinders instead of sand) |
| 7 | Partition walls and parapet walls | 1:3 | 1:1 (moderately hydraulic lime) |
| 8 | Stone masonry | 1:6 | 1:2 (eminently or mod. hyd. lime) |
| 9 | Arch work | 1:3 | – |
| 10 | Pointing work | 1:1 to 1:2 | – |
| 11 | Brickwork plaster (external) | 1:5 to 1:6 | – |
| 12 | Brickwork plaster (internal) | 1:5 to 1:6 | – |
| 13 | Ceiling plaster | 1:3 to 1:4 | – |
| 14 | RCC tank and retaining walls | 1:1 | – |
| 15 | RCC works—beams, slabs, and columns | 1:2 | – |
| 16 | Highly stressed structures | 1:1 | – |
| 17 | Laying fire bricks | 1:2 (aluminous cement and crushed fire bricks) | – |
