Chapter 2: Mortar and Lime
2.1 Bulking of Sand
The term mortar is used to indicate a paste prepared by addition of required quantity of water to a mixture of binding material like cement or lime and fine aggregate like sand. Mortar is similar to concrete but does not contain coarse aggregate.
The presence of moisture in sand increases the volume of sand. Moisture creates a thin film of water around the sand particles. For moisture content of about 5 to 8%, increase of volume may be as much as 20 to 40%. This phenomena is known as the bulking of sand.
Fig. 2.1 — Bulking of Sand chart. Fine sand shows maximum bulking (~35–40% increase at 5–7% moisture). Coarse sand peaks ~15%. All curves converge near zero at full saturation. Bulking is more with fine sand and less with coarse sand.
2.2 Mortars
Mortars are classified on the basis of: (i) Bulk density, (ii) Type of binding materials, (iii) Nature of application, (iv) Special mortars.
- Heavy Mortars: bulk density ≥ 15 kN/m³; prepared from heavy quartz or other sands
- Light weight mortars: bulk density < 15 kN/m³; prepared from light porous sands, pumice and other fine aggregates
2.2.1 Type of Binding Material
Fig. 2.2 — Four types of mortar by binding material. (i) Lime Mortar; (ii) Surkhi Mortar; (iii) Cement Mortar — proportion 1:2 to 1:6; (iv) Gauged Mortar — cement:lime = 1:6 to 1:8.
2.2.2 Special Mortars
| Type | Preparation | Use |
|---|---|---|
| Fire Resistant | Aluminous cement + finely crushed powder of fire bricks (1:2 proportion) | Lining furnaces, fire places, ovens |
| Light Weight | Adding saw dust, wood powder, asbestos fibres to lime/cement mortar | Soundproof and heatproof construction |
| Packing Mortar | High homogeneity, water resistance, forms solid waterproof plugs | To pack oil wells |
| Sound Absorbing | Portland cement, lime, gypsum; bulk density 6–12 kN/m³ | To reduce noise level |
2.2.3 Properties of a Good Mortar
- Should be capable of developing good adhesion with building units such as bricks, stones etc.
- Should be capable of developing the designed stresses; should be cheap, durable, easily workable
- Should set quickly so that speed in construction may be achieved
- The joints formed by mortar should not develop cracks and they should maintain their appearance for a sufficiently long period
2.3 Tests for Mortars
2.3.1 Adhesiveness Test
Two bricks placed at right angles; mortar placed to join them forming a horizontal joint of 9 cm × 9 cm = 81 cm². Ultimate adhesive strength = maximum load / 81 cm² area.
2.3.3 Tensile Strength (Briquette Test)
Fig. 2.3 — Standard briquette: central cross-section = 38 mm × 38 mm = 1444 mm². Load applied at 0.7 N/mm² in 12 seconds. OPC: ≥2.0 MPa (3 days); ≥2.5 MPa (7 days).
2.4 Some Basic Definitions
| Term | Definition |
|---|---|
| Calcination | The heating of limestone to redness in contact with air |
| Hydraulicity | Property of lime by which it sets or hardens in damp places, water or thick masonry walls where there is no free circulation of air |
| Lime | Product of calcination of limestone. CaCO₃ → CaO + CO₂↑. Chemical composition: CaO |
| Quick Lime | Lime obtained by calcination of comparatively pure limestone. Chemical composition: CaO. Also known as caustic lime or lump lime. |
| Slaking | When water added to quick lime → cracks, swells and falls into powder form (calcium hydrate Ca(OH)₂). CaO + H₂O → Ca(OH)₂ + Heat |
| Slaked Lime | Product obtained by slaking of quick lime. White powder. Chemical composition: Ca(OH)₂. |
2.5 Classification of Lime
Fig. 2.4 — Classification of lime and comparison between Fat Lime and Hydraulic Lime across 8 properties (from source table).
2.5.3 Poor Lime
- Also known as impure lime or lean lime; contains more than 30% of clay
- Slakes very slowly; forms thin paste; sets or hardens very slowly; poor binding properties
- Can only be used for interior work at places where good lime is not available
2.6 Impurities in Limestones
| Impurity | Effect |
|---|---|
| Magnesium Carbonate | Irregular properties of calcination, slaking and hardening. Upto 5% of MgO imparts excellent hydraulic properties. |
| Clay | Mainly responsible for hydraulic properties. 10–30% clay produces hydraulicity. 20–30% clay → excellent hydraulic properties — suitable for aqueous foundations. |
| Silica (free form) | Has a detrimental effect on properties of lime |
| Iron Compounds | Pyrite or iron sulphide is highly undesirable. For hydraulic limes, 2–5% of iron oxide is necessary. |
| Sulphates | Slow down the slaking action and increase the setting time of limes |
| Alkalies | Undesirable when pure lime required. However, up to 5% of alkalies in hydraulic lime have no ill effect. |
2.6.7 Classification of Mortar on Strength Basis
| Grade | Min. Compressive Strength |
|---|---|
| H₁ Mortar | 10 N/mm² (min) |
| H₂ Mortar | 6–7.5 N/mm² (min) |
| M₁ Mortar | 3–5 N/mm² (min) |
| M₂ Mortar | 2–3 N/mm² (min) |
| M₃ Mortar | 1.5 N/mm² (min) |
| L₁ Mortar | 0.7 N/mm² (min) |
| L₂ Mortar | 0.5 N/mm² (min) |
- C Mortar richer than 1:3 not used in masonry — prone to high shrinkage
- By adding Surkhi to Fat Lime → Artificial Hydraulic Lime is obtained
- Calcification of “Kankar” gives Hydraulic Lime
- Modulus of Rupture @ 28 days of Mortar: not less than 1.5 N/mm²
Chapter 2: Mortar and Lime — Civil Engineering · Construction Materials
All technical data as per IS specifications and source material