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Mortar and Lime – Chapter 2

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Chapter 2: Mortar and Lime — Civil Engineering

Table of Contents

  1. 2.1 Bulking of Sand
  2. 2.2 Mortars — Classification and Types
  3. 2.3 Tests for Mortars
  4. 2.4 Some Basic Definitions
  5. 2.5 Classification of Lime
  6. 2.6 Impurities in Limestones
  7. 2.6.7 Classification of Mortar on Strength Basis

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.

  • Building mortar = mixture of cement, sand and water
  • Mortar is similar to concrete but does not contain coarse aggregate
  • Mortar is used for filling joints as a binder in stone and brick masonry

The presence of moisture in sand increases the volume of sand. Moisture creates a thin film of water around the sand particles which results in increase of volume of sand.

  • For moisture content of about 5 to 8%, increase of volume may be as much as 20 to 40%, depending upon the grading of sand
  • The liner the material, the more will be the increase in volume for a given moisture content
  • This phenomena is known as the bulking of sand
  • The graph below shows variation of percentage increase in volume of sand with moisture content
Figure 2.1 — Bulking of Sand: % Increase in Volume vs % Moisture Content (Chart from source)
Bulking of sand chart from source showing percentage volume increase versus moisture content for fine medium and coarse sand Three curves: Fine sand peaks at approximately 35 percent increase at 5-7 percent moisture content. Medium sand peaks at approximately 25 percent. Coarse sand peaks at approximately 15 percent at lower moisture. All curves converge back near zero at full saturation around 20 percent moisture. 0 5 10 20 30 40 Percentage Increase in Volume 0 5 10 15 20 Percentage by Weight of Moisture Fine Medium Coarse Fine sand Medium (Coarse) sand Coarse sand Fig. Showing bulking of sand — source: Construction Materials, CE
Fig. 2.1 — Bulking of Sand chart (from source). Fine sand shows maximum bulking (~35–40% increase at 5–7% moisture). Medium sand peaks ~25%. Coarse sand peaks ~15%. All curves converge near zero at full saturation. Bulking is more with fine sand and less with coarse sand.
Do you know? The bulking of sand affects the volumetric proportioning of sand to a large extent. It 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 of 15 kN/m³ or more; prepared from heavy quartz or other sands
  • Light weight mortars: bulk density less than 15 kN/m³; prepared from light porous sands, pumice and other fine aggregates

2.2.1 Type of Binding Material

Figure 2.2 — Types of Mortar by Binding Material
Classification of mortars by binding material type Four types: Lime Mortar fat or hydraulic lime, Surkhi Mortar surkhi instead of sand, Cement Mortar cement as binder proportion 1 to 2 to 1 to 6, Gauged Mortar lime plus cement ratio 1 to 6 to 1 to 8. Type of Binding Material Classification of Mortars (i) Lime Mortar Fat or hydraulic lime (ii) Surkhi Mortar Surkhi instead of sand (iii) Cement Mortar Cement as binding (iv) Gauged Mortar Lime + cement Fat lime or hydraulic lime • Fat lime shrinks; needs 2–3× volume of sand • Lime must be slaked • Unsuitable for waterlogged areas or damp situations • Hydraulic: lime:sand=1:2 • High plasticity; easy placing; shrinks little • Hardens slowly Surkhi mortar • Surkhi instead of sand or half of sand (fat lime mortar case) • Used for all masonry in foundation and superstructure • Cannot be used for plastering/pointing (surkhi disintegrates) Cement mortar • Cement as binder • Proportion cement:sand varies 1:2 to 1:6 or more • Used where high strength and water-resisting properties are required • Underground construction • Water saturated soil Gauged mortar • Improve quality of lime mortar by adding cement (gauging) • Cement:lime = 1:6 to 1:8 • Also called composite mortar / lime-cement • Makes lime mortar economical, strong, dense • Used for bedding, thick walls
Fig. 2.2 — Four types of mortar by binding material: (i) Lime Mortar — fat lime or hydraulic lime; (ii) Surkhi Mortar; (iii) Cement Mortar — proportion 1:2 to 1:6; (iv) Gauged Mortar — lime + cement, cement:lime = 1:6 to 1:8.

2.2.2 Special Mortars

TypePreparationUse
(i) Fire Resistant MortarAluminous cement + finely crushed powder of fire bricks. Usual proportion: 1 part aluminous cement to 2 parts powder of firebricksFire-resistant; used with firebricks for lining furnaces, fire places, ovens etc.
(ii) Light Weight MortarAdding saw dust, wood powder to lime mortar or cement mortar. Other materials: asbestos fibres, jute fibres, coir etc.Used in soundproof and heatproof construction
(iii) Packing MortarSpecial mortar with high homogeneity, water resistance, ability to form solid waterproof plugs in cracks and voids of rocksTo pack oil wells
(iv) Sound Absorbing MortarBinding materials: Portland cement, lime, gypsum. Bulk density of such mortar = 6–12 kN/m³To reduce the noise level; sound-absorbing plaster formed with this mortar

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
  • Should not affect the durability of materials with which it comes into contact
  • The joints formed by mortar should not develop cracks and they should be able to maintain their appearance for a sufficiently long period

2.2.4 Uses of Mortar

  • To bind the building units such as bricks, stones etc.
  • To carry out pointing and plaster work on exposed surfaces of masonry
  • To form an even and soft bedding layer for building units
  • To form joints of pipes
  • To hide the open joints of brickwork and stonework
  • To improve the general appearance of structure

2.2.5 Functions of Sand in Mortar

  1. Bulking
  2. Setting
  3. Shrinkage gets avoided due to sand
  4. Strength gets imparted due to sand but doesn’t often binding strength and shrinkage

2.3 Tests for Mortars

2.3.1 Adhesiveness to Building Units

  • Two bricks are placed at right angles to each other
  • Mortar is placed to join them so as to form a horizontal joint. If size of bricks is 19 cm × 9 cm × 9 cm, a horizontal joint of 9 cm × 9 cm = 81 cm² will be formed
  • Ultimate adhesive strength of mortar per cm² area is obtained by dividing maximum load with 81 cm² area

2.3.2 Crushing Strength

  • Brick masonry or stone masonry laid in mortar to be tested are crushed in compression machine
  • The load at which the masonry crushes gives the crushing strength

2.3.3 Tensile Strength

Figure 2.3 — Standard Briquette for Tensile Strength Test of Mortar (as per source)
Standard briquette dimensions for tensile strength test of mortar as per source Dog-bone shaped briquette shown in elevation and plan view. Elevation: overall 152mm long, central waist 38mm wide and 76mm high. Plan: width 76.20mm at ends, 50.80mm at waist. Cross-sectional area at central portion 38mm by 38mm equals 1444 mm squared. Load applied at 0.7 N per mm squared in 12 seconds. Elevation Overall length: 36.10 mm to 38.10 mm 44.50 mm 38 mm × 38 mm = 1444 mm² 50.80 mm (waist) Plan 70.20 mm 76.20 mm (width at ends)
Fig. 2.3 — Standard briquette for tensile strength test. Central cross-section = 38 mm × 38 mm = 1444 mm² = 14.44 cm². Six briquettes tested; average tensile strength calculated. Load applied at 0.7 N/mm² in 12 seconds. OPC tensile strength ≥ 2.0 MPa (3 days); ≥ 2.5 MPa (7 days).

2.3.4 Lime

  • The use of lime as a cementing material has been in use since ancient times
  • At present, cement has replaced lime to a great extent; but where lime is locally available and there is shortage of cement, lime provides a cheap and reliable alternative

2.4 Some Basic Definitions

TermDefinition / Description
CalcinationThe heating of limestone to redness in contact with air is known as calcination
HydraulicityProperty of lime by which it sets or hardens in damp places, water or thick masonry walls where there is no free circulation of air
LimeDue to calcination of limestone, moisture and CO₂ are removed. Remaining product = lime. Chemical composition: CaO (oxide of calcium). Reaction: CaCO₃ → CaO + CO₂↑
Quick LimeLime obtained by calcination of comparatively pure limestone. Capable of slaking with water; no affinity for carbonic acid. Chemical composition: CaO. Also known as caustic lime or lump lime. Quick lime from kilns = lump lime.
SettingThe process of hardening of lime after it has been converted into paste form is known as the setting. Quite different from drying.
Slaked LimeProduct obtained by slaking of quick lime. White powder. Chemical composition: Ca(OH)₂ (hydrated oxide of calcium). Reaction: CaO + H₂O → Ca(OH)₂ + Heat
SlakingWhen water is added to quick lime in sufficient quantity, a chemical reaction takes place — quick lime cracks, swells and falls into a powder form which is calcium hydrate Ca(OH)₂ = hydrated lime. This process = slaking.
Do you know? A thin pourable suspension of slaked lime in water is known as the milk of lime. Quick lime as it comes out from kilns is known as the lump lime.

2.5 Classification of Lime

  • (i) Fat Lime  |  (ii) Hydraulic Lime  |  (iii) Poor Lime or Lean Lime
Figure 2.4 — Classification of Lime with Fat Lime vs Hydraulic Lime Comparison
Classification of lime and comparison table of fat lime versus hydraulic lime Top shows three types of lime branching from Classification of Lime root. Below is a seven-row comparison table between fat lime and hydraulic lime covering composition, slaking action, setting action, hydraulicity, colour, strength and uses. Classification of Lime (i) Fat Lime High calcium/pure/rich/white lime (ii) Hydraulic Lime Water lime · sets under water (iii) Poor Lime Impure/lean lime · >30% clay Table: Comparison between Fat Lime and Hydraulic Lime Item Fat Lime Hydraulic Lime 1. Composition Pure carbonate of lime; impurities <5% Limestone with clay 5–30% + ferrous oxide 2. Slaking Slakes vigorously; volume increased 2–2.5×; heat & sound evolved Slakes slowly; volume slightly increased; not accompanied by sound/heat 3. Setting Sets slowly in air; absorbs CO₂; forms CaCO₃ Sets under water; combines with water; forms crystals of aluminate and dicalcium silicate 4. Hydraulicity Does NOT possess hydraulic property Possesses hydraulic property 5. Colour Perfectly white in colour Colour not so white as fat lime 6. Strength Not very strong; cannot be used where strength required Strong; can be adopted where strength is required 7. Uses Plastering, white washing, mortar with sand/surkhi; metallurgical industry Mortar for thick walls, damp places; extreme care for plaster work 8. Impurities Less than 5% impurities Clay 5–30% + some amount of ferrous oxide
Fig. 2.4 — Classification of lime and comparison between Fat Lime and Hydraulic Lime across 8 properties (from source table).

2.5.1 Fat Lime

  • Also known as: high calcium lime, pure lime, rich lime, white lime
  • Slakes vigorously; volume increases to about 2–2.5 times the volume of quick lime
  • Percentage of impurities: less than 5%
  • Characteristics: (i) Hardens very slowly (ii) High degree of plasticity (iii) Soluble in water — changed frequently (iv) Perfectly white (v) Sets slowly in presence of air (vi) Slakes vigorously
  • Uses: White washing and plastering of walls; with sand forms lime mortar which sets in thin joints; manufacture of cement and metallurgical industry

2.5.2 Hydraulic Lime

  • Also known as water lime — it sets under water
  • Contains clay and some amount of ferrous oxide
  • Classified depending on % of clay: (a) Feebly hydraulic, (b) Moderately hydraulic, (c) Eminently hydraulic
  • Increase in percentage of clay → makes slaking difficult → increases hydraulic property
  • Can set under water and in thick walls where there is no free circulation of air

2.5.3 Poor Lime

  • Also known as impure lime or lean lime
  • Contains more than 30% of clay
  • Slakes very slowly; forms a thin paste with water; sets or hardens very slowly
  • Poor binding properties; colour is muddy white
  • Makes a very poor mortar; used for interior work at places where good lime is not available

2.6 Impurities in Limestones

2.6.1 Magnesium Carbonate

  • Magnesium limestones are hard, heavy and compact in texture
  • Display irregular properties of calcination, slaking and hardening
  • Upto 5% of magnesium oxide imparts excellent hydraulic properties to the lime

2.6.2 Clay

  • Mainly responsible for the hydraulic properties of lime
  • Percentage of clay to produce hydraulicity: usually 10–30%
  • 3–5% of clay: no hydraulic property displayed
  • 20–30% of clay: excellent hydraulic properties, most suitable for aqueous foundations

2.6.3 Silica

  • In its free form, it has a detrimental effect on the properties of lime

2.6.4 Iron Compounds

  • Iron occurs in small proportions as oxides, carbonates and sulphides
  • Pyrite or iron sulphide is regarded to be highly undesirable
  • For hydraulic limes, 2–5% of iron oxide is necessary

2.6.5 Sulphates

  • If present, slow down the slaking action and increase the setting time of limes

2.6.6 Alkalies

  • When pure lime required, alkalies are undesirable
  • However, up to 5% of alkalies in hydraulic lime do not have any ill effect

2.6.7 Classification of Mortar on Strength Basis

GradeMin. Compressive Strength
H₁ Mortar10 N/mm² (min)
H₂ Mortar6–7.5 N/mm² (min)
M₁ Mortar3–5 N/mm² (min)
M₂ Mortar2–3 N/mm² (min)
M₃ Mortar1.5 N/mm² (min)
L₁ Mortar0.7 N/mm² (min)
L₂ Mortar0.5 N/mm² (min)
Notes:
  • Use of C Mortar richer than 1:3 are not used in masonry work — provides no gain of strength and prone to high shrinkage
  • By adding Surkhi to Pure Lime (Quick Lime/Fat lime): the Artificial Hydraulic Lime is obtained
  • Calcification of “Kankar” gives Hydraulic Lime
  • Modulus of Rupture @ 28 days of Mortar should not be less than 1.5 N/mm²
Do you know? The setting action of mortar is affected by the presence of frost. It is therefore advisable to stop the work in frosty weather or to execute it with cement mortar which will set before it commences to freeze.

Chapter 2: Mortar and Lime — Civil Engineering · Construction Materials
All technical data as per IS specifications and source material

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