Building Materials – Comprehensive Notes on Lime

Introduction to Lime

• Lime (chemical symbol $CaO$) is one of the oldest binding materials, historically used in palaces, monuments, and other masonry works.
• Never occurs in free state; always derived from limestone (primary source) through calcination.
• Ideal where:
  • Workability is paramount.
  • Required compressive strength $< 3\,\text{MPa}$.

Geological & Industrial Sources

• Main natural source: Limestone ($CaCO_3$) – also called calcite.
• Other geological varieties:
  • Magnesite $MgCO<em>3$ • Dolomitic limestone $CaCO</em>3 + MgCO_3$
  • Kankar (nodular impure limestone found underground)
  • Shell deposits (sea‐shells, corals, exoskeletons of sea animals).

Lime Cycle: Major Chemical Transformations

• Calcination (burning) at $\approx 800^\circ\text{C}$: $CaCO<em>3 \xrightarrow{\Delta} CaO + CO</em>2 \uparrow$
  • $\approx 49\%$ weight loss (escape of $CO<em>2$). • Energy required $\approx 42.52\,\text{kcal}$ per $CaCO</em>3$ unit.
• Slaking (hydration) – exothermic: $CaO + H<em>2O \rightarrow Ca(OH)</em>2$
  • Volume increase = $1.0\text{–}2.5$ times original lime.
  • Requires water = $32\%$ of lime weight for complete slaking.
  • Heat evolution raises slurry to $\approx 120^\circ\text{C}$.
• Carbonation (hardening): $Ca(OH)<em>2 + CO</em>2 \xrightarrow{air} CaCO<em>3 + H</em>2O$
  • Regenerates limestone within the set mortar/plaster.

Fuels Commonly Used for Calcination

• Coal, charcoal, lignite, firewood & similar carbonaceous fuels.

Key Properties of Lime & Mortar

• Fat lime (pure) exhibits:
  • Brilliant white colour, high plasticity, vigorous slaking, bulk volume expansion.
  • Hardens slowly by atmospheric $CO_2$ absorption.
  • Mortar strength low (≈ $0.5\text{–}1\,\text{N/mm}^2$).
• Hydraulicity: ability to set under water or in saturated/damp conditions.
  • Caused by chemical combination of lime with clay, silica, alumina, iron oxide, sulphates, alkalies, etc.
  • Expressed in three grades: feebly, moderately & eminently hydraulic.

Impurities & Their Influence (Maximum desirable % values given)

• $MgCO_3$ (≤ $5\%$): improves strength but delays slaking if excess.
• Clay (Al$<em>2$O$</em>3$, SiO$_2$) $10\text{–}30\%$: increases hydraulicity, but prolongs setting when > $25\%$.
• Silica (free) > $10\%$: produces poor/lean lime with weak binding.
• Sulphates: reduce slaking rate; excessive amounts undesirable.
• Alkalies $> 5\%$: detrimental (efflorescence, volume instability).

Classification of Lime (IS 712: 1984)

1. Class A – Eminently hydraulic
   • Clay ≈ $25\%$, $CaO = 60\text{–}70\%$.
   • Grey colour, slakes with great difficulty.
   • Initial set: $\approx 2\,\text{h}$; final set: $\le 48\,\text{h}$.
   • Use: high‐strength mortars & concrete (foundations, water works).
2. Class B – Semi-hydraulic
   • Clay ≈ $15\%$.
   • Use: general mortar, plaster in ordinary construction.
3. Class C – Fat (high‐calcium)
   • $CaO = 90\text{–}93\%$, clay $5\text{–}7\%$.
   • Pure white; used for finishing coats, whitewashing, lime–pozzolana mortars.
4. Class D – Magnesium / Dolomitic
   • $(CaO + MgO) \approx 85\%$.
   • Slow setting; finishing plaster coat.
5. Class E – Kankar lime
   • $CaO \approx 20\%$, $MgO \approx 5\%$, rest miscellaneous impurities.
   • Derived by calcining kankar nodules; source of hydraulic lime.
6. Class F – Siliceous dolomitic
   • High silica with magnesia; used as under-coat plaster.

Conventional (Impurity-Based) Classification

• Pure/Fat/Rich/Quick lime: impurities < $5\%$.
• Hydraulic lime (5–30 % impurities) sub-graded:
  • Feebly hydraulic: $5\text{–}10\%$ impurities; set ≈ 21 days.
  • Moderately hydraulic: $10\text{–}20\%$; set 7 days.
  • Eminently hydraulic: $20\text{–}30\%$; set 2 h (initial) – 48 h (final).
• Poor/Lean lime: impurities > $30\%$; virtually non-hydraulic and weak.

Slaking (Hydration) Practices

• Quick (lump) lime is spread in thin layers and sprinkled or submerged.
• Correct water dose: $0.32$ kg per kg of $CaO$ (≈ $32\%$ by weight).
• Reaction generates heat & steam – termed “boiling” or “hissing.”
• Under-burnt lime – slakes sluggishly; over-burnt – may fail to slake.

Manufacturing Flow (Modern Continuous Plant)

1. Crushing & screening limestone.
2. Proportioning with bituminous coal (fuel) on belt conveyor.
3. Burning in vertical/rotary kiln ($\approx 1000\text{–}1300^\circ\text{C}$): $CO_2$ evolution.
4. Cooling; separation of over-burnt nodules.
5. Hydration in hydrator.
6. Grinding & air-classification to obtain hydrated lime powder (passes through dedicated screen).
7. Storage in silos or bags (moisture-proof).

Testing of Lime

• Visual & Field Tests

• Colour: pure lime = brilliant white; hydraulic = grey; presence of unburnt cores shows under-burning.
• Touch/feel: earthy smell, gritty particles indicate impurities.
• Workability test: 1 : 3 (lime–sand) mortar thrown onto a vertical surface; area covered qualitatively gauges plasticity.
• Slaking sound/temperature evolution: vigorous for fat lime.
• Impurity (weight-loss) test: dissolve lime in water; filter & dry residue.
  • $\text{Loss} < 10\%$ → good; $10\text{–}20\%$ → fair; $> 20\%$ → poor.

• Laboratory Tests (IS 6932)

1. Fineness: sieve analysis on 100 g sample; coarse residue measured.
2. Workability (Henri rolling-table): 500 g lime paste rolled until 190 mm spread reached; strokes counted.
3. Chemical analysis:
   • $\text{Cementation value} = 3.8SiO<em>2 + 1.18Al</em>2O<em>3 + 0.7Fe</em>2O<em>3$ — compared against $CaO + MgO + 1.4H</em>2O$ to assess capability for hydraulic set.

Typical Uses & Practical Considerations

• Fat lime: whitewashing, distemper, lime putty, finish coats, low-strength cement–lime mortars.
• Semi/Eminent hydraulic: structural masonry mortar, foundation concrete, damp-proof works, guniting.
• Kankar/magnesium lime: base or under-coat plaster to resist shrinkage.
• Poor lime: only for inferior, temporary or filler works.

Ethical & Environmental Notes

• Lime production is energy-intensive and releases $CO<em>2$ (≈ 0.79 t $CO</em>2$ per tonne of quicklime) – contributes to greenhouse gas emissions.
• Historic lime mortars, however, are more breathable and compatible with heritage masonry than modern OPC, reducing long-term deterioration.

Objective Questions Extracted (Self-Check)

1. Mixing water with quick lime is called slaking (hydration).
2. Rate of slaking depends on: burning degree, agitation, atmospheric conditions, chemical composition (all listed).
3. Lime is commercially obtained by burning limestone.
4. IS–712 classifies lime into 6 classes.
5. A property of fat lime: high plasticity / vigorous slaking.
6. Lime from pure limestone is termed quick (fat/pure) lime.
7. Specialty of hydraulic lime: sets under water due to impurities (clay, silica, etc.).
8. Slaking within a few minutes: moderately hydraulic lime.
9. Non-existent type: Roman lime (deprecated term).
10. Process of heating limestone in absence of air: calcination.
11. Chemical composition of quick lime: $CaO$.
12. Slaked lime is hydrated oxide of calcium – $Ca(OH)_2$.
13. Hydraulicity needs both alumina and silica in combination with lime.
14. Fat lime does NOT readily harden under water (sets slowly in air).
15. Poor lime has clay > 30 %.

Quick Reference Equations

$CaCO<em>3 \xrightarrow{800^\circ C} CaO + CO</em>2$ (Calcination)
$CaO + H<em>2O \rightarrow Ca(OH)</em>2 + \Delta H$ (Slaking)
$Ca(OH)<em>2 + CO</em>2 \rightarrow CaCO<em>3 + H</em>2O$ (Carbonation)
$\text{Cementation value} = 3.8SiO<em>2 + 1.18Al</em>2O<em>3 + 0.7Fe</em>2O_3$

Connections to Later Topics

• Hydraulic lime’s chemistry foreshadows the clinker composition in Portland cement (both rely on silicates & aluminates for strength).
• Lime–pozzolana mixes (Roman concretes) are historical precursors of modern blended cements.
• Understanding slaking & carbonation is essential for diagnosing heritage plaster failures and specifying compatible repair mortars.