LIME

Lime is a more or less impure calcium oxide (CaO) and obtained by the calcination (heating of
shells corals, limestone, kankar, and other substances).
 Lime acts as a binding or cementing material in engineering constructions.
Uses of Limes: Lime is used for the following purposes-
i. White washing
ii. Manufacturing cement
iii. Making concrete (Lime concrete)
iv. Making mortar (Lime mortar and surki mortar)
Qualities of good lime
1. It should be free from ashes, unburnt particles, and other impurities.
2. It must pass through Sieve No.64
Constituents of Lime: The following are the main constituents:
Calcium oxide
 This is the principal constituent of limestone. It acts as a binder.
Clay
 Clay in small quantity retards slaking
 It causes the setting of lime and renders it insoluble in water.
 Generally, 10% of clay presence in limestone is recommended.
Silica and Alumina
 They impart hydraulicity in lime.
Magnesium Carbonate
 It slakes lime more slowly
 Evolves less heat and expands less
 Set more slowly but finally gains greater strength
Alkalies and Metallic oxide
 They impart hydraulicity in lime
 Small quantity of sulfate retards the slaking action and increases the rapidity of setting
Iron Pyrites
 The presence of iron pyrites is highly objectionable as they reduce the strength of lime.
Classification of Limes: They are: (i) Fat lime, (ii) Hydraulic lime and (iii) Natural cement
Fat Lime
 This is also known as High Calcium Lime
 This is a purer type of lime; it swells two to three times its volume when slaked
 Fat lime is obtained by calcination of pearly pure limestone, chalk, and sea shells
 It is nearly white and free from other substances
 It does not set under water but dissolves
 It is generally used for finishing coat in plastering white washing
Hydraulic Lime
 It possesses the property of setting and hardening under water This is obtained from kankar
or clayey limestone
 It is not white because it contains impurities of clay and magnesium carbonate
 This is used for mortar for masonry in foundations and for thick walls
Natural Cement
 These are hydraulic binding materials, almost similar to hydraulic limestone but containing
a higher percentage of clayey matter (40%).
 These natural stones are the complication; the products exhibit a high value of hydraulicity.
Tests of Limestone: For the manufacture of lime and cement engineers are to carry out some tests
on limestone. The tests are discussed below:
1. Physical Tests: The sample of limestone showing a slightly earthy surface indicates the presence
of clay and will be suitable for producing hydraulic lime. The white colour indicates pure
limestone.
2. Chemical Tests: The following are the different chemical tests that are generally performed on
limestone.
(a) Heat Test: Weigh a piece of dry limestone. Heat it to redness in an open fire for about six
hours. Since the bond between calcium oxide and carbon dioxide in limestone is very weak the
following reaction will take place:
Limestone (CaCO3) + Heat = Lime (CaO) + Carbon dioxide (CO2)
Allow the product to cool and weigh again. From the loss of weight, the amount of calcium
carbonate in the specimen can be determined. For every loss in weight of 44 parts, there are 56
parts of lime by weight.
This is adopted if the sample does not contain magnesium carbonate. Clayey and siliceous
impurities require higher temperatures to drive out carbon dioxide and this indicates the extent to
which the lime is hydraulic.
Example: One pound sample of limestone was heated in a lime kiln at a temperature of 1500 oC.
After heating the sample weighed 0.325 lbs. Calculate the percentage of quick lime that will be
available from this sample.
Solution: Loss of wt due to heating = l.00 - 0.325 = 0.675 lbs.
0675 × 56
Percentage of quick lime = × 100 = 86%
44
(b) Slaking Test: Quick lime is an anhydrous product of calcinations or heating. When water is
added to it, the following reaction takes place.
Quick Lime (CaO) + Water (H2O) = Hydrated Lime [Ca(OH)2]
During the process of hydration, heat is given out and the energy thus liberated is often enough to
split it and to make it crumble to powder. A vigorous slacking indicates high calcium content in
the sample i.e., fat lime, while with hydraulic limes, the slacking action is very slow. Again,
sufficient water is added and if the sample sets under water, it is hydraulic lime, if not, fat lime.
From this test, it is possible to determine whether or not a particular sample of limestone will yield
fat lime or hydraulic lime

(c) Acid Test: This test is carried out for two purposes which are discussed below:
(i) To determine the presence of calcium carbonate in a given sample of limestone:
Dilute hydrochloric acid is added to the specimen. This will cause effervescence with the liberation
of carbon dioxide and the formation of calcium chloride. The reaction is as follows:
Limestone (CaCO3) + Hydrochloric Acid (2HCl) = Calcium Chloride (CaCl2) + Carbon-dioxide
(CO2) + H2O
Effervescence will be vigorous if the presence of calcium carbonate content of the sample is very
high and thereby the residue will be less. If the effervescence is not vigorous, calcium carbonate
content is 35 less and the residue will be more.
(ii) To determine the rough composition of limestone:
Powder a sample of limestone and dry it gently to remove moisture. Weight it, let the weight be
W gms. Put the powdered sample in a tumbler and add dilute hydrochloric acid gradually. Stir and
add hydrochloric acid until effervescence ceases. Strain the content through filter paper. Carefully
dry the residue on the filter paper and weigh it. Let the weight be W1 gms. Then (W-W1) will be
the weight of calcium carbonate in the sample. To separate the clay from the sand in the residue,
stir up the residue with water. The sand being heavier will settle at the bottom, while the clay
particles will be in suspension. Decant the water. Dry the sand and weigh it. Let the weight be W2
gms. Then (W1-W2) will be the weight of the clay.

Example: The initial wt. of a sample of powder limestone is 445.45gms and the wt. of the dried
residue on filter paper is 120.23gms. The residue after decantation and drying weighed 68.42
grams. Calculate the percentage of carbonate, sand, and clay present in the sample of limestone.

Solution: Here, W = 445.45 gms, W1 = 120.23 gms, W2 = 68.42 gms.

Wt. of calcium carbonate = W-W1 = 445.45 - l20.23 = 325.22 gms.
325.22
Percentage of calcium carbonate = × 100 = 73%
445.45
Clay = l20.23 - 68.42 = 5l.81 gms.
51.81
Percentage of clay = × 100 = 12%
445.45
Percentage of sand = l00 - (73+12) = 15%

3. Chemical Analysis of Limestone: The engineers are rarely called upon to carry out detailed
chemical tests on ironstones but the selection of limestone will not be proper if the following
important things are not known to the engineers. By detailed and routine chemical tests, the correct
values of the five principal components of limestone (calcium oxide, magnesia, alumina, silica,
and iron oxides) must be known to determine the following:
Silica + Alumina + Oxides of Iron
Hydraulic Index or Ratio =
Lime + Magnesia
It may be noted from the above equation that there is a possibility of getting the same ratio though
the percentage of the ingredients change in them. This indicates that two different samples will
have the same value of the ratio.
 2.8 Silica + 1.1 Alumina + 0.7 Oxides of Iron
Cementation Index =
Lime + 1.4 Magnesia
The hydraulicity of limestone is always expressed based on the hydraulic index and cementation
index.
Example: The chemical analysis of a sample of limestone gave the following results.
Calcium oxide = 70%
Silica = 20%
Alumina = 6%
Magnesia = l.5%
Iron Oxides = 2.5%
Determine the hydraulic index and cementation index.

Solution:
20 − 6 + 2.5
Hydraulic Index = = 0.399
70 + 1.5
2.8 × 20 + 1.1 × 6 + 0.7 × 2.5
Cementation Index = = 0.892
70 + 1.4 × 1.5
Testing of lime: The following tests are generally carried out to determine the strength of lime:
Adhesive Strength Test
 Prepare a paste of lime and sand in the proportion 1:3 with the requisite amount of water
 Two standard-sized bricks are placed flat in a cross-fashion one over the other with a joint
of " thick by the prepared mortar
 The two joined bricks should be kept wet with the help of a gunny bag for 24 hours and
then they are kept immersed in water for 7 days
 The two cemented bricks are then tested. The force (pull) required to separate them at the
joint should not be less than 30 psi
Tensile Strength Test
 A lime mortar of ratio 1:3 is prepared and a briquette is made
 Kept the briquette underwater for 7 days
 This is tested in the briquette testing machine for tensile strength of lime, this should not
be less than 45 psi after 7 days and 90 psi after 28 days
Compressive Strength Test
 With the help of the same mortar of a 1:3 ratio, 2" cubes are made
 Kept this cube for 7 days in water
 The cubes are then tested for compressive strength of lime and this should not be less than
200 psi after 7 and 450 psi after 28 days
Soundness Test
 The lime is mixed with the necessary quantity of water to form a paste
 The paste is made into a small ball 1" in diameter by hand and then allowed to set under a
wet cloth for 24 hours
 Then it is kept underwater for a further period of 24 hours
 Thereafter, it is exposed to steam for 6 hours
 At the end of this period, the ball should not show any sign of cracking or warping
Questions
1. What is lime? What are the uses of lime in engineering construction?
2. Discuss the different constituents of limestone.
3. What are the different types of limes? Give their properties.
4. What are the characteristics of good lime? Name the tests that are generally performed to
determine the strength of lime.
5. A sample of limestone weighing 1 lb was heated in a lime-kiln at a temperature of 1500 oC.
After heating, the sample weighed 0.254 lbs. Calculate the percentage of quick lime that will be
available from the sample.
6. The initial wt. of a sample of powdered limestone is 330.50 gms and the wt. of the dried residue
on the filter paper is 100.25 gms. The residue after decantation and drying weighed 60.50 gms.
Calculate the percentage of calcium carbonate, sand and clay present in the sample of limestone.
7. Explain hydraulic index and cementation index. The chemical analysis of a sample of limestone
gave the following results.
Calcium oxide 72%
Silica 18%
Alumna 5%
Magnesia 1.5%
Iron oxides 3.5%
Determine the hydraulic index and cementation index.