Mortars

Mortars

There are many types of mortars used in brick work. The type and mix of mortar has to be decided taking into account the strength required, and, the availability of materials and skilled labour etc. In general, the strength of the mortar shall not be greater than that of the masonry unit.

Materials

Water

Water used shall be clean and reasonably free from deleterious materials like oils,acids, alkalies, salts etc. Potable water is generally considered satisfactory. Water should be tested for the following characteristics :

(a) Limits of Acidity
To neutralise 200 ml sample of water, it should not require more than 2ml of 0.1 normal caustic soda solution.

(b) Limits of alkalinity
To neutralise 200 ml sample of water, it should not require more than 10 ml of 0.1 normal hydrochloric acid.

(c) Percentage of solids

It shall not exceeed the following limits for various solids :

Organic  200mg/l

Inorganic 3000mg/l

Sulphates  500mg/l

Chlorides  20000mg/l

Suspended matter 2000 mg/l
 

(d) The pH value

The pH value of water shall generally be not less than 6.

Cement

Cement shall conform to any one of the following specifications :

(a) 33 grade ordinary portland cement, IS : 269 - 1989

(b) 43 grade ordinary portland cement, IS : 81 12 - 1989

(c) 53 grade ordinary portland cement, IS : 1269 - 1987

(d) Rapid hardening portland cement, IS : 8041 - 1990(e) Low heat portland cement, IS : 12600 - 1989 Bnilding construction

(f) Portland Pozzolana cement, IS : 1489 - 1991

(g) Portland slag cement, IS : 455 - 1989

Lime

Lime shall conform to standards given in IS : 712 - 1984.

Building lime shall be classified as follows :

Class A - Eminently hydraulic lime used for structural purposes.

Class B - Semi-hydraulic lime for masonry.

Class C - Fat lime used for finishing purposes; it can be used for masonry mortar with addition of pozzolanic material.

Class D - Magnesium lime used for finishing coat.

Class E - Kankar lime used for mortar.

Carbide lime obtained as a byproduct in the manufacture of acetyline meets the requirement of class C lime and can he used for mortar.

Fine Aggregate
This cosists of natural pit or river sand, or crushed stone, most of which passes through IS Sieve 4.75 mm. It shall not contain harmful organic impurities in such form or quantities (5%)) to affect the strength of the mortar. Sand is generally classified as fine or coarse.

Fine Sand

This shall be river sand and the grading shall be within the limits of grading Zone IV of Table .

Stone Dust

This shall be obtained by crushing hard stones and the grading shall be within the limit for Zone I11 of Table.

Coarse Sand

This shall be either river sand or pit sand and shall conform to the grading of Zone I11 of Table 

The silt or organic content in fine aggregate should not in any case exceed 8%. The silt content is tested by placing a sample of sand in a 200 ml measuring cylinder.The volume of sample will be such that it fills upto the 100 ml mark. Clean water shall be added upto the 150 rnl. mark. Before adding water dissolve a little salt(one teaspoon per half litre) in the water. Shake the mixture vigorously. Allow the contents to settle down for three hours. The height of the silt visible as a layer above the sand shall be expressed as a percentage of the height of sand below.

Sand having more than the allowable percentage of silt shall be washed to bring down the silt content within the specified limits.Cement Mortar This shall be prepared by mixing cement and sand in the specified proportion for the given work. For load bearing construction coarse sand is used in the mix of the mortar. The proportion of cement and sand in cement mortars varies generally from 1 cement to 3 to 8 of sand, the strength and workability improving wit. increase in the proportion of cement. Mortars richer than 1 : 3 are not used in masonry because of high shrinkage with no appreciable gain in the strength of the masonry. Mortars leaner than 1 : 6 proportion tend to become harsh and, hence,unworkable.

Lime Mortar
This consists of lime as a binder and sand, surkhi, cinder as fine aggregates,generally in the proportion 1 : 2 or 1 : 3. Lime is slaked and used as lime putty.Hydrated lime available in powder form can also be used. Lime mortar g m s strength slowly. The main advantages of lime mortar are its good workability, high water retentivity and low shrinkage.