Surkhi is made by grinding to powder burnt bricks, brick-bats or burnt clay; under-burnt or over-burnt bricks should not be used, nor bricks containing a high proportion of sand but Surkhi for plaster may be made from slightly under-burnt bricks, and ground very fine; this will improve the hydraulicity of fat lime (Hydraulicity is the property of a binder to harden in contact with water.)
Surkhi makes cement mortars and concretes more waterproof, more resistant to alkalies, and to salt solutions than those in which no surkhi is used. Surkhi mixed-in cement concrete has been utilized in a number of large dams and different huge works in India. This admixture is understood to lessen the temperature upward thrust all through hydration in a mass cement concrete and decrease cracking. It is likewise beneficial in sea-water construction, in systems which can be a situation to assault from competitive groundwaters or business waters, and in hydraulic systems wherein water tightness is the primary attention Surkhi is used instead for sand for concrete and mortar and has nearly the identical characteristic as of sand however it additionally imparts a few power and hydraulicity.
- the bulk volume of mortar is increased which results in a reduction of cost, It does not increase the strength of mortar
- If building material is fat lime, the carbon dioxide is absorbed through the voids of sand and the setting of fat lime occurs effectively.
- It prevents excessive shrinkage of the mortar in the course of drying and hence the cracking of mortar during setting is avoided.
- It subdivides the paste of the binding material into a thin film and thus more surface area is offered for its spreading and adhering.
Fineness modulus of Sand Coarse sand 2.9-3.2 medium sand 2.6 to 2.9 Fine Sand 2.2 to 2.6
- In the case of masonry exposed frequently to rains and where there is further protection by way of plastering or rendering or other finishes, the grade of mortar shall not be less than MM 0.7 but shall preferably be of grade MM 2. Where no protection is provided, the grade of mortar for external walls shall riot be less than 0-5 MM
- In the case of load-bearing internal walls, the grade of mortar shall preferably be MM 0.7 or more for high durability but in no case less than MM O-5.
- In the case of masonry in foundations laid below the damp-proof course, the grades of morter for use in masonry shall be as specified below:
- Where the soil has little moisture, masonry mortar of grade now no longer much less than MM 0.7 will be used;
- Where soil may be very damp, masonry mortar of grade ideally MM 2 or greater will be used. But in no case shall the grade of mortar be much less than MM 0’7; and
- Where soil is saturated with water, masonry mortar of grade MM three will be used however in no case shall the grade of mortar be much less than MM 2.
- For masonry in buildings’ concern to the vibration of machinery, the grade of mortar shall now no longer be much less than MM three.
- For parapets, where the height is greater than thrice the thickness, the grade of masonry mortar used shall not be less than MM 3.
- In the case of low parapets, the grade of mortar shall be the same as used in the wall masonry below.
- The grade of mortar for bedding joints with large concrete blocks shall not be less than 3MM
Cement Mortar for different work
Normal Brick Work and Internal Plaster (1:6), External Plastering Work and Ceiling (1:4), Grouting the cavernous rocks (1:1.5), Guniting (1:3)
Cement Mortar of hight Grade is not required because there will be no gain in strength of masonry after that but shrinkage will increase due to excessive cement used.
Dry volume of mortar required for plastering = 1.27 X Wet volume of mortar