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
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 used in some of the big dams and other massive works in India. This admixture is known to reduce the temperature rise during hydration in a mass cement concrete and reduce cracking. It is also useful in sea-water construction, in structures which are subject to attack from aggressive groundwaters or industrial waters, and in hydraulic structures where water tightness is the first consideration Surkhi is used as a substitute for sand for concrete and mortar and has almost the same function as of sand but it also imparts some strength 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 not less than MM 0.7 shall be used;
- Where soil is very damp, masonry mortar of grade preferably MM 2 or more shall be used. But in no case shall the grade of mortar be less than MM 0’7; and
- Where soil is saturated with water, masonry mortar of grade MM 3 shall be used but in no case shall the grade of mortar be less than MM 2.
- For masonry in buildings’ subject to vibration of machinery, the grade of mortar shall not be less than MM 3.
- 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