Written by pushkar

Definition:
Cement sand plaster is a layer of cement and sand mortar which is applied over walls made of brickwork, cement block work, RCC walls or RCC columns and beams.  

The plaster can be divided into 2 basic categories:
1. Interior plaster (as specified by interior designers) is usually smooth finish and 12mm thick (can have rough keyed surface to receive another material like tile or stone cladding)
2. Exterior plaster which is applied on building facades, compound walls etc. is usually 24 mm thick and is applied in 2 coats of 12 mm each. The top coat is usually rough finish with a sand grain texture.

Mortar Mixes:
The cement – sand mortar mix can be in the range of 1:3 to 1:6. A ‘rich’ mix would be 1:3 which consists of 1 part cement and 3 parts sand. This kind of mix will have good adhesion but will crack easily if not cured properly. The usual mix specified by architects and interior designers is 1:4. A liquid waterproofing compound may be added to the mortar mix to make the plaster water-proof.

Quality of materials:
The cement used for plastering has to be 43 grade Ordinary Portland Cement. Although 53 grade of cement is stronger; it causes cracks in plaster and is used for RCC work. As per your interior designers specifications the sand used for plastering has

to be mineral free sand with silt percentage not exceeding 8%. Sand with higher percentage of silt is unfit for plastering. If silt free sand is unavailable locally, the sand would be washed with water before adding to the mortar mix.

 

Thumb rule for interior designers to calculate quantity:
Cement sand plaster. Ratio 1:4 (Avg. 12 mm thick)
Cement required for 1 cu. Ft. = 0.0083333 cu. Ft. =  0.006666 bags
Sand required for 1 cu. Ft. = 0.03333 cu. Ft. = 0.0003333 brass

Important check points:
- Check that the sub surface is sprayed with water before plastering.
- Check that the newly plastered surface is protected from direct sunlight by using plastic sheets or bardaan (jute).
- Check that the plaster is cured for at least 3 days
- Check the plumb-line (olamba) at intervals of every 3’
- Cement has to be 43 Grade and NOT 53 grade (Although 53 grade is stronger it is not suitable for plaster mortar)
- Sand used in mortar should be have minimum silt i.e. 7% to 8% silt. Silt percentage of more than 8% is not advisable for any purpose like mortar, plastering or RCC.
- A mild steel chicken-mesh can be fixed at corners and joints to avoid cracks at joints.
- Get the work inspected by your interior designer for faults by checking for ringing plaster (ringing sound due to loose plaster