Cement Mortar – Facts Every Site Engineers Should Know
Mortar is a homogeneous mixture, produced by intimately mixing cementitious materials, water and inert materials, such as sand, to the required consistency for use in building together with masonry units.
Quality of brickwork or blockwork made with cement mortar largely depend on quality of mortar used. However, most of the time proper care has not been taken during preparation phase of mortar.
The following guidelines will help site engineers understands important concepts related to cement mortar.
Mortar made with contaminated sand will always have less strength compared to mortar made with clean sand.
Contaminated sand will not bind with cement, hence is weakening the mortar.
Also sand with high percentage of clay or silt will weaken the mortar, because the clay or silt contains too many fines that needs to be covered by cement for proper binding.
Mortar made with fresh batch of cement will have higher strength compared to mortar made with stored cement.
The more is the storing time period of cement, the less strength will it remains. Cement which has been stored for about 6 months will show 30% less strength as compared to fresh cement.
The table below shows the loss of strength of cement with time
Period of storage
% Loss of strength of cement
Strength of mortar is very important for a good masonry work as it influences the overall quality of the building.
Quality of mortar mix improves when ingredients are first dry mixed and then with water.
Always instruct your construction workers to mix sand and cement in dry condition before adding water to it.
Wet sand particles have the tendency to stick together. This makes it difficult for cement particle s to coat around sand.
This results in a non-uniform mix that is reducing the mortar quality, because each sand particle should ideally be fully covered with cement.
Further, adding water together with sand and cement, makes mixing the mortar extremely difficult for the laborers.
The water should be enough to maintain the fluidity of the mortar during application, but at the same time it shall not be excessive leading to segregation of aggregates from the cementitious material.
Mortar should always be mixed over a nonabsorbent platform and away from win rain and sunshine.
Normally the working time of cement mortar is generally 30 minutes. After which it becomes difficult for application.
Because of wind and sunshine mortar tends to loss it’s water content and makes it less workable. Also, during high temperature mortar tends to set rapidly making it difficult to work with.
This makes the mortar useless for any application.
Rain adds extra water and mortar becomes unusable too. Also, addition of rain water reduces strength of mortar.
At a time, only as much cement mortar as would be required for 30 minutes should be mixed.
In hardened mortar, the hydration process of the cement has started. When the same mortar is remixed, it destroys the bond between cement and sand. This bond cannot regain strength again by simply adding fresh water to the mortar.
As cement mortar sets relatively quickly (approx. 30 minutes), it should never be mixed in huge quantities.