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.
Why Use Fresh Cement for Mortar?
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. Strength of mortar is very important for a good masonry work as it influences the overall quality of the building.
Why Mix Dry Ingredients (i.e. sand & cement) First Before Adding Water?
Wet sand particles have the tendency to stick together and are therefore hindering that cement can cover them. 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 I one go makes mixing the mortar extremely difficult for the laborers.
Why to Protect Mortar Mixing Place from Wind, Rain & Sunshine?
Wind and sunshine is entraining the water from the mortar and is accelerating the hardening process before it is being used. This makes the mortar useless for any purpose. Rain adds water and mortar becomes unusable too.
Why Not Use or Re-use Mortar That Has Already Set?
In hardened mortar, the hydration process of the cement has started and re-mixing of it is destroying 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 mixed in huge quantities.