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Water Loss a Warm-Weather Concern for Mortar
 Unlike concrete, adding water to mortar is not only allowed, it’s encouraged, especially during warmer weather when evaporation can leave the material with a consistency that’s difficult to work with and inadequate for proper bond development.
The water that’s lost should be replaced by mixing additional water into the mortar; the process is called retempering.
There are also seven steps that can be taken to reduce water loss from the mortar:
- Cover or shade mortar materials from direct sunlight, whenever practical. You can avoid unnecessary retempering by keeping materials as cool as possible.
- Sprinkle sand stockpiles with water to restore moisture and increase evaporative cooling.
- Use cool water to mix the mortar. Water stored in a light colored, open barrel is cooled to some extent by surface evaporation. Store the barrel away from direct sunlight.
- Avoid using water from an unshaded water hose of any significant length. When exposed to sunlight, long water hoses become effective water heaters.
- In extreme heat, add ice to the water.
- Cool mixers, wheelbarrows, mortar pans and other metal equipment by flushing them with cool water. Mortar can absorb heat from contact with metal equipment.
- Flush wooden mortar boards with water prior to contact with fresh mortar to reduce absorption.
As always, mortar should be mixed for three to five minutes in a mechanical mixer using the maximum amount of water consistent with good workability. It should be retempered as needed and thoroughly remixed.
Retempering can affect the appearance of mortar joints, particularly where colored mortars are used. The use of retarding admixtures in conventional mortar systems is not recommended. Retarders delay the amount of time it takes the mortar to set, but they do not reduce evaporation rates.
Mortar that hasn’t been used within two hours should be discarded.
More information is available from the Portland Cement Association publication, “Trowel Tips: Hot Weather Masonry Construction.”
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