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Hot Weather Mortar
 Concrete and mortar both contain cement, water, and aggregates. But unlike concrete, adding water to mortar, is not only allowed, it is encouraged. This process is referred to as retempering. Adequate water content in mortar is necessary to maintain a workable consistency. This allows time to place units, and allows for strength and bond development. It is vital to understand this concept to get good results in masonry construction, especially during warmer weather.
As summer temperatures soar, water evaporates out of mortar. That water should be replaced by mixing additional water into the mortar, or retempering. And though retempering is accepted practice, there are things you can do on the job to reduce the need to add water to mortar.
Seven Tips for Preventing Water Loss from 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.
- For extreme heat, add ice to the water.
- Cool mixers, wheelbarrows, mortar pans, and other metal equipment by flushing 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 between 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. Be aware that 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 set time of the mortar, but they do not reduce evaporation rates. Discard mortar that has not been used within two hours and mix fresh mortar.
Other Resources:
Trowel Tips: Hot Weather Masonry Construction
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