This section is from the book "Cyclopedia Of Architecture, Carpentry, And Building", by James C. et al. Also available from Amazon: Cyclopedia Of Architecture, Carpentry And Building.
To obtain a satisfactory result, bricks should never be laid in freezing weather. If the temperature is much below 40° F. during the day, so that it is likely to freeze during the night, salt may be mixed with the mortar and the top of the wall well covered with boards and straw, and if the upper courses are found to have been frozen over night they must be taken down and re-laid, as the alternate freezing and thawing will materially damage the strength of lime mortar and will entirely ruin mortar made of natural cement of the Rosendale type. "Mortar made of one part of Portland cement and three parts of sand is entirely uninjured by freezing and thawing." If it is absolutely necessary that brickwork should be laid in freezing weather with natural cement mortar, it may be done by mixing the mortar "with water to which salt has been added in the proportion of one pound of salt to eighteen gallons of water, when the temperature is at 32° F. and, for each degree of temperature below 32°, add three additional ounces of salt. Mortar mixed with such a solution does not freeze in ordinary winter weather, and hence is not injured by frost." In addition to this, the bricks should be warmed to remove any ice or frost.
These methods may be used in emergencies, but the laying of bricks in freezing weather is not to be recommended, if it can be avoided.