The bond strength of mortar is considerably reduced when mortar is frozen prior to hardening. The chemical reaction between water and cement (hydration) progresses very slowly below 40℉. Protection is necessary if the outside air temperature is 40℉ and falling.
Admixtures or anti-freezes should not be used to lower the freezing point of mortar. The effectiveness of most of these compounds is due to the calcium chloride they contain acting as accelerators.
Calcium chloride cannot be used on limestone. Salts cause efflorescence and may cause spalling or flaking through re-crystallization (crystal growth).
Heating all materials must be considered. Sand contains some moisture that will form ice when stored in freezing temperatures and must be heated to thaw the ice. Sand must be heated slowly to prevent scorching. Mixing water should not be above 160℉ to prevent the danger of flash set with cement. The mortars should be between 40℉ and 120℉ when used.
Stone should be covered with tarpaulin, felt paper, or polyethylene, and heating units used to warm the stone. Caution must be used to prevent smoke under the covering from salamanders.
Never set stone on a snow or ice-covered bed. Bond cannot develop between the mortar bed and frozen supporting surfaces. If stone is to be set during cold weather the cold weather masonry construction recommendations of the International Masonry Industry All-weather Council should be followed.