If you use Type III portland cement, can you repoint brick masonry when the wall temperature is below freezing?
I do not recommend repointing when the surface temperature of the brick masonry is 40 F or less. Although Type III portland cement will perform slightly better than Type I in this situation, the advantage may not be enough to prevent freezing. Type III portland cement performs better because it hydrates faster. All portland cement contains both tricalcium silicate and dicalcium silicate. Tricalcium silicate hydrates faster than dicalcium silicate. Type III portland cement contains a greater concentration of tricalcium silicate; therefore, it generally hydrates faster. Furthermore, Type III cement is finer, and will also hydrate faster for this reason. The faster the portland cement hydrates, the less time the mortar must be kept warm. When mortar hydrates faster, it generates more heat. And the warmer the mortar, the faster it will hydrate. Most Type I portland cement sold today, however, contains fairly high concentrations of tricalcium silicate and is ground more finely than in the past. This brings it close to the requirements of Type III portland cement, so there may be little practical difference between Type III and Type I portland cements. There is a greater risk of mortar freezing when repointing than when installing new masonry during cold weather, because the volume of repointing mortar compared to the volume of masonry is so small. If the wall's surface temperature is less than that of the mortar, the repointing mortar will lose heat quickly regardless of the type of portland cement used. When new masonry is installed during cold weather, the entire mortar joint is new and more of the wall may be warm. Using Type III portland cement raises other questions as well. Repointing mortar typically is prehydrated for 1 hour before it is used. I am not aware of any studies that indicate whether or how long Type III mortars should be prehydrated. Also, depending on the mortar's temperature, its board life may be reduced. The normal working time of 2 1/2 hours may be far too long for mortars made with Type III portland cement.