Cold weather does more than just make masons uncomfortable. It also reduces the strength development of freshly laid masonry. There are two reasons for this: slower hydration of the portland cement in the mortar and destructive expansion of the mortar as the water in the mortar freezes. If the mortar contains more than 6% water, the expansion due to freezing will be great enough to crack the mortar. SELECTING MATERIALS The contractor may decide to speed hydration by using a high-early cement (Type III) or by using an accelerator. Calcium chloride can be used as an accelerator, but because it corrodes metal, don't use it in masonry that contains joint reinforcement, wall ties, anchors, or other metal. HEATING MATERIALS When laying masonry in cold weather, always follow this rule: Keep the mortar temperature a minimum of 40 degrees Fahrenheit. The easiest way is to heat the mix water. At temperatures below 32 degrees Fahrenheit, also heat the sand. Proprietary steam heating units can be used. PROTECTING THE WALL Depending on how cold it is, wind screens, insulated blankets, temporary enclosures, or auxiliary heat may be needed to protect the newly constructed wall sections. PROTECT WORKERS TOO Cold weather also slows workers and makes them less productive. Consequently, the more that workers are protected from the cold, the more masonry they can lay.