Masonry Magazine November 1965 Page. 23
Cold & Hot Weather Aspects of Mortar
(continued from page 10)
Help the masons be more efficient. Consistent workability and consistent rate of hardening will help the mason properly plan and efficiently accomplish his work. For this reason, in cold-weather construction as well as conventional construction we strongly recommend the use of masonry cements for mortar.
Since masonry cements are factory prepared under high-quality controlled conditions, they are consistent bag to bag, consequently, consistent batch to batch at the job site. This consistency holds true for every physical property be it color, workability, rate of hardening, or any other physical property we may desire to measure. In addition, masonry cements contain air-entraining agents that entrain air in the mortar mix. In the hardened mortar this produces a mortar that is much more durable during freeze-thaw cycles than a conventional mortar mix.
To obtain full air-entrainment, the mortar should be mechanically mixed in a mechanical mixer, and the material should be mixed for a full five minutes after the last shovelful is added. Frequently on the average job, we see the mixer-tender start timing when the first shovelful is added. Oftentimes, the mix is continued for only a minute or so after the last shovelful is in the mixer. Full mixing time is necessary because the agitation that the mechanical mixer provides is necessary. A full five minutes is necessary to provide the mixing action that entrains air.
Another important point is that if the mixer blades are full of hardened material, they are not providing the mixing action that the mixer-designer designed into his mixer. Mixer blades that are clogged with hardened material can act no better than clubs being pulled through the mix rather than mixer blades that provide proper agitation. A faulty engine with inadequate power can also result in a mix without proper air content.
An air-entrained mortar mix also has an additional advantage and that is air-entrainment helps improve water retention. Water retention is important in providing good bond and reducing plastic shrinkage cracking. This ties in with the third point necessary to accomplish proper hydration, presence of H2O. Rapid loss moisture from a mortar mix can also cause plastic shrinkage cracks and poor bond at the point of contact between the mortar and the unit. Consequently, a watertight wall is not obtained if this occurs.
In one of the earlier discussions today the subject accelerators was brought up. We recommend that calcium chloride be used only as an accelerator in quantities of up to one percent for masonry cement and up to two percent for portland cement and that they not be regarded as anti-freeze agents. A two percent content of calcium chloride will lower the freezing point somewhere between two and three degrees. We all know that weather forecasting is not accurate to the point where we can predict temperatures to within plus or minus two degrees. We are concerned with temperatures (continued on page 25)
ASONRY November, 1965
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