Q: As fall encroaches upon us with its cold weather, a question that is going to be popping up with more frequency is… Is it too cold to start or finish my project with NHL?
A: Using natural hydraulic lime in cold weather. This may be one of the most difficult questions to answer, where the wrong answer can result in a lot of lost time and damage. The simple and safest answer is, do not perform work with NHL when temperatures will fall below 40 degrees Fahrenheit (including wind chill) for at minimum 7 days after placement of the mortar. Even following this model answer can result in frost damage to the mortar in some cases. So what is an applicator to do, to ensure that their work will last a reasonable amount of time? Well, care, caution, attention to detail, and patience is the answer.
If you can wait until spring to complete the project then you probably should play it safe. If however, you need to complete the project before winter and do not mind playing with fire, then tenting and heating the scaffolding is an option. Tenting and heating can cost a lot of extra money in labor and fuel, so make sure that there is enough money budgeted aside to warrant this approach.
By completely encasing the scaffolding envelope with heavy-duty plastic and being sure to affix the uppermost part of the plastic to either the roof or its eave, one can create a tight enclosed space for heating. When using a heater make sure that it is in a safe place and slightly raised off the ground. You should refer to any local building codes to make sure that you comply and most importantly safe.
Of course, good masonry practices should not be skipped over just because the work is tented in and heated. Damp curing with burlap is still recommended and when repointing work is being executed good compaction of the mortar against the background mortar is still a must.
There are a few other tricks available to the applicator that can help prevent frost damage. Using natural hydraulic lime in cold weather is tricky. One is the use of air entrainment in the mortar. Careful dosing of an air-entrainer can help but not eliminate frost damage due to improper curing practices. Adding air entrainment must be done with caution, as too much air in a mix will make the mortar weak and friable. When using proprietary admixtures, proper testing should be conducted to make sure that there are no adverse side effects.
Lowering the water content of the mix and increasing mixing time will help reduce the amount of water available to freeze without sacrificing too much workability. The use of warm mixing water, preheated sand as well as preheating masonry units will help slow the development of frost. Winter accelerators associated with working with Portland cement mixes should not be used, such as calcium chloride or any nitrates. Depending on the properties of the surrounding masonry units NHL 5 can be appropriate to use in a masonry mortar to help withstand frost, due to its faster setting time.
When in doubt about working in possibly freezing conditions or allowing recently completed work to be exposed to freezing temperatures, you should probably trust your gut and call it a season. Dancing with the weather can be costly and should be avoided.
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