Its well known that mortar needs a temperature above about 4-5 degrees C to set properly.

Various websites state that protection, covering, or other tricks may be used for lower temperatures, but they are very nonspecific - none of them seem to have a table that says "at X temperature, these precautions are sufficient". I need a more specific answer, on the lines of "this will be good enough to ensure the mortar sets properly".

I'm laying a small amount of structural masonry. The masonry is 2 courses of dense concrete block on foundations (the foundations have already cured), and is outdoors. The mortar is 1:3 sulphate resisting cement to sharp sand, with 1:10 of sika-1 waterproofer in the water (there's an adjacent overflowing drain, I can't fix it so I'm doing what I can to keep it from permeating!).

Eventually the blocks will support a 1 storey external wall of a building, but that part will be done by bricklayers. I need to get some of the base blocks in early to let me put a neighbour's fence back though :)

Here's a picture to give an idea:

enter image description here

I don't plan to use any other accelerants or additives, as they probably don't solve the basic problem and may provide false comfort, or not react well with the waterproofer. I'd rather rely on protection/warming only.

I have polythene sheet (DPM), hessian, and towels laying around. I also have some 3kW warm air blowers if needed, although that would mean setting up an enclosed shelter safe for electrical appliances around the block work, and a risk of over warming/drying, so I'd really prefer to avoid that.

Given temps around 0- 3 degrees (but not likely to be lower, maybe at worst -1 or -2 at night if very cold later on), what level of protection is appropriate to be sure the mortar sets nicely, and for how long should I use it?

  • What is "hessian"?
    – isherwood
    Commented Nov 20, 2018 at 14:31
  • On concrete pours we would have calcium added when the temps were getting close to freezing 2% was what we normally ordered. Tenting the area may be enough.
    – Ed Beal
    Commented Nov 20, 2018 at 14:41
  • Isherwood - hessian is a rough cloth used for various things in construction, including keeping moist things moist. Not sure how good a thermal insulator it is, because of the open weave, but it is used in this context.
    – Stilez
    Commented Nov 20, 2018 at 15:50
  • Ed Beak - that's concrete, tends to be more bulk than mortar so it generates some of its own heat too. Not sure how applicable that is to ordinary mortar and blockwork, especially as I'm reluctant to use additives as I'm unsure of any side effects or interactions with the existing mix ingredients.
    – Stilez
    Commented Nov 20, 2018 at 15:52

1 Answer 1


Just tent the project for a few days. The heat from the ground and solar gain will keep the mortar from freezing. Cure will be slower, but that's ok if you aren't in a hurry. You could introduce a small electric heater, but don't go beyond that.

Be wary of running heaters inside the enclosure. It's very easy to dry out the mix to the point where it doesn't cure at all. I've seen front porches, beautifully formed, crumble under foot traffic after several days in a heated tent. The mix was completely dry near the heater.

  • Exactly, I could heat but very wary of the risks of doing so. I'm not in a hurry, and tenting would be easy. So would just covering with something and polythene on top, held down at the edges. But is this ok, and how much insulation/what degree of tenting/how sure is this? I don't want to have much risk of it going wrong, and can't find any kind of "formal" reference for how much of what covering is enough.
    – Stilez
    Commented Nov 20, 2018 at 15:53
  • Just a sheet of poly should do if you're not expecting air temps much below freezing. I wasn't thinking of any insulation at all. Just try to seal it up fairly well along the ground.
    – isherwood
    Commented Nov 20, 2018 at 16:25

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