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Will a mortar made of hydrated lime and sharp sand (no portland in the mix) adhere to a fully cured portland cement substrate subject to freezing temperatures in winter?

Context is bedding sandstone slabs upon an existing concrete slab. Work would probably be done late summer, early fall, a couple of months before freezing temperatures arrive.

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I doubt it will adhere well at all, hydrated lime is a pretty weak binder.

Sandstone is notoriously difficult to bed as it's so porous. Regions vary of course, but I've always used a cement and sand mortar (usually 1:5 ratio) AND a slurry applied to the back of the sandstone before laying. Special primers/adhesives are available for this purpose.

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  • If the bedding mortar doesn't adhere well, what is the result? Does the stone work itself loose or tend to crack? It's light foot traffic only.
    – mr blint
    Commented Aug 1, 2023 at 21:49
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    The mortar will probably stick to the concrete below better than the sandstone. The slabs will become loose. Does depend a bit on how you point up the joints... It is possible that a proper mortar in the joints (rather than sand or some brush in rubbish) can hold everything in place. But gaps or de-bonded slabs can be lifted by frost etc. over time
    – handyman
    Commented Aug 3, 2023 at 6:58
  • I had to use portland cement sand topping mix to create a drainage slope on the existing slab (installed decades ago) because it was level, and tech support at the company that sells the sand topping mix told me it would retain a lot of moisture. The reason I was tending towards the hydrated lime cement for the bedding mortar (and a richer hydrated lime mortar for the joints) is that I'd read the hydrated lime mortar "breathes" and would allow moisture to escape from below. But that's only based on my reading; I have no experience with masonry and mortars (except for indoor tile).
    – mr blint
    Commented Aug 3, 2023 at 13:06

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