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I recently did some outside brick wall patching and tuckpointing. Since the old mortar was whitish, I didn't use the gray ready mix from the Home Depot but I mixed my own, the same what I used for my exposed indoor brick wall restoration: 1 part masonry lime (white powder), 2.5 parts masonry sand, plus water. I also used the same mix for some indoor plaster patches (house is old and has plaster over brick).

But I noticed that that recipe doesn't hold very well on the outside, crumbles and dissipates, unlike on the indoor walls where it's been rock solid and beautiful several years now.

Is lime based mortar not weatherproof and should there always be cement for outdoor applications?

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    What happened is either it got too wet or more likely too dry before it had a chance to fully cure. – Dan D. Jun 4 '16 at 2:58
  • Not unlikely. But it's not a complete failure, will hold some years for sure – amphibient Jun 4 '16 at 3:00
  • Does cement based mortar not need to cure that long? – amphibient Jun 4 '16 at 3:00
  • For a white mortar , use white cement. – blacksmith37 Apr 18 '18 at 15:31
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Yes, there should always be cement for outdoor mortar applications.

Lime is a cementitious ingredient in mortar which will increase workability and water retention, resulting in increased bond strength. Lime also increases plastic flow, allowing mortar to take up slight movements without apparent opening of joints.

Requirements for hydrated lime for masonry are specified in ASTM C 207. Other types of lime should not be used in masonry construction.

There are 2 types of HYDRATED lime: 1) Type S, and 2) Type N.

Type S is Special Hydrated Lime which has high plasticity, high water retention, and a maximum of 8% for unhydrated oxides.

Type N is Normal Hydrated Lime and is not recommend for mortar applications.

Mortar bond strength increases as mortar flow increases. The consistency of mortar must be controlled for brick / block suction or absorption limited. This is done with optimum ratio of high plastic lime (Type S) and use of well graded sand with no voids.

Cements come with and without air entrainment. (Air entrainment reduces bond, so usually not used in mortar.) Type I cement is usually used for mortar, unless high early strength is required due to cold weather...then use Type III. Masonry cements are proprietary and mixes are seldom discussed. You’ll usually just see that it complies with ASTM C 91.

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Lime has it's tricks to working with it.

It needs to stay moist to cure the CO2 in the air has to dissolve in water before the Ca(OH)2 + CO2 => CaCO3 reaction can proceed.

Frost interferes with the reaction.

So:

  • Wet the brick before applying Brick will pull water out of the mortar mix. This is true for lots of permeable stone too.
  • Optimum weather for lime work is 50 F, overcast, high humidity.
  • There are agents to add (casein powder, some glues) that slow down evaporation to keep the surface from drying out.
  • Mist daily, or more often in warm weather.

Lime plasters are often used in strawbale construction. May find more tips there.

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