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?
Best Answer
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.