Is lime based mortar weatherproof?
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.