Mortar is used for holding masonry or rock materials in place -- it is composed of Portland cement, hydrated lime and sand combined with water. The proportions of these three ingredients vary depending on the type of mortar. Mortar types differ in strength and best-use situations. Three of the most common types of mortar are M, S and N, though types O and K may be suitable for historic purposes and repair work. The best type of mortar for a rock wall depends on whether the wall is freestanding, serving as a retaining wall or is being built to imitate historic rock walls.
Recommended Mortar Type
For freestanding rock walls, type N mortar is often the best and most widely-available mortar to use. The Portland Cement Association recommends as a general rule of thumb that type N mortar is suitable for most masonry work. Type N, which consists of one part each Portland cement and hydrated lime and six parts sand, is used for exterior above-grade walls that will be exposed to weathering. Type O mortar can be utilized in non-load bearing, above-grade situations. Also known as "pointing" mortar, type O mortar is a mix of one part Portland cement to two parts hydrated lime and nine parts sand.
Rock walls that are built in slopes may benefit from the use of a mortar suitable for at or below-grade applications. The recommended mortar type for this purpose is S. Type S mortar consists of two parts Portland cement, one part hydrated lime and nine parts sand.
Type M mortar is a suitable alternative for types N and S in above-grade, load-bearing walls and below-grade walls. Type M is the strongest type of mortar and is composed of three parts Portland cement, one part hydrated lime and 12 parts sand.
Historic or Repair Considerations
Type K mortar, a mixture of one part Portland cement, three parts hydrated lime and 10 parts sand, is useful for historic preservation purposes where load-bearing strength is unimportant. Straight lime mortar was utilized to construct rock walls before Portland cement was available and is only used now to recreate historical methods. Types O and K, though weaker than M, S and N, have better sealing and adhesion, often making them better-suited for repairs than the stronger mortars.
Proper mortaring techniques can be just as important as selecting the best type of mortar for the rock wall. Add enough water that the mortar will have a consistency that allows it to be packed between rocks but not run out wherever placed. When mortaring a wall, especially a retaining wall, it is important to include weepholes spaced regularly throughout the wall to allow for drainage. Cover mortar work at the end of each day and moisten old masonry before attaching new mortar to allow proper curing.