Differences in Masonry Mortars
By Robert Wilkin, P.E.
There are three types of mortar mixes for masonry wall construction:
- Site mixed Portland cement, lime and sand
- Premixed and bagged Mortar Cement
- Premixed and bagged Masonry Cement
Site mixed mortar is the traditional preference for mortar. However, it can have variations in mix and color due to human input in getting the proportions consistent. A number of mix proportions can be specified including:
- Type N (normal, general purpose mortar mix and can be used in above grade work in both exterior and interior load-bearing installations)
- Type M (used for below grade load-bearing masonry work and for chimneys and brick manholes)
- Type S (used for below grade work and in such areas as masonry foundation walls, brick manholes, retaining walls, sewers, brick walkways, brick pavement and brick patios)
- Type O (lime rich mortar, also referred to as “pointing” mortar)
Premixed cements are more consistent than site mixed mortar since they are bagged in the plant and added to the sand onsite. This makes mixing simpler and is preferred by masons. They also contain air entraining additives to increase weather resistance of the mortar which is only added to site mixed mortar if specified.
The formulation of Masonry Cement replaces the lime in traditional site mixed mortar with workability additives, primarily finely ground limestone and other additives to provide a workable mix. The limestone is inert and does not add to the strength of the mix. The lack of lime can also make the mortar less sticky than traditional mortar allowing more water seepage through less tightly adhered and sealed joints to the masonry units.
Therefore, there is a significant difference between Mortar Cement and Masonry Cement. Mortar Cement is specified in ASTM C 1329 to have a better bond strength, similar to traditional site mixed mortar. This is preferable for exterior masonry and structural masonry making Mortar Cement a better choice for weather resistance through lower permeability of the masonry, and for seismic and load bearing masonry. Masonry Cement specified under ASTM C 91, with lime has lower bond strengths.
Remember, Masonry Cement is generally better for the mason and Mortar Cement provides a better mortar for the Building.
When installing masonry on flat surfaces or on steps where it will be subjected to water flow and penetration, avoid the use of lime in the mix. The lime that does not combine with the cement and remains free in the set mortar will leach out and form significant white staining on the masonry. This free lime bleed out can even be sufficient enough to plug drain lines. It is better to use a sand/cement grout or even Masonry Cement for this type of application.