When repointing a stone arch bridge using the mortar mixes that are sold in bags, one is faced with a variety of mortar choices. This, of course, begs some questions, such as what is the difference and why does it matter. In this post, we will look at what makes the mortar types different and how this relates to stonework.
The Types
The basic types of mortar are M, S, N, O and K. These letters are not entirely arbitrary. Curiously enough, they were derived from the word masonwork with every other letter being used as a mortar type: MaSoNwOrK. These letters designate the relative compressive strength of the mortar, with M being the strongest all the way down to K, which is the weakest. Of course, be aware that there can be considerable variation in mortar strength from batch to batch; for instance a runny batch of a given type of mortar will tend to be significantly weaker than a properly prepared batch of the same type.
What Makes the Difference?
The basic difference in the mortar types is the relative ratio of lime and Portland cement. The more lime relative to Portland cement in the mortar, the weaker the mortar will be, yet the better it will “take” to a surface. Furthermore, while there are several other factors at play here, broadly speaking lime is vulnerable to precipitating out of the mortar over time, making the more limey mortars less durable. That said, lime mortars tend to be more elastic and forgiving of shifting within the structure. And, as an added plus, since lime is at least mildly water-soluble, cracks in mortars high in lime can actually “heal” over time. This is because any lime dissolved by rain action will tend to accumulate and harden in cracks
Properties of the Mortar Types
The compressive strength for mortar is the amount of pressure the mortar should be able to handle after being cured for 28 days. Below is the compressive strength of some of the mortar types:
- M mortar: 2500 psi.
- S mortar: 1800 psi.
- N mortar: 750 psi.
- O mortar: 350 psi.
K mortar is very soft and does not provide much compressive strength at all; it is rarely used save for restoring historic structures, which need a soft mortar to prevent damage to the masonry units. Among the more common mortar types, M mortar is ideally suited for subgrade work, while S mortar is well suited to a variety of applications, above and below ground. N mortar is best used for above-grade general-purpose use. O mortar is ideally suited for use with soft stones.
Why Does it Matter?
The key to selecting mortar in stonework is that the mortar needs to be softer than the stones used. If the mortar is harder that the stones, severe deterioration of the stones themselves can occur, as the stones will yield to the strains of the environment before the mortar will. If, on the other hand, the mortar is softer than the stones, the mortar will end up taking most of the abuse. To be sure, as the mortar will eventually crumble away, the structure will need to be repointed, but repointing is far easier than repairing sections of damaged stonework.
Also important is that the mortar should be more porous than the stones. Porous mortar will tend to absorb water instead of the stones. By this means, porous mortar helps to prevent freeze-thaw and salt damage to the stonework. Salt damage is similar to frost damage; salts that dissolve in rainwater recrystalize in the masonry when dried, expanding and potentially damaging the stones.
When porous mortar is used, the mortar will suffer over time from frost and salts, but again, damaged mortar is more easily repaired (and less serious as well) than damaged stones. Softer mortars tend to be more porous than hard mortars, which is another advantage in using soft mortars for historic stonework. Incidentally, these difficulties with water are far, far less of an issue with mortarless work, which is one major advantage with this type of construction. Mortarless construction does not hold water like mortared work does.
Mortar and Historic Bridges
Many historic structures were built using soft mortar, which is why K mortar, the softest of them all, finds considerable application for historic restorations. However, soft mortar is not always the best solution for historic structures. Relatively newer stone bridges built after the advent of Portland cement will likely have originally used a harder mortar; for instance, Walter Sharp stone bridges were intentionally built with hard mortar to make the arch angles. Thus, in a Walter Sharp bridge, a soft mortar will be an inappropriate choice for arch repairs. The upshot, then, is that it is best to determine the type of mortar originally used when restoring a historic structure. Beyond that, softer mortars are often the safer choice, as they are less likely to result in damage to the masonry. However, it may still be acceptable to use one of the harder grades of mortar on an old structure if it can be determined that the mortar is softer than the stones used in the bridge.
Stone Arch Bridges 