Mortar Is Not One Material

The mortar takes up the small differences between units, spreads load, holds the wall together and is meant to be the sacrificial part. Joints weather back over time, and some erosion is normal and even intended in a wall designed to be sacrificial at the joint. Erosion that begins to expose the units is the point at which attention is needed.

The joints are also a large proportion of the wall's surface, so their behaviour is not a detail. They are usually the most permeable path through a masonry wall, and how readily they let moisture move and evaporate is part of what keeps the units themselves from staying saturated.

Soft handmade brick and many building stones were bedded in lime mortar, which is weaker, more permeable and better able to accommodate small movements. Repointing such a wall in a hard cement-rich mix traps moisture behind the joints and concentrates damage in the historic fabric.

The visible result is counter-intuitive and easy to misread. A wall that looks sound with crisp joints can be quietly shedding the faces of its bricks, and a hard modern mortar can destroy soft reclaimed bricks while leaving the joints themselves intact.

Specifying lime mortar without saying which lime leaves the most important decision unmade. Air lime sets by taking carbon dioxide back out of the air, working inward from the exposed face and needing air and moisture present to continue at all. Natural hydraulic lime gets part of its set from reaction with water, and carbonates afterwards over a much longer period.

Hydrated lime added to a cement mortar is doing something different again: it is there to make the mix more workable and more forgiving rather than to act as the binder. Because the mechanisms differ, so does the protection needed, the weather work can be done in, and when the work can be built on.

The mortar has to suit the suction of the units. A very absorbent unit draws water from the mix quickly, and a dense low-absorption unit does the opposite, and both change how the joint sets. Reclaimed bricks add unpredictability, because absorption is unknown until they are examined and can differ within a single load.

Unit families also move in different directions. Fired clay tends to take up moisture and expand slowly after the kiln, while calcium silicate and cement-bound units tend to move in the drying direction. That is why mixing families within one leaf, or repairing one with the other, is a design question rather than a matter of matching the look.

A concrete repair mortar is built onto an open, prepared, often vertical face with exposed reinforcement, so it has to stay in place against gravity and move with the surrounding concrete. A non-shrink bedding grout goes into a confined gap between two elements that is transferring load, and filling it completely is the whole job. The confinement is what separates them.

External paving bedding and jointing is a third case again. A paving joint is horizontal, holds water, takes abrasion from feet and tyres, and is what usually fails first in a paved area. Carrying interior tiling habits outdoors, where the exposure is completely different, is a recurring source of trouble.

Colour comes mainly from the sand, with the binder and any pigment modifying it, so matching a repair means matching the sand as much as the binder. Sand from a different pit changes mortar colour noticeably even when the recipe is unchanged, and a change of mixing practice shows as bands across a wall.

The mix used should be recorded, including the sand source, because a repair made years later can only be matched if someone knows what went into the original. That record costs nothing at the time and is almost never made.

Mortar is designed to be the renewable part of a wall, and repointing is ordinary maintenance rather than an intervention. The difficulty lies in matching the original in colour, texture and behaviour rather than in the work itself, and in not making the wall worse in the process.

One mechanical point is worth stating plainly: repointing carried out with a grinder can cut the edges of the units and leave damage that cannot be undone. That is a workmanship question to settle before anyone starts rather than after the first elevation.