Lime is central to building conservation and has been used for centuries in historic construction. We use lime exclusively as the binder in our mortars and renders.
Lime mortar offers many advantages over modern cement. Unlike cement-based mortars, lime mortars are softer and more porous, allowing moisture to evaporate through the joints rather than through the surrounding bricks or stone. This prevents excess moisture from building up within the walls and reduces the risk of harmful soluble salts forming in the masonry — salts that often lead to deterioration, such as the spalling of bricks and stone.
For most of our work, we use non-hydraulic lime in putty form for mortars. In certain situations, we add a pozzolan (such as dust from soft, low-fired bricks) to assist the setting process. In some cases, hydraulic mortars based on Natural Hydraulic Lime (NHL) are used. Lime putty benefits from being matured — the longer it is left to stand, the more plastic it becomes, improving workability and enhancing the bond between mortar and masonry by resisting suction from the substrate.
Much of our work involves replacing cement mortar with lime mortar. Old mortar must be carefully raked out to a good depth using the correct tools, as working with precision is essential to avoid damaging the building fabric. Before applying new mortar, joints are dampened to prevent the masonry from absorbing too much water, which could cause the lime mortar to dry too quickly and lose strength. Care is also taken to avoid smearing or staining the masonry. Mortar should be as dry as practical for pointing, allowing maximum compaction in the joints, reducing shrinkage cracks, and limiting smearing.
A typical mix might be 2.5:1 coarse sharp sand to lime putty, with pozzolan added where needed. The quantity of pozzolan varies depending on the hardness of the masonry, the degree of exposure, and the time of year. Weather conditions play a significant role in the choice and performance of lime mortar. Colour is also critical, controlled by the type of aggregates, lime, and pozzolan used, to ensure new mortar blends with the original.
We apply mortar by forcing it into the joints with handmade pointing keys or a metal trowel, working from a hawk. Every cavity is filled to eliminate air pockets, reducing the risk of failure. As the mortar begins to set, we monitor it closely, spraying with water to keep it moist as required. We often use hessian to retain moisture and protect the wall surface, with plastic sheeting to shield the work from sun or wind. These measures help regulate the drying process and ensure the mortar achieves the desired strength.
Once the mortar has “gone green” but remains firm, we finish by compacting it with a churn brush. This not only strengthens the bond with the masonry but also creates a traditional textured finish. The brushing highlights the aggregates, giving the mortar a natural, less uniform appearance that blends sympathetically with the character of the building.
