Stone Repair Training Course

Course Outline for our Stone Repair Training Day: Using Lime-Bound Repair Mixes, including lime putty, hydraulic lime, natural cement and premixed Restone, Socli, Lithomex and St. One Lime-Based Stone Repair Mortars

The Theory

Training begins with identifying why stone or brick is decaying and how to address the causes before repairing historic brick or stone. Salts, wind, iron, organic growth, incompatible mixes of stones on a façade and human activity can damage historic fabric. We will consider options such as piecing in new stone or in-depth repairs, as well as mortar-based repairs. The appropriateness of new stone compared with mortar repairs for particular works involving buttresses, string courses, hood-moulds, arches and almost horizontal surfaces.

Training covers the theory and practice of how stone repair using lime-based materials has developed since it was advocated in the late Victorian Era. You will learn the main properties required of the mortar, plus pozzolanic reactions, NHL chemistry, carbonation vs hydration, and why excessive cement is avoided.

Lime putty alone hardens by carbonation, a slow process that depends on atmospheric CO₂ penetrating the mortar. In deep stone or brick repairs, carbonation can be too slow to develop adequate strength. One option is the use of pozzolans, which are siliceous or alumino-siliceous materials that, in finely divided form and in the presence of moisture, chemically react with calcium hydroxide (the active component of lime putty) to form calcium silicate hydrates (C-S-H) — the same strength-giving compounds found in cement, but formed at ambient temperature and without the damaging properties of Portland cement. The pozzolanic reaction provides faster and greater strength development than carbonation alone, more predictable setting times compared to pure lime putty, and increased resistance to erosion and weathering, but unlike

cement, lime-pozzolan mortars remain vapour-permeable and compatible with historic masonry.

Other options are also available after decades of European and then UK-based lime suppliers manufacturing premixed stone repair mixes that avoid excessive use of cements or resins. Different lime-bound stone and brick repair brands will be presented during the training day.

The trainer, Dr Mark Womersley, will guide attendees through substrate preparation, cleaning, pre-wetting control, reinforcing/anchoring for deep repairs, edge preparation, mixing & application theory, water ratios, layering limits, compaction, shaping windows, and tooling. The training day uses samples on walls prepared the previous day and those applied during the training days, providing attendees with the full range of skills for finishing traditional ‘plastic’ mortars and solely lime-based repair mortars.

The Practical

After ensuring attendees understand safety precautions, the corrosive nature of lime, dust control, and first aid procedures. The group will carry out substrate preparation, cleaning, and misting. Before work starts, soak the masonry thoroughly, wet several times, with the final wetting immediately before applying mortar; the surface should be damp but not glistening with surface water

A range of different repair mixes will be prepared, layered, compacted, shaped and finished. Adequate curing arrangements will be set up to retain moisture and offer protection from sun/wind/frost.

Practical experience will be gained in surface repairs, stone simulation, brick and stone patching, deep reconstruction and architectural ornament repairs. The group will gain experience with repairing sandstone, limestone, brick, and terracotta. Colour matching will be carried out using aggregates and mineral-based iron oxide pigments. Always test colour and texture match on a small, inconspicuous area before full application; keep records of mix ratios, water addition, application dates, and environmental conditions; and consider using lime washes, fresco work, and water glass-bound pigments.