How Does Lime Mortar Work? A Chemical Cycle That Returns to Its Start

The chemistry runs in three stages and returns to where it began. Heating limestone, which is calcium carbonate, to around nine hundred degrees drives off carbon dioxide and leaves calcium oxide, called quicklime, which is caustic and reacts violently with water. Slaking, meaning adding water, converts it to calcium hydroxide, which is lime putty or hydrated lime and is the workable material, and the reaction releases considerable heat. Mixed with sand and applied, the mortar then carbonates, reacting slowly with carbon dioxide from the air to become calcium carbonate again, which is chemically the limestone it started as. The last stage is the slow one, proceeding from the surface inward at a rate that means a thick wall may still be carbonating decades later, and it requires access to air and to moisture, so lime does not set underwater and does not set in a sealed joint. That slowness is a limitation and the source of the material's characteristic behaviour.

The properties follow from the chemistry and from the material's softness:

• •It is vapour permeable, so moisture entering a wall can evaporate through the mortar joints rather than being trapped, which matters enormously in buildings with no damp-proof course
• •It is softer than most historic stone and brick, so movement and salt crystallisation damage the mortar rather than the stone, and mortar is far cheaper to replace
• •It accommodates movement, since a lime-bedded wall flexes slightly and redistributes stress instead of cracking at a fixed point
• •It self-heals to a degree, because free lime dissolves in water passing through a hairline crack and redeposits as carbonate, closing it
• •It is reusable, since old lime-bedded bricks can be cleaned and used again, which cement-bedded ones cannot
• •It absorbs carbon dioxide while curing, which recovers part of what was released during burning, though not all of it

Portland cement, patented in 1824 and dominant from the late nineteenth century, sets by hydration rather than carbonation, which means it hardens in days rather than months, sets underwater, reaches far higher strength and works in cold conditions. Those are decisive advantages for modern construction and are the wrong properties for a historic wall. Cement mortar is harder than most old brick and stone, so when movement or freezing occurs the stone spalls instead of the joint. It is far less permeable, so moisture that enters the wall cannot escape through the joints and instead leaves through the face of the stone, carrying dissolved salts that crystallise and blow the surface off. It is rigid, so a building that previously flexed now cracks. The result is a well-documented pattern of damage, recognisable as eroded stone faces beside intact cement pointing, and the conservation consensus for over half a century has been that historic solid-wall buildings should be repointed in lime.

Lime is not one material. Non-hydraulic lime, from pure limestone, sets only by carbonation and is the softest and most permeable. Hydraulic limes, made from limestone containing clay impurities, contain compounds that set by reacting with water as cement does, giving a range of strengths classified by how hydraulic they are, which allows a specifier to match the mortar to the strength of the masonry rather than defaulting to the hardest available. Pozzolanic additions, including volcanic ash, brick dust and calcined clay, add hydraulic behaviour to a non-hydraulic lime, a technique the Romans used extensively and which explains the durability of Roman concrete, with recent research suggesting that lumps of unmixed lime in Roman material provided a self-healing reserve. Working with lime demands different practice: protection from frost and from drying too fast, keeping the work damp for days, and patience, since strength develops over months. The revival of these skills has required deliberate training programmes, because the knowledge was very nearly lost within two generations.

Burning limestone drives off carbon dioxide, slaking with water gives a workable putty, and the mortar then slowly reabsorbs carbon dioxide and becomes limestone again, a cycle taking months or years and requiring air and moisture. That leaves a permeable mortar softer than the stone around it, so moisture escapes through the joints and damage falls on the cheap component. Cement sets hard and impermeable, which traps moisture and destroys the stone face, which is why historic walls are repointed in lime.