Lime mortar

Limestone burnt to quicklime and slaked with water gives lime putty, which sets by absorbing carbon dioxide from the air and turning back into calcium carbonate. The material returns to the rock it came from, and the whole cycle is closed.

It is much weaker than cement mortar, and in masonry that is an advantage rather than a compromise. A mortar joint should be the sacrificial element: softer than the brick or stone, so movement cracks the joint rather than the units, and repointing repairs the wall. A cement mortar stronger than a soft historic brick does the opposite — the brick faces spall off and the joint survives, which is why hard repointing has damaged a great many old buildings.

Lime is also permeable, so a wall built with it lets moisture evaporate out. Sealing a solid wall with impermeable cement traps water inside it, which is a common and expensive mistake in building conservation.

And it self-heals: fine cracks fill with calcium carbonate dissolved and redeposited by water moving through the joint, which is a real observed behaviour and part of why medieval walls are still standing.

Limestone is calcined at around 900 °C to quicklime, slaked with water to calcium hydroxide, and matured as putty — traditionally for months, and the maturation genuinely improves it. Mixed with sand and used.

Carbonation is slow and proceeds from the surface inwards at a rate measured in millimetres per year, so a thick lime wall can take decades to fully set. Hydraulic limes, which contain clay impurities and set partly by reaction with water like cement, are the middle ground and are what most conservation work uses.

Masonry mortar, render and plaster in traditional building, and in conservation of it — which is now the principal use, and is a requirement rather than a preference on most historic fabric.

Limewash as a breathable exterior finish. And, historically, essentially all masonry construction before Portland cement.

Lime plaster is Neolithic. Roman concrete is lime-based, and its durability comes from adding volcanic ash — pozzolana — which reacts with the lime to give a hydraulic set that works underwater. Roman marine concrete has survived two thousand years of seawater, which modern Portland cement concrete conspicuously does not.

Portland cement from 1824 displaced lime almost entirely within a century, on speed and strength. The rediscovery of lime for conservation from the 1970s onward followed the visible damage that cement repairs had done to historic buildings.

Burning limestone releases carbon dioxide from the rock, exactly as cement production does — and lime mortar then reabsorbs a large part of it as it carbonates over the following years. That reabsorption is real and is the basis of the claim that lime is the lower-carbon mortar, though it is slow, incomplete, and does not recover the fuel emissions.

Its stronger environmental argument is that a lime-built wall can be taken apart and its units reused, because the mortar is weaker than the brick. A cement-mortared wall generally cannot, and becomes rubble.