Hot-mixed vs NHL lime: what we actually use on Somerset cob

There are two limes in common use for historic masonry repair in the South West, and they are not interchangeable. Hot-mixed lime — made on site by slaking quicklime directly into damp sand — and natural hydraulic lime (NHL), supplied dry in a bag. Each has its place. Choosing the right one for the wall in front of you is most of the job.

What hot-mixed lime actually is

Hot-mixed lime is the traditional method. You take a measure of pure calcium oxide (quicklime, CaO), tip it into a gauging box of damp sand and aggregate, add water, and stand back. The quicklime slakes with a violent exothermic reaction — steam, heat, the whole mix expanding — and within an hour you have a hot, soft, sticky, intensely workable mortar that is mostly calcium hydroxide with a tiny excess of quicklime running through it. It sets slowly by carbonation: reabsorbing atmospheric CO₂ over weeks and months, hardening as it does so.

It is soft. It is breathable. It is sacrificial. It is unbeaten on the most delicate historic fabric — cob, soft sandstone, lias rubble, hand-made brick. The original mortars in most pre-1900 Somerset walls were hot-mixed, and a hot-mixed repair is genuinely indistinguishable from the original after a few seasons.

What NHL is, and where it belongs

Natural hydraulic lime is fundamentally different. It is fired from a naturally argillaceous (clay-containing) limestone, ground, and bagged. When you add water, the clay component reacts and gives the mortar an initial hydraulic set — a measurable strength within days, before any carbonation has happened. NHL is graded by strength: NHL 2 (feebly hydraulic, soft), NHL 3.5 (moderately hydraulic, medium), NHL 5 (eminently hydraulic, harder than most historic walls need).

NHL is faster to work with, more predictable in cold or damp conditions, and gives an earlier mechanical strength. It is the right choice on exposed elevations, on hard stone (granite, hard limestones), and on works where you cannot afford to protect new mortar from rain for two months. NHL 2 is appropriate on soft historic walls; NHL 3.5 on harder stone; NHL 5 almost never on historic fabric and never on cob.

How we choose, in practice

For Somerset cob, the answer is always hot-mixed lime, with no NHL component at all. Cob is so soft and so movement-prone that any hydraulic set will crack off the substrate within a winter. We use hot-mixed haired mortars for both the structural repair (in lifts) and the render coats over it.

For soft red sandstone cottages around Wellington and the Blackdowns, hot-mixed lime again. The stone is too soft for an NHL — even NHL 2 is harder than the stone face.

For lias rubble walls on the Somerset Levels and around Glastonbury, hot-mixed for most domestic and listed-building work; NHL 2 occasionally for very exposed elevations or autumn-season pointing where carbonation will be slow.

For harder stone — Ham stone, Doulting limestone, harder oolites — NHL 2 or NHL 3.5 is often the right choice. It still moves with the building. It still breathes. It just sets faster.

The honest summary

Most of our Somerset work uses hot-mixed lime. NHL has its place — and we use it where it belongs — but it has been over-specified in the conservation world for the last twenty years, often by people more comfortable with a bag than a slaking box. If your specification calls for NHL 3.5 on a cob cottage, ask the question. It is almost certainly the wrong call.