An East Rock owner brought me a photo of two bags from the builders’ merchant and asked which one was lime mortar. Neither was. One was masonry cement, the other was a Type S bagged mix with a small lime content. The distinction sounds academic until you put the wrong one on an 1890s wall.
The family tree, briefly
Non-hydraulic lime — often sold as lime putty or hydrated lime. It sets by reabsorbing carbon dioxide from the air, slowly, over months and years. Very soft, very permeable, self-healing to a degree. Used on the softest historic work and interior plaster. It cannot set under water and it hates frost before it has carbonated.
Natural hydraulic lime (NHL) — burnt from limestone containing clay impurities, so it sets partly by chemical reaction with water and partly by carbonation. Sold in three grades: NHL 2 (feebly hydraulic, soft), NHL 3.5 (moderately hydraulic, the workhorse for most repointing), NHL 5 (eminently hydraulic, for chimneys, copings and anything exposed or wet).
Lime-cement mixes — Portland cement, lime and sand. Designated by ASTM type: Type O around 350 psi, Type N around 750, Type S around 1,800. Type O is the right answer for a lot of soft historic brick; Type N for harder units and moderate exposure.
Masonry cement — a bagged blend of Portland and finely ground limestone or other filler with air-entraining agents. It is not lime mortar and it does not behave like it, whatever the name on the bag suggests.
Choosing for a New England wall
Around here the housing stock that needs this decision is mostly 1870 to 1910 soft handmade brick or fieldstone, and both are weaker than any modern unit. The default I use is NHL 3.5 for general repointing, NHL 5 where the wall is a chimney, a parapet or below grade, and NHL 2 or a Type O only on very soft brick that a key marks easily.
Two local factors push the choice: 40 to 60 freeze-thaw cycles a winter means the mortar has to be carbonated or set before the first hard frost, and salt air off the Sound argues for a sacrificial joint that gives up its surface rather than a hard one that pushes the damage into the brick. If you are unsure whether the wall in front of you is soft-brick territory in the first place, the scratch test on brick and joint together settles it in about a minute.
Sand is half the mortar
People argue about binders and ignore the aggregate, which is usually 65 to 75 percent of the mix by volume and does most of the work on appearance.
- Sharp, well-graded sand — a range of particle sizes from coarse down to fine. Uniform sand makes weak mortar.
- Washed, not builders’ sand with clay fines. Fines weaken the mix and cause shrinkage cracking.
- Match the original sand for color and grain. This, not pigment, is what makes new pointing disappear into an old wall. A local sand pit is usually a better match than a bagged silica.
A mortar analysis on a sample of original joint runs $150 to $400 and gives you the binder ratio and a sieve grading of the sand. On a whole-house repointing job it is a rounding error against the cost of getting the mix wrong, and the National Park Service preservation guidance on repointing historic masonry, available through the National Park Service, is the standard reference most conservators work from.
Frequently asked questions
How long does lime mortar take to cure? NHL mortars gain workable strength in days and continue for months. Non-hydraulic lime carbonates from the surface inward at roughly a quarter inch a month, so a deep joint can take a year. Both need protection from frost, driving rain and hot sun during that period.
Can I add cement to lime mortar to speed it up? You can, and that is what a Type O or N mix is. What you cannot do is add cement on site to a bagged NHL to make it set faster — the proportions matter, and improvised additions typically overshoot into a mortar harder than the brick.
Where do I buy NHL? Building conservation suppliers and some masonry yards carry it, usually as an imported product in 55 pound bags at $25 to $45 each. It is more expensive than bagged Type S but it is a small part of a job that is mostly labor.