For most old brick walls, the right repointing mortar is one that is softer and more vapor permeable than the brick and no harder than the original mortar. On walls built with lime mortar, that usually means a lime-rich mix rather than a dense portland cement mortar. The exact mix depends on the brick, the old mortar and the exposure.
That is the short answer to lime mortar vs portland cement, and it comes from the National Park Service’s Preservation Brief 2. True Course Masonry Newark repoints older brick and stone walls across Newark and Essex County with mortar chosen for each wall, with the work carried out by licensed and insured contractors. This guide explains what each binder is, why hardness matters, how the standard mortar types differ, and how a mix gets chosen. The brick pointing page covers the repointing work itself.
Lime mortar: what it is
Lime mortar is lime and sand. Traditionally, quicklime was slaked with water on site to make a putty that aged for weeks or longer, then mixed with local sand, commonly about one part lime putty to three parts sand. Preservation Brief 2 notes that crushed shell, brick dust, clay, natural cements, pigments and even animal hair were sometimes added.
Lime hardens by slowly absorbing carbon dioxide from the air and turning back toward limestone. That makes it slow to cure, soft compared with cement, somewhat flexible, and very permeable to water vapor. It also has a useful property in old walls: small cracks can partly close as lime is dissolved and redeposited by moisture. The trade-off is care during placement. Lime mortar needs protection from frost and from drying too fast while it hardens, which in Newark means planning around late fall freezes and hot summer walls; the cold weather masonry guide covers timing.
Portland cement mortar: what it is
Portland cement was patented in Britain in 1824 and first made in the United States in 1872. According to Preservation Brief 2, it was not in common use across the country until the early twentieth century; before then it was mostly a minor additive to speed up lime mortar’s set. By the 1930s most masons used mixes of roughly equal parts portland cement and lime, and bagged masonry cement arrived in the same decade. The brief notes that mortar in buildings from about 1873 to 1930 can be anything from pure lime and sand to a wide range of lime-cement blends.
Portland cement sets by reacting with water. It cures fast, reaches high strength and is much less permeable than lime. Straight portland and sand mortar is very hard and hard to work, which is why lime is added to modern mixes for workability. The more cement in the mix, the harder and less permeable the mortar.
Why hard mortar damages soft brick
Mortar and brick in an old wall are a system. Rain soaks into both, then dries out through both. When the joints are softer and more open than the brick, they take the brunt: water leaves through them, salts deposit in them, and over decades they erode and get repointed. The brick survives.
Repoint that wall with mortar harder and denser than the brick and the roles reverse. Water that used to evaporate through the joints now has to leave through the brick faces, and salts crystallize just under the face. Freezing works on the wettest, weakest material, which is now the brick edge. Brick also expands and contracts with heat and moisture; a rigid joint cannot give, so stress concentrates on the brick edges instead. Preservation Brief 2 notes that salt-driven deterioration results when mortar is less permeable than the masonry units. The result is brick edges breaking away while the hard joints stand proud, a pattern the spalling brick guide describes.
Brownstone is an especially vulnerable case. A cement-rich joint or patch on soft, layered sandstone pushes water out through the stone around it; the brownstone repair and matching guide covers it.
Strength vs compatibility
ASTM classifies mortars by approximate compressive strength. Preservation Brief 2 lists them, from hardest to softest, as Type M (about 2,500 psi), Type S (1,800 psi), Type N (750 psi), Type O (350 psi) and Type K (75 psi), with straight lime and sand mixes softer still. The letters come from every other letter of “MASON WORK.”
| Type | Approximate strength (PB2) | What it describes |
|---|---|---|
| M | 2,500 psi | Highest strength, most cement |
| S | 1,800 psi | High strength |
| N | 750 psi | Medium strength, general-purpose modern mix |
| O | 350 psi | Lower strength, more lime |
| K | 75 psi | Highest lime content of cement-containing mixes; seldom used outside preservation |
| Lime and sand | Lower still | No portland cement |
These letters describe a property range, not a quality ranking or a stamp of approval for structural or historic work. A high compressive strength is useful where a wall is designed for it, such as hard stone carrying heavy loads. Preservation Brief 2 makes the comparison directly: strong mortar may suit a granite bridge pier, while a softer, more permeable lime mortar is preferable for a historic wall of soft brick. On an old wall, compatibility matters more than strength, because the joint only has to be strong enough for the wall, and any extra hardness is transferred to the brick as damage.
How a mix gets chosen
Preservation Brief 2’s rule is that repointing mortar should match the historic mortar in color, texture and tooling; be more vapor permeable and softer than the masonry units; and be as permeable and as soft or softer than the historic mortar. Working to that rule usually involves:
- Testing the existing mortar and brick. The brief describes scraping the mortar with a screwdriver and the brick, more gently, with a fingernail, as a starting comparison of hardness. On important or uncertain walls, a laboratory mortar analysis identifies the binder and sand.
- Checking the exposure. Chimneys, parapets, copings and grade-level courses are wetter and freeze more than a sheltered wall, which can shift the choice within the compatible range.
- Matching sand and color. Sand gives mortar most of its color and texture. Matching it matters as much as the binder; the matching mortar and brick guide explains why.
- Making a trial area. A sample is placed, cured and approved before the whole wall is done.
Some walls already hold a mix of mortars from different repairs, and stone walls and foundations bring their own joint widths and stone types, covered under stone and brownstone repair. Where hard cement pointing has been used on soft brick, it often needs to come out with hand tools before compatible pointing goes back, which takes care and time.
Making the decision on your wall
Write down what you know: roughly when the house was built, whether it has been repointed and with what, and whether any spalling sits next to hard grey joints. If a contractor proposes a mortar, ask what it is, why it suits this brick and this old mortar, and whether a trial area comes first. The tuckpointing vs repointing guide and the when to repoint brick guide help you judge the rest of the scope, and the historic masonry restoration page covers walls where a commission may review materials. When you are ready, send photos through the contact page.