Chimney Mortar: Lime, Portland and Refractory

One word, several materials

"Mortar" covers any paste that binds masonry units together, and the binder in that paste decides how it behaves. In a chimney you are most likely to meet four families:

• Lime mortar: lime and sand, the traditional mix for most of masonry history.
• Portland cement-lime mortar: portland cement, hydrated lime and sand, made in standard proportions known as Types M, S, N and O.
• Masonry cement and mortar cement: factory-blended bagged cements that are mixed with sand on site.
• Refractory mortar: a heat-resistant mortar for flue liners and firebrick.

The crown on top of a chimney is a separate case. It is better thought of as a small concrete slab than as a mortar joint; see crown coating vs. crown rebuild.

What the age of the house suggests

The National Park Service's repointing brief gives a useful timeline. Mortar made mainly of lime and sand was the norm for centuries. Portland cement was first manufactured in the United States in 1872 but was not in common use across the country until the early twentieth century. Buildings put up between 1873 and 1930 can contain anything from pure lime mortar to many combinations of lime and portland cement, and by the 1930s most masons were using a mix of equal parts portland cement and lime putty.

So a chimney on an older New Jersey house may well have been laid in a soft, lime-rich mortar, and a postwar chimney in a portland cement-lime mix. That is a starting assumption, not a finding. Chimneys get repointed and rebuilt, and the top of a stack is often newer than the bottom. The NPS brief describes a simple field check of relative hardness: scraping the mortar with a screwdriver, then tapping progressively harder with a cold chisel. Laboratory analysis gives a fuller answer when it matters, such as on historic buildings.

Lime mortar: soft by design

The NPS brief describes lime mortar as soft and porous, changing little in volume as temperatures swing, and slightly water soluble, which lets it reseal hairline cracks. The Brick Industry Association's mortar note explains the chemistry of that healing: water and carbon dioxide entering a fine crack react with calcium hydroxide in the mortar to form calcium carbonate, which seals it.

Those traits make lime mortar forgiving of soft, old brick. Its trade-offs are practical: it gains strength slowly, it needs to be kept damp while it cures, and it wears away faster in exposed spots, which on a chimney means it needs renewing more often than the brick around it. That is the intended order of wear.

Portland cement-lime mortars and the letter types

Modern mortars are specified by type under the ASTM C270 standard: M, S, N and O, from strongest to weakest in compression. BIA's guidance on choosing among them makes two points that run against intuition.

• Stronger is not better. BIA's mortar note says compressive strength is not the primary consideration for brick mortar; bond strength, workability and water retention matter more. Its selection note says to choose the type with the lowest compressive strength that meets the project's requirements.
• Chimneys have a recommended type. BIA's selection table lists Type N as recommended and Type S as the alternate for exterior parapets and chimneys above grade.

For repair work, the same BIA note says Type O mortar is often used for repointing older brickwork, while Type N may be suitable for repointing newer brickwork. A straight portland cement and sand mortar with no lime sits at the far end of the scale: the NPS brief describes it as extremely hard, resistant to the movement of water, prone to shrinking as it sets, and subject to relatively large thermal movement.

Masonry cement and bagged mixes

BIA defines masonry cement as a hydraulic cement made from portland or blended cement plus plasticizing materials such as limestone or lime, along with other additives. It is convenient and workable. BIA notes that portland cement-lime mortars typically have higher flexural bond strength than masonry cement mortars, and that mortar cement aims for a performance level similar to portland cement-lime mortar with the convenience of a single bag.

On older houses, the NPS brief is more cautious. Because masonry cements are not required to contain hydrated lime and generally do not, it says they produce high-strength mortars that can damage historic masonry and are generally not recommended for it. When someone proposes a bagged mix for repointing an older chimney, it is reasonable to ask which product and type, and how it compares with the brick and the original mortar.

Refractory mortar: for the joints that face heat and gases

The joints inside a chimney face conditions the outer brickwork never sees. Model code text from the 2003 International Residential Code, as reproduced by Buckley Rumford Fireplaces, calls for fireclay flue liners to be laid in medium-duty refractory mortar conforming to ASTM C199, with tight joints left smooth on the inside. The same text requires firebrick in a fireplace to be laid with medium-duty refractory mortar meeting the same standard, with joints no wider than 1/4 inch. BIA's fireplace design note also says refractory mortars must be used with thin joints.

Thin, tight joints matter because the joint is the weak point in a heated lining. A wide joint of any mortar inside a firebox or flue is where heat, moisture and acids get through first. For more on firebrick and refractory products, see inside the firebox and firebrick, refractory mortar and firebox panels.

Which mortar belongs where, top to bottom

Your chimney's original builder may have made different choices, and a local building official decides what applies to new construction today. The table shows the logic: the mortar follows the exposure.

What goes wrong when the wrong mortar is used

• Hard mortar in soft brick. The NPS brief explains that a mortar stronger than the masonry units will not give, so stresses are relieved through the bricks instead, causing permanent damage. On a chimney this can show up a few winters after repointing as flaking brick edges. See why chimney brick flakes.
• Ordinary mortar where refractory belongs. Code text specifies refractory mortar for flue liner and firebox joints. Patching a firebox or flue with general-purpose mortar substitutes a material the standard does not call for.
• Skim coats over old joints. New mortar smeared over eroded joints without raking them out does not bond well and tends to fall out. See repointing vs. tuckpointing.
• New mortar over an unsolved leak. Any mortar fails early if water keeps soaking it. NPS guidance is to fix the cause of deterioration before repointing.

When we plan masonry repairs, the mortar choice is part of the assessment, alongside the condition of the brick, the crown, cap and flashing, and the flue. We share those findings, with photos, before any work is scheduled. See masonry repair and repointing.

Questions homeowners ask

Sources

1. National Park Service, Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings (PDF)
2. Brick Industry Association, Technical Note 8: Mortars for Brickwork (PDF)
3. Brick Industry Association, Technical Note 8B: Mortars for Brickwork, Selection and Quality Assurance (PDF)
4. 2003 International Residential Code, Chapter 10, Chimneys and Fireplaces (text reproduced by Buckley Rumford Fireplaces)
5. Brick Industry Association, Technical Note 19: Residential Fireplace Design (PDF)

This guide is general information. It can't diagnose a specific chimney; an inspection can.