Mortar for Block Walls: Type N, S, M and O Compared

Which mortar type for a concrete block wall?

The decision comes down to three questions, in this order.

Does a code provision remove the choice? Some applications are prescribed and there is nothing to decide.

Is the wall loadbearing, below grade, or holding back soil? If yes, Type S is the normal answer and Type M is the answer where the exposure is severe.

Is it an interior partition carrying nothing but itself? Type N covers it, and Type O is permissible where conditions are mild and the wall is genuinely non loadbearing.

Everything below explains why those answers are what they are. For how many bags the job needs, use the block mortar calculator.

The four ASTM C270 mortar types

ASTM C270 recognizes four mortar types. Under the property specification, each carries a minimum average compressive strength at 28 days.

The strength numbers get quoted constantly, and they are the least useful part of the standard for choosing a type. Two other requirements in the same table matter more day to day.

Water retention has a minimum of 75 percent for every type. That is the property that keeps mortar workable against an absorptive block face long enough to get the unit placed and adjusted.

Air content has a maximum that varies by cementitious system, not by whether the mortar is strong. Cement lime and mortar cement mortars are capped at 12 percent for Types M and S and 14 percent for Types N and O. Masonry cement mortars are capped at 18 percent for Types M and S and 20 percent for Types N and O.

There is an exception worth knowing. Where structural reinforcement is embedded in the mortar, the cap tightens to 12 percent for cement lime mortars and 18 percent for masonry cement mortars. Air improves freeze thaw durability and workability, but it works against bond, and bond is what holds reinforced masonry together.

Why stronger mortar is usually the wrong choice

The most common mistake on a block job is specifying up.

Compressive strength is not the property that governs how a mortar performs in a wall. Bond strength, workability and water retention do more work. A mortar that tests high in a cube and bonds poorly to the unit produces a worse wall than a weaker mortar that bonds well.

There is also a maintenance argument. A joint can be cut out and repointed. A cracked unit cannot.

Type M at 2,500 psi is not a safer default than Type S at 1,800 psi. It is a different specification for a different exposure, and using it where it is not called for buys nothing.

When the code makes the decision for you

Three situations remove the choice entirely.

Empirical design of foundation walls requires Type M or Type S mortar.

Glass unit masonry requires Type N or Type S mortar.

Seismic Design Categories D, E and F require portland cement lime mortar or mortar cement mortar, Type S or Type M. Masonry cement mortars and mortars containing air entraining admixtures are not permitted in the seismic force resisting system in those categories.

Check the project documents before applying any general rule from this page. Where the structural drawings name a type, that is the specification.

Mix ratios: what ASTM C270 actually specifies

Search for a mortar mix ratio and you will find a single set of numbers repeated everywhere: one part cement, one part lime, six parts sand for Type N, and so on.

Those figures are estimating recipes. They sit inside the specification, but they are not the specification.

Under the proportion specification, C270 sets portland cement at one part and gives hydrated lime as a band by type. Type M takes one quarter part. Type S takes over one quarter up to one half. Type N takes over one half up to one and one quarter. Type O takes over one and one quarter up to two and one half.

Aggregate is also a band, not a number. Under the proportion specification it runs from two and one quarter to three times the sum of the separate volumes of cementitious materials. Under the property specification the upper limit is three and one half times.

That is why two sites can publish different mix ratios for Type S and both be correct. One is quoting the low end of the band and the other the high end.

Proportion specification or property specification, never both

C270 offers two routes to compliance and they are alternatives, not a checklist.

The proportion specification is followed by batching the specified volumes. No testing is required, because meeting the proportions is the compliance.

The property specification is followed by demonstrating the required properties through laboratory testing. The proportions are then whatever produced the tested result.

Project documents should name one route. Specifying both creates a conflict, because a mix batched to the proportion specification may not hit the laboratory numbers of the property specification, and a mix designed to the property specification may sit outside the proportion bands. If neither is named, the proportion specification governs by default.

Can you use concrete mix instead of mortar?

No, and the reason is not strength.

Mortar is designed to hold water against a unit that wants to pull it out, and to stay plastic under the trowel long enough to be worked. That is the 75 percent water retention requirement doing its job. Concrete mix has no such requirement, and it contains coarse aggregate that will not compress into a three eighths inch joint.

The same distinction runs the other way with grout. Grout is fluid by design so it flows into cells around reinforcement, which is why it is specified at a high slump. Mortar at that consistency would not hold a block. Three materials, three jobs. If you are filling cores rather than bedding units, see the block fill calculator.

Masonry cement, mortar cement and cement lime

Three cementitious systems produce the same four mortar types, and the difference between two of them is easy to miss.

Portland cement and lime mortars are batched on site from separate materials. Cement meets ASTM C150, hydrated lime meets ASTM C207, and the aggregate meets ASTM C144. C207 recognizes four lime types, N, S, NA and SA, of which Types N and NA are not generally used in mortar.

Masonry cement under ASTM C91 arrives as a proprietary blend. Composition is not disclosed by the manufacturer, and the standard sets no minimum bond strength.

Mortar cement under ASTM C1329 is similar in handling but adds a minimum bond strength requirement. It is also one of the two cementitious systems permitted in Seismic Design Categories D, E and F, where masonry cement is not.

If bond strength matters to the structural design, the cementitious system matters, not just the type letter.

What field testing can and cannot tell you

ASTM C780 covers field testing of mortar, and its results are routinely misread.

C780 exists to verify consistency of materials and procedures during construction. It contains no minimum compressive strength requirement, and its compressive strength results are not comparable to the C270 laboratory values. A field cube that comes in below 1,800 psi does not mean a Type S mortar failed.

Laboratory preparation under C270 uses a controlled flow and a non absorptive mold. Field mortar is mixed wetter so it can be worked, and the unit pulls water out of it after placement. The two are not the same test and were never meant to produce the same number.

Working time and retempering

Mortar must be placed in its final position within two and one half hours of original mixing, unless a set retarding admixture is used. That limit comes from TMS 602.

Retempering, meaning adding water to restore workability, is generally not harmful when it is done before hydration has begun. Mortar that has started to stiffen because of hydration rather than evaporation should be discarded.

Mortar makes up roughly 7 percent of a block wall's face area. It is a small fraction of the material and a large fraction of the performance.

Sources

CMHA TEK 09-01A, Mortars for Concrete Masonry. ASTM C270 mortar types, proportion and property specifications, code restrictions, field testing and retempering.

CMHA TEK 04-02A, Estimating Concrete Masonry Materials. Mortar estimating recipes and quantities.