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What is Mortar?

posted by Bob Wednesday, January 12, 2011 at 4:34 PM

What is Mortar?

Mortar is a mixture of sand and cements that is most often used to build brick or block walls. In my July blog on cement and concrete I dealt exclusively with portland cement products and uses. I also promised that at a later date I would talk about the masonry world. For those of you on the edge of your seats since then, well today is your lucky day.

While portland cement concrete is certainly one of the most widely used building product in the world, masonry mortar is close behind. It is doubtful that you live or work in a building that doesn’t have mortar in it somewhere. The 3/8” gray line of material that separates the brick or block is mortar. Mortar is a very different animal than concrete. Concrete is designed to be used in thicker applications and to reach very high strengths. It achieves its durability through brute force. Mortar is also designed to be durable but achieves its goal through finesse. Its strengths are quite low compared with concrete and it is never used in thick applications. It is much creamier and more workable than concrete. If you play tennis, think of concrete as your most powerful serve when you are trying to smash the ball into or through your opponent so that they can’t return the serve. Then think of mortar as a very gently placed lob close to the net that gets your opponent leaning the wrong way with no chance of returning the ball. Both achieve the desired result of earning you a point. The point of this long winded analogy is to let you know that using concrete and mortar interchangeably will lead to disaster.

Mortar can be made in one of two ways. The older method is to take portland cement, add hydrated lime and mix with fine sand. The newer method is use masonry cement and fine sand. Masonry cement is simply a material made by most cement companies where they intergrind the portland with lime or other ingredients designed to produce high air content in a kiln. The results are the same. If you go to the store and buy a bag of mortar you will not be able to tell which method was used to make the masonry cement. There are some parts of the country where masonry cement is simply not available and you will only get portland/lime mortar. The good thing is that it doesn’t matter. In my thirty years of doing this type of work I have found that masons are the most determined group I have ever run into. Some will absolutely swear that you must use portland/lime while others insist masonry cement is the way to go. Just chalk it up to personal preference.

There are three things that make mortar very different from concrete. Mortar has lower strengths, must have the ability to retain water and have a high air content. There are three widely produced types of mortar. Types M, S and N. Type M will achieve a compressive strength of 2500 psi at 28 days. Type S will yield 1800 while Type N yields 750. By way of reference most general concrete is in the 4000 psi range but can go as high as 8000 psi for special applications. The easy way to remember the strengths is by spelling out the word MASON WORKS.

                                        M       A -----          O ----            W ----    O       R ----    K       S ----
                                                    2500                 1800                750                     350                   75

Types O and K are not made commercially and can only be used for non structural applications such as fixing old mortar that has fallen out.

Why not make mortar as strong as concrete? In America isn’t bigger better? Not with mortar. When you build a wall you want the block or brick to be the strongest part of the wall. When the wall shifts, and almost all do over time, the weakest part is going to break. If the brick or block were to break, then the fix would be building a new wall. If the mortar crumbles, provided you don’t let it go on for years, you simply need to repoint the wall (that means sticking mortar back in where the old mortar fell out). This is a simpler and more inexpensive fix. One important reason why masonry cement or a portland/lime blend works well in a mortar joint and straight portland does not is the high air content. Many walls are exposed to rain and water. In the winter many of these are in area where temperatures often dip below freezing. The mortar contains a certain amount of water. When water freezes it takes up more space than water. This means that it will expand and crack whatever is next to it. By intentionally putting lots of tiny air bubbles in the mortar, the expanded ice has somewhere to go without causing damage. Concrete generally has enough mass that this isn’t a problem.

I previously mentioned that mortar must also be able to retain water for a certain period of time. Most masons mix up enough mortar to be able to lay a good amount of brick or block without having to stop and mix more. If the mortar they mixed doesn’t retain its flowable nature for at least an hour the mason will simply add more water to bring it back to life. While this is a really bad idea, it is done in the field all of the time. The problem with this is that the strength of the mortar is designed around certain water to cement ratio. If the ratio is skewed, then so are you when the walls fail due to weak mortar.

So which mortar should I use? Commercial jobs will usually state which you are required to use. For none specification work, Type S is usually a safe bet and is what most of you would use on your projects. For those who have a quest to know more, the best general rule is that if the wall is non load bearing, such as a divider wall inside of a building, then Type N is sufficient. If the wall is below grade or carrying the load of the roof or several stories of brick or block use Type S. Type M is rarely called for and only used in certain large commercial projects.

Bob Schmidt
Product Manager
Sakrete of North America

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69 USER COMMENTS

Scott, for grouting in between your granite steps you may need to use an unsanded tile grout. Are the steps polished, what are the joint widths and depth, and is this interior or exterior? Please contact technical support for more information. Thank you we are happy to help.
- Chris Technical Services
Tuesday, September 23, 2014 at 2:19 PM

I have custom made granite steps on the front and rear entry of my house. The material (mortar?) in between each piece of granite on the horizontal part of each step is loose and breaking apart. What should I use to repair all the horizontal joints of these steps?
- Scott
Thursday, September 18, 2014 at 10:24 AM

Jono, mortar mix is only used for vertical surfaces. To resurface your driveway you will need to first prep the surface by removing any paint, sealer, dirt, loose material, etc. anything that will inhibit the bond from old to new. If there are cracks present you will want to address them first (correct product depends on the dimensions of the crack being filled). To resurface your concrete driveway we recommend using our Sakrete Flo-Coat Resurfacer. It’s polymer modified for superior bonding and adhesion. Flo-Coat is applied with a squeegee and can be used from ½” down to a feathered edge.
- Chris Technical Services
Monday, July 21, 2014 at 9:42 AM

I want to resurface my Driveway, skim coat or slurry etc. I have bought a bag of premixed mortar, will this give a hard wearing finish?
- Jono Watkins
Sunday, July 20, 2014 at 7:48 PM

Herb, given the information you have posted, we would recommend that you to contact a structural engineer for guidance in this matter.
- Chris Technical Services
Monday, July 14, 2014 at 3:12 PM

I have a 100 year old bluestone foundation, 18" thick. I have and coming out of the walls. I have dug out sand in some spots up to 12" deep and maybe 8" square around it. For sake of explaining this, let's say a couple of the voids are as big as a basketball, most are softball sized or a little larger. I bought type 's' mortar, but I'm reading here that mortar is not for volume applications. What would be my best bet for filling these voids? Thanks!
- herb fellows
Monday, July 14, 2014 at 8:01 AM

Jim, Sand mix should be used in a dry pack consistancy for your preslope. Refer to the TCNA handbook for shower installation instructions.
- Chris Technical Services
Monday, June 30, 2014 at 3:47 PM

I am putting in a thin layer of mortar mix under my shower pan liner, to achieve a good pre-pitch. Above the liner, I plan to use sand mix before the ceramic tile. Are these choices correct?
- Jim
Saturday, June 28, 2014 at 11:04 PM

Anna, There are 4 types of Mortar, O=350 psi, N=750 psi, S=1,800 psi, M=2,500 psi. The cement quantity and sand mixture is what makes up the types of mortars. Type M is fine to use above and below grade, however Type N can only be used for above grade applications. Please call us at 1-866-725-7383 if you have any other questions so we can better assist in this matter.
- Chris Technical Services
Thursday, June 26, 2014 at 10:57 AM

Mary, we recommend using our Sakrete Sand Mix and Sakrete Concrete Bonder & Fortifier to set your pavers. Make sure to prep the existing concrete slab by removing any loose material, dirt, debris, paint, stain, etc. anything that will inhibit the bond from the existing concrete to the new concrete. The Sakrete Concrete Bonder & Fortifier can be used to prime the area (let dry clear before applying any products first) and also as an ad mixture. The ad mix should be half the suggested amount of water and the other half Sakrete Concrete Bonder & Fortifier (1:1 mixed separate then added to the Sakrete Sand Mix). The Sakrete Sand Mix is made for ½” up to 2” in thickness.
- Chris Technical Services
Thursday, June 26, 2014 at 10:16 AM

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