What is Mortar?

posted by Bob

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 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.

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.




Robert, unfortunately this is going to fail and it does need to be torn out. The cracking will just continue until everything starts to comes loose from the surface. it would be best just to tear it out and start over. Be sure to remove all of the cement before starting over because it will be too soft to support your new mortar.
- Lee-Technical Service
Tuesday, November 15, 2016 at 9:39 AM

Jeffrey, no the mortar mixes are not the product to use in that application. Mortars are used for wall applications only. It does not have the wear resistance needed to last. Depending on the size and depth of the cracks, you could use either Top'n Bond or Fast Setting Cement Patcher. Top'n Bond can be placed from 1/2" down to a feather edge. Fast Setting Cement Patcher can be placed from 2" down to 1/4". Keep in mind that the slab has cracked for a reason. Whether it be due to settling, erosion, or improper bedding. So there could still be movement within the slab. If it moves, the crack will telegraph through the patching material.
- Lee-Technical Service
Tuesday, November 15, 2016 at 9:34 AM

I installed 4 capstones for a fireplace hearth. Felt paper, lath then mortar. I thought I was using premixed, turns out it was type S with no sand. I also grouted with the same mix. I am seeing shrinkage and fine cracks as it is drying. Is it going to fail, should I attempt to tear it out?
- Robert
Saturday, November 12, 2016 at 6:30 PM

I had a 4-inch thick concrete walkway poured around the side of my house and it developed cracks inside a few of the joints, and on one slabs. The cracks aren't wide, but go all the way through the concrete. Could I mix up a thin batch of mortar and put it in a squeeze bottle and fill in the cracks to the full depth? Which Sakrete product should I use?
- Jeffrey
Thursday, November 10, 2016 at 2:18 AM

Rod, yes it can be used in a mortar bag. Sometimes the opening on the bag is too small and needs to be cut back to give room for the material to come out. Try to cut it back to where the opening is the size of your joint. You can also add a small amount of water even after you cut the nozzle to make the material come out easier.
- Lee-Technical Service
Tuesday, November 8, 2016 at 9:09 AM

can mortar mix type N be used in a mortar bag easily or at all? I tried but I think it was too dry. however, when I added water it didn't seem to help - still clogged up and had water coming out as I squeezed the bag.
- rod
Wednesday, November 2, 2016 at 6:51 PM

I just poured a new concrete walk with intentions of reinstalling the flagstones previously used in that location. Are there thickness (joint and/or bed) for the mortar? What mortar mix should be used for bed and joints? What temperature is it too cold to install?
- Lee
Friday, October 21, 2016 at 12:48 PM

John, the best thing to do would be to just simply add more rocks. If you add mortar or any type of cement based products the ph will become very high and kill anything living in the ponds. Just adding clean rocks should not have as great an impact on the ecosystem.
- Lee-Technical Service
Monday, October 10, 2016 at 9:42 AM

I placed on a brand new rough concrete slab a set of mosaic tiles made of small cuttings of flagstones. Each tile surface ends up 2 to 3 inches above the slab and this thickness is made up of the flagstone itself and type S mortar. Each tile is spaced a minimum of 2.5in. from the other. I would like to fill this space to the top of the stones with a colored material. Should I use concrete, mortar or some combination for this filler.
- Laurent
Saturday, October 8, 2016 at 10:26 PM

We have a stream leading into a pond. There are individual pools that fall over lip stones. The pools are all lined with basalt rock and mortar., that has been painted with Drylok paint (for swimming pools). Several of the pools are too deep and we want to fill them in up to 10 inches with more mortar and rocks. Do you have some advice on how to best do this?
- John
Thursday, September 8, 2016 at 11:20 AM



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