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Why do masons hate type-O?

jeffsm | Posted in Construction Techniques on October 2, 2006 07:13am

The house was built around 1890 – soft bricks and softer mortar. The bricks are in good shape, but the mortar is all but gone.

The masons who have stopped by, all very reputable, have all talked about using type-N mortar for the re-pointing. Articles on historic structures and older bricks talk about using type-O. When pressed, responses tend to be in the vein of, “I have always used type-N and I have never had a problem. Type-N is the way to go.”

Can anyone help me figure out what the resistance is to using type-O mortar? Is there a workability or a time issue involved? An education issue (on one of our parts)? Or is this a common disagreement between specifiers and practitioners?

Thanks for any input, Jeff

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  1. brownbagg | Oct 02, 2006 07:56pm | #1

    type n has a little bit of lime in it, so it make its it a little creamy, easy to work with.

  2. john_carroll | Oct 02, 2006 10:53pm | #2

    There is a lot of confusion concerning masonry mortars. First of all the term mortar is a generic term. Mortar holds masonry units together and fills the spaces in between. Although portland cement is the basic ingredient in most modern mortars, it did not exist prior to the 1820s. The first mortar was clay and, for thousands of years, lime has been the primary mortar in Europe. Today, many special-use mortars have no portland cement; these include epoxy mortars and refractory mortars. When someone says that mortar is made of lime and portland cement, they're using the word "mortar" wrong. Mortar can be made of many things.

    There are actually three different kinds of mortar appropriate for general masonry work. All three of these can be used to make the four basic Types: M,S,N,O. The Types are performance standards based on the compressive strength of the given mortars; this compressive strength is determined by the proportion of portland cement is the mix.

    The three kinds of mortar are portland cement/lime mortar, masonry cement mortar, and "mortar-cement" mortar.  Portland cement/ lime mortars developed gradually in the nineteenth century and are still used today. They are made from a combination of portland cement and lime. Masonry cement mortars developed in the 1920s. They are made from portland cement and proprietary ingredients that take the place of lime. (Contrary to what was written in What's the Difference in this month's issue of FHB, the most common types of mortar found in masonry supply outlets aren't composed of portland cement and lime, they are masonry cement mortars which substitute the proprietary ingredients for lime). Masonry cement mortars are sold at Lowes, Home Depot and masonry supply stores. They are used extensively in residential construction. Mortar-cement mortars were developed in the 1980s. These were developed to compete with portland/cement lime mortars. Because portland cement/lime mortars out-perform masonry cement mortars (mainly in terms of water infiltration), they have been specified far more often in commercial construction. In other words, mortar-cement mortars use the proprietary ingredients of masonry cement (and were developed by that half of the industry) and perform like portland cement/lime mortars.

    Your masons probably use masonry cement mortars and may not be aware of portland cement/lime mortars. Masonry cement comes pre-mixed in Types S and N. (To make Type M, which has the highest compressive strength, mix one part portland to one part Type N masonry cement.) Everything that I see here, and I have several sources, simply says to use Type N masonry cement to make Type O mortar. Again, the Type designation is a performance standard which can be achieved in several ways. I've never seen Type O masonry cement offered pre-mixed at a masonry supply house.

    If you don't want to use Type N masonry cement, you can make Type O portland cement/lime mortar by mixing 1 part portland, 2 parts hydrated lime and 9 parts sand. One way to do this is to use a bucket or other container to measure each part.  

     

    1. ClaysWorld | Oct 02, 2006 11:25pm | #3

      I hate it when my head explodes with to much mud.

      Great reply, now I'll have to let it settle a little. Sometime my head cant handle a full bucket of mud in one thinking.

      1. john_carroll | Oct 03, 2006 03:32pm | #5

        Clay, This is laid out nicely in the 2000 International Residential Code in Table R607.1, "Mortar Proportions." It's on p. 174 in my admittedly dated copy of the IRC. I'd be very surprised if it's not included in more recent editions.

    2. DaveRicheson | Oct 02, 2006 11:26pm | #4

      Excellent.

    3. jeffsm | Oct 04, 2006 11:11pm | #6

      Mudslinger, Thanks for the detailed response. I am not sure I understand why [one can use], "Type N masonry cement to make Type O mortar." Sorry if I am being a bit dense.I was asking the masons about type-O because from what I understand the mortar should be the weakest link - softer and more water permeable than the masonry unit. From the Park Service:Permeability, or rate of vapor transmission, is also critical. High lime mortars are more permeable than denser cement mortars. Historically, mortar acted as a bedding material--not unlike an expansion joint--rather than a "glue" for the masonry units, and moisture was able to migrate through the mortar joints rather than the masonry units. When moisture evaporates from the masonry it deposits any soluble salts either on the surface as efflorescence or below the surface as subflorescence. While salts deposited on the surface of masonry units are usually relatively harmless, salt crystallization within a masonry unit creates pressure that can cause parts of the outer surface to spall off or delaminate. If the mortar does not permit moisture or moisture vapor to migrate out of the wall and evaporate, the result will be damage to the masonry units
      ...
      For this reason, they [masonry cements are] generally are not recommended for use on historic masonry buildings."
      <http://www.cr.nps.gov/hps/tps/briefs/brief02.htm#Mortar>Then again, I’ll turn around and read something like this:"Type O should not be used if it will be subjected to freezing and thawing in the presence of excessive moisture. Many building codes do not allow Type O mortar for construction. However, some local building codes allow Type O to be used for load-bearing walls of solid blocks when the compressive stresses do not exceed 100 pounds per square inch. Before using this type mortar, check local building codes."I am not quite sure what to make of these two (seemingly opposing) points of view or if I should press the issue and insist on one type or the other.Thanks again, Jeff

      1. john_carroll | Oct 05, 2006 03:52am | #11

        Jeff,

        As I've said, the mortar Type is a specification, not a product. It is akin to the performance standards in the wood panel industry. A panel that is rated exterior wall sheathing by the APA can be plywood, osb or composite panels. Portland cement/ lime mortars are as different from masonry cement mortars as plywood is from osb. But, like engineered panels, they can both meet the same performance standards (i.e., Types) . There has, in fact, been a heated debate in the masonry industry for years over the relative merits of portland cement/lime mortars versus masonry cement mortars. It is very similar to the debate in the lumber industry over the qualities of plywood versus osb.

        This is covered in "Mortar: How to Specify and Use Masonry Mortar." It's a Hanley Wood publication, printed in 1990. http://search.barnesandnoble.com/booksearch/results.asp?WRD=Mortar%3A+How+to+Specify+and+Use+Mortar&z=y

        Christine Beall also covers this in her excellent book, "Masonry Design and Detailing."

        I don't know why the masonry cement (and the mortar cement) manufacturers don't make Type O by the bag. Both the books mentioned above and, indeed, everything I've read simply lists Type N masonry cement as the product to use when Type O is specified. It's in the IRC. (If anybody following this thread has a scanner, it would be helpful to this discussion to post the chart in the IRC providing the proportions for mortar. Unfortunately, my scanner doesn't work with my new computer.) 

        But, to get to the problem at hand, the basic principle, repeated over and over again in industry literature, is that the mortar should not be harder than the bricks. You have a nineteenth century house. If the bricks seem light and porous and the mortar is soft and easy to rake out, I would recommend Type O portland cement/lime mortar. Get a bag of portland cement and a bag of hydrated lime. Collect 12 equal-sized containers (16 oz. drink cups would work). Fill nine with masonry sand, two with lime and one with portland. Mix these together with water and you'll have enough mortar to keep a couple of masons busy tuckpointing for two hours--the recommended board life of masonry mortar. The mix, by the way, will be as smooth as butter and very easy to work with.

        1. User avater
          Dinosaur | Oct 05, 2006 04:19am | #12

          I have learned more in this one thread tonight than in several years of trying to get a straight answer out of any of the local masons I deal with. You just ranked yourself up there with 4Lorn, Boris, Piffin, and a few others, Mudslinger. Excellent work.

          Not wanting to hijack the thread, could I slip in a related question about refractory parging? I've got a brick bread oven to re-build next week. What kind of a mix am I looking for?Dinosaur

          How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

          1. stevent1 | Oct 05, 2006 05:25am | #13

            I carried the hod almost 40 years ago for about 2 months before they put me on the line. Now I know.Thanx for the great, imformative thread. ( Been eating ust for the last 30)Chucklive, work, build, ...better with wood

          2. john_carroll | Oct 05, 2006 05:54am | #14

            Thanks, Dinosaur. Nice article in FHB, btw. I don't have any experience with bread ovens. Here's some pretty good info, though:  http://www.superiorclay.com/oven/

          3. User avater
            Dinosaur | Oct 05, 2006 06:12am | #16

            Thanks for that link. It appears what I will need is refractory insulating concrete; the parging has to be 2" thick.

            IIRC, you said refractory mortar contains no portland cement. What does it contain? And how would mortar differ from insulating concrete?

             Dinosaur

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

          4. User avater
            zak | Oct 05, 2006 06:58am | #17

            There are cements made with alumina, I believe just called high-alumina cements.  They'll be good up past 2000º F.  It might need to be mixed with portland cement, in small quantities.  A refractory supplier or a good pottery supplier will have it.

            Rather than using sand for your mix, you should use grog- crushed, fired clay.  It will have better expansion/contraction characteristics.  I wish I could remember specifics better- it's been a few years since my kiln building days, and I wasn't very good at record keeping back then.  If you search around for something like "castable refractory recipe" you should get some hits.  They'll do the job, but the workability for a parging material will be variable. 

            The basic ingredients are usually something like this: coarse grog, fine grog, fire clay, refractory cement, portland cement.  I wouldn't use more than about 5% portland cement.zak

            "When we build, let us think that we build forever.  Let it not be for present delight nor for present use alone." --John Ruskin

            "so it goes"

             

          5. john_carroll | Oct 05, 2006 03:29pm | #19

            Refractory mortars are made from fire clay, with calcium aluminate or sodium silicate as a binder. These mortars fuse with the masonry as they get heated. They usually come premixed and are available at most masonry supply houses. I don't think portland cement is a good idea. As Jim Buckley, the Guru of Rumford fireplaces has said, portland cement breaks down when exposed to high temperatures. If memory serves, the most damaging temperatures are mid-range temps as the fire builds and cools down. Check out his web site. Also google refractory mortar. This will bring you to Rutland, which has a lot of info on refractory mortars.

          6. john_carroll | Oct 05, 2006 03:43pm | #20

            Here's an article by Buckley and a co-author which covers the pitfalls of using a portland cement-based mortar: http://www.rumford.com/articleRefmortar.html

          7. User avater
            Dinosaur | Oct 06, 2006 07:13am | #21

            Excellent article, Mud. I thank you again. We're gonna bake bread in that thing yet!Dinosaur

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

          8. john_carroll | Oct 06, 2006 03:13pm | #23

            Dino, Let us know how the oven--and the bread--turn out.

          9. User avater
            Dinosaur | Oct 07, 2006 02:48am | #27

            Dino, Let us know how the oven--and the bread--turn out.I'll try to remember to take along my camera for that job.Dinosaur

            How now, Mighty Sauron, that thou art not broughtlow by this? For thine evil pales before that whichfoolish men call Justice....

          10. User avater
            txlandlord | Oct 06, 2006 04:35pm | #25

            Why do masons hate type-O?

            Because most of them here in TX can't read english, and type-Os make things worse.

          11. JohnSprung | Oct 06, 2006 06:43pm | #26

            No, no, it's because they don't like getting blood in the mud, and most of it is type "O".  Now put a little A-B negative in the mud and you got something....  ;-) 

            (How many days has it taken to come up with a lame blood type joke....? ) 

             

            -- J.S.

             

          12. junkhound | Oct 06, 2006 08:09am | #22

            Mudslinger, you should submit your responses on this thread to FHB for an article, best educational thread for me in quite a while, esp on brick and mortar.

            Have to admit I have never laid more than a few common brick, but have done a bit of stonework with basalt and granite based 'river rock', all with 100% portland cement/sand/gravel vs any lime/proprietary mix as that seems to bond best.

            Have laid a few thousand firebrick,  always used just fireclay (or dry) for those.  

        2. jeffsm | Oct 05, 2006 07:39am | #18

          Thanks to Mudslinger and everybody else for all of the information.  You have been a big help in sorting this out.  I'll see what the local masons have to say tomorrow.

          For those interested, this particular house got some press coverage recently when a neighbor (a plaster by day) asked to stage a production in there during construction: <http://www.post-gazette.com/pg/06261/722761-53.stm>.

          Thanks again, Jeff

    4. JohnSprung | Oct 05, 2006 12:06am | #8

      Thanks for the info.  I've printed it out, and will file it with my concrete and masonry books. Perhaps it could be expanded into an article for FHB.  

       

      -- J.S.

       

      1. john_carroll | Oct 05, 2006 06:00am | #15

        Check out the National Lime Association for one side of the debate and the Portland Cement Association for the other.

    5. User avater
      txlandlord | Oct 06, 2006 04:31pm | #24

      WOW! I am impressed at your response Mr. Mudslinger, and I learned something too. Thanks.

    6. blue_eyed_devil | Oct 08, 2006 06:01pm | #28

      Thank you Mudslinger for your excellent and informative response regarding mud. Responses such as yours is the primary reason I prowl this forum although sometimes I lose sight of that reasoning.

      Thanks for the followup responses too.

      blueOur Skytrak is for sale. It has 500 hrs on it. We want  50k (you pay the freight) and we'll finance it. Drop me an email; it's a good buy.

  3. MSA1 | Oct 04, 2006 11:29pm | #7

    I'm nowhere a mason but I heard (actually on This Old House) that if you use modern mortar with the old bricks it could damage the brick.

    Take it for what its worth, i'm just passing along whay I heard.

    1. johng45 | Oct 05, 2006 01:25am | #9

      "... I heard (actually on This Old House) that if you use modern mortar with the old bricks it could damage the brick."I heard the exact same thing.JohnG

  4. scooleen | Oct 05, 2006 01:36am | #10

    Jeff,

    I work in historic restoration work, almost exclusively. Typically type N mortar has a lower PSI then other mortars, when old brick are in need of re-pointing, the worst thing you can do is surround it with portland. Portland has so much compressive force that it actually spalls the face off the brick. Mortar is and should be a sacrificial material. Old brick are too soft for anything else. Portland also does not have the same permeability of moisture as Lime based mortars.

    Stick with type N on old brick. Lime mortar is easy to work with.

  5. user-3976312 | Aug 18, 2020 11:32pm | #29

    I have always used the formula that John Carroll mentions to mix a lime mortar to use on antique, low fired bricks. The part that will confuse most people is the need for hydrated lime. This is something that is not sold. It has to be made by mixing quick lime with water and keeping it from ever drying before use in a mortar, or for lime plaster. The lime hardens by absorbing carbon dioxide from the air, a process that is slow, thus the addition of a small amount of Portland cement for a quicker set. Garden lime is sometimes referred to as hydrated lime, but it has been dried and ground up for use.

    The easiest way to store active hydrated lime is in a container with enough water to cover the lime, which settles into a fairly solid substance that has a plasticized, spreadable, fairly sticky quality.

    This makes a mortar that is a bright white. Working on antique restorations that originally had pure clay mortar I've used cement coloring agents to tone down the brightness. I believe there was a time when Portland was not available and hydrated lime was added to clay to give the clay mortar a bit more weather resistance. In New England you still find "Lime Kiln Road" in some towns, dating to a time when limestone was fired to create quick lime. If you search you tube there are even videos demonstrating low tech ways to do it.

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