Mortar and Methods of Building with Cordwood
Rob Roy is Director of the Earthwood Building School, which has specialized in cordwood masonry instruction since 1980. Rob and his wife, Jaki, have built four innovative cordwood homes for themselves since 1975, including the Earthwood home where they have lived for over three decades. Earthwood is a 2400 SF two-story round, load-bearing cordwood home, earth-bermed and earth-roofed. Details of construction are in Rob's Earth-Sheltered Houses (New Society, 2006) and Cordwood Building: The State of the Art (New Society, 2003) two of fourteen books he is written in the alternative building field. Rob and Jaki have taught cordwood masonry in Earthwood Building School all over North America, as well as in Chile, New Zealand, Australia and Hawaii. They have helped scores of owner-builders with their cordwood projects, including homes, saunas and outbuildings. Earthwood has produced a 3.25-hour DVD on cordwood construction, which, with his books, can be accessed through the Earthwood website, or on the Cordwood page here. Rob is considered to be one of the leaders in the field of cordwood construction and earth sheltering. He does individual consultations at a flat rate of $75/hour, but answers questions here without charge. See also Earthwood Facebook. In July of 2015, Rob and Jaki, will be host the 5th Contintental Cordwood Conference at Earthwood CoCoCo/15, returning the conference to where the first one was held in 1994. Go to Earthwood Building School for information. See also Earthwood Facebook.
Q: I am interested in cordwood construction, and was wondering at the possibility of papercrete as a mortar, and most importantly, is it true about shrinkage between the mortar and the wood leading to infestation of all sorts? I am wanting to start building but after hearing about rodent and pest problems, plus loss of insulation value, I am skeptical. Please advise, thank you!
A: Several individuals have been working with papercrete and cordwood during the past couple of years, and early returns are very positive. Former Earthwood student Jim Juczak and his wife Krista have just moved into their 18-sided cordwood and papercrete home near Watertown, NY and are very pleased with the combination. My wife, Jaki, and I were there near the beginning of the project and found that papercrete can be pointed quite well. You can email Jim at jsjuczakATgiscoDOTnet Another former student, Alan Stankevitz, is building a 16-sided building of cordwood and papercrete (which he more accurately calls P.E.M. or "paper-enhanced mortar") in southeastern Minnesota. Alan also operates the excellent Daycreek website (www.daycreek.com) which tells of his own building project and serves as the best forum on cordwood masonry on the web. Tom Huber in Michigan has also done successful experiments in cordwood and papercrete. All three have contributed their experiences to my new book on cordwood masonry which will appear in January of 2003. The title will be Cordwood Building: The State of the Art. (New Society Publishers, www.newsociety.com ) In the meantime, check out the Daycreek website for Alan's latest cordwood and P.E.M. experiences.
Q: I am in the process of digging the foundation trench for what was originally intended to be a 16x16 two story cob structure with a one story [wooden] 20x8 addition, which will serve as the kitchen/wash area, attached to the north side. Realizing that I have several standing dead pines that were killed in an ice storm 2-3 years ago I am trying to figure a way to use them in the wooden addition to keep down costs. The trees are perfectly seasoned already. I am wondering if there are any hybrids around using a combination of cob and cordwood, and if so, what is the most viable solution to using the two together? How would you build so that the differing shrinkage rates and settling rates will not pull apart the structure? Can one build the cordwood up to a height of about 3 feet then do the remaining height of the one story part in cob? Or can one do the opposite: build the cob up about 3 feet then do the remaining height of the wall in cordwood? Also, even if I settle on doing an entirely wooden structure on the north side, will there be any problems where the cob walls and the wood structure are joined? I know the first thing most people say is to attend a workshop, but that is not feasible for me.
A: You can definitely combine cordwood and cob, and in a variety of ways. Ianto Evans and Linda Smiley visited Earthwood a couple of years ago, and, while they were here, we built a panel of our garage using cob in place of mortar in a cordwood wall, called "cobwood." It has been a good success. Others have done the same, including builders in Denmark, Wales, and New Mexico. You can do either of the two methods you suggest in your letter - cob first, then cobwood; or cobwood first, then straight cob - but I tend to lean towards the first method.
I am not a cob expert and any questions you may have about that should be directed towards Ianto or Cob Cottage Company. Their new book, The Hand-Sculpted House: A Practical and Philosophical Guide to Building a Cob Cottage by Ianto Evans, Michael G. Smith, and Linda Smiley is excellent. I do not see a problem where the cob meets the wood.
For my part, I think that cobwood has a couple of advantages over straight cob walls. In northern climates, the wall will be better insulated, particularly if you build the cross-section as cob-insulation-cob, as we do mortar-insulation-mortar in regular cordwood work. Secondly, mixing cob is very labor intensive. I believe that cobwood will reduce you cob-mixing to about 20 to 25% of the amount required of an all-cob wall of the same thickness. Of course, you will have to cut all that cordwood, but that has to be a lot faster than making cob. Your cordwood will not "pull apart the structure." The log-ends may shrink a little, but one of the beauties of using cob as mortar is that you can just spray it down near the log-end and repoint it into any shrinkage gaps that form. Incidentally, your standing dead trees will still shrink a little more after they are cut into log-end length, They will check, if they have not done so already. It sounds like great wood that you've got, but I'd still give it two months of air-drying at log-end length. By the way, cobwood is discussed more fully in my big new book, just out, called Cordwood Masonry: The State of the Art (New Society, 2003). You can get it from Earthwood, 366 Murtagh Hill Road, West Chazy, NY 12992 at $29, including book rate postage. It is so new that I haven't even had time to put it on our website yet.
Q: My husband and I purchased a cordwood home 2 years ago. The house is approximately 50 years old. We have been very busy this spring cleaning up the walls, both inside and out, which were encrusted with about 20 years of dust, dirt and patching. We discovered that the previous owners used many and varied materials to patch the walls around the shrinking logs - all of which have turned to dust. The walls look quite wonderful now and are a lovely honey color. We would like to seal the spaces between the mortar and the logs but we are unsure of which product would be best. We would prefer NOT to use a clear caulking as this looks very shiny next to the logs when it dries. My husband thinks that the best method would be to somehow inject the logs with a product similar to the expanding foam used to seal masonry. However, we can't find anything to use for the "needle" part - everything on the market has too small an aperture. We live in the Georgian Bay, Ontario area and, needless to say, we would like to have the job finished before winter sets in. Any suggestions and / advise you can give would be greatly appreciated.
A: My strong suggestion is that you apply a good quality log cabin chinking to the entire mortar joint, right up to the edge of the log-ends. These caulks will span a quarter inch with no problem and they have the flexibility to move with any additional expansion or contraction of the log-ends, although my guess is that you will have very little expansion or shrinkage because if the age of the building. With spaces between the log-end and mortar exceeding 1/4", you might want to stuff some 3/8" backer rod into the space. This is a foam insulation that comes in a - usually - 25'-long coil.
Q: I am building a shed using traditional "stack cord wood" technique. I have built one already using basic cement between the log ends. I would like to try a different material than regular concrete. The concrete tends to crack and the log ends shrink a little leaving a gap. The primary function of the cement in my new design will be to act as fill between the log ends. Support of the structure will come from the framing around the "log end" panels. I am then planning on using log cabin type "Chinking" to go over whatever material I use for fill. The chinking will provide a flexible seal to the log ends and address the log shrinking issue. What would you recommend for a replacement to regular concrete in this application? If there was a material that would act as fill and also have "flexibility" to it that would be even better (eliminate the chinking process).
A: Here are two different non-shrink mortars (not concrete, which contains stone aggregate):
Q: I am in the beginning stages of planning a cordwood addition to our home. Some friends "warned" me to have big eaves around the outside to protect the wood. No problem, we are building a wrap-around porch. But, they had heard to use "anti-freeze" on the outside log ends or in the mortar. Have you heard of anybody doing this? I have ordered your "state of the Art" book and cannot wait to read it!
A: When I used to work as a mason's laborer in the North of Scotland, almost 40 years ago, we would occasionally use an "antifreeze" in the block or stone mortar, if we thought there was a danger of freezing damage to the mortar over the next night or two. It was a glycol product of some kind. As you know, it is still available. But ...
Q: I live in the Bella Coola area of BC. I'm planning on building a cordwood home far from the beaten path.(20km from any road) I'll be using cottonwood to build with. My major obstacle is the mortar mix. I'll be packing in my materials. Lime and portland cement would slow me down to say the least. What materials could I possibly find closer to the site that I could use instead?There is lots of river sand and clay.
A: A number of people in Europe and America have been experimenting, quite successfully, with something which has become known as cobwood or cobwood masonry, where cob is used in place of mortar in a cordwood masonry wall. Linda Evans and Ianto Evans of Cob Cottage Company in Cottage Grove, Oregon, visited Jaki and me at Earthwood a few years ago, and, together, we built a fine panel of cobwood at our garage. Basically, cob is about 80% sand and 20% pure clay, with some natural fiber, usually straw, as a reinforcing binder. We didn't have straw available and used coarse hay instead, which has worked out well in this application. Mix your cob as described in various books, the best of which is probably Ianto and Linda's The Hand Sculpted House.
Q: Last summer/fall we built a cordwood shed over our pool pump in central Nova Scotia, Canada. We used 6" juniper and your recipe for mortar using sand, portland cement, soaked softwood sawdust and lime. It is a wonderful building and was great fun to put up. There is minor (about 1/16 - 1/8" shrinkage this summer...the logs are loose. I wonder if we should let it go over this winter and see how things are next year before we consider a chinking compound?
A: Sounds like wood shrinkage. Wait over the winter, as you said, to let the wood shrink as much as it will. Then, at a time when the wood is dry (not swollen), apply Log Jam (Sashco Industries) log home chinking to the mortar joints. It come in 5 colors, but I like Mortar White, which will brighten your wall at the same time, but still look like mortar. The Log Jam will close up to 1/4" shrinkage cracks and remains flexible. You can go quite thin on the mortar joints themselves, as the stuff is expensive. Put it on with a pointing knife, then draw it along the mortar joints with a 3/4" or 1" brush, dipped in water. Buy it in 5-gallon tubs, not caulking tubes, which would be very much more expensive.
Q: We are in the process of building a cordwood house. I took a workshop recently in Berea, KY (Clarke Snell) While we were there an expert in the restoration of masonry buildings happened to show up. (Dr. James Adams) In his restoration work they use a lime putty. He highly recommended the use of lime putty over any portland cement based mortar. I read somewhere that you built a lime putty cordwood wall. What are your thoughts on a pure lime putty mortar? ....I have also read your underground house book....we are building or basement with drystack block and will apply surface bonding cement. The reason I bring this up...I was thinking that perhaps adding the the same AR glass fiber to the cordwood mortar would be helpful in preventing cracking?
A: Thanks for your two excellent questions.
(1) Lime putty mortar. Yes! We have been very happy with lime putty mortar, and have used it quite a bit at workshops for the past 2 or 3 years. We also continue to use our regular Portland-based mortar. Both work well, and it is too early to say that one is "better" than the other. (Maybe we'll never be able to say that; it might be a preference thing.) We have made the lime putty with an expensive specially mixed product made for the purpose - it comes in 5-gallon pails - and we have made our own from ordinary Type S Hydrated Builder's Lime. I can't say that I observe much difference between the two, but making your own from 50-pound bags of Type S is a quarter to a third of the cost of the lime putty in pails. I make my lime putty by distributing four gallons of water into two 5-gallon plastic pails, two gallons in each. Then I carefully pour half of a 50-pound bag of Type S lime into each bucket and let it percolate down into the water. It might take a half hour, but that is okay, as you won't be used it for three days, anyway. After it has stopped bubbling, stir it with a stout stick, such as a broom handle. You are looking for the consistency of a very thick milkshake, not window putty."Putty" may be a misnomer here. Let the putty hydrate for three days, and stir it again before use. Always use rubber gloves and a face mask when handling lime, wet or dry.
To make lime putty mortar, mix three parts masonry sand with one part lime putty by volume. Best way to do this is to use a one gallon plastic bucket as a measuring device, as a shovel will not hold the lime putty. We have found that it all mixes together better if you use a paddle drill to first mix a gallon of lime putty with a gallon container of dry sand. Then add the remaining two gallons of sand and mix with a hoe in a wheelbarrow. You are looking for the same consistency as regular Portland cordwood mortar, that is: fairly stiff, more like "stone mortar" than "brick or block mortar."
The advantages of lime putty mortar are: it is very light in color, it doesn't shrink, it points nicely (even two or three days later), it is inexpensive, and it doesn't use any Portland cement. The disadvantages are: it is sloppy to mix and work with and it takes ten days or more to fully cure. I would not advise it when freezing at night is a possibility. A neighbor went a little too late into the fall season last year and the outer half inch of his lime putty mortar did not properly harden and cure and is easily scraped away. But the rest of the building is perfect. Lime putty mortar can be just as hard and durable as Portland-based mortar. Incidentally, we still use a lot of Type S lime in our Portland mix, too, but in dry form.
You can see my article on Lime Putty Mortar in the Continental Cordwood Conference Collected Papers for 2005 (CoCoCo/05 Papers, for short). This fine collection of 27 illustrated papers (many in color) is available from Earthwood at $30, postage included. Go to www.cordwoodmasonry.com
(2) Glass fibers in mortar. I have heard this suggested several times, but never heard of anyone actually doing it. It seems like an unnecessary and expensive step. The fibers themselves won't stop cracking - slow curing is the secret to that - but they will hold the mortar together if and when it cracks. But I have never seen even cracked mortar come tumbling down, so again, what is the advantage? And, if you ever wanted to remove a cordwood wall, such as to build an addition or install a door or window, tearing out a fiber-reinforced section would be a nightmare. But, having said all that, if you want to try it, go ahead. But be sure to report your findings here in this column.
Q: I am about to build a cordwood shop at my house and it will be on a concrete slab. Since I live in N.C. my wood will be Eastern Red Cedar. It will be one story with a metal roof. My question lies in the mortar. I would like to use a mixture of one part type S builders lime to two and a half parts sand. No cement. This formula known as a lime putty mortar has been used for thousands of years in stone work and appeals most to me because it is self healing where spaces develop. It has a lower psi strength of around 200, but if a wall is two feet thick with two 8 inch mortar beds you end up with 3200 pounds of compressive strength per inch of wall length. I would love a second opinion about all of this. I am planning on pushing the system by using lomax corners and not post and beam construction.
A: This is my second lime putty mortar question in a 24-hour period! First, please read my reply to Luke, above. Specific to your question, I must call attention to the need to hydrate the Type S Builders lime before using it. Ianto Evans, of Cob Cottage fame, advised me to soak it for three days, as I describe in my reply to Luke. Longer is even better, but three days has worked very well for us. A ratio of 2.5 to 1, by volume, as you suggest, will certainly work. We have had good results with a ratio of 3 to 1. I can't verify your psi figures, but can tell you that our lime putty mortar walls are real strong and appear to be as hard as adjacent Portland mortar walls built at the same time.
You can try mixing sand and dry lime, and then adding water. I don't think it will work very well, but we would all learn something. Please report on your efforts.
Lomax corners are good. The only drawback is that you are at the mercy of the elements until all the cordwood walls are built. Only then can you get the roof on, with its protection from sun and rain.
A: Concrete is a mixture of sand, stone aggregate and Portland cement. I would not use it with cordwood masonry. If you mean lightweight mortar, Jim Juczak has used what he calls papercrete as mortar successfully on his cordwood home near Watertown, New York. You can read of his results in Cordwood Building: The State of the Art and updated in the CoCoCo/05 Coillected Papers, both available from Earthwood at www.cordwoodmasonry.com . It would take 1000 words to rehash his comments in this column and I have very little personal experience with papercrete anyway. Alan Stankevitz in Minnesota has also done extensive work with what he calls PEM, paper enhanced mortar, and his written about it in the same works already cited. The main difference between his mix and Jim's is that Alan adds sand to his recipe. See also his excellent website at www.daycreek.com . Finally, a fellow in Quebec had good success years ago by making cordwood mortar with "mortar aggregate vermiculite." His recipe was 3 parts mortar aggregate vermiculite and 1 part masonry cement. Very light in weight, non-shrink, highly insulative, and strong. Probably a wee bit on the expensive side.
Q: A friend of mine built a beautiful round cordwood home in Flagstaff, AZ. With a framed second story pony wall. This inspired me. As I own my own tree service, wood is not a problem. My question is: I want to build one in Northern New Mexico near Taos and can't find any literature. Every question asked seems to be from someone in colder climates. Any suggestions? The papercrete seems to be a good start as that is the direction I am leaning towards.
A: (Kelly) There is no reason why cordwood wouldn't be a good choice for the Taos area. Actually it gets plenty cold there in the winter. Most of the information on this forum pertains to your area as well. Papercrete (or paper enhanced mortar) might be a reasonable choice for mortar on the outside, but I would still recommend using standard mortar on the inside to give the home more thermal mass inside.
Q: I live on Manitoulin Island where we have a number of stackwall buildings and find them a beautiful alternative building option. I have begun building a small 16' X 14' outbuilding, post and beam/cordwood. I have the footing down the posts up, but am I too late in the year to continue building. Temperature is dropping, still above freezing. What are the issues with mortar and colder temperatures?
A: Cordwood masonry is not much fun when you're working in temperatures in the mid to low 40s F (5 to 8 degrees C), but you can do it. Our standard portland cement cordwood mortar mix begins to act peculiarly in the high 30s F (3 - 4 degrees C), like it is beginning to freeze, although it isn't. We have to work it over again with our gloved hands to bring it back to the nice plastic consistency. In my view, you can do cordwood work down to about 35 degrees F (2 C), but you will want to protect the day's work from freezing for at least 48 hours. Once, in Colorado's high country, at the 9000-foot level, we did cordwood work on a fine day in the 60s, but we knew from the weather forecast that a severe cold front was moving in that night. We covered our work with a woolen blanket topped with a sheet of 6-mil plastic, all weighted down with log-ends. The temperature got down to 10 degrees F (minus 16 C) and stayed below freezing for two days, before rising again into the 60s. Because of our protective measures, the only "damage" was a small piece of spalled-off mortar, about the size of silver dollar, no problem. And that was where the blanket was not covering quite as well. Portland mortar actually makes a little heat during the chemical curing process, which helps. But keep it protected for 2 to 3 days as long as there is danger of freezing.
When you are working in colder weather, retarding the set is much less important. Portland mortar slows its set in cold weather anyway. With wet softwood sawdust as the retarding agent, you could cut the sawdust to 1 part instead of 3. If you have been using a commercial cement retarder, you could cut that to one-third the usual amount, one ounce instead of three ounces per batch, for example. In real cold weather, leave out the retarding agents altogether.
We like lime putty mortar, too, but one of its drawbacks is that you need to be able to protect against frost damage for a much longer period of time, up to four weeks. And it does not help itself by creating any significant heat as it is curing, unlike the portland mix. Neighbors here in New York built a lovely little home of cordwood masonry using lime putty mortar, but suffered some damage in the outer mortar joint when they worked too late into the fall season. Freezing nights caused a weakening of the still-setting lime putty mortar. The outer half-inch was crumbly and had to be redone the following spring.
But it is worth saying again: Cordwood masonry is no fun in cold weather. And if it is not fun, why do it? Build quality will suffer and you will not be a happy camper. My advice is to tarp the building and resume in the spring when the weather gets better.
Q: Taking your advice I have waited for warmer weather to begin my post and beam cordwood shed. I have laid my first day of cordwood and wonder if there is a preferred way to end for the day. Should all the logs be encased in mortar at day's end? Should a latex bonding agent be used at the start of a new day? Any other insights?
A: That is an excellent and important question. Jaki and I take extra time at our workshops to show students how to leave the cordwood masonry for overnight or longer.
Leave the top of today's work with either wood exposed or the mortar between wood, what we call "filling the teeth." (Filling the teeth between log-ends gives them support, so that they are not displaced by "things that go bump in the night.") Do not finish with the regular one-inch mortar joint. If you do, you will need to lay down another mortar joint when you resume work, creating a "double-thick" mortar joint.
Just as important is to leave the full width of mortar joint on the mortar between log-ends. Do not leave a chamfered edge on this mortar towards the outside (or inside), as this curved edge will not lend good support to the new mortar which you install when work is resumed. For example, say you are building a sauna wall with 8" long log-ends (an 8" thick wall.) The "M-I-M" stick (Mortar, Insulation, Mortar) that we use as a guide would look like a Mexican or French flag with each field being 2.5" wide. (The other half-inch is divided into two quarter-inch reveals at each end. That is to say: the wood is one quarter inch "proud" of the mortar background.)
With your pointing knife and gloved hand, sharpen the edge of the mortar between log-ends to a 90-degree angle, one-quarter inch back from the end of the log. By this method, you will have a full flat 2.5" of mortar to support the next day's work. This is important. A chamfered edge, like a droopy shoulder, does not give the required support. Don't over point the top surface. Leave it rough, for better grab to future work. Some people even scratch the top of the mortar for better adhesion.
Try this visualization: picture the last course of wood as a row of equally sized 6-inch diameter cylindrical log-ends. The mortar just comes up to the tangent point between the log-ends, sharpened to a right angle at the edge. When work recommences, you would run the new bed of mortar, about an inch thick, following the little hills and valleys of the previous course.
Cover your work to protect against rain. Cover the top of the wall to shed water, but not the sides of the wall (especially with plastic) which can create a steamy greenhouse situation and even cause swelling of the wood.
If you are building every day, no other treatment to the mortar is necessary. If a few days has gone by, brush the work, wood and mortar, with a damp brush, to get rid of dust and powder which further diminishes the already weak paste bond between wood and mortar (and mortar and mortar!). You can even brush on a little Acryl-60, Dap, or equal bonding agent to improve the bond between successive courses laid a week or more apart.
Q: We live in Bella Coola BC and want to build a cobwood house. We have a few books about cob and intend to attend a cob workshop. We wonder which cordwood book/books/video we should buy bearing in mind that we probably won't attend a cordwood workshop.
A: For other readers, it will be useful to say that "cobwood" is a word that has been coined for cordwood masonry where cob is used in place of mortar. Ianto Evans - the well-known cob advocate, teacher and writer - and I (with our wives) did a nice little cobwood panel here at our garage in West Chazy, NewYork a few years ago. It has certainly stood the test of time. Quite a bit of work has been done in this field in Germany, Wales and Denmark. The only published references that I am aware of are two chapters 19 and 20 of my book Cordwood Building: The State of the Art (New Society, 2003) To my knowledge, the topic has not been covered in video. But you will use the cob very much like you use mortar - you can even point it - so our Complete Cordwood DVD (New Society, 2007) will be useful. Both of these are available from us at Earthwood, www.cordwoodmasonry.com and book stores.
Q: I have a 20 year old cordwood house on Vancouver Island, British Columbia that is in great condition. It is built with 24" pieces of Red Cedar within a post and beam frame. The previous owners (who built the house) covered the interior walls with a plastic vapour barrier and drywall, which I am removing in favour of the cordwood underneath. It looks great but there has been some shrinkage around the wood (less than ¼") and in some areas you can see through to the outside. I like the idea of applying the log chinking product but am wondering whether I should use this on the exterior, interior, or both? We have cool and very damp winters here.
A: The log chinking products (such as Log Jam and Perma-Chink) are excellent for this purpose, but expensive and time-consuming to apply. Therefore, I would suggest doing the inside first to get a sense of how long it takes and how far it goes. I would also recommend a light-colored chinking, such as Log Jam's Mortar White or equivalent. Clean the mortar joints of dust with a damp brush prior to application. Apply a thin coat to the entire mortar joint. You can apply the product with a pointing knife and draw it out with a damp one-inch paint brush. (Jog Jam is water compatible; maybe Perma-Chink is, too). These products will easily fill your quarter-inch shrinkage gaps and remain flexible. If you like the results, you can decide if it is worth doing to the exterior. By that time, you will be much more of an "expert" on this than I am.
I hope your log-ends are "proud" of the mortar joint by a quarter inch or more. This will make it much easier to apply the chinking, as the log-ends will then help to guide your brush. Try to keep the chinking off the ends of the logs, which looks tacky.
Oh, one last thing: I have said that these products are expensive, but they are even more expensive if you buy them in caulking tubes. Buy a 5-gallon pail and see how far it goes before ordering more. Work right out of the pail and cover it tightly after use. It will stay fresh.
Q: I have access to Devlo Stabilizer. The sheet advises 4+/- 1 fl oz/cwt of cementitious materials. Do you have any experience with this product for cordwood construction using dry Western Red Cedar logs? I will try 3oz/ cwt as you advise for 17 as a starting point.
A: I have had good luck with Daratard-17 (W.R. Grace) and Sika Plastiment cement retarders, using about 3 ounces per batch of mortar, a wheelbarrow load. I am just guessing, but our 10 sand, 2 Portland, 3 Type S hydrated lime recipe (medium shovelfuls), probably weighs just under a hundredweight.
There are a lot of cement retarders on the market. I have heard of Devlo but have no personal experience with it. You should test a batch just as you have described it in your email question. You might also Google-search Devlo to see if you can find a spec sheet for their Devlo Stabilizer.
Here is (ideally) what you should observe: The next day after building, you should easily be able to scratch the mortar with your fingernail, a hardness test. The second day, you should still be able to scratch it, but with some difficulty. By the third day, you will probably not be able to scratch it with a fingernail. If mortar shrinkage cracks are going to occur, you should know about them by Day 5.
Q: I am building a 12' X 16' timber frame/cordwood garden shed. I plan on using your suggested mortar mix of 9 parts sand, 3 sawdust, 3 builder's lime & 2 Portland cement. I'm trying to estimate how much of each to get to complete the project.
There are two important points here:
As a point of interest, when Jaki and I are building 8-inch walls, we get about four square feet (4 SF) of wall done from a wheelbarrow batch. We get 6 batches from a bag of Portland, therefore we can do 24 SF of wall from a bag. With the lime, we can do about 16 SF of wall per bag. For a common denominator, you could say that we can build 48 SF of wall with 2 bags of Portland and 3 bags of lime. This is a full six foot by eight foot section.
Q: I was thinking about putting up 12" square corner posts and then connect them with plywood 2'x8' on one side, 4'x8' on other to create a slipform. Taking 12" Poplar (Aspen) cordwood and slipping them in to the necessary depths/placements and doing one layer at a time and holding them in mid air, flush with the plywood, with screws and washers through the plywood. These screws would replace the wires/ties needed in slipforming. Then pouring in a cement (cordwood mortar mix listed on your site) mix over the layer and then vibrating the mix to make it settle. Repeating this over and over till the height desired. Another 4'x8' on the low side to create another 2' working depth but having to access it on the other side. If someone wanted to put in bottles they could let them "set" on screws that are screwed in the plywood and then retracted when set. In theory this should work. The questions however are in the insulative value and the moisture retention of the cordwood.
A: I just can't see any upside to your interesting idea, not in time-saving, aesthetics or thermal performance (no insulation in the mortar joint.) Vibrating the cordwood mortar would require a wetter batch than I would feel good about, although I could be wrong on this. All I can suggest is that if you want to do it, do it. I hope you prove me totally wrong.
Q: Wondering if you have any advice on how cold we can work in before it affects the integrity of the mortar/wall and/or how to prepare the walls if they need to last the winter before being completed?
A: With lime putty mortar (LPM), you should stop building now. LPM is subject to frost damage for a couple of weeks after building. I presume you are using a more traditional Portland-based cordwood mortar. In this case, you can cut way back on whatever cement retarding agent you might be using (soaked sawdust or cement retarder), as the mortar cures slowly anyway during cold weather. You night eliminate it altogether.
Portland-based mortars are subject to frost damage for about 48 hours, so cover the work for the night with a blanket and plastic. During the curing process, a small amount of heat is created. In combination with covering, this generally prevents frost damage. Personally, I do not like working with the mortar in temperatures less than 40 degrees F (5 degrees C.). The fingers get cold, the mortar performs oddly, and the build quality goes down.
Once you decide to quit for the season, just protect the top of the wall from rain. Let the sides of the wall breathe. If mice take up residence in your wall, you'll have to eradicate them in the springtime. Pretty hard to keep them out of the wall over winter, unless you completely finish a panel within a timber frame.
Q: If I use a lime mortar (no cement) for my cordwood, should I still use the sawdust or not?
A: No, you do not need the sawdust with the lime putty mortar. Be sure to read up on the making of the lime putty and its use with cordwood mortar. You need to be very careful about what you do, in several different ways.
Q: I have 2 big loads of red sand from India I will use in mortar for a cordwood building. It is the consistency of baby powder. Will it harden OK mixed with lime and wet sawdust and cement?
A: Something with the consistency of baby powder does not sound like sand to me, more like silt. I am not optimistic about this. All you can do is test it. Wait a week or two after laying up a section and see if the mortar has strength.