A previous article, Best wood for cordwood building, discussed how to choose from the types of wood available for cordwood. The next decision is what to use as mortar to hold the wood together in the walls.
The mortar choice will be different from that used for conventional brick walls since it makes up so much of the wall's content, filling in the spaces between wood pieces, approximately forty percent of the wall surface area. It must be strong enough to stand up to shrinkage and expansion of the wood, and to provide more insulative power.
The Romans used something similar to the latest lime putty mortar (LPM), where type S hydrated lime is soaked for a few days, then mixed in with mason's sand, 3 parts sand to 1 part LPM. See Rob Roy's article on Lime Putty Mortar in the Continental Cordwood Conference Collected Papers for 2005 available from Earthwood.
Adding soaked sawdust, perlite or vermiculite to mortar keeps it wet longer and slows the cure to prevent cracking. Sawdust should be from light, airy softwood, passed through a half-inch screen, and can be obtained from saw mills or chainsaw dust. Presoaked in water before using, it acts like a sponge for the mortar to draw moisture, drying more slowly. There are chemical cement retardants like Daratard -17 by W.R. Grace and Co. or Plastiment by Sika Corporation, which should not be mixed together, but any chemical is not as good for the environment as natural fibers. Sawdust and straw also help increase mortar's R-value.
Some people, like Alan Stankevitz, are using slurried paper or papercrete mortar, about twenty percent Portland cement and eighty percent paper, or sixty percent paper, twenty percent light sand, and twenty percent Portland cement. In one instance, a cordwood builder used about forty percent slurried recycled newspaper in the mortar mix. Avoid Portland cement that has fly ash, the byproduct of burned coal, already added to it.
Rob Roy's book, Cordwood Building: The State of the Art, recommends a mortar mix of 9 parts sand, 3 parts soaked sawdust, 3 parts builder's lime, and 2 parts Portland cement. The lime must be burned and rehydrated, usually is in 50 pound bags, and can be called autoclaved mason's lime, builder's lime or hydrated lime. It is not the same as agricultural lime. Roy says the best sawdust is from low shrinkage softwood from a mill, not too fine from a table saw nor too thick mill chunks. Mix the ingredients in a wheelbarrow, first dry, then adding water slowly until you can throw a ball of it into the air and catch it without it falling apart. It will be thicker than normal masonry mortar.
In Richard Flatau's book, Cordwood Construction: Best Practices (2012), he suggests a mortar mix of 3 parts sand, 2 parts soaked sawdust, 1 part Portland Cement and 1 part Hydrated Lime for non-load-bearing cordwood used as infill, in post and beam framing for example. Use sawdust to cause it to cure more slowly with less cracking. Flatau says to protect the masonry from full sunlight and cover it at day's end.
The book Living Homes: Integrated Design & Construction By Thomas J. Elpel has a good section on mortar mixes. It suggests alternatives for the energy intensive and polluting Portland cement, but also explains that adding materials like sawdust, vermiculite and straw weakens the mortar. It is still strong enough in the right applications.
Vermiculite is native to both North and South Carolina, so it would seem like a good choice for mortar mix in this region. One drawback is that it tends to shrink more as it cures. A man in Quebec used three parts mortar aggregate vermiculite to one part masonry cement for cordwood mortar and it was very lightweight, non-shrinking, highly insulative, and strong, but was more expensive.
Cob mortar made with soil from the building site is a better environmental choice than high energy Portland Cement transported distances, but a high foundation and large eaves to keep water away from the walls must be added. It is a shame to cover the beautiful cordwood exterior, but with cob it is safer to waterproof it with lime plaster or earthen plaster covered with linseed oil, and Earthbind™ 100. Ianto and Linda Evans, cob experts of Cob Cottage Company in Cottage Grove, Oregon, use a cob mix of eighty percent sand and twenty percent pure clay, with natural fiber like chopped straw thrown in for a reinforcing binder.
Builder’s lime in the mortar takes longer to completely set than Portland cement resulting in a more flexible, breathable, self-healing wall. The Portland cement, either type I or type II, chemically binds the mortar, but having high-embodied-energy, the less used, the better for the environment.
Some builders "point" the mortar, smoothing it out around the logs with relief between the wood and the mortar. It takes longer to build that way, but can make the wall look more finished. Some prefer the mortar flush with the log faces. Some like the wood sticking out from the mortar a bit. It is an aesthetic issue and a matter of personal preference.