Stabilized Mud -- Tinkering of the Worst Kind
I've been told that adobe is ephemeral, that it is too vulnerable to the elements to be lasting and so assailable by roots and rodents as to be unsafe. I have also been told that evolution is theoretical, heretical bunk and that the Earth is hollow - postulations that in the face of the evidence, I have chosen not to believe. In the case of adobe the evidence of its success and longevity is so overwhelming that I might, perhaps, accept the hollow Earth theory before acknowledging the fugitive nature of earthen buildings. As evidence I can point to the walls of Jericho (ca. 7000 BCE) which, though subjected to a biblical cataclysm are so well preserved that they still inform us about such critical issues as drainage and footings, and the church of San Esteban del Rey at Acoma, standing solidly since 1629, here in New Mexico. By all indications, adobe is a pretty stable material.
Nevertheless, it is human nature to tinker with things -- even when there is no need. The notion of "stabilizing" mud seems to have first occurred and gained currency in the 1950's. In California where with each earthquake another 10% of their earthen patrimony disappeared, the tendency was to use Portland cement to make the buildings stiffer and more durable. Seismic retrofits included, among other things, replacement of "traditional," or un-amended, adobe blocks with a newer version that contained ten percent or more Portland cement in the mix.
The Portland-stabilized blocks, though demonstrating slightly more compressive strength, proved equally as brittle during an earthquake as the standard variety. No gain.
In New Mexico where there is little seismic threat, adobe was, and is, amended for different reasons. My friend Jerry Sanchez of Rio Abajo Adobes in Belen has the best reason; the soils he has to work with have practically no clays in them so he uses emulsified asphalt (about 6% by volume in the water) as a binder. In the parlance of latter-day adoberos Jerry's blocks are semi-stabilized. A block that is fully stabilized will contain 12% or more asphalt emulsion, not in an attempt to bind the aggregates, but to waterproof the block.
I believe there are many reasons why you can not build properly with stabilized mud. This position is not speculative. Many years and millions of dollars have been spent by organizations such as the Getty Conservation Institute in Los Angeles, The Catholic University of Perú and the Ecole d'Architecture de Grenoble, France, researching the efficacy of amended muds (I will happily provide a bibliography upon request). Ultimately, most agree that doing things the old fashioned way results in a better looking, safer and more durable product than one that has been riddled and smeared with everything from lacquers to ethyl silicates, discarded motor oil to acrylic polymers, floor sealants to modified polysiloxanes. I've had a field day watching and, truth be known, participating in some spectacular failures. The only caveat to apply to "the old fashioned way" is that, as with any building material, you must design and construct attentively and correctly.
Code writers and enforcers see adobe as inherently weak and made weaker yet if exposed to water. Well, they're right -- sort of. A wood-frame building is as susceptible to moisture as mud. A wet adobe is weaker than a dry one, no argument there. But what the code people, and many engineers for that matter, are often slow to acknowledge is that by trying to make adobe waterproof through amending the mud or sheathing adobe in "waterproof" plasters, they really decrease strength and longevity. They also encourage false confidence in a material that, like any other, needs maintenance.
Adobe can be kept dry without "stabilization," by doing things as simple as keeping it eight inches off the ground and providing protection at the top. If your design doesn't call for a pitched roof with overhang, then cap the parapets -- give me a call and I'll provide the detail.
A moment ago I "sort of" agreed that a wet adobe is weaker than a dry one. The qualifier is based on solid, empirical evidence that you can get adobe wet, even very wet, without hurting it -- provided it can dry out. Much of that evidence is spread over northern New Mexico where we see earthen walls that have been exposed to the elements for decades without any attention whatsoever, much less a regular maintenance regime. They are remarkable.
On your next trip to "O'Keeffe country" check out the walls of the Santa Rosa de Lima church just this side of Abiquiu, on the right-hand (east) side of the road. Ruins, yes -- and I have been watching them now for fifty years or so and am impressed with the indolent, almost imperceptible, rate at which they are eroding -- and they don t even have a roof. Then drive through Guachupangue, Tajique and Santa Clara Pueblo. In those places you will see old adobes, abandoned for the most part, neglected to be sure, still standing after decades of neglect.
If I can make the case that unamended adobe is durable, then I would conversely argue that "stabilized" adobe is unnecessary, and worse. Stabilized mud is intended to be waterproof or, at minimum, water resistant. That means that in order to plaster it you must first lath it because neither mud nor any other plaster can bond to an impermeable substrate. In order to lath stabilized adobes you must drive nails into it, usually 16d or 20d nails. Do that to an adobe brick and watch it shatter. It is remarkable to me that one would take the time to build with adobe, only to weaken it significantly in an effort to protect it.
And, if you dislike the idea of formaldehyde in insulation and pressed board because it off-gasses potentially toxic fumes, think twice about living in a house that has been saturated with toxic sludge.