When salts attack
Preserve your adobe home
By Marissa Bond
The Las Cruces Bulletin
Southwestern architecture engages with the landscape, reflecting the natural shapes and colors around it. Adobe, made of the sun and soil itself, is a historic form of southwestern architecture, brought to the area by the Spanish and flourishing in the clay-heavy soil of the Mesilla Valley. Many new residents and old families of the valley both appreciate the aesthetic appearance, historic connections and energy conservation of adobe homes.
“A lot of our historic buildings are adobe,” Eric Liefeld, president of Mesilla Valley Preservation, Inc., said. “All of our towns are mud towns – Doña Ana, Las Cruces, Mesilla. That was the technology here. … It was this very complete technology by the 1850s, when a lot of these buildings were built.”
However, adobe has acquired a reputation for being difficult to maintain. Due to well-intentioned – but misguided – reparative measures that do more harm than good, many historic structures and older homes have been lost to disrepair and torn down.
Liefeld and Mesilla Valley Preservation has come to understand many of the problems with older adobe homes are the results of damage from salt crystals, known as salt attack.
Salt attack is not new, nor is it particular to adobe. The effects of salt attack can be seen in sites both ancient and modern. From the ancient limestone of the Sphinx, pockmarked by damage the Greeks and Romans tried to repair with masonry, to modern cement-block walls decaying at the base, salt attack can be seen wherever the right cocktail of conditions are in place.
“This is very much a natural process,” Liefeld said. “It really has nothing to do with adobe at the end of the day. It has everything to do with our soils and our arid climate.”
Salt attack occurs, to some degree, in all climates, but the most severe damage happens, surprisingly, in arid climates with alkaline soils where irrigation occurs.
“You find the really bad problem in agricultural valleys,” Liefeld said. “Why? Because you are irrigating and evaporating. So you are constantly concentrating salts in the upper layers of the soil They are naturally occurring, to start with, but we’ve sort of exacerbated the problem by our agricultural processes.”
How salt attack works
Adobe homes often have adobe foundations. This is not commonly a problem as long as they are kept dry.
When water saturates the soil around the foundation and base of the house, it dissolves salts present in the ground. The walls wick the water absorbed from the soil upward, bringing with it the dissolved salts.
Evaporation usually takes place on level with the surface of the soil. As the water evaporates, the salt crystallizes and is left behind. Sometimes salt crystallizes on the surface, but many times it ends up inside the pores of the adobe.
Liefeld said salt crystals could grow inside the pores for a long time before they become a problem. However, the crystal, which grows more with the dendric searching of a root system than the cubist angles of table salt, eventually reaches the boundaries of the adobe pore it occupies.
“As it crystallizes, it grows with a crystallization pressure of over 8,000 pounds per square inch,” Liefeld said. “So, over four tons per square inch, outward. Nothing can resist this, including concrete, brick and stone. So again, this is not just an adobe problem. It’s all of our masonry structures here. As those crystals reach, essentially, the little edges of the pore they are in, they essentially explode it. That outward pressure turns it to dust.”
This process creates the coving seen on some adobe walls as the growing salt crystals eat away at the bottom of the wall. The crystals, freed from the adobe, fall back to the ground, where they re-enter the cycle, increasing the concentration of salt in the adobe.
Evidence of salt damage can appear in different forms, making it difficult to diagnose without specialized tools.
Efflorescence appears as a salt rime on the surface of a wall, as salt crystals migrate out of the material. This is the less destructive form. The destructive phase is called subflorescence, where the building material has been blown apart on the microscopic level.
The salt molecules are hygroscopic, or water-attracting. On humid days, they can draw moisture out of the air and dissolve while in the pores of the adobe, giving the salt-saturated areas the appearance of being damp, even when it has not recently rained. “They will literally dissolve themselves in moisture out of the air,” Liefeld said. “They are deliquescent – they will dissolve themselves in this moisture they’ve attracted. And then as the humidity drops, they’ll recrystallize. So we have a little microscopic freeze-thaw cycle happening.”
Compounding the issue
This process only occurs on walls that have a continual moisture cycle. Often times, Liefeld said, a home can have one wall with extreme damage, while the others are relatively unscathed.
The problem is compounded with the ways people have tried to mitigate the process. Usually, it is treated as a moisture problem and not a salt problem. Building cement collars and sidewalks around the base of buildings only drives the moisture toward the wall and prevents evaporation from taking place away from the building.
“Moisture finds the path of least resistance,” Liefeld said. “If you put a sidewalk here, you are just going to drive that moisture up into the wall even faster.”
Cement or polymer plasters also exacerbate the problem. The cement is attached to the plaster by a metal lathe, usually chicken wire, which will rust and eventually crumble to dust, along with the wall behind it. As the mesh crumbles, the cement falls away, exposing the adobe behind. This however, is still preferable to driving the salt further to the interior side of the wall, causing the damage to take place in interior walls.
“Because of them worrying about the outside so much, they’ve forced the moisture inside, and with the moisture come the salts,” Liefeld said.
Before attempting to heal salt damage, you have to be certain you are addressing the right problem.
Mesilla Valley Preservation has been awarded a grant from the National Center for Preservation Technology and Training, an office of the National Park Service, to study salt attack. The focus of the grant will be on developing tools and measuring techniques to better assess where salt attack is occurring, and on education for contractors to have more effective means of addressing the problem.
“We’re trying to put together essentially field tools so a contractor can look at these things and go, ‘Okay, that is a salt problem,’ or ‘That’s not a salt problem,’” Liefeld said. Some ways for healing salt attack include applying a sacrificial plaster, usually made of mud or lime. The salts will move into the plaster, which will then fail. However, when the plaster is removed it will take the salts with it, in effect healing the wall.
“The first time we saw this was one of our projects here,” Liefeld said. “We had a horribly degraded wall. We didn’t really understand the salt process. We threw a coat of plaster on it. We came back a month later, and it had just fallen off. We thought, ‘Well, we screwed up. We must have had bad mesh or bad plaster.’ “We, for some reason, had the foresight to get rid of it. We put another coat of mud plaster on, and that lasted much, much better. In hindsight, okay, what we did was we put that mud plaster on. It pulled the salts out of the wall and fell apart in the process. We got rid of it, and in so doing, we got rid of a huge concentration of salt. So we did the right thing accidentally.”
A sacrificial plaster allows the homeowner of contractor to see the areas of damage, and also keeps the damage on the surface, rather than in the structure of the wall. The damaged plaster can be removed from the problem areas, taking the salt with it.
Another technique – one that has been used historically – involves putting an adobe collar around the base of buildings. Even though it is the same physical dimension as the problematic cement collar, it behaves differently, taking damage for quite a while without impacting the load-bearing structure. Like the plaster, it is still a sacrificial element – made to be destroyed, or to allow the homeowner to see the damage that is taking place.
An ounce of prevention
The best way to protect your home from salt attack is the simplest – prevention.
Because the process is driven through evaporation, managing water around your home will be the easiest and most economical way to prevent salt attack. Make sure the grading around your home works – water should flow away from the home, not toward it or pooling against it. Check the canales or gutters to make sure the water is flowing far enough away from your house. Don’t plant flowers and install irrigation near the walls of the house. Check for leaks in air conditioners and swamp coolers, and repair issues quickly when you have them.
Adobe homes with cement collars or plasters are not doomed to salt attack, particularly if homeowners are conscientious about how water drains around the home.
“I don’t think necessarily that cement plasters are something we have to remove from all of our buildings,” Liefeld said. “I happen to live in a very old adobe house that has concrete all around it, that is covered in cement plaster. So I can’t get too religious about this. But my house sits on a old bank of the river. It’s probably the bestdrained site for an old adobe house that you can imagine. I don’t have salt problems because my house is so well-sited. So it really depends.
“This is not something that every house is going to have, but if it is over a certain age, there is a good chance you’ll see some of it.”
While salt attack is a problem compounded by time, it should not be dismissed as relevant only to old adobe.
“This is a more than a preservation issue,” Liefeld said. “If you are building a home here, you are dealing with our climate, our soils. … A big part of this is just getting that understanding out there, because once you have the understanding, it changes how you address the problem.”