Cement Plaster Fig 1418

About 75 to 100 years ago, cement became available to the general public. It was supposed to be the endall to the desire for a quick and hard setting, low maintenance plaster and mortar that would replace the softer, slower-setting traditional lime mortars and plasters. We flocked to it like moths to a flame, only to be burned in the long run. Many historic missions in the Southwest have been brought to their knees due to the substitution of cement for traditional lime-based renders.

As we mentioned in Chapter 4, the problem with cement stucco over earthen substrates is that cement is impermeable. That is, it does not allow water vapor to transpire through it. The funny thing is, it wicks water readily thus accumulating water in the underlying layers. Water passes through cement, but water vapor does not. This is the main reason that so many adobe missions in the Southwest that had their traditional lime plasters replaced with cement stucco in the last century are now showing signs of imminent failure (Fig. 14.19).

We are obviously not big proponents of using cement. As noted earlier in this book, cement manufacturing contributes a significant amount of pollution to our environment. But situations arise in this big world that sometimes require us to eat a little crow

14.19: Even cement plaster over stabilized adobe has continued to crack on the walls of this high-end Southwest-style home.

and admit there are some very good uses for a material that we normally wouldn't be caught dead working with. There are some situations where cement plaster is not only a good plaster, but also the best choice, like building in places where clay simply does not exist.

While we were in the Bahamas helping our friends with their earthbag project, we were often vexed with the unavailability of some construction materials. We take for granted a trip to the hardware store for something we don't have, but when the nearest store is over 200 miles (320 km) away by boat, it's necessary to simply make-do. What they did have an abundance of was cement and cement-based products. Even though the old ruins on the island contained lime mortars and plasters that had undoubtedly been produced locally, these remote locations had also fallen prey to the inscrutable promotion of cement in the 20th century.

If you live in an area that is rarely or never subjected to freezing weather (like the Bahamas), it is probably all right to use cement-based stucco over earthbags. Cement-based plasters are most suitable over earthbags filled with coarse, sandy, well-draining soils, as sandy soils are less apt to harbor moisture and therefore to remain stable in wet or freezing conditions.

Chicken wire, stucco mesh, or extruded plastic mesh can be used in conjunction with cement plaster. The mesh provides tensile strength for the cement, which is likely to separate from the surface of the earthbags without it.

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