Adobe vs. Cement
Portland cement has replaced traditional plasters as an interior and exterior finish for many earthen buildings, new and old. However, there are great differences between old construction methods and new, and what is right for one is not always right for the other.
Why is cement plaster bad for adobe buildings?
There are several disadvantages to applying a cement plaster to an earthen structure, based primarily on the fact that cement plaster is a rigid material and adobe is a soft one: Adobe and cement plasters expand and contract differently. Cement does not "give" like mud does and therefore cracks. Cement does not naturally adhere to an adobe surface and therefore it needs the aid of wire lath that is nailed to the surface of the wall. Nailing the lath to the walls may cause damage to the adobe fabric, especially when the old plaster is removed in order to apply a new one.
Experience has indicated that after about 30 years, adobe walls cased in concrete show damage that is frequently irreparable. Though cement became very popular during the 50's and has been applied with all the best intentions, it is not a sustainable substitute for traditional materials
Is cement compatible with adobe?
No. Cement used as plaster on historic earthen structures may eventually cause irreversible damage. It is advisable to use compatible, permeable and traditional plasters even if they need cyclical maintenance. It is inaccurate to think that if an earthen wall is not protected by an impervious substrate, such as cement, that the building will disintegrate when exposed to weather.
How does cement cause damage?
- When adobes have a permeable, "breathable" plaster such as mud or lime, walls are in a constant cycle of getting wet and drying out. As long as the traditional plaster is maintained, the building can last for hundreds (even thousands) of years. When a cement plaster, elastomeric coating or foam insulation covered with a cement plaster is applied to many of these historic buildings, problems may start to appear due to the moisture that accumulates in the walls.
There are two main ways that water invades a cement-covered adobe wall:
- Capillary Action - Many old adobes were constructed with stone footings and, in some cases, no footings whatsoever. Walls that are built straight up from the ground and then covered by a cement coating are susceptible to capillary action. Like a drinking straw, moisture is "grabbed" and "pulled" up the wall.
- Cracks - Moisture also enters through minute cracks in the cement. When this happens, the water tends to rise up, looking for a way out. Because the cement is not permeable, water is trapped in the walls.
Once saturated, a wall begins to erode. However, damage goes unnoticed because it hidden behind the rigid cement shell. It is not until the plaster is removed that large gaps and cavities are noticed on the wall surface. When the moisture exceeds 12%, the bottom portion of the wall loses stability and cohesiveness to support its upper loads. The lower part of the wall not being able to support its upper load will begin settling, bulging and slumping.
- During cold weather walls may suffer from freeze/thaw cycles, which will eventually cause the cement plaster to delaminate and in some cases the adobes to crack.
- Another common, yet not so noticeable, problem is the accumulation of salts on a wall surface. These salts, which are present in cementitous products and also seep up along with water from the ground, crystallize on the surface of a wall. Salt deposits can cause the plaster and adobe surfaces to delaminate and fall apart, and the wire mesh used to adhere the plaster to rust.