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What holds this stuff together ?

A clear understanding of what holds plaster or stucco mortar together is important in why this material works, and why it fails.

The three kinds of bond are:

1. A mechanical key.
An example of a mechanical key is mortar on metal lath. Mortar squishes through the holes in the lath and sets up, locking the mortar in place. The mortar that squishes through is called a "key".

Keys on wood lath

An example of mechanical keys is this old

wood lath and plaster application. The mortar that squishes through the gaps in the lath forms keys that lock the mortar in place when it sets up.


2. Chemical bond.
Chemical bond is used in several ways, through paint on bonding agents, or bonding admixtures. In paint-on bonding agents, such as plaster weld, the surface of the bonding agent partially dissolves when wet plaster is applied.
The wet plaster mixes with this dissolved portion, and hardens, forming a bond. This isn't voodoo, like some may believe.
Another kind of chemical bond is formed using bonding admixtures. These add adhesive properties to the mortar, sticking the mortar on the substrate.

3. Suction bond.
    Suction refers to the absorption of water. A example of a suction bond is laying bricks. The bricks are porous and absorb water rapidly. It is this suction that absorbs the mortar into the brick, creating a permanent bond. This suction also holds plaster together. Too much suction, or not enough suction, are the two main reasons that plaster, interior plaster or exterior cement plaster (stucco) fails.
     An example of a high suction substrate is cement block, also known as cinder block or CMU (concrete masonry units). Blocks absorb water very rapidly, making it desirable to wet the blocks before applying mortar. The idea is to slow down the suction, so the mortar is absorbed into the block before the water is absorbed out of the mortar. The more water that is sucked out of the mortar before it sets, the weaker the bond.
     This it what is referred to as a high suction surface if you see this on a bag of
plaster or a manual on stucco. The bag of plaster suggests adding more sand to
the mix for a high suction surface. The idea is the same as wetting the block. The additional sand retains more water in the mix, allowing more mortar to be absorbed, and less water.
    Other examples of a high suction substrate is a scratch coat on metal lath, where the scratch coat has set and partially dried. The scratch coat should be wet down thoroughly before the brown coat, or double up coat is applied.
   The same goes for brick or other surfaces before the mortar is
applied. The suction is what holds the mortar on, but high suction can be
controlled by wetting the surface. or adding more sand to the mix on the next coat.

Wall is wet with a hose to slow down the suction


Photo shows the brown coat  wet with a hose to slow down the suction

before the application of the finish coat. This assures a good bond and extends the working life of the mortar.

 Too much suction will cause the wall to fail, or the mortar to pop off. A couple of examples of too much suction are old soft porous bricks, or an old stucco scratch coat that has set up for way too long. These surfaces seem like they can't be wet enough. How we deal with these surfaces is by using a chemical bond. On cement plaster (stucco) we use an acrylic admixture in the mortar.  We use acrylic mixed half and half with water. The acrylic retains the water in the mix to keep the water from being absorbed too quickly, as well as forms a chemical bond by adhesion. This works on other exterior surfaces, or interior surfaces that receive cement plaster.
 Other examples : Old blocks that have had the mortar stripped off. The pores are filled with dry mortar that have too much suction, Other examples of too much suction are gypsum block, old scratch or brown coats whet the mortar has been torn off for repair, and existing finish coats (color coats). On interior plaster, we use paint on bonding agents to kill the suction by sealing the pores and creating a chemical bond.
 Plaster Weld. Euco Weld, and USG plaster bonder work well for this. I have had failure from other brands, which seem too watery. These paint on bonding agents should never be used for outside stucco, because they will get wet and fail.
  White Portland won't bond to white Portland, contrary to popular belief.
Readers have written to explain they have had their house re color coated,
and the new color coat is flaking off, or falling off. A popular method for
re-color coating a house is to sand blast the color coat back off, at least most of the way before applying the color. This also removes any loose and spalling areas.
 My sandblasting days are over. We use the same 50-50 mix of acrylic admixture when re color coating existing stucco. If the surface has a rough texture or has nonuniform suction, such as a wall that has patches,it may be desirable to apply a bonding coat first.
 A bonding coat consists of the same acrylic mixed in grey cement mortar, or mixed with pure Portland cement (with no sand), and applied with a stiff brush or a whisk broom. This leaves the surface rough and the pores open for suction to support the finish coat.
 A good example of a low suction surface, or too little suction is concrete, both cast in place or pre-cast. Another example is old terra cotta tile.
The concrete does absorb water, but very little and slowly. Concrete is porous, but not porous enough for a good bond.  Cement mortar applied directly to concrete will fail. Sometimes it won't fail immediately, but it generally starts to fail within about six months. This is why readers have written to me about their stucco falling off, and why so many of these fake stone applications fail on new houses. The small amount of suction and few open pores will hold the mortar on,
but not good enough. We have torn off and re-done fairly new stucco applications that were done this way. Our solution is the same: acrylic admixtures. There are some old fashioned methods that work well for this, and some applications where there is too much suction, but I'll cover these later in this article. If you aren't too bored, please read on.
 Our mix is the same, half and half acrylic with water in the mix. We put this in the scratch coat for a typical three coat, 3/4" application, or if the the concrete is real straight, a thin base coat that is roughed up using a float or a stiff brush,
before applying the finish, or color coat.

Scarifying the scratch coat


Scarifying, (or raking, or scratching) the scratch coat

not only provides a mechanical key for the brown coat, but increases the suction by opening the pores and increasing the surface area.
This is a homemade scarifier I made from an old refrigerator rack.

    Increasing suction may be desirable for ceilings, but with caution.
For inside plaster on metal lath ceilings, suction can be increased by reducing
the amount of sand in the mix. For example, instead of 20 shovels of sand to
a bag, the amount of sand can be reduced to about 12 shovels of sand to a bag.
This allows the ceiling to be brown coated or doubled up the next day. Less sand and more plaster makes the mortar more porous and increases the suction. The idea
is to be able to double the ceiling the next day, instead of letting it set and dry
the normal two days. Once we did this on some ceilings in Alexandria, and let them set all weekend. The ceilings had so much suction, we had to soak them with a hose, and work in small sections because the mortar was taking up too fast. Taking up is when the mortar becomes firm, before the initial set. It is probably better to just use a standard mix, and let it set up for two days.
    Suction in the scratch coat can be reduced by adding more sand to the brown coat, but this also should be done with caution. Too much sand can also weaken the mix. Poor mortar, that is mortar with too much sand, is also slippery, and a lot of the mortar goes too waste from dropping it on the ground.
    I have to comment on posts I have seen in places like the wall and ceiling forum and other places on the web. I have seen posted that a good mix for stucco ceilings is 3:1, that is about 30 shovels, or 6 buckets of sand for a bag. Please don't believe this.  I doubt anyone who says this has ever put mortar on a ceiling all day. Our standard mix is about 2:1. Much more than this and most of the mortar will end up on the ground.
 Straw Bale: I have never done any straw bale, and it isn't used around here. What I've heard is that mud plaster is thrown (dashed) onto the bales and allowed to dry for a week or so. This provides suction for the brown coat. If this is wrong, please let me know. There are other methods of straw bale.
 Adobe: I haven't done any adobe, and it isn't used in Virginia, but it used heavily used in third world countries. Stucco will fail before long because the cement and sand won't bond to the mud adobe. Lime is used for a link. Soupy lime is dashed on the adobe block and allowed to harden and dry. The lime is firm and very porous and provides a good suction bond for stucco.
   Bonding mortar to concrete: This also applies to other low suction substrates.
I have written about this before, which you may find here:   Question2002

 An old fashioned method I learned from an old timer and I have used before
is using a Portland cement paste. This method was used in plastering swimming pools and on concrete years ago, and still works today. A paste in made using pure Portland cement and water, to a consistency it can be applied with a brush. First, the concrete is wet by splashing with water. The paste is then put on
with a brush, leaving the surface rough. This paste will set up immediately, and start to dry quickly. It is IMPORTANT to put the scratch coat of mortar before this paste dries. This can be done in two ways. The paste can be applied to a small area at a time, say about 3 feet by three feet, and then the mortar scratched on. Another method is to have two people working, one painting on the paste and the other scratching mortar on right behind him. The mortar is the scarified or raked. and allowed to set up and partially dry at least overnight. There is now good suction to support the brown coat.
 Peppering: This is also a good method if have seen being done in Mexico, and have seen foreigners doing here. First the concrete is wet down and rich mortar (mortar with less sand) is splattered on the surface, leaving the splatters as rough as possible. After this sets firm and partially dries, a scratch coat of mortar can be put on and scarified (raked, notice I don't say scratched)
. This scratch coat can be browned after it has set up and dried. I have seen concrete soffits done like this that were three stories high. Just think that someone could be killed if a piece of this stucco mortar fell off and hit them on the head. These ceilings will not ever fail if this was done using this method.
 Peppering is also used for using old welded wire or woven wire lath, where there is no paper behind the lath to support the scratch coat until it sets up. The holes on the lath are too big or the wire can't hold the mortar on. First, the metal lath is peppered by splashing soupy mortar on. Better suction is created by using rich mortar, or less sand. After this peppering sets and partially dries, suction is created for better support on the lath, and the lath can then be filled in with a scratch coat. I have never pumped mortar on a ceiling, but I have heard that peppering is used before pumping the scratch coat on a metal lath ceiling. This suction  prevents "drop outs" when the scratch coat is pumped on.
I mentioned I would bring up plastering on old bricks without using chemical bonders. There is no real good method, except first soaking the bricks at least twice before beginning. One person can then splash water on the wall while another immediately applies mortar. If this mortar starts to dry before it sets, more mortar should be troweled over the drying mortar before it dries. The idea is prevent the mortar from drying before it sets, avoiding "dry-outs". These "dry outs" will leave a soft powdery surface unsuitable for applying more mortar. It is good to use a lot of sand in the scratch coat for old bricks (meaning 1800's bricks). , say as much as 30 shovels per 94 pound bag of Portland. The additional sand retains water in the mix longer to counteract the suction.
No matter what, on old soft bricks, the scratch coat will have tons of road map shrinkage cracks due to the high amount of suction. These will fill in with the brown coat and should be of no concern unless the edges of the cracks curl up, indicating failure to bond. There is nothing else to do in that case but tear it off and start over.
More about suction some other time. Suction is also used in the brown and finish coats to provide firmness and and workability.
 I want to devote a following article to chemical bonding. Use of
chemicals has improved the quality of our product dramatically, but should
be done with caution. Please check back

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