Stucco and the blind man
Chapter 5 - Mortar and sand for plaster and stucco
My interest here is mainly explaining mortar and sand for cement stucco. I also will cover plaster, and also lime and sand. I'll cover plaster more in depth in a later chapter.
When sand is used in cement mortar, or in plaster basecoats, or sand finishes, the sand is the most important ingredient. Sand makes up most of the mortar, so the using right sand, and the right amount of sand is critical.
I've never really worked with it lime and sand base coats. Lime and sand was used for most plaster walls, including stucco until the late 1920s. Lime and sand was used in most outside stucco until the late 1920's, until cement mortar took over.
Lime and sand is a lot weaker than cement mortar or plaster base coats. Usually on interior plaster if you see wood lath, the plaster is just lime and sand. The finish coat plaster is lime with molding plaster or gauging plaster, or another form of plaster of Paris. Interior plaster lime and sand mortar was generally reinforced with animal hair, and is usually weak and crumbly. On the exterior, lime and sand was used on both wood lath and metal lath. Metal lath became popular after 1905. The oldest house I've seen with Portland cement stucco was in Washington, DC. The house was built about 1910. Lime and sand base coats were used in houses up to about the late 1920s. However, gypsum plaster was used widely in commercial buildings such as train stations, hospitals, and schools about 1890.
Lime base coats are fairly hard, however we have found them where they were so weak and crumbly could pull the mortar off the wall with your hands and crumble it with your fingers. Very weak mortar might be attributed to the fact that it had too much sand in the mortar or "dead mortar" was used. What I mean by dead mortar is lime and sand doesn't really really harden like cement or plaster, but has a drying process much like drywall compound or paints. Lime and sand was frequently crushed up and re-used day-by-day. After so many times of reusing the leftover mortar it simply had no strength left in it.
Lime will dissolve in water and can be water damaged easily.
There are claims, and I sort of believe, that lime base coats are self-healing. Since the mortar dissolves in water, cracks are filled in when it rains. There is some truth to this, but when stucco has been painted it doesn't dissolve much. Lime base Coats were considered more crack resistant because they were soft and moved with the building more than a more brittle cement basecoat.
Why Portland cement stucco didn't catch on sooner was probably due to the fact lime and sand was easier to find locally and cheaper. I think another reason the public was slow to catch on to cement stucco was because old buzzards were too hard headed to catch onto the new.
The only time I've used lime and sand was in a brick-laying class I took many years ago. At the end of the class when we were finished with the retaining walls, or BBQs whatever we would disassemble the brick work scrape all the mortar back off into a pile. At the beginning of the next class we would mix this mortar with water in a mortar mixer and use it over and over again. Of course you couldn't do this with cement mortar.
On bricks, lime base coats were really hard probably because it had a stronger substrate than wood framing. Many times lime base coats were rubbed down with a wood float, which added to the strength of the mortar. The float packed down the mortar and made for a denser for a stronger base coat.
Old floats like this were used for rubbing down lime and sand basecoats.
When I say sand, I mean aggregate for plaster, or stucco. For example, a mortar that is portland cement, lime and sand, has portland cement and lime for a binder and sand for an aggregate, Concrete has portland cement for a binder and sand and gravel for aggregates. When we refer to sand usually we think of masonry sand. Masonry sand is usually quarry sand, or bank sand or river sand. Beach sand should never be used for mortar because the high salt content deteriorates the mortar. This Old House once had an episode about deteriorated blocks in New Jersey that were made years ago with beach sand. In California, desert sand is scooped up off the ground and used as is. Desert sand is great sand for mortar because usually it is coarse and sharp. River sand is also used, and was used a lot years ago. River sand is usually not a good choice because it is generally real fine and has a lot of dirt. We have used Potomac river sand to duplicate historic colors and appearance, and Shenandoah River sand because that was all we can get nearby, and for the nice color for finishing.
Sand dredged out of a quarry is usually screened to various sizes. The coarse gravel is screened out first, then the pea gravel, then the concrete sand, which is coarse sand with some small gravel. The last is masonry sand, which shouldn't have any rocks, but may have a few teeny weeny rocks. Bank sand is scooped out of the ground from what was an old river bed. It is screened and washed if necessary. We also use masonry sand that is crushed sandstone from West Virginia. This is excellent sand for a stucco basecoat.
The ideal sand for a stucco basecoat is a coarse sand with jagged, or sharp edges. Mortar with coarse sand shrinks less, and the sharp edges retain water in the mix longer. The water retention allows mortar to set without drying out. Also, the coarser the sand, the less the sand separates or the mortar board, or mortar box. Fine sand sinks to the bottom and requires constant turning over. Mortar with fine sand shrinks and cracks more, and is never sets as hard. Also, mortar with fine sand sets faster, not allowing time to do as satisfactory a job in hot weather.
There are other kinds of sand, or aggregate, for stucco or plaster. For example, marble dust, crushed quartz, and perlite or vermiculite. Perlite and vermiculite are lightweight aggregates for plaster, and are sometimes used in cement mortar for specialties like relining chimney flues, or insulated block. Perlite is made from dust from mica rocks. The dust is heated until it expands, like popcorn, to several times it's original size. A 4 cubic foot bag of perlite only weighs 20 pounds, as opposed to 200 pounds for 4 cubic feet of sand.
The labor saving of light weight aggregate is obvious. The main disadvantage is less strength. On government work, state of Virginia, Washington, DC and the federal government we could not use light weight aggregates, only sand. Sand plaster basecoats are much harder, have better sound resistence, and better security. Premixed perlite and plaster basecoats are sold in building supplies and even Home Depot. These are convenient for the DIY and Handy Andy types that just want to fill a hole. The two main brands of pre-mixed perlited plaster basecoats are Structo-lite, by United States Gypsum, and Gypso-lite by National Gypsum, known as Gold Bond.
Structo-lite used to have a warning on the bag, "not recommended for use on metal lath." I am not sure why they took the warning off. I only use sand basecoats on metal lath, or white mortar for patches. more about white mortar later. When I refer to light weight aggregate I am referring to perlite, or vermiculite. I havent seen vermiculite plaster aggregate is over 40 years. I do have to make an exception for perlited mortar on metal lath. We used structolite on metal lath a few years ago for fireproofing. We put in suspended meatal lath ceilings in two school kitchens above the finished ceilings. Since these ceilings don't really matter what they look like, it doesn't matter if they excessively crack. Perlite mortar even has a better fire rating than sand mortar, because perlite absorbs water. I didn't publish pictures of these ceilings on my web site because they sure weren't beautiful, but just fire proof.
Since perlite absorbs water, it had to be coated for insulated block fill, which I think may be obsolete. One needs to be careful to use coated perlite for block fill, and not perlite plaster aggregate, because perlite absorbs water. I just need to comment if one needs perlite mortar, it is a lot cheaper to mix the perlite in the mortar, instead of buying pre-mixed mortar.
Unaggregated gypsum plaster for plaster basecoats, that is plaster that isn't mixed with sand or perlite, is called "hardwall", after the gold bond brand of basecoat plaster, or sometimes is called "red top plaster", after the USG brand name for plaster products. If you hear someone talk about hardwall and sand, this is what they mean.
One thing that really bugs me is I when see online a ratio of cement to sand is 3 parts and 1 part cement. Our mix Portland cement 1/4 bag of lime and 20 shovels of sand. Five shovels of sand is roughly one cubic foot. A 94 lb. bag of Portland is two cubic feet. So our ratio is more like 2 to 1. For a patch we mix 2 parts sand to one part cement, and a little lime. Anyone that says the ratio should be 3 to 1 has never put mortar on a ceiling, because 3 to 1 won't stay on the ceiling. It may be possible to put 3 to 1 mortar on a side wall, but it must be built up slowly and is a complete waste of time, not to mention it is a weak product.
I knew a plasterer many years ago that had a stucco job at a military base here in the Washington, DC area. I am not going to mention names. The specifications called for a maximum of 6 cubic feet of sand for a bag of Portland cement, a 94 pound bag of Portland. He made a 1 cubic foot box, 12 inches cubed and put exactly 6 cubic feet of sand in every batch. Remember my formula is 4 cubic feet per bag.
Mortar without enough sand is referred to as rich mortar, and mortar with too much sand is referred to as poor mortar.
The reasons for sand in mortar: 1. Adds bulk. More bulk allows more mortar to be picked up and spread at a time.
2. Economy. Sand is cheaper than cement. Adding too much sand to save money not only results in a weaker product, but more mortar goes to waste. Poor mortar is slippery and more mortar is dripped on the ground. More mortar on metal lath walls with no backup falls behind the wall. The money saved by adding too much sand is wasted by mortar dropped on the floor or the scaffold. More mortar dropped also creates more cleanup.
3. Reduces shrinkage. Sand doesn't shrink like cement. Less shrinkage means fewer shrinkage cracks in the basecoat or finish. One expression used to justify poor mortar is " Have you ever seen a sand pile crack ?". Oversanding does weaken the mix. Shrinkage cracks in the basecoat are filled in by the finish coat. We contol shrinkage cracks in the finish by using chemical admixtures.
4. Retains water in the mix. The important thing is sand with sharp jagged edges retains water longer in the mortar, improving strength. Quarry sand produces much stronger mortar than river sand.
The reason mortar sets up, concrete and plaster ,too, is because of crystalization. If you are making a statue out of plaster in a rubber mold, you mix the plaster with water, pour the mix into the mold, and after about an hour the plaster hardens and you can pop your statue out of the mold. When the plaster hardens microscopic and sub microscopic crystals form transferring your liquid mix into a stone like hardness. Why this happens is that plaster is made from gypsum rock. Rocks in nature have a high H2O content. When gypsum rock is crushed and heated water is released from the rock. This is how plaster is made. When water is added to the plaster to make your cast statue, the plaster forms crytals and returns to rock form. When cement stucco goes through wet and dry cycles, the stucco improves strength. This is caused by smaller molecules being joined together with H2O molecules, creating a denser molecule and a denser crystal.
Portland cement is made of different kinds of rocks, like slate and shale, that have been heated to release water in the rock and crushed. Gypsum rock, that is plaster, is the main ingredient that forms crystals and accerates the set of the other types of rock. This is why plaster is a good accelertor for setting up cement mortar. Molding plaster is used as an accelerator in pre-cast concrete, allowing several "stones" to be cast in a day instead of one a day.
Grading of sand, that is the use of different sizes of sand, can dramatically improve the strength of mortar.
Sand used for basecoat stucco is usually natural sand, that is, quarry sand, bank sand, or river sand. We sometimes use sand that is made from crushed sandstone from West Virginia. This is excellent sand. The sand for our stucco finish coat is mainly manufactured sand, usually called silica sand. Silica sand is made from crushed quartz, or crushed marble, also known as marble dust. Other aggregates such as crushed limestone are sometimes used in stucco.
Lately, crushed quartz has been specified in plaster basecoats. These ASTM rated sands are made of different grits, or sizes of the screen used to sift the sand. Sand is graded from coarse down to fine, the finer sand filling the voids in the coarser sand.
Different sizes of sand also improve crack resistence and reduce shrinkage. For our stucco finish, we sometimes used industrial quartz, which is different sizes from coarse down to fine powder. Lately we have been using mixtures of graded sand such as half number one (fine) to half number two (medium), or part number three (coarse). California pre-mixed stucco finish is sold by sand size, 16-20, or 20-30. 16-20 is a mix of coarse to medium, 20-30 is so many shovelfuls of medium to fine. All fine sand is used sometimes for smooth trowel finishes and other reasons. Fine sand in the finish sets fast, so it may not allow time to do a decent job. Also mortar with fine sand is weaker, and not as crack resistent. Mortar that has fine sand needs to be constantly turned over on a hot day, because the sand separates. We just used fine sand, or number one, for an exposed aggregate finish, that is, we threw small rocks into the finish.
We use crushed quartz for nearly all our stucco finish coats. Our formula is 1 94 lb. bag of white portland 1/4 50 lb. bag lime 200 lb.s white sand (crushed quartz) We also use masonry sand or river sand for historic work to match historic appearance, or for someone who likes that look. Masonry sand with white portland has a color and patina that is just unbeatable. There are different kinds of crushed quartz or marble dust that can be used. White sand is usually called silica sand, but really all sand is silica. When someone says silica sand they are referring to crushed quartz or marble that is used for an aggregate in cement plaster. We sometimes use Q-rok as and aggregate for our stucco finish. Q-rok is quartz that has been screened or graded into different sizes, 1 is fine, 2 is medium, and 3 is coarse. There is also 00 which is super fine. Sometimes this is called silica fines. More about fines in my chapter on plaster. For a fine finish we mix number 1 and number 2 together, and for a coarse finish we mix 2 and 3 together. Sometimes we mix 1, 2 and 3. Mixing the different grades add a lot of strength to the mix, and a nice deeper appearance. Number 1 alone is way to fine and sets too fast. Also pure fine sand can create a lot of shrinkage cracks.
The strength of plaster basecoats can be increased two-fold or more by using graded silica sand for the basecoat.
I met someone when I was out west who plastered boats, known as ferro-cement boats. The boats were plastered in the same operation, or the same day. The formula was 7 different grits of sand in portland cement. That is seven 100 lb. bags of different grits of sand to 4 94 lb. bags of Portland cement. Let's say just under 2 to 1 sand to cement. After the boat was plastered, the boat was steamed at a temperature of about 120 degrees for a few days.
These boats were guaranteed to withstand 8000 psi in a crush test. Some of these boats withstood over 10,000 psi, and sometimes even broke the testing machine. To put this in perspective, the footers under a new house are usually rated at 2,500 psi.
Why is the word pound abbreviated lb. instead of pd. ? In the grocery store it says 3 lb.s of bananas, and not 3 pd.s. Even in England the symbol for the currency, or the Pound is an "L" and not a "P". The reason is the latin word for pound is Libra.
Taking this another step, how about if graded silica sand is used on mud floors ? Mud floors are for tile installation, or stucco floors.
The main types of cement mortar, according to strength are, type O, type N, type S, and type M. Don't ask me why these letters are in order. It is not for me to wonder why.
Type O is very weak and is a mixture of cement with a lot of lime and usually a lot more sand than a standard mix. Type O is mainly used for for masonry when use in old, soft bricks. When I say old, I mean before 1900. This weak mix is to prevent bricks "blowing up", that is bricks crumbling in the wall. The idea behind the weak mortar is it has more give. Before 1920, most masonry mortar was just lime and sand, with no cement.
Type N is used for brick veneer, or applications where bricks don't support anything. The advantage of type N is that it is easy to clean off. Acid wash cleans it off easily, and sometimes it can be washed off sidewilks, windows, etc. with just a hose and a broom. Type N is sold in 70 lb. bags and is typically mixed with 16 shovels of sand per bag. An equivalent can be made using 1 bag of portland and one bag of lime.
Type S is structural strength made for residential or light commercial building. Type S is used for example, block basements or foundations, or small 2-3 story block or masonry buildings. Type S also comes in 70 lb. bags and is usually mixed with 16 shovels of sand to a bag. The equivalent using portland is 1 94 bag Portland and a half a bag of lime.
Type M is used for stucco and for structural masonry on commercial buildings. Not only is it used for block, but for brick veneer going up more than two stories. The formulas for type M is one bag of portland and one forth bag of lime, or one 94 lb. bag of portland and one 70 lb. bag of type S masonry cement.
Retempering is adding water to mortar and mixing it up after it starts to set. Cement mortar can be retempered or shaken up, but plaster must be thrown away. Still, cement mortar should not be retempered much because retempering weakens the mix. If mortar is chunky, it should be thrown away.
Work should be planned to avoid retempering. Besides killing the strength of the mortar, retempering is a huge waste of time.
A few rules can be followed, particulary in the summer, to prevent mortar from setting fast.
Clean water should always be used. Dirty water makes mortar set too fast. The worst is water that was used to wash shovels, tools, etc. This water with mortar in it accelerates the chemical reaction causing mortar to set. Water barrels should be rinsed out time to time. It isn't a bad idea to have at least 2 barrels, on for mixing water and one for washing tools.
The longer mortar is mixed, the faster it sets. This is true even for plaster mixed on a mortar board, and is true for every method of mixing mortar. If using a mixer, it shouldn't be allowed to run a long time. No matter how mortar is mixed, with a hoe, a shovel a drill, whatever, it should be mixed until it is barely good enough.
Work should be planned to work on the shady side in the summer, or starting at 6 am, if allowed by the noise ordinace. It isn't difficult to build a shade tent over a mortar box with scaffold and drop cloths. Tyvek works good for shade tents. We have even shaded the whole wall with tyvek.
I have to tell a story about bricklayers that were working on a school we were working on in Fairfax County, Virginia. This was a huge masonry contractor here in the Washington, DC area. They tempered up their mortar 5-6 times, where we didn't retemper at all. They mixed mortar in the hot sun with a mortar mixer. The mixer ran all the time. When there was a batch in the mixer the mortar man drove the forklift loading down the pans. After mortar is mixed in a mixer 15-20 minutes it sets almost instantly when it is poured out.
After the mortar was poured into the forklift mortar tubs, the mortar man retempered the mortar because it was already chunky. He would pour water in the batch and mix it with a shovel. There were buckets of water and shovels on the scaffold for retempering. The mortar was like chunky soup. There were drools all over the face of the brick to be cleaned off, and the mortar joints could be scratched easily with a nail. The forklift tub and the metal pans were never cleaned real well so the old mortar absorbed water out of the new mortar accelerating the set more. The brickwork sure didn't look top notch.
Changing a few habits would lead to not only far better work, but saving a lot of time. Time equals money, right ?
The ideal sand for cement stucco finish is crushed quartz or marble dust, mixed with white portland cement and lime. Graded sand just means screened to size.
Crushed industrial quartz is available that is coarse down to fine powder. This sand is sharp and interlocks well, with the finer sand filling the voids between the larger particles. Another good sand for stucco finish is white Q-rok, or quartz graded to size. This works well when the grades are mixed together, such as number 1 (fine) mixed with number 2 (medium), or number 3 (coarse) mixed with number 2 or number 1. Using all fine sand should be avoided altogether. Fine sand makes the mortar set fast, not allowing time to finish well. Also, finish mortar with real fine sand is weaker, and prone to shrinkage cracks.
Industrial Quartz. often used for sandblasting, is excellent for cement stucco finish.
Industrial quartz is crushed quartz graded from coarse to fine powder. The edges are sharp for good water retention and interlocking.
Q-rok is an example of quartz graded to different sizes.
Our formula for cement stucco finish is: 1 94 lb. bag white portland. (Lehigh is my preference) One fourth 50 lb. bag lime (masonry lime or finish lime) 200 lb.s quartz or marble dust Liquid is half acrylic bonding admixture and half water.