Instead of integral color in concrete we are going to talk about mortar mix design for the April Newsletter. You can blame Jeff Datin of Concrete Artists Network. Jeff graciously let me be at his concrete carving class at the Concrete Décor Show in Phoenix and I met a great bunch of people who I would describe as free spirits. Am I right? Are carvers by nature free spirits, self taught, marching to their own drum? At the very least concrete carvers are pioneers and are taking the decorative concrete arts in new directions. They do some things with coloring and finishing that freak me out but new ideas incline back to the center as good inventions become tried and true. A true pioneer’s highest motivation is to give the customer good value and something unique. Anyway, several people have asked me about “scratch” mix designs. I hope this helps.
Reinforcement: As with concrete slab on grade so with the structural base for carving, whether on grade or a span, proper reinforcement is critical. The best information about reinforcement without getting a degree in civil engineering is available on a CD by Jeffrey Girard and Lane Mangum from the Concrete Countertop Institute. To order call them at 888-386-7711. You can trust their information completely. They are the real deal for straight stuff. A generalization should cover most situations. Whether you create a shape or basic structure for your project with EPS (expanded polystyrene), expanded metal lathe, paperback lathe, a wood structure, earth to be removed later, or other ingenious method, the reinforcing bar is required to be exterior to the contoured structure and fully embedded in the mortar or concrete. Do not put the steel behind the lathe, even if you can more or less embed it. Conventional concrete, from scratch mortar, sack concrete or packaged masonry mortar may be used to create the basic structure or “scratch” coat but the steel, whether #3, #4 or larger diameters must be embedded in the mortar. Spacing, 40 diameter laps or other engineered connectors will apply. Be sure to consider plan approvals, permitting and inspections.
Mix Designs and History: In the mortar design literature there are only two classes of mortar you should consider, Type S and Type M. Type S can yield 1800 psi and Type M 2500 psi. The difference is lime content and sand ratio. Do not confuse Type S mortar with Type S Lime. Either Type S or Type M mortar is adequate for your scratch coat in most cases.
A word about Type S lime: Up until about 1950, maybe 1955, masonry and plaster mortars were made using lime putties. Lime (CaO) was mixed in a pit with water to hydrate or slack the lime to get rid of the chemical heat of reaction and make it safer to use. This chemical reaction yields Ca(OH)2 as part of the lime or lime plus water so to speak. The bond is highly reactive as is the calcium oxide. The resulting product was delivered to the job as putty and dumped near the mixer. The hoddy cut a cubic foot with his shovel or measured in a box for accuracy. The quality of these mortars cannot be matched with modern bagged products. Type S lime is referred to as pre-slacked or slacked because the slacked lime or putty is dried and bagged. It is much “cooler” and safer to use but does not have the “slick” wonderful trowel properties of the old lime putty mortars. Look for the lime bag with the green print rather than the blue print. The cubic foot of putty dried is 2 cubic feet or one bag of lime. This is important because mortar designs are by volume. Treat all lime with respect. It can cause chemical burns.
To make a Type S mortar, mix 1 cubic foot or one 94 pound bag of Type I-II or I-IV Portland cement with ¼ bag Type S lime which is ½ cubic foot or 12.5 pounds. Add 360 pounds of washed masonry sand or about 24-30 #2 shovels. Use about 40 percent of the cement weight for mix water or 40 pounds which is approximately 1 and 1/3 five gallon pails. You will find the mortar fairly stiff but if you can apply it in a fairly uniform ¾ inch layer you can achieve 1800 psi. If you add the powders to the water it is easier to get a homogenous mix, same as cookin’ in the kitchen.
To make a Type M mortar, reduce the Type S lime content to 1/3 cubic foot or 1/6 of a bag of Type S lime, just 8.3 pounds. You will also reduce the water to 1 five gallon pail plus 2 quarts. Again, the mortar will be rather stiff but will yield a scratch coat harder than your head. Use the same amount of washed plaster or masonry sand. Notice that these modifications increase psi almost 40% to 2500 psi. That’s a lot. Usually there is not a lot of fuss over flexural strength which is a much more important structural quality, but psi is assumed to imply flexural strength for convenience and economics, a poor assumption.
Other Mix Designs: The two most common variations of these two mixes are using Plastic Cement and using bagged mortar. Plastic cement is common in the Southwest or other places where exterior plaster is a big industry. Plastic cement was developed as part of the history of asbestos in mortars because if lime could be reduced or eliminated while adding more sand and maintaining strength it saves money. Asbestos fibers were magic, enabling plaster to be pumped through plaster gun lines for several hundred feet. To maintain workability plastic cement has considerable air entrainment and plasticizing and usually contains large amounts of flyash to improve workability. If it works well for you, use it.
The other common product is bagged mortars. I hate them. Sand and lime have water loosely attached to them. When you mix Portland cement with sand, etc. as you must to bag a mortar, hydration begins because the cement has higher energy potential and will pull the water off the sand and lime. After three days of this slow hydration in the bag the mortar has lost its workability and a good deal of strength. Unless you are getting bagged mortars fresh from the bagger stay away from them. If you make your own mortars so you are used to good flow and trowel properties you won’t be able to stand bagged mortar.
Some Tricks for Strength, etc.: Workability or trowel properties are a huge issue for carving, kind of like overhead plastering. You need it to hang and stick. You have two choices with unmodified mixes, increase the lime and or decrease the sand. In general you don’t want to decrease the sand, it just gets too sticky and will be prone to shrinkage cracks. Increasing the lime can quickly reduce strength. (If you have trouble with shrinkage cracks you need more sand, less water or better sand meaning less clay contamination and fewer fines.)
The most effective alternative is to reduce the water. Decreasing water content by 30% can double the strength of mortar designs. The cheapest way to reduce water is to use either sodium lignin sulfonate or neutralized phenolic resin. These are low cost unsophisticated chemicals originating from stump pine tars in the South. They are the only high range water reducers that don’t turn your mix to heavy creeping syrup. Use ¾ ounce dry of either per sack of cement and you will probably reduce water for a given slump by up to 30%. Don’t use more than one ounce. You will have too much air and a serious drop in strength. You will be amazed at the strengths you can achieve. These products enable you to keep the lime ratios up for workability, reduce plastic shrinkage, use enough sand to avoid shrink cracks and still give you all the end product qualities you desire. The carboxylate polymers should be used if you have technical requirements.
In a past life I did custom masonry. I would tell the electricians, “You better get your boxes set before I cover you up.” They would tell me not to worry, that they would just cut them in. They only tried that once. When they could not cut the mortar joints they learned to get the boxes ready before I laid the brick.
Secondary or Supplemental Cementitious Content: The quickest way to get increased strengths is to use silica fume. Problem is silica fume is thirsty and tends to cut your open time to nothing. Better choices are VCAS and metakaolin. Both VCAS and metakaolin offer the additional benefit of reducing efflorescence. They accomplish this by consuming the excess Ca(OH)2 byproduct of hydration to form calcium silicate hydrate and calcium aluminate hydrate in the capillaries of the concrete matrix, thus densifying the concrete. The capillaries come from excess water in the concrete. Concrete and mortars typically require about 25% of the cement weight to fully hydrate. For workability and placement issues the amount used is usually around 40%. After secondary hydration the hydroxide radical is not available or able to leach to the surface, react with carbon dioxide in the atmosphere and form the insoluble carbonate stains you know as efflorescence. Both calcium silicate hydrate and calcium aluminate hydrate are stable components of hardened concrete and actually improve the long term stability by reducing swelling and other volume changes in concrete associated with shortening the long term life. A concrete specialist would say that secondary or supplemental cementitious materials are not needed if we correct the amount of water. Problem is we are using excess water for trowel and workability so we need at least some extra water.
VCAS and metakaolin are both aluminosilicates but VCAS comes from the glass industry and metakaolin comes from the clay industry. Both are fired at high temperatures in a combustion chamber to form amorphous or glass phase and then quenched to create their unique characteristics with respect to size and reactivity. If you read the literature you will find that both are presented as cement replacements up to 30%, sometimes more. For mortars I disagree. As a safe guideline I recommend using no more than 8% and not as a replacement but an add-on. In other words if you have 100 pounds of cement use up to 8 pounds of metakaolin or VCAS in addition to the 100 pounds. Don’t try to get more complex. To sum up, these products can increase your strengths to offset the reduction in strength from using more lime and also reduce efflorescence so your creation looks better over the life of the piece. In a word I think they are good for the end product for a long, long time.
To find a local source for VCAS call Vitro Minerals, 678-990-2658. For a local source of metakaolin call Powerpozz, 800-595-7552. I have found these people make a good effort to be helpful.
More tricks: One way to give increased fatness and some more workability to your mix is to add clay. This phenomenon is called false body. I recommend Amador 200 at a 2% dose or about 2 pounds Amador 200 per sack cement. I discourage fire clays. Fire clays work great but they are not good for color, acting like a wick to pull water from the atmosphere and give you rude surprises in color changes. Amador 200 is available at stucco manufacturers if you are located in California, Arizona, Nevada, New Mexico or Texas. If not, Amador 200 is usually available from a pottery or ceramic supplier.
Just a note on polymer additives: Generally they are a waste of money for scratch coats. If you use them avoid acetates (like Elmer’s Glue or anything that smells like it.) They blush if they get wet again. If you use them keep the dosage extremely low, about one cup, two at the most, per sack of cement, assuming about 30% solids. We make an acrylic additive we call Acrylic Activator for overlays, mortars, art work, logos, etc. It could be used as the mix water after blending 50/50 with water. I have used it before when I wanted a dramatic increase in flexural strength. Most cases it would waste your money.
Fibers: If you use fibers and there is any chance they will be exposed use only ¼ inch nylon. You will still get “hair” if you scrape the surface but they are easily burned off so you have a nice finish surface. Remember, we are talking scratch coat and base here. Keep the dose low. I would not use more than 100 grams or ¼ pound per cubic yard of mortar so that translates to about 18 grams per sack of cement. That is only 2/3 ounce, not much. Using the larger stiff fibers in a mortar can cause you problems trying to trowel into a mesh or stucco netting. The fibers sometimes form a grid you cannot easily push through the wire to get good embedment.
Using Color: Most carvers I have talked to don’t want color in the base. I think it is because they usually don’t know where they are going until they get there so they see color in the base as a limitation. If you learn to use small dosages of color you can save a lot of time coloring the finished shape. Try a ½ percent dose of red iron oxide in natural cement. You will find it knocks the green out of the cement and gives you a nice pallet to work on. If you need a light background to color try using a slag cement component up to 50%. It is a lot cheaper than white cement. Of course if you need a dark brown or black background you can dose up the color to about 4% of the cement weight. Increasing the dose beyond 4% won’t give much bang for your buck.
Don’t forget that Absolute Concrete Colors has dry pigments, mortar colors, acid stains, and other products for coloring, antiquing, cementitious paints, textures, sealers, releases and we can make a custom texture mat if you need one.
Summary: While there is a lot of monkey business you can do with mortars always start with the basics. If you are going to modify the basics don’t do everything at once. Add one thing. Then you can assess the difference. Then you can add something else if you want and assess that. It is easy to get too complex and just spend money and time. Get sophisticated if you want. Mortars are fun and interesting. Just keep the guidelines in mind and you will be okay.
Good concreting and have fun.
