Without getting into the chemistry about why one of these types of products is better than the other I have just started testing them in my pieces to see which ones I like. Personally I am overwhelmed by the many different types of cements that are available at my local hardware stores, not to mention the mortar isles, I haven't even been able to start in that section. Truly I don't know the difference between mortar and cement, other than one must be made for adhesion and the other for durability and strength. The mixtures between portland cement, sand and additives seems to largely make up the differences between the materials. Price is my concern and the ability of the cements to make detail in a mold, so I haven't found many workable solutions that include any type of aggregate when it comes to the portland cement products. Mortar has no aggregate, but the cracking when it is poured at 1/2" or more increases, so I have decided to stay away from the mortars for now.
The biggest problem with portland cement based products verus the plaster gypsum based products is shrinkage. Cement shrinks, even the expensive stuff. I've been working with "Cement All" at $18 for a 55 lb. bag and although the cracking is down to a minimum, it still appears to shrink around the edges. I have anchored the pieces before I pour the Cement All mixture, as well, I have put in a reinforced wire mesh to strengthen the casting, but I don't want to have to rely on the mesh and anchors as a way to keep my pieces in place and even with an epoxy coating on top to help with the bonding, I find myself back to square one which is why use durable materials if I have to seal them in a clear bullet-proof case to make them work?
Ideally the concrete may get it's strength from the shrinkage as the water is displaced and the molecules harden into durable structures that can exist outside in the elements. Also the strength may allow the material itself to resist cracking in normal temperature fluctuations, so there are some good factors about portland based cement products that have kept me from giving up. Stucco was another material that I have been playing with, but as I use it more and more I don't see really the advantage of it over other acrylic polymer based gypsum based products and I don't seem to get the detail out of the stucco that I do with the plasters. I have also forgot to mention the setting times are longer with the cements, even the quick ones, as compared to the plasters.
This brings me back to the Gypsum plaster based products and the one that seems to work like cement, but expands like plaster is Duracal. Grey in color I have mixed it with red cement powder and have gotten a nice brick color. Duracal seems to be strong and sets very quickly, unlike the cements. The detail when casting with the Duracal is as good as with the other plasters like Hydrocal, Forton MG and Tufstone. Tufstone is my favorite white based plaster that has microfibers mixed in and an acrylic polymer, but neither of these items may be as durable as cement without coating them and covering them with sealants.
Having completed my first Wall of Photo Fresco pieces, as shown in a prior post, I realize that weight may be a larger factor than strength. The pieces I have been making with the cement were made with the idea of being about to be placed in the ground, like bricks for a walkway, only with images molded and cast into them. I took these same items that can hardly be picked up and used them as a the pieces in my photo fresco wall and the weight of the wall scares me. I can see this think falling over and killing me as some sort of artistic justice in that I keep trying to make something that will last a long time, so it inadvertently shortens my lifespan proportionately. Now I live in fear of my wall as I have tied it to the strongest thing in my yard, and unused weight lifting device that is too heavy to get rid of. I may not lift weights, but lifting and moving these photo fresco castings all day is indeed a workout.
The weight of the wall is something that I must deal with and this seems to lead to the obvious conclusion that the thickness must be reduced. I spoke with an engineer about how thick concrete needs to be to be able to be walked on and 2 1/2 inches is a basic measure, so I have been back filling these castings with thicker layers of aggregate based cement to give them the option of being able to be used as counter tops on tables or a walking bricks, sort of like the Hollywood stars in LA, but my style. This added weight also means more material, which in the end offsets any savings that come from the cheaper Portland cement based products. Unless I can find a cement that does not shrink and is truly crack resistant then I need to go back to the high tech plasters, like Duracal and make thinner walls that weigh less once they are made.
My son has finally found a practical use for this wall as he thinks that he can use the wall as a practice board for throwing lacrosse balls at, well, this may not work with the thinner structures, but if the wall can withstand that type of abuse then I can use that as a measure of success. At least as he propels the rubbery heavy balls to the wall he won't be standing under it, so the risk of the wall falling on him is less. Still I don't trust the weight and I can't see any way to make the photo fresco wall without using lighter materials, even if they require additional exterior coats to make them durable and resistant to the elements.
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