Sticky Stucco

Parge over Slip over CMU

We’ve been doing a lot of experimenting with lime for our stucco parging of walls, ceiling coats, mortar on glass bottle walls and brick arches. Figuring out the right formula for a very sticky mix has been a challenge. At this point we’re using by volume 1 lime, 2 portland cement, 6 sand, a handful of PVA fibers per 5 gallon bucket of concrete and then water to make the mix right. In the mix water we have hand dish soap as water reducer, Aquron 300 and Accelaguard (Calcium Nitrate) as a accelerant for when I want to work fast. The water volume varies a little depending on conditions but generally comes to about 0.72 gallons of water (one half gallon + two cups of eight ounces each) for a five gallon bucket of concrete in our mortar mixer. That is about 0.36 water to cement ratio by weight.

In addition to getting the mix right we’ve found that wall prep is very important. We start with a clean wall and lightly wet it with a sprayer. If it is a little rough, so much the better. If a bit porous that also helps. For foam board we scratch up the surface to be parged. We are mostly going on to concrete blocks (Cement Masonry Units = CMU) as well as onto pink foam, glass, granite and marble. Today for the first time we painted the pre-wetted walls with a neat slip of portland cement and water prior to applying the parge. This worked great. The parge concrete sticks wonderfully, no more pealing or avalanches. We are able to get a much more even and thinner layer with less effort. The trick is to paint the slip on in small areas just before parging so that it doesn’t have time to dry before the concrete parge is applied. The slip is the white in the photo above.

Another important technique is working from the bottom up and vibrating the spatula, trowel or float as we apply the fresh concrete to the wall to make good contact and drive out air along the interface.

Paul of the FerroCement.net discussion list cautions:

While adding lime seems like a good idea as far as workability goes, it is a bad idea for ultimate perfomance of FC as any ‘free lime’ attacks and ultimately corrodes steel reniforcement. One of the goals in curing mortar used in FC is to lock up ‘free lime’ that is always present during curing of Portland cement. The additions of pozzolans such as flyash and silica fume help with this.

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This is good to know. In the areas we’re doing right now we have no steel so this should not be an issue. On pozzolan and flyash, I have not been able to find them locally – if anyone knows of sources around Vermont, please do let me know. I do have some hesitation on flyash from a health stand point.

Weds-Fri Outdoors: 70°F/42°F Sunny
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Tiny Cottage: 72°F/65°F

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About Walter Jeffries

Tinker, Tailor…

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