competing info on stucco mix ratios
At this website it says that the cementitious part of a stucco mix includes both the portland AND the lime:
"It is important to note that the term “cement” includes all cementitious materials, such as cement plus lime. So if 1 part cement is used with one-half part lime, that equals 1-1/2 parts cementitious materials, and that total is then multiplied by the sand number. For the finish coat, for instance, the range is 1-1/2 to 3 parts sand: 1-1/2 times 1-1/2 is 2-1/4 and 1-1/2 times 3 is 4-1/2. So if we have 1-1/2 total parts of cementitious materials, the sand parts would range from 2-1/4 to 4-1/2"
But, at this website, a finish coat stucco mix ratio is defined as having 1 part portland, 1 part lime, and 1.5 to 3 parts sand without any mention of the cementitious relationship between the portland and lime:
"Now the sand in the mixture is cut back so that the mud is creamy: 1 part portland cement; 1 part hydrated lime; and 1 1/2 to 3 parts sand. The key to putting on the top coat is that it must be kept moist to avoid cracking. In this process you can color the mixture but mix the dye with water and stir well first, then add it to the top coat bucket as you are stir it with the drill and wand. Wet the surface before applying the coat and then trowel on."
My question is: which one has it right?
Here's another link with detailed information on stucco application, and it seems to agree with the first paragraph quoted above. See page 16 of the document and note the table column heading labelled "Volume of aggregate per sum of cementitious materials"
You are missing the point that those are just ratios to start from. A big unknown is the gradation of the sand (size of the particles, which can vary locally) and the shape of the particles (round, oval, harsh or semi crushed in some areas). Perfectly round sphere of sand have less area and need less cementitious material and also less water. On the other end end of the scale, harsh sand will require more cementitious materials and consequently more water to get a usable mix. More water also means more shrickage(cracks?).
It is the same as mixing mortar for masonry units.
Lime and portland (or masonry) cement are all considered to be cementious in this context, so the final ratio is 1/1/4-6 depending upon the sand gradation.
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