Point 7 of my 12 point list of methods for reducing cracking in portland cement stucco



             Point seven - Gradation of sand in the finish coat

 Another secret shared by Don Miles- using several grades of sand in the
finish coat. This not only dramatically increases strength in the mortar, but 
adds crack resistent properties.
 One large reason for excessive cracking in the finish coat is the sand is
too fine. Your western style stucco finishes are sold in a variation of two
grits, such as 40-60, or 16-20, for example. These means the ratio of
coarse sand to fine sand. Even though these finishes look nice, they are
on the right track. But they still aren't good enough.
 I met someone in Seattle that used to plaster boats. The formula they
used for boats was 7 bags of 7 grits of sand, for 2 bags of portland.
He had to guarantee the boats would withstand 8,000 psi, which is
incredibly strong. Samples were taken to break them in a concrete
tester. Sometimes the samples would break at 12,000 psi or more.
To put this into perspective, the footers under your house are probably
required tp withstand 2,500 psi.
Sand sold here in Northern Virginia for stucco finish, comes from West Virginia.
It is crushed quartz, graded into 3 grades, no.1, no. 2, and no. 3, number three
being the coarsest. A grade finer than number one is available from the factory,
and is called silica powder.
The disadvantage of this sand is the sand tends to be shaped in round balls,
instead of sharp jagged grains. The sharper and more jagged grains are better
for retaining water in the mix through setting and curing. Also, the sand is graded
as no. 3 is number 3 only-nothing finer and nothing coarser. Seven grits is
What do we use ? No. 2 sandblasting sand. No. 2 sandblasting sand has a
coarse sand, like no. 3 quartz, but is graded differently. No. 2 is coarse
and everything below, providing a uniform mixture of everything from coarse
down to powder. Also, Sandblasting sand,which is also crushed quartz, has
a nice jagged grain, which looks like Southern California desert sand.
 Believe it or not, this fact is one of the largest improvements we have made to crack resistence. In other uses of portland cement mortar such as mud base
floors or laying block, the strength of the mortar can be more than doubled,
simply by using various grits of sand.

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