Stucco News

Question and answers on Stucco and Plastering

March, 2015

Graded sand for plaster-
ASTM C 897, “Standard Specification for Aggregate for Job-Mixed Portland Cement-Based Plasters”
ASTM C 926, “ Standard Specification for Application of Portland Cement-Based Plaster"


sandblasting sand is graded
                    for a stucco finish coat.
This sandblasting sand is graded from medium (about 1-2 mm) down to a very fine powder. The varied grits, or different particle size adds increased strength and crack resistance to stucco.

When used in a cement basecoat for
high strength applications such as stucco floors or cement boats, the various grits can quadruple the compressive strength of Portland cement mortar.

The reason is that the smaller grains fill in the voids when the mortar is packed down.

This is what we usually use in our stucco finish coat.

Q-ROK is graded quartzite
Q-ROK is graded quartzite, which means crushed quartz or crushed white sandstone. Sand like Q-ROK is used in stucco finishes, but
the grading is different. Q-ROK comes in 3 grades, #1, or fine; #2, or
medium and #3, coarse. Fine powder is packaged as silica fines.
The main difference is that number 3 is all the same size (about 3 mm.) Number two is all medium, and number 1 is all fine.
When used as a stucco finish, different grades are usually mixed together to vary the size for improved strength and appearance.

ASTM C 897, Standard
                    Specification for Aggregate for Job-Mixed Portland
                    Cement-Based Plasters

What this means in human readable terms:

Silica sand (usually crushed quartz or sandstone) is
passed through various screen sizes and measured
as to the percentage of what size the grains are.

Everything above 4.75mm is thrown away. The sand
is then graded from less than 2.36mm (coarse or # 3)
down to a fine powder of 75 micrometers (µm) or
75 thousandths of a millimeter.

This chart is completely ambiguous if you do the math,
but the idea is that the sand is graded from coarse
to fine powder.
Also, since the sand is irregularly shaped, larger width
particles can pass through the sieves.

ASTM C 926 to the rescue.

"Aggregates failing to meet only the gradation limits of this specification shall
be permitted to be used, provided the supplier furnishes satisfactory
documentation to the specifier that the plaster made with the aggregate has an
acceptable demonstrated performance record."

In other words, if you don't have ASTM C 897 sand prepackaged in bags, or don't happen
to have a 75 micrometer screen, and if you can demonstrate the performance of graded sand,
you can use whatever sand you have.

What I am trying to say with all of this is that mortar can develop incredible strength by
simply varying the grit of the sand, and using silica sand.
I know somebody out west who
plasters boats. His formula is 7 bags of varied grits of sand, that is one bag each of #1, #2, etc.
 up to #7, mixed with 2 bags of Portland cement. The breaking strength of these boats was well
over 10,000 PSI. To put this into perspective, the footers under your house are probably
about 2,500 PSI.

Imagine how strong other high abuse applications can be, such as mud floors, simply by
spending a few bucks for graded silica sand.

A plasterer armed with this knowledge can dramatically improve
 the quality of his product.

More about dollars and cents:
ASTM C 826 is currently being specified a lot for interior basecoat plaster in Federal
government work.
ASTM C 826 is sold in bags for about 11 dollars a 50 lb. bag, which works out to about
$ 440 per ton. When I did a lot of federal government work in the 1990's, I used masonry
sand, which cost about $ 20 per ton then, and about $ 40 per ton now.
Back in 1998 I was paying between $ 9-12 for a 100 lb. bag of hardwall (gypsum basecoat
plaster). Now, gypsum plaster is only avaiable in 50 lb. bags, for about $ 22 per bag.
As an illustation,  I could brown out 300 square feet at 3/4" thick for a material cost of
 $ 120 in 1998. The same area today would be $ 900.

Another mystic secret revealed: Have you ever wondered why the word "pound" is
abbreviated lb. instead of pd. ? Even in England. where the currency is a Pound, pound
is abbreviated with an "L" and not a "P" ?
The word "pound" in Spanish, Italian and Latin is "Libra".