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FAQs > Determining depth of cement-treated soil
Because there is no noticeable color change, how
can I tell how deep my roadway materials were treated with cement?
Basically, when cement is added to the soil material,
the resultant mixture experiences an increase in pH—usually
into the moderate to strongly alkaline range from 8.0 to 9.0. This
is a measurable difference from the untreated soil which commonly
has a lower pH. Therefore, knowing the soil pH provides various
clues about soil properties (such as the presence of calcium-rich
materials like portland cement) and is easily determined. A hole
dug through the completed cement-stabilized work (post-hole diggers
work very well for this) can be tested at various depths to determine
at what point the pH increase ceases— thereby indicating the
depth of treatment. The most accurate method of determining soil
pH is by a pH meter.
A second method, which is simple and easy but less accurate then
using a pH meter, consists of using certain indicators or dyes.
Many dyes change color with an increase or decrease of pH making
it possible to estimate soil pH. In making a pH determination on
soil, the sample is saturated with the dye for a few minutes and
the color observed. This method is accurate enough for most purposes.
Both pH meters and pH kits containing the necessary chemicals and
color charts are available from garden supply stores. Here are some
sites that you might find useful:
http://homeharvest.com/soiltesting.htm
http://www.biconet.com/testing/st-t.html
More sophisticated (and usually more expensive) testing
devices are also available through laboratory supply companies.
Some helpful sites include:
http://www.omega.com/pptst/ITK_RITK.html
http://www.gemplers.com/a/shop/product.asp?T1=RLM100&src=21TL001
A commonly used chemical (although strictly controlled
in some areas of the country) to determine the presence of calcium
in cement-stabilized materials is phenolphthalein. While very effective
for the intended use, it has been reasonably considered by many
to be a human carcinogen. It is, however, still available for purchase
and I offer the following site:
http://secure.sciencecompany.com/product1.aspx?SID=10&Product_ID=6364
Another method that I have heard a great deal of for
determining the presence of portland cement in stabilized soils
is through the use of "natural indicators." Nature provides
us with pH indicators in the form of plant pigments known as antocyanines,
which change color over different pH ranges, depending on the source.
The juice from a red cabbage will function over a wide pH range,
from as low as 1.0 up to 12.0, while the juice from red beets will
change from red to yellow somewhere between 11.0 and 12.0. The following
preparation information comes from the website referenced below:
"The indicator solutions are prepared by chopping up the red
beets or red cabbage into small pieces, preferably by running them
through a blender. Then the juice is strained off from the resulting
mush, and the mush can also be extracted with water to yield even
more of the pigment. The solutions are filtered to remove any remaining
plant matter, and can then be used as is, or diluted if the color
is too intense."
http://www.rhodium.ws/chemistry/equipment/ph-indicator.html
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