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Frequently Asked Questions
Q: What are the different forms of sulfate in portland
cement and how can I analyze cement for SO3?
A: Sulfates in portland
cement can be broadly categorized as:
Although normally reported as SO3 (% by mass) for consistency,
sulfur can be found in any combination of these forms. Elemental sulfur
is almost never found in portland cement, except in trace amounts,
as it is normally produced in an oxidizing environment.
- added sulfates—gypsum, hemihydrate, anhydrite, several
synthetic forms of sulfates (typically by-products like flue gas
- clinker sulfates—including arcanite, aphthitalite, calcium
langbeinite, and thenardite.
The added sulfates are blended with clinker during the finish grinding
of the cement in amounts needed to control early setting properties,
as well as shrinkage and strength development. The amount needed varies
from cement plant to cement plant, depending on the chemistry and
fineness of the cement, but is typically on the order of 5% by mass.
The most common form of sulfate added to portland cement is gypsum,
some of which is intentionally dehydrated by the heat of grinding
to form hemihydrate, which is more soluble and therefore available
earlier to control early hydration reactions.
Clinker sulfates form naturally during clinker production (sulfates
typically are part of the raw materials as mined). These sulfates
tend to volatilize at the temperatures of cement kilns (up to about
1450ºC) and tend to condense on the outer surface of clinker
nodules as alkali sulfates during the last stage of clinker production
(rapid cooling). Again, the amount depends on the chemistry of the
raw materials and kiln operating conditions, making every cement somewhat
unique. These alkali sulfates also are soluble enough to help control
early hydration reactions. Some clinker sulfate is also incorporated
into other cement phases.
Since every cement is unique, chemical analyses are the best method
of determining the SO3 content of cements. Typically
the total SO3 content is measured (or elemental S measured
and converted to SO3) through methods in ASTM C114 (or
AASHTO T 105). XRF analysis is probably the most common technique.
PCA report SN2079, The Distribution of Sulfur in Present-Day
Clinkers of Variable Sulfur Content (1996) provides several
techniques for further analyzing forms of sulfate in cement. Please
PCA Library if you would like a copy of the report. Email firstname.lastname@example.org.