The
Problems with Pack Set
 The
laws of physics have not changed since their discovery—and
neither has the problem of pack set. Primarily affecting bulk
cement shipping, but also present in storage facilities, pack
set is the resistance of cement particles to the onset
of flow.
When finely ground particles are stored and exposed to vibration,
they will consolidate. Bulk cement shipped from manufacturing
plants by rail car or truck provides a classic example. During
discharge, the consolidated, bulk cement may resist flowing.
The initial resistance to the onset of flow is called pack
set. The biggest problems with pack set are the time delays
in handling bulk cement shipments. These delays can be significant
enough. But even without the delays, customer misperceptions
related to pack set can strain well-established business relationships.
Add to this the further confusion between pack set and warehouse
set. Warehouse set is a condition arising from surface hydration
of stored cement particles in the presence of moisture.
When did this problem begin?
With the increasing transition to bulk cement shipments beginning
in the 1970’s, there has been an increased awareness
of this problem. However, relatively little research has been
conducted. Why? The simple reason is that pack set exists
more as a phenomenon than as a problem plaguing the industry.
Some manufacturers experience pack set regularly in storage
silos on site; some only occasionally with bulk shipments.
Others see it regularly with a specific cement type. Still
others hear of problems only from the occasional customer.
With this kind of sporadic occurrence, tracking the cause
of the pack set is difficult.
In 2003, ASTM C1565 Standard Test Method for Determination
of Pack-Set Index of Portland Cement was adopted. Long utilized
by manufacturing and testing facilities (at least since the
1950’s), this method provides a measure for examining
the potential for pack set for a cement. The procedure uses
a flask filled with cement and applies a consolidation effort
using a measured vibration voltage. The flask is then rotated
in the horizontal position until the compacted bed collapses.
The number of half-turns yields a pack-set index indicating
the relative force required to overcome the consolidation
effort. This index can be used by manufacturers to assess
the tendency for their product to experience pack set with
vibration. The higher the number; the greater the tendency
for pack set. A pack set of 3 or more is generally considered
problematic.
But why does this phenomenon occur?
The reason for pack set is not completely understood. The
current perspective centers on a complex relationship between
the surface energy of the individual cement grains and the
flow of those same particles. Surface energy is affected by
many factors in the cement manufacturing process: fineness
of grinding, type of grinding equipment, kiln temperature,
clinker temperature at quenching, the rate of cooling, and
the type and use of grinding aids. When large surface charges
exist (due to unbalanced valence forces) and compactive effort
is supplied (as with bulk shipment), pack set is more likely
to occur.
What can be done to reduce pack set?
The use of cement additives dates back to the 1930’s
and has proven very effective in enhancing the manufacture
and handling of bulk cement as well as the performance of
cement in concrete. Grinding aids act to satisfy the surface
charges developed on the cement particles. This action then
provides resistance to pack set and improves flowability by
reducing particle agglomeration.
References
Bhatty, J.I., Miller F.M., and Kosmatka, S.H., Innovations
in Portland Cement Manufacturing, SP400, Portland
Cement Association, Skokie, IL, 2004, 1370 pages.
“Pack Set: Cause and Prevention”, Processing
Additives Technical Bulletin 3, Grace Construction Products,
September 1977, 4 pages.
Strohman, Nile R., “Additives from TDA to Polyfon T”,
ASTM Standardization News, November 2002.
Strohman, Nile R., Private communication with the author
September 6, 2005.
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