Recently, we have received a complaint regarding soft lumps, i.e reverses back to powder form when finger-pressed, and because of that the affected cement were not used due to fears that quality has been severely affected. Was the quality personnel right in not using the affected cement? What are the possible causes of these "soft lumps" and preventive measures in the manufacturing line? Thanks.
Ted Krapkat
Hello Norew,
I assume that the cement you are talking about is reasonably fresh, and has not been stored for too long before being sent to the customer.
Soft lumps are often due to a phenomenon called "pack-set" which can have a couple of causes;-
- Electrostatic charging of particles and subsequent compaction (particulalry in bulk cement)
- Dehydration of gypsum in hot cement resulting in syngenite (K2SO4.CaSO4.H2O) formation and partial pre-hydration of the cement via moisture lost from the gypsum.
- Pre-hydration of the cement caused by exposure to external humidity/moisture during storage or transport.
Electrostatic charging can be prevented by using certain grinding aids such as W.R. Grace's HEA2 .
Syngenite formation and partial pre-hydration can be prevented by tighter control over the maximum exit temperature of the mill. In extreme cases it may also be necessary to reduce the alkali levels in the clinker.
Prevention of pre-hydration due to humidity or moisture during storage is simply a matter of ensuring that the storage and transport conditions are as moisture free as possible.
If the cement is less than a couple of weeks old and the lumps are soft, then the cement is probably okay. However if the lumps are hard and the cement is more than a few weeks old or there is any visible signs of moisture contamination then the cement should not be used unless re-tested.
Regards,
Ted.
Silastman
The pieces can be also formed due to the high Blein's specific surface. Surface of cement's particles is very active and freshly ground particles stick together (aggregated). In this case cement also faster take moisture from air and formated aggregates.
norew
Thanks Sir Ted. How is the alkali content of clinker related to the formation of soft lumps?