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Heading: 1.2Transparent Aggregate
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Dear Technical People on

Our company manufactures chemicals for the cast concrete industry. One of our customers is experiencing a strange phenomenum. They cast pavers in polyurethane rubber moulds. Upon demoulding we find that there is a spot associated with each stone below the surface which is darker than the surrounding area. We thought that this was due to differential drying out of the surface and that once the whole tile was completely dry these marks would disappear. However the spots remain. Sometimes the spots are lighter than the surrounding area!

I have been told this is called "Transparent aggregate " but no explaination for its occurance has been given. We call it Leopard spot !

Would you be able to assist me in any way giving an explaination of this even which now seems to plague us more and more.

Best regards,

John Souter
Redec Chemicals
Cape Town
South Africa

Tania Hudson

A petrographic study comparing the light and dark areas could be useful. One study several years ago for a precaster found that the dark spots were related to significantly lower w/cm paste overlying highly flattened coarse aggregate particles located quite close to the cast surface. Another study of dark spots on a flagstone-textured precast found that the dark spots were caused by severe cement retardation where a release agent had pooled in low spots in the mould.
Laura Powers
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