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Heading: 1.5Transparent 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

The best resource to go to is "Surface Discoloration of Concrete Flatwork" by N. Greening & R. Landgren, PCA 1966. The explanation is somewhat complex, but on pp. 40-41 attributes it to the ratio of alkalies to chlorides in the water at the discolored surface. The publication can be obtained from PCA at The current item code is RX203.

Bob Neal
Robert Neal
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