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Heading: 1.3Transparent 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

Mr. John Souter,

What an interesting phenomena. I've never seen this happen consistently. I wanted to toss out a couple questions that may help the more experienced members deconstruct this:

1. Does this phenomena occur across different aggregate sources that your customer uses?

2. Does this occur across different cementitious sources, and different w/c ratios?

3. Do all surfaces of the pavers exhibit this effect? Or is this isolated to the upper surface in the vertical direction?

4. What is the description of the sands used in the pavers?

5. Oddball question: What is the water source? Recycled water, gray water, water from other operations, etc.? Coupled to that inquiry would be the presence of any organics in the sand itself.

If someone isn't drafting a response now that explains the Leopard spots, hopefully this may help!

Best regards,

Brian Litts

Brian Litts
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