Concrete flags - discoloured

[mo-uk]

I had these basic concrete slabs installed before having a dispute with the tradesman and taking them up.

https://imgur.com/a/wyoynfW

As you can see some have dark patches on. They were stored for a couple of years before being used and I am 99% sure they did not have the dark patches on them at the time.

I have seen this page which suggests the patches can be down to storage (and probably permanent). Although not sure its the same thing.

https://www.pavingexpert.com/faq_patio_circularmarks


When I took the slabs up I keep the really dark ones in a separate pile.

I have noticed today when looking at my 'good pile' that some of these also have gone dark which suggests its something that has happened whilst they have been stored for the past few months.

Any ideas?

[Tony McC]

That looks very much like Differential Curing, which is as described on the page you quote.

It's not that uncommon with that type of flag.