Strange circles on patio slabs

[Merlin5]

I had these front garden slabs put down a couple of years ago but apart from a single slab, they've all got these big spots. Any idea what they are and how to get rid, if it's even possible at all? I thought it was damp showing through but I don't think it is? Sand was put down over the old cracked black and white diamond tiles (same as next door in first photo)and the new slabs cemented over that.

https://i.postimg.cc/1tjPczpF/20180922_144424.jpg
https://i.postimg.cc/g0LfFKRx/20180922_144145.jpg
https://i.postimg.cc/52hsvWYt/20180922_144127.jpg

 

[Newboy]

I'd hazard a guess at the pointing staining the edges of the slab. Was any cement dye used in the pointing as it looks quite dark?

 

[footprints]

My guess would be water leaching in at the edges washing away the colouring some slabs are coloured all through others have a surface colouring only they are more prone to fading.
Laying on dabs can have a similar effect did you just lay them on a bed all around the edges by any chance?

 

[foxhole]

I would say the slabs were laid on one heap of cement which spread to a circle and therefor shows thru the slab as it slows slabs drying unlike the edge which only has grout around and probably hollow, if you tap the corners does in sound hollow?

 

[Ian H]

My neighbour has strange shapes on his concrete 2x2’s, all exactly that same but at different angles

 

[lonner]

There a cheaper quality pressed slab ,in time they will all fade to the outer colour.
The outer edges tend to fade first. Or they go blotchy as they fade.
Dearer slabs tend to be coloured with aggregate so its a natural colour,this doesn't fade but will weather and obviously will go green and get litchen stains ect and suffer the usual outdoor dirty look.

Not a lot you can do with your slabs im afraid.

 

[Merlin5]

Yep they were cheap. Apparently it's a waiting game, but in two years there's no improvement on mine so I think it's either permanent or I won't see a change in my lifetime. I didn't do the work myself so I cant comment on the details of how it was done it or what was added, but I found this article which suggests it's all down to the stacking and outer edges of the stone drying quicker than the centre.

http://www.pavingexpert.com/faq_patio_circularmarks.htm

 

[RayCaister]

It's called patina,it makes them more valuable.

 

[foxhole]

My neighbour has strange shapes on his concrete 2x2’s, all exactly that same but at different angles
View attachment 148915

Looks like limes scale staining.

 

[Ian H]

Looks like limes scale staining.

But all ecacltly the same, i’m Saying crop circle, alien abduction.