Cleaning porcelain garden slabs - any ideas

[waltzingparrot]

I have cleaned the slabs in both these pictures this week. The pale grey ones are the colour they should be and came up beautifully with just a jet wash.

The marked ones have then also been cleaned with a light bleach solution (1 part bleach/4 parts water) which didn't work and have just used Spear & Jackson patio cleaner. Obviously didn't work either.

I have no idea what caused the marks. They are the slabs at the bottom of the garden next to the fence, so were heavily stained with green algae, which has come off, but has left these marks.

They've been down for four years and I jet wash them every spring and it's the first time they've been marked like this. No new trees/plants in the area.

Any ideas what I could try next, or is it a pointless exercise?

I suspect that the algae has dyed them. I can’t guarantee it will work, but one suggested method for cleaning patio slabs is to sprinkle them with bio washing powder, use a watering can to dampen it, leave it a while and then brush it all off with a stiff brush. The enzymes in the washing powder are supposed to be good at lifting off the grot.

Cloris

@NineOClockOnASaturday Thanks, I'll try that next.

@liverpool1981 What's Cloris please? Googled it but nothing coming up.

Sorry cholorus I think

@liverpool1981 Thank you

I’d try dilute bleach, but try not to wash in into any flowerbeds
Patio Magic is also worth a try (works on green algae type growth so may work on the stains from it??)

DH used a box of Daz on ours this week and it's come up a treat!
We are going to try the steam cleaner on any stubborn marks - quite reluctant to use strong chemicals because of the cat!
I'll let you know how that goes! 🤣

Great to hear that the washing powder method works! I’d be very wary about bleach, as it’d be hard to stop it running off into flowerbeds and it’s noxious stuff.