what is this slab? looks natural stone

cherry2017

Thanks, I want to clean it.
is it ok to use pressure cleaner?

twopenny

I thought at first it was concrete but some of the surface is layered and coming off.
Also is that fossils in there or manmade pattern?
If it's fossils then that's expensive stuff - but worn.

Look online at suppliers. Some have information about care - or email them about cleaning.

Also depends on the strength of your pressure washer. 

njkmr

It should be fine to jet wash it. Should clean up nicely.

Bookworm105

whether it is natural stone, reconstituted stone, or plain concrete does not alter the fact it is safe to clean using a pressure washer.

No domestic pressure washer is going to damage either material.
(Whether it actually cleans them without using chemicals is a different question)

cherry2017

twopenny said:

 is that fossils in there or manmade pattern?

Thanks for your reply, the fossils are natual, not manmade. the video link below has more details of the slab.
can I use sander to sand off the black spot if I cannot clean it easily?

https://youtube.com/shorts/fzAAnotWdzI

the_lunatic_is_in_my_head

We have Indian sandstone slabs and yours look very similar to that but I'm no expert. 

I think the black spots are lichen, if so bleach and water with a stiff brush should remove, whatever you decide to do test on a small piece somewhere out the way to see the result before doing the whole lot  

cherry2017

Bookworm105 said:

No domestic pressure washer is going to damage either material.
(Whether it actually cleans them without using chemicals is a different question)

Thanks. we tried detergent washing powder this afternoon, it seemed ok, I checked the detergent should a little bit alkaline, but the black spot is hard to get rid of, I will buy the karcher k4 to try it again, pressure is about 130 bar. I tried steam cleaner, it is very helpful for cleaning, but it is so time consuming

Nobbie1967

Be careful using a pressure washer near the joints as they look in pretty poor condition and you could end up with a lot of gaps where the grout comes out.

ashe

Sodium hydrochlorite. 

It's nasty stuff, you mix it 3:1 or 4:1 with water (add the hydro to water. Not the other way around in case it splashes)

I wore gloves, a proper mask not a diy dust mask, full outfit no exposed skin and sealed goggles. 

Did the job in about 30 minutes then rinsed 

mask did a very good job; as soon as I took it off it smelled like I was in a pool but about 5x stronger 😂

much less effort then pressure washing - got rid of black spot and algae and general muck, but it's obviously a strong chemical 

Bookworm105

cherry2017 said:

Bookworm105 said:

No domestic pressure washer is going to damage either material.
(Whether it actually cleans them without using chemicals is a different question)

Thanks. we tried detergent washing powder this afternoon, it seemed ok, I checked the detergent should a little bit alkaline, but the black spot is hard to get rid of, I will buy the karcher k4 to try it again, pressure is about 130 bar. I tried steam cleaner, it is very helpful for cleaning, but it is so time consuming

a K4 will do nothing at all to black spot, but for domestic garden cleaning is a reasonable choice.

Washing powder is an expensive way of using detergent to loosen dirt!

black spot can only be removed by very strong chemicals, not a washing powder. The relevant one being Sodium Hypochlorite ("SH", "hypo", or "bleach"), but at a stronger concentration that you can buy in a supermarket.

how much you dilute it by (3:1, 4:1 as mentioned above) depends on what concentration it was to start with!

Supermarket bleach is around 4.5%, you need to start with SH concentration at > 10% (mostly sold as swimming pool "chlorine") before diluting, even then spots may need to be treated with neat "hypo"

You MUST wear a mask when using "hypo", not a dust mask, a chemical mask.

Albermarle

ashe said:

Sodium hydrochlorite. 

It's nasty stuff, you mix it 3:1 or 4:1 with water (add the hydro to water. Not the other way around in case it splashes)

I wore gloves, a proper mask not a diy dust mask, full outfit no exposed skin and sealed goggles. 

Did the job in about 30 minutes then rinsed 

mask did a very good job; as soon as I took it off it smelled like I was in a pool but about 5x stronger 😂

much less effort then pressure washing - got rid of black spot and algae and general muck, but it's obviously a strong chemical 

Presumably you have to rinse it off at the end, and any run off going on the lawn or plants will kill them ?