Cleaning paving slabs - what works?

[Richard_C]

Each year I pressure wash, sometimes twice, the large patio area in the 'L' of my L shaped house. It was built* when the house was, 35 years ago, I've been here 20 of those years. Pressure washing cleans off the slippy slimy stuff, but each year it ends up a bit darker than the year before, black blotches which grow. Its made of what I call moan (mock stone, concrete really) which has a deliberately uneven surface as if a mountain stream had gently eroded limestone pavement .... at least I bet that's what the brochure said.

So - what can I do to give it a really good clean and bring it back to something a bit brighter? Gardening magazines are full of adverts for magic potions, many of them expensive - must be 50 sq m in all with the bits that go round the house - and look to me like snake oil and magic elixirs.

What, if anything, works?

*whenever I do serious work like fitting the new kitchen I grumble that the person with the spirit level, square or whatever must have been off work that day because a lot of it is a bit 'off'. Spirit level person was definitely in work when the patio was laid, its dead level and rain doesn't run off. Grrrr. Could be worse though, could slope towards the house

 

[SammyQ]

Stone cleaner. Hydrochloric acid under another name.

Sam

 

[Sheffield Tony]

Hydrochloric acid aka brick cleaner will presumably etch the surface of the concrete a bit. That might not matter, indeed might be good. It completely destroyed some terracotta plant pots / feet I tried to clean limescale off - became soft and flakey.

Jeyes fluid used to be amazing for killing algae. That was when it was a phenolic almost black stuff that turned cloudy in water. As with so many things, the modern safer alternative formulation is not at all the same. IIRC, the old formulation was a coal tar derivative, so there probably aren't readily available supplies of that anymore.

 

[Rorschach]

Bleach seems to do a fairly good job but needs a couple of applications the first time. It has the advantage of being cheap and fairly innocuous environmentally wise (after some exposure to UV).

 

[SammyQ]

Tony? It does etch a bit, sure, but not to a serious extent if you follow the guidelines and use antediluvian amounts of water after'timezup'.

Sam

 

[Tris]

Anything containing didecyl dimethyl ammonium chloride should give you good results. The one I used is sold under the brand name sapphire, but aldi sell a patio cleaner with it in although at about one tenth the concentration.

 

[The_Yellow_Ardvark]

Dish washer or pool Salt (1/2 pint)
Water (1/4 pint)
Distilled White Vinegar (5%, 3/4pint)
Washing up soap (1/4 pint)
Measurement cups.
Dispensing bottle.

Mix the ingredients together. If the salt has not dissolved add more water or vinegar.

This works well on Ivy and other waxed leaves plants as well as grass.
Plus it is harmless to pets, it will also discourage neighbourhood cats doing their business.

Pressure wash the slabs off.

Then apply that mix with a stiff brush and leave.

 

[sneggysteve]

Stone cleaner. Hydrochloric acid under another name.

Sam

Shouldn't that be hypochlorite acid - https://www.youtube.com/watch?v=hDQu5pJAmdQ

 

[Rorschach]

Stone cleaner. Hydrochloric acid under another name.

Sam

Shouldn't that be hypochlorite acid - https://www.youtube.com/watch?v=hDQu5pJAmdQ

Sodium Hypochlorite is bleach.

 

[Glynne]

Swimming pool chlorine solution works well.
Sounds a bit exotic but lots of local suppliers will stock it. I dilute it ~ 25% and it will shift grime, moss etc far better than a pressure washer and lasts a lot longer.

 

[Rorschach]

Swimming pool chlorine solution works well.
Sounds a bit exotic but lots of local suppliers will stock it. I dilute it ~ 25% and it will shift grime, moss etc far better than a pressure washer and lasts a lot longer.

That is also bleach.

 

[SammyQ]

Nope Sneggy, hydrochoric is wot it ses on the cannister.

Sam

 

[Richard_C]

Both previous posts are right. Its coming back to me now, we used to make pool chemicals in part of a factory I worked in in Widnes.

Bleach is a generic name, mostly chlorine based but Hydrogen Peroxide is a wholly different chemical also sold as bleach for some applications.

Most liquid bleach we use is Sodium Hypochlorite.

Pool chlorine agents are normally Calcium Hypochlorite, often in granular form which dissolve easily. Calcium Hypochlorite is a touch more stable in storage and use than Sodium Hypochlorite and in pool chemistry releases its chlorine more slowly. The 'branded pool treatments' often include other stuff but its the hypochlorite bit that does the work.

Both should give similar results in this sort of use.

Although bleach is readily available and cheap it can be hazardous - for instance adding an acid (vinegar even) will release chlorine gas.

Will probably try bleach on a paving slab or two and see what happens, mainly because I've got some so it seems sensible to try that first.

 

[Rorschach]

Adding vinegar increases the antimicrobial effectiveness of bleach massively but also potentially releasing chlorine gas. Of course this is very dangerous if you don't know what you are doing.
For something like cleaning patio slabs outdoors though, a very small quantity of vinegar added to decrease the pH could make it much more effective and in the concentrations used at household level it is pretty safe as the chlorine would be minimal and you would be outside as well.

 

[Glynne]

Stating the obvious, watch out for splashes on your clothes unless you want to re-live the bleached jeans era. DAMHIKT.

 

[AndyT]

We've got some paving slabs at the bottom of the garden, under the apple tree. It's a nice shady spot on a sunny day. I recently noticed a big clean area on one of the slabs where the dirt/algae looked as if it had been scrubbed away. I've certainly not been splashing caustic chemicals about, but we do often get foxes in the garden. I reckon the clean slab is from the same cause as patches of brown grass.
Whether you want to train foxes to pee on your patio is up to you!

 

[Woodchips2]

I use sodium hypochlorite (Deosan Red Label £11.76 for 20 litres) from a farming supplier. It is used for disinfecting dairies and I dilute it with water 50/50. Spray it on and leave for a couple of hours and then hose off. Also good for cleaning down plastic gutters and downpipes and is effective on timber decking. Don't get it on your clothes unless you like the white speckled effect.
Regards Keith