Ash in cement,Ratio?

[damduck-egg]

Its that time of year again when operating the multi fuel stove banked up with an anthracite based briquette product means there is some Ash to be disposed of. Wood ash is no problem as it can be used in the garden but I am reluctant to dispose of coal ash in this way though I believe it may have been used by some in the past so it get bagged up and taken to the dump or sometimes put in a bag in the non recyclable bin which often means double handling as it is too hot for plastic bags.

Next spring we will need to replace some border edging so killing two birds with one stone I'm thinking of getting one of those moulds designed for use with cement to cast some edging and use the ash as a constituent in place of the some of the cement or sand and cast some edges over the winter as and when I feel like it. Anybody have any suggestion as to what ratios would make a reasonably robust mix.

G.Harman

[Chris Hogg]

Fly ash, aka pulverised fuel ash (PFA), is pozzolanic and often used as a replacement for Portland cement in mortar. Yours won't be pulverised, though, and won't have reached such a high temperature as PFA so will be coarser and less effective, but still worth using in that way. See

, which says up to 30% of the cement can be replaced by PFA. In your case, I wouldn't use that much, but I see no reason why you shouldn't just add say 10% of your ash to a conventional mortar mix, although I'd sieve out the obvious coarse bits of partially burnt coal etc.

If you want to go down the cement replacement route, if a 3:1 sand:cement mix is replaced by a 9:2:1 or a 12:3:1 s:c:ash, that would give you 33% and 25% replacement of the cement by ash, respectively, but seems rather complicated. I'd stick with just adding it. For border edging, it's not as though you're looking for a high strength result. Plenty more on cement replacement in mortars and concretes, here

[Max Demian]

You could use it to dress a cinder path if you need another footpath and don't want it concrete or asphalt.

[The Natural Philosopher]

According to wiki it is in itself a sort of cement, so I'd mix it 30:70 with cement and use normal 2:1 style mixes.

[charles]

on visiting a mediaeval house in Cheshire, we saw what looked like a cement floor. Apparently made of coal ash & cowdung.

[tabbypurr]

I can't find anywhere it says that

nor anywhere that recommends that. I dare say it would work, but there are possibly more optimal mixes.

NT

[The Natural Philosopher]

Try looking a bit harder mate.

[Bob Eager]

When the old railway tunnel under my PPoE collapsed (partially) they pumped it full of a cement filler made partly from ash from the local power station. Presumably it's nice and liquid and can be pumped well.

[Chris Hogg]

You haven't looked very hard! Like I said in my reply to the OP, it's pozzolanic. See the Wiki link.

[Andy Burns]

Isn't a pozzolan something that helps a cement to set/bind, rather than being a cement itself?

[The Natural Philosopher]

There is calcium oxide in ash. And sulphurs

That sets into calcium carbonate and sulphate eventually.

But even without that, pozzolans reduce the amount of cement needed.

[harry]

[Chris Hogg]

The Romans used pozzolans extensively in their constructions. In their case, they used ground up volcanic ash which, combined with lime, sets to a solid product similar to what you get with Portland cement.

Fly ash / PFA has a composition not too far removed from natural pozzolans, i.e. it contains finely divided silica and alumina, much of it in the form of calcined clay and fine sand, from the original coal, or before that the swamp that the carboniferous vegetation grew in. The lime comes from the Portland cement. The alkaline solution from the lime attacks the aluminosilicates to form complex calcium silicate hydrates and calcium aluminosilicate hydrates that crystallise and bind the sand or aggregate particles together as it all sets, in much the same way as happens with the setting of Portland cement.

Pure clay, calcined at around 700-800C, when mixed with lime in the right proportions, makes a good pozzolanic cement. In a sense, Portland cement is a pre-mixed lime-pozzolan combination, being made from lime (usually chalk) and clay mixed together and calcined at very high temperatures. Just add water!

[Cynic]

Fly ash from the precipitators of coal fired power stations was the raw mat erial for the company Pozzolanic Ltd. Some power stations were better than others as sources. Fiddlers Ferry, Iro nbridge, West Burton and Longannet were regular sources. These produced a p ale, low residual carbon ash with small particle size. 50% substitution of the cement content was used in the concrete of the Thames Barrier as it mad e a dense concrete which was slow to cure without the vast damaging heat re lease of straight ordinary Portland cement during curing. The majority of t he ash was transported from West Burton by dedication trains. Pozzolan was also used extensively to combat "concrete cancer" as Alkali Si lcate reaction became known. Fly ash is a fascinating subject if you research it.

[tabbypurr]

aterial for the company Pozzolanic Ltd.

ronbridge, West Burton and Longannet were regular sources. These produced a pale, low residual carbon ash with small particle size. 50% substitution o f the cement content was used in the concrete of the Thames Barrier as it m ade a dense concrete which was slow to cure without the vast damaging heat release of straight ordinary Portland cement during curing. The majority of the ash was transported from West Burton by dedication trains.

Silcate reaction became known.

Sure... but when used as pozzolan it needs to be extremely finely ground. W ood ash from your fireplace does not cut it.

If not so ground it acts more as a filler, replacing sand, making black mor tar.

NT

[tabbypurr]

so you didn't find that statement on the wiki either

[Cynic]

The Romans used volcanic and other ashes as pozzolans. I don't have any data as to it's "fineness".

[Chris Hogg]

What makes you think that ash from a domestic coal fire isn't fine enough? Granted, coal burnt in a power station to give PFA is milled before being blown into the furnace to get it to burn rapidly and completely, but this diagram suggests the ash from domestic coal fires has a similar PSD to fly ash, with a maximum at about 10µm.

or

You're not thinking just of the cinders, are you?

[damduck-egg]

As the OP most of the coal ash I have does appear to quite fine. Today in what turned out to be a successful attempt to find the entrances to a rats nest where the rodent had burrowed into a raised flower bed where the soil is kept between two 2ft high brick walls I poured some of the ash into one entrance while at the same time directing a flow of air from a small garden leaf blower into the hole and as I hoped some emerged from the other holes it had dug. It actually looked like smoke was emerging not unlike an old style diesel exhaust.

G.Harman