I’m not sure if I have a correct understanding of the term “pozzolans” but would I be correct in thinking that these are materials for example brick dust, which when added to a sand and hydrated lime mix or to a sand and lime putty mix change the properties of that mix to that of hydraulic lime or feebly hydraulic lime? Indeed isn’t hydraulic lime just hydrated lime with natural pozzolans in it? My main concern is that a pure 3:1 sand and hydrated lime mix or sand and lime putty mix can be very light in colour when cured and even glaringly white despite having used a relatively dark concreting sand. On one occasion I found that my lime cured to a pink colour after having used a specified Westerham sand. I therefore sometimes find it necessary to darken the mix down in order to match up with old. In my case I found I could get the right shade of colour by adding a small quantity of coal dust into the mix i.e. tiny shards of coal that you find at the bottom of a coal pile which I broke down with a mallet. Would this coal dust be considered a pozzolan and if so will it affect the properties of my mix? Is there something else I should be using instead of coal dust to attain the right colour without affecting the properties of the lime? I had heard that iron oxide (rust) is quite good but again wouldn’t this also act as a pozzolan? Sorry if I’ve got it all wrong and I’m just talking nonsense.
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biffvernon
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That's right, Pozzolano is a type of volcanic rock that the Ancient Romans used to make their concrete, turning hydrated into hydraulic lime. Brick dust and certain other materials, do the same job as you say, but coal dust and iron oxide will NOT work. They're all right to use as colourings though I just add a bit of earth (or, rather, use dirty sand) to stop the pointing looking too bright white.
Nemesis
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http://www.buildingconservation.com/articles/pozzo/pozzo.htm
I notice that the article from Building conservation.com provided by Nemesis states:-
“Ashes of organic origin. Coal cinders generally have an acceptable balance of silica and alumina, and have been used historically as a pozzolanic additive, but their physical structure tends to weaken the mortar and to absorb excessive water. Coal ash is widely used, in the form of PFA (pulverised fuel ash) as an additive to cementitious mortars and in lime-based grouts. The use of coal-based products carries a risk of sulphate contamination and the materials should always be selected from low sulphate coals. The residue of fuels from lime burning, whether from coal-, coke-, or wood-fired kilns, known as lime-ash, is well known historically as a pozzolan and is still available. Other vegetable ashes, such as rice husk ash, are used as pozzolans in other parts of the world. Bone ash is also known to have been used.”
I know that I have been using coal dust rather than coal ash but I guess coal in its raw form must also contain silica and alumina i.e. active pozzolans that I guess must be turning my hydrated lime into hydraulic. I’m also worried about the risk of sulphate contamination mentioned in the above article.
Considering almost everybody on this site supports the use of either hydrated lime or lime putty over hydraulic lime,then perhaps I’ve been making a big mistake in adding coal dust to my mix?
“Ashes of organic origin. Coal cinders generally have an acceptable balance of silica and alumina, and have been used historically as a pozzolanic additive, but their physical structure tends to weaken the mortar and to absorb excessive water. Coal ash is widely used, in the form of PFA (pulverised fuel ash) as an additive to cementitious mortars and in lime-based grouts. The use of coal-based products carries a risk of sulphate contamination and the materials should always be selected from low sulphate coals. The residue of fuels from lime burning, whether from coal-, coke-, or wood-fired kilns, known as lime-ash, is well known historically as a pozzolan and is still available. Other vegetable ashes, such as rice husk ash, are used as pozzolans in other parts of the world. Bone ash is also known to have been used.”
I know that I have been using coal dust rather than coal ash but I guess coal in its raw form must also contain silica and alumina i.e. active pozzolans that I guess must be turning my hydrated lime into hydraulic. I’m also worried about the risk of sulphate contamination mentioned in the above article.
Considering almost everybody on this site supports the use of either hydrated lime or lime putty over hydraulic lime,then perhaps I’ve been making a big mistake in adding coal dust to my mix?
Nemesis
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We have used ash and had no problems.
Nemesis
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Very, very weakly - it was used for a particular reason in that case.
biffvernon
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NT said:
It isn't. I left out the word 'not'
