This is a continuation of this thread:

http://www.greenbuildingforum.co.uk/forum114/comments.php?DiscussionID=9069

... but with a more appropriate subject.

The short version is that at about the same price, without specifying anything differently you can get very similar concretes from suppliers in the same town, but the embodied CO2 may be almost twice as high from one supplier compared to another.

http://www.ukcsma.co.uk/sustainability.html

e.g. for a 10 cubic meter pour, three different suppliers might supply concretes which have resulted in the release of 1.7 tonnes, 2.5 tonnes, and 3.1 tonnes of CO2 respectively.

... and if you don't mind a little research (and possibly an extra day or two's wait before heavily loading) it might well be possible to get the same pour down to below 1 tonne, by specifying for example a 75% GGBS to 25% portland cement blend in your concrete. 85%/15% blends are not unheard of.


Here's my impression (accurate or otherwise) of the state of supply in my corner of the UK (Brighton), and some background on the materials in question:

100% Portland cement (AKA OPC or CEM I) - typical production CO2 is 1 tonne CO2 per 1 tonne portland cement end-product.
GGBS - typical CO2 0.1 tonne per tonne GGBS.
PFA - typical CO2 is difficult to come by, but is probably similar or less than 0.1 tonne per tonne produced.

PFA is normally a coal-fired power station by-product (and otherwise goes to landfill).
GGBS is a steel manufacturing by-product (and otherwise goes to landfill too).

Both are a bit cheaper on the wholesale market than portland cement is.

All of the local volumetric mix truck companies (i.e. the services which turn up with a lorry which mixes the concrete there and then) in this area (Brighton) only supply 100% OPC based concrete. This is mainly because they only have one silo, and it's cheaper for them to get 100% OPC (from a local importer who come into Shoreham docks) than it is for them to get a blend of OPC and PFA. The same importer does use 28% PFA by default if you buy their batch mixed concrete from them. "$%^"*!

In general, PFA is widely available, and you can sometimes get it in small bags if you're doing your own mixing (e.g. it's sold for "dry-mixes" when doing underfloor heating in a timber floor), but it's easier to get in bulk bags (try your local batch mix company - they probably use it, and may well sell you some). A lot of volumetric mix trucks do it.

The majority of batch-mix (i.e. the lorries with the turny-roundy bit on the back) use 25% to 30% PFA in their mixes, and you can normally spec more (the exception is if they get the portland cement and PFA preblended from their supplier e.g. "Lafarge Phoenix", instead of buying it in separately).

A few companies in the UK supply GGBS, the main producer (and the only one I could find who would supply to my area) is Hanson (they market their GGBS as "Regen") - they also have reasonable coverage of mixed concrete supply locations across the UK (some areas are more patchy than others tho'). In my area, they seemed to be the only ones who routinely use GGBS for normal concrete mixes. If you buy concrete from Hanson (and don't tell them otherwise), then in most parts of the country, what you get normally has 50% of the portland cement substituted with GGBS - so 45% or-so less CO2 than the equivalent 100% portland mix.

It's often quite practical to specify 70% or even up to 85% GGBS substitution, and I think I'm likely to go for 75% GGBS substitution for my floor slabs - a saving of approx two thirds of the CO2 production from the cement element of the concrete. The significant differences (or rather the ones which matter in my circumstances) will be:

. Longer initial set times (useful in my case)

. Lower strength after 2 days (although I'm thinking of mitigating or eliminating this by keeping the slab warm during curing - by temporarily putting insulation on top of the slab - this may have the side effect of reducing thermal micro cracking too - resulting in a stronger slab in the long term), and slightly lower 7 and 28 day strengths.

. Increased long term strength (up to twice as strong as a 100% portland cement slab after a year or two).



Quite a bit of general technical info here:

http://www.ecocem.ie/technical,working.htm

... albeit from a company which flogs the stuff.

My most practical way of getting GGBS concrete at the moment appears to be to get a local bloke with a tipper lorry to pick some up (mixed there and then) from my local Hanson Ready Mix concrete location (about 5 miles away) - subject to confirmation.

I'll post more info when I've got things firmed up a bit more.

Tim.

good info , what was the price like relative to the standard mix per m2 ?

Posted By: jamesingramgood info , what was the price like relative to the standard mix per m2 ?

Same price - the GGBS wholesale price is a bit lower than Portland cement one. The price I got wasn't especially good, but was in the middle of the local mix truck prices (not including the mix truck call-out fees).

If I was buying more (and had opened an account with them), I get the impression I could have negotiated it down a fair bit.

From the point of exit from the steel works until it goes into the cement mixer, the embodied energy of GGBS is 50 to 90kg CO2 per tonne.

For portland cement, it's about 900kg per tonne.

Now you could perhaps argue that it's use subsidises steel making (itself a CO2 intensive industry) to some extent, but I understand that this effect is slight, and the majority of steelworks slag still goes to landfill.

Posted By: fostertomthe chemistry requires a certain (large) energy input in order to 'pump' the endothermic reactions 'uphill' - that is irreducible for OPC and poss for manuf of substitutes.

Yes, but GGBS is a waste product of steel production, and won't be going away in the foreseeable. GGBS also has a lower exothermic reaction on curing than portland cement does.

Turning limestone into portland cement releases CO2 as a chemical by-product, not just from the fuelling of the reaction, so even if you make portland cement in a solar furnace, you still end up producing 580kg CO2 per tonne of portland produced (i.e. more than half of what the industry currently achieves with conventionally fuelled portland cement production).

If you use a cement such as the (not actually available yet, but due for pilot use in concrete block production last I heard) magnesium based cements (e.g. Novacem), then the overall reaction is slightly carbon negative, as no CO2 is evolved during the manufacture reactions, but CO2 is absorbed from the atmosphere during cure.

In the meantime, GGBS seems like the best mainstream bet (e.g. I can go down the road and buy some GGBS concrete, and that goes for most of the UK too).