I'm planning an extension to my home and one of the many decisions is whether to go for a standard concrete slab over hardcore, or limecrete over blown glass with concrete strip foundations (e.g. option 2 here).

Money saved here can go on extra insulation/solar panels/whatever elsewhere, so I want a rough order of magnitude idea of the carbon savings vs the cost. But I can't find any good data on the embodied carbon of limecrete vs. concrete. There are lots of sites with opinion (usually people selling one or other product), but no numbers anywhere. Ideally I would like two comparable numbers for carbon emissions (or embodied energy) per m3 for concrete and limecrete. Has anyone done this comparison?

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    This case study is site specific but at least adds a data point -- their analysis showed that a polished lime floor had about 45% less embodied CO2 than a polished concrete floor. – LShaver Mar 5 '19 at 15:44

A paper on the St Astier website (not independent - they quarry and produce lime building products) gives the following figures in kg CO2/tonne for Natural Hydraulic Lime grade 5 vs. cement:

product  |  CO2 emitted during manufacture | CO2 reabsorbed in setting |  net
NHL5     |  635                            | 220                       |  415
cement   |  819                            | 0                         |  819

So, making some reasonable assumptions (similar embodied CO2 for the aggregates, 1:2 ratio for limecrete and 1:6 ratio for concrete, dry lime bulk density 0.7 kg/l, dry cement bulk density 1.2 kg/l) I get roughly 260 kg CO2/m3 for concrete and 230 kg CO2/m3 for limecrete.

I am no building expert and I could easily be a factor of 2 out, but this does seem to bear out the verbal opinions I've read: yes, lime has lower CO2 emissions and re-absorbs some but it's weaker so you use more of it and in the end they are comparable.

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Well, suppose that everyone went to lighter weight building materials or lighter weight construction methods. That would reduce carbon dioxide in the world.

So consider using lighter weight construction.

In fact most high-rise buildings are now dead-weight concrete instead of the previous lightweight steel frame.

But I build a house with 304L joist holders set directly in concrete footings. Then the joist-holder might as well also hold the stud such that both the stud and the floor-joist-end are bolted to the joist-holder. If the ceiling-joist or roof truss is to be bolted then, strangely enough, two studs are needed at each stud location so that the ceiling-joist can sit on one stud and bolt to the side of the other stud. Finish the frame section with a bolted blocking, using brackets, near the ceiling-joist.

Oh, a "joist holder" is a 2" square or round 304L tube with a 0.120" wall thickness. But a 304L flat plate is welded to the bottom of the tube and a 304L L-angle is welded to the top of the tube. A floor-joist-end sits on the L-angle and bolts to the side of it. For taller tubes out of the concrete footings just increase tube diameter. Make 304L joist-holders with a truck-load of wholesale 304L, a metal-cutting bandsaw, and an arc-welder.

Or, use a 304L joist-holder as a 4 1/2" diameter tube with a 0.120" wall thickness and then set every 7' to 8' apart. Run double joist-size lumber between the 304L tubes to make a beam for floor-joists to set cross-ways on. Bolt the floor-joists to the beam ? That requires two 90-degree brackets.

Now any bolt, nut, or washer that touches a 304L joist-holder should be 18-8. Otherwise lumber can be bolted together with galvanized bolts.

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  • Only your first 3 sentences seem to address the question, but it's not backed-up with any data. – THelper Mar 6 '19 at 6:51
  • I think it is a sufficient self-evident logic that if fewer pounds of material were shipped or transported then there would be less carbon-dioxide generated by the shipping industry. In fact light-gauge-steel joists are available and even entire houses are framed with light-gauge folded steel. Personally, I would use 430 for folded-steel structural building material but as for bolting not for welding. – S Spring Mar 6 '19 at 8:06
  • Lighter weight is not the only consideration - embodied carbon in the materials, plus how far they have to travel. The general point is valid (think about design not just materials), but I'm specifically asking about foundations and floor slab. – aucuparia Mar 6 '19 at 8:32
  • I have a light-weight crawl-space foundation design. In fact the crawl-space height could be just a couple of inches if wanted. I went on and mentioned light-gauge steel framing material but I'm not forgetting that lumber has its own carbon offset. – S Spring Mar 6 '19 at 8:38

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