This will be of interest to many of you, I think - an alternative to concrete, called Ferrock that uses much less energy to make, and is ~5X stronger, and I think it looks very good, too. It actually absorbs CO2 as it is made.
[youtube]https://www.youtube.com/watch?v=Di4b-7_Vp8c[/youtube]
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[youtube]https://www.youtube.com/watch?v=Di4b-7_Vp8c[/youtube]