Quote:
Originally Posted by thrash
Marble, I would think. There are also naturally occurring materials that mimic concrete, with varying degrees of efficiency.
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Concrete is a conglomerate (it's got sand and little stones in it). Some cements are calcium carbonate, so marble would be plausible for those, but funny looking marble with all those inclusions. Most cements are calcium silicates and aluminates though, so probably feldspar for those.
Clearly the planet had some really *weird* depositional environments back when concrete formed as a sedimentary rock. Probably whatever produced all the volcanic ash that went into these deposits also contributed to the co-occurring Q-N mass extinction.