Where exactly is the difference between expanded clay and pumice?
Oops, well, now you've really "caught me". How do I explain this to my child... how do you explain it when you're neither a geologist nor a chemist, but a commercial clerk and decades-long experienced jack-of-all-trades, in a way that is "generally understandable and simplified" but not "wrong"? — well, I'll try, aiming to embarrass myself only moderately:
So, in the beginning there were two basic ideas that led to three products.
On the one hand, there was the desire for a building material as stable as concrete but without its poor thermal "insulation". So one took a concrete mix, but without the gravel-like aggregates, and invented a technique that is conceptually similar to a cheese culture with a sort of pyrotechnical leavening agent. This yeast-dough-like concrete was named aerated concrete and got its brand name (which today, like "Tempo" for paper tissues or "Tesa" for adhesive tape, is used by the public as a generic term) after the initial letter of the city Yxhult. So much for "aerated concrete".
The second basic idea was simply "what to do with it, and how to turn this useless stuff into money". The Laacher See volcano had — eleven thousand years ago, long before anyone began building stone houses — like a popping popcorn machine, rained volcanic rock fragments over the "Neuwied Basin". The farmers thus had huge amounts of it under their fields, and the seed crows did not eat it. But if you mixed it with concrete, you could build stable and fireproof field barns from it. When "pumice" is talked about in construction, it means this pumice concrete stone or "drift stone" or "lightweight concrete stone". Strictly speaking, "drift stone" is something else though, because it is much more often made with industrial rather than volcanic slag.
In the other direction, not far from Koblenz in the Westerwald region lies the Kannenbäckerland, also very blessed with soil that depending on what you make from it can be annoying or useful; the treasure in this case is called "clay". "Kannenbäcker" means "potter". "Expanded clay" is again such a shortened term when referring to a building material. Then it is likewise a similar type of concrete stone, except that here a lightweight concrete stone is "macroporous" through a "bonding" of expanded clay. A "macroporous structure" is — do not forget the preliminary remark! — a loose bulk of "macromolecules".
As a fundamental difference between pumice and expanded clay one could — see the preliminary note: explained most simply — say that in both cases the industrial stone products are a kind of cement stones, whereby the "popcorn" in the pumice was formed very hot in a volcano but after millennia of cooling is now fused together in a cold manufacturing process; whereas with expanded clay, clay pellets (currently, industrially, not from a long time ago in a volcano) are expanded in a firing process, with these pellets resembling the smaller grain sizes of hydroponic sack goods (but of course gray concrete-colored due to the other ingredients).
In the end, in all three cases you have stable "artificial stones" with structurally high strength but decent thermal conductivity values, whereby "pumice" and "expanded clay" despite their different "genetics" show a very close relationship in their properties, while "Ytong" (careful, brand name!) even for laypeople with the naked eye is clearly distinguishable as a different category, with much lower density, and is even DIY-friendly because it can be worked with wood saws.
So, dear children, before you start dreaming nonsense from this: the uncle explaining is not
a natural scientist and the descriptions are necessarily imperfect attempts at explanations that should also be compatible with East Frisian blonde hairdressers — in the Bible one would call them "parables" ;-)