Cement in Ars is an Alchemical Reagent (A&A p.75) of the same name - hence advanced but non-magical medieval chemistry. Some experimental philosophers and advanced building workshops know it. Most other people in the middle ages don't.
A&A p.70 box Replicating Formulae with Hermetic Magic describes, how Hermetic magi use craft magic to replicate it, needing high Finesse scores.
Just Creo-ing medieval concrete or Roman opus caementicium with Hermetic magic requires (ArM5 p.77) familiarity with it, and Finesse too. I would have it treated as 'stone' (ArM5 p.152) for that purpose.
The players in our troupe were discussing population of ancient Rome, and 5 story apartment blocks, and that Roman cement might be a better building material than anything a Conjuring the Mystic Tower might provide.
A&A is the book on philosophy, science, engineering and arts of Mythic Europe. It abbreviates and simplifies for play, but is pretty well researched.
Make them look at this and begin to imagine their covenant buildings:
Medieval concrete was often used for hydraulic engineering, but crops up in places where you wouldn't have expected it. Medieval workshops still knew of some of the sites, where Roman engineers found the volcanic ashes for their hydraulic-setting cements.
The problem with concrete is that a rock performs better and last more (just look at any mountain); in fact basically concrete is used just because it allows to create rocky stuff in any shape required, problem that any crafty Terram specialist won’t have.
There are different kinds of rock and different kinds of concrete, each with different characteristics and qualities. Some concrete is excellent for structures, and some rock is really bad.
Also, mountains aren't necessarily a great point of comparison. They're huge, so take a long time to erode, but erode they do, and form cracks and fissures. Unlike many of our structures, mountains are just a big pile of stuff. The design requirements for that are far less onerous than for, say, an aqueduct.
Well, we have plenty of these around here in Spain, at least fifteen listed in Wikipedia. The most famous one and the one I'd seen closely more times is Segovia's aqueduct, which still stands in pretty good shape after 18 centuries. But take a close look at it:
Zoom it and you'll find that the missing parts are... the concrete parts. Which means that concrete is far more vulnerable to erosion than rock. True, rocks crack and such, but they do it in an exponential way: tiny cracks must exist before a bigger one opens. And if you have a building made from a single, solid, crackless stone, then erosion will take milleniums to crack it (and probably sooner than later in geological times some magi will cast some crack-sealing spell or add some weather-protection --and probably pidgeon-bane-- enchantments to their mystic towers).
About different rocks hardness, of course you could summon a Mystic Tower of mudstone, but I guess the default rock for a Terram magus would be some igneous rock, which are the most common ones, like basalt (which name by the way seems to come from the romans themselves, meaning "very hard stone") or granite. But if I could choose the material, I'd have my tower made of quartz. And these towers would probably overlast any roman construction (if you don't make much PeTe enemies or poke dragons, elementals, giants and such, or have a magus blowing it because of a lab accident, of course. Magi constructions' main problem is that they are infested by magi, after all).
I spent two weeks in Spain in 2016. Fabulous. We couldn't fit Segovia into our plan, sadly.
Are you sure that's not rock missing? Because there are rocks and there are other rocks, and they don't degrade at the same rate.
Some concrete, some rock. Not sure what the rest of this means.
Well, if I were a magus going about building towers, I'd rather build them than summon them. Costs no vis. At that point, I'd certainly choose some kind of basalt (glass, igneous rock, slag, etc), which I could make myself from existing Terram material. Rammed earth can be extremely resilient and strong, however, as can brick. (None of this stuff is weak, especially when you make 50 ton blocks of the stuff...) Magi do have better ways to fabricate custom rocks than concrete.