I'd like to be clear that concrete was not lost at any point, and the Romans did not invent it.

The particular technology I believe you are speaking of is what is referred to as "Pozzolan" concrete, which uses a recipe that found popularity first in Latium (usually accredited to the area around Napoli, specifically).

"Roman concrete" was merely their particular recipe that involved using lime and pumice binder material from the greater Italy region. Historically, Germany and Greece also have similar deposits that can produce the same effect, but I won't pretend to be a geologist to say that 100%

A quick google search says that the recipe was rediscovered in the 15th century, which is well beyond my time-frame of expertise, but I assuee you concrete was used between 476 and ~1400.

Ok, I'm not a historian, so I can't talk about when, the "opus caementicium" aka "Roman concrete" stopped being used, and I am loathe to rely upon wikipedia. However, I do have training as a geologist and material scientist, so I'd like to talk about what is meant, from material science perspective, by "Roman concrete" and how it is both similar to and different from modern construction aggregates based on the use of the "Portland cement".

A Brief Overview on the Composition and Properties of Concretes
What people today coloquially refer to as "concrete" is a litho-ceramic composite that is created by the selective mixing of three main ingredients:

• A silici-clastic "aggregate" that gives body and strength to the composite -- in modern times this is usually a mixtures of crushed rock, usually a fine-grained sedimentary rock like shale or some kinds of limestones, in a mixture of sizes ranging from sand (avg diameter <2mm) up to large pebbles (avg diameter <4cm). The exact particle size-range distribution of the aggregate may vary considerably based on engineering needs and source materials, and the aggegate clasts may be angular or rounded, again depending on a mix of engineering and logistical factors.
• A "binder" or cement that reacts with water and CO2 to harden and set, locking the aggregate in place, and allowing transport of the aggregate and cement as dry bulk goods rather than dimension stone. The other reason that cement is useful is that it means that a significant portion of the mass of the final structure being made of water and CO2, does not have to be transported large distances to the construction site, saving on costs. Generally speaking, the cement or binder is made of up mostly of a mixture of calcium-silicate and aluminum-silicate minerals, as well as some other stuff (more on this in a minute).
• Water, added as a liquid, and CO2 from the atmosphere, which reacts with the calcium oxides (and other substances) in the cement to form a sort of artificial limestone. This chemical reaction can be oversimplified to be viewed as the reaction of calcium oxide (CaO) with water (H2O) and carbon dioxide (CO2) to form calcium carbonate (CaCO3). I'm going to ignore the role of aluminum-silicate hydrates in this process, since this is /r/AskHistorians, not /r/askscience.

Modern concretes may also use a variety of other additives to modifiy the physical and chemical properties of the resulting concrete: these include mineral and chemical "admixtures" and "reinforcements". The exact identity of these addtives and admixtures is largely beyond the scope of this discussion, but it's important to note that the Romans did use, reinforcements and mineral admixtures in their concrete, even though they probably didn't realize at the time what they were doing and why certain ingredients were important in determining the resulting structural chemical properties of the concrete they were creating.

Ok, now that the underlying science lesson is largely over with let's discuss the composition of original Roman concretes. Roman concretes were mainly in the category of modern concretes that use what is known as "pozzolan cement". This means that unlike modern portland cement, which is what most sidewalks are made of, the pozzolan cement used by the Romans had a much lower percentage of calcium oxides and calcium hydroxides (CaO and CaOH of +21% in modern portland cement vs CaO & CaOH of <12% in modern fly-ash cements), and much higher amounts of aluminum-silicate oxides (<5% in modern portland cement vs 15-30+% in modern pozzolan cements).

So the Romans were actually manfactuing what is even by modern standards a fairly sophsisticed geopolymer. But, here's the catch -- the Romans didn't actually understand what they were doing. They understood that they could create strong water-proof concretes if they used certain types of pozzolan (a kind of partially-weathered volcanic ash deposit) in their cements, but they didn't understand why what they were doing worked. Once access to particular localities that supplied the needed types of pozzolan were disrupted, the knowledge of how to make they sophisticated concrete mixtures was probably lost.