@CCNA
The Eastern Roman Empire did not fall until around the 15th century, which means concrete was not in use for at most around a century if at all, though in western Europe where most history was transcribed at the time, it was not in use because of (what I personally think) the disarray of the society at the time; and while the Byzantines did not use concrete as much as their forebears, they did occasionally delve into the technology for certain things, such as the Ayia (Hagia) Sophia. The technology was never truly lost, it only became scarce.
An excerpt from Sir Banister-Fletcher's
A History of Architecture on the Comparative Method:The system of construction in concrete and brickwork introduced by the Romans was adopted by the Byzantines. The carcase of concrete and brickwork was first completed and allowed to settle before the surface sheathing of unyielding marble slabs was added, and this independence of the component parts is characteristic of Byzantine construction. Brickwork, moreover. lent itself externally to decorative caprices in patterns and banding, and internally it was suitable for covering with marble, mosaic, and fresco decoration.
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This general use of brickwork necessitated special care in making mortar, which was composed of lime and sand with crushed pottery, tiles, or bricks, and much of it remains as hard as that in the best buildings of Rome, while the core of the wall was sometimes of concrete, as in the Roman period.
After the fall of the Byzantine Empire, structures in Finland began to use concrete circa the 16th century, and the Canol du Midi was built of concrete in 1670. It wasn't until the Eddystone Lighthouse in England was made a century later that it began to become modern and widespread again.