When I read about Roman concrete I understand its use was forgotten after the fall of the Roman empire. But what about the Eastern Roman empire, the Byzantine Empire? Did they still make buildings with Roman concrete? This Wikipedia page, for example, does not refer to the use of concrete.
The use of concrete did fade after the fall of the western Roman Empire, though at least some aspects of the use of concrete was held over into the Byzantine Architectural style, on some early Byzantine works. A web page here shows an excerpt from a book on Architecture, A History of Architecture on the Comparative Method, by Sir Banister-Fletcher, New York, 1950, pp. 238, 240, 242. This says:
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.
The style of decorative facing over the structural element was continued, but the structural concrete was phased out and replaced with structural stone or brick elements.
An earlier version (published 1905) of the above book can be read here and it has dozens of references to the use of concrete, several mentions concerning the use of concrete in Byzantine construction:
Page 212, in a discussion of characteristics of Byzantine Architecture:
D. Roofs. The method of roofing these buildings was by a series of domes formed in brick, stone, or concrete, with frequently no further external covering.
In fact no church was founded during this period in which mosaic was not intended to be employed, and the decoration of S. Sophia and the churches of Nicaea and Thessalonica show the perfection to which this was carried out. The core of the wall was generally of concrete, as in the Roman period, but the manner in which the bricks of the casing were arranged contributed greatly to the decoration of the exterior.
Also on page 194 is given an idea concerning the time frame for this transition from Roman towards more Byzantine construction styles:
The change from the old Roman forms was of course gradual, but in the course of 200 years the East asserted itself, and under Justinian, the Church of S. Sophia (A.D./532-537) was erected, and remains the greatest achievement in the style the interior being perhaps the most satisfactory of all domed examples.
Concrete was no longer necessary.
The use of concrete was at its peak during the construction of all the Roman 'mega' projects, things on the scale of the huge temples such as the Pantheon, public works such as the aqueducts, grand palaces for emperors, or displays of wealth and power such as the Colosseum. The Byzantines did not build on such a vast scale such that the need to construct something so huge, so quickly, was not there.
The book Ancient Building Technology, Volume 3: Construction (2 vols) By Wright, pg 235 seems to sum this up with a line:
For socio-economic reasons Roman Concrete construction was not appropriate or practical in Constantinople
Over the following two hundred years of the Empire, scale and material experimentation continued, though perhaps less markedly, with notable examples found in the Basilica of Maxentius (306; Figure 1.1) and the so-called ‘Temple of Minerva Medica’ (c.325).19 For roughly the last one hundred years of the Empire, the disintegrating central organization and shrinking building industry of the Empire could no longer provide the resources necessary for large-scale concrete construction. (Note says:Lanciani 1897 identifies re-claimed materials, particularly bricks, in both the Baths of Diocletian and Basilica of Maxentius.)
This implies that the Western Roman Empire itself had very limited use of concrete, for its last 100 years or so, and the Temple of Minerva in 325 may have been the last major concrete construction. (which agrees with the date of 330 in comments by @LangLangC). Again 'the disintegrating central organization and shrinking building industry' would agree with 'socio-economic' issues making the use of concrete impractical.
The lack of volcanic ash.
One of the special components used in Roman concrete is volcanic ash, or pozzolana. The Roman source for this was close and convenient, Vesuvius. Without such a easily accessible source, it is possible concrete production was less economically viable.
The lack of pozzolana is also mentioned in a discussion of the transition from concrete to brick structures on page 97 of the 2014 book Earthen Architecture: Past, Present and Future
Byzantines probably didn't trust their concrete without pozzolana
Whatever the cause, the use of concrete was not as prominent in the Byzantine Empire as the earlier, Western Roman Empire.