In some implementations, a cement testing system includes an upper end module and a lower end module. Casing-emulating tubing couples to the upper end module and to the lower end module and emulates a wellbore casing. A plurality of intermediate well-wall-emulating modules is configured to couple end-to-end and to couple to the upper end module and the lower end module to form an annulus around the casing emulating tubing. Each of the plurality of intermediate well-wall emulating modules is configured to emulate one or more characteristics of a well wall.