**4. Conclusions**

The e ffect of the hydration processes on the mechanical strength of five types of the porous gypsum plasterboards was investigated throughout this research. Three-point bending tests have revealed the strength loss of 70%–80% for all tested specimens due to hydration. Subsequent Ultrasonic (UT) measurements revealed that longitudinal wave velocity *c*L decreased after hydration. Based on the plasterboard porosity volume [27], four types of approximated formulae were proposed in which the UT wave velocity (km/s) dependence on the moisture content (%) was described.

The dedicated UT transducers with low acoustic impedance and a solid-state polymer jelly interface were capable of achieving the required propagation parameters of UT waves and further determined their velocity in investigated plasterboard. The registered changes of cL values were remarkably greater than the standard deviation of single series of measurements, included in the range of 2%÷3% from the average value of the readings. It was found that the ultrasonic method could be successfully used for plasterboards hydration processes monitoring.

In authors' opinions, more research work is required to determine the relationship between ultrasonic wave velocity and mechanical properties. That relationship strongly depends on the plasterboard type. For the standard indoor installation board type, it can be concluded that approx. a 70% loss of mechanical strength denotes a 10% loss of UT wave velocity. For I and K types, an 80% loss of mechanical strength denotes an 8% loss of UT wave velocity.

**Author Contributions:** Z.R. prepared the UT instrumentation, analyzed UT test results and performed paper editing; P.R. performed the optical microscopic observations; T.D. prepared the specimens; A.B. performed the SEM observations; M.K. performed the specimen hydration procedure and proofreading; M.R. analyzed UT test results, K.F. prepared the specimens; M.S. completed the experiments; E.K. completed the experiments; K.S. conceived and designed the experimental work. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Conflicts of Interest:** The authors declare no conflict of interest.
