## 3 Materials and Experimental Apparatus  
Cement mortar cubic specimens of 75 mm edge dimensions were prepared. The cement mortar specimens' preparation process is described in Reference [26]. Preliminary tests were conducted in order to estimate their axial compressive strength. The compressive stress strength was found to be of the order of 53 \(\pm\) 2 MPa. An Instron 300 DX Static Hydraulic Testing Machine loading frame of 300 kN capacity was used for the specimens' loading. Also, an electric strain gauge was placed at the center of the specimens' front side in order to measure the axial deformation. After estimating the expected uniaxial stress strength, the following loading protocol was conducted. Axial compressive stress control was applied at a constant rate 0.4 MPa/s up to a stress level in the vicinity of failure. Sequentially, the stress was maintained at a constant, and the specimens were left to collapse due to severe internal damage accumulation and consecutive crack extension caused by the excessive stress. For the first of the two experiments that will be presented next, after the applied stress was increased linearly up to 54.5 MPa approaching the specimen's total strength, it was then kept constant, awaiting the final fracture. Indeed, the specimen collapsed after the 180 s.  
Figure 1 is a sketch of the experimental installation. It shows the locations of all the sensors on the specimen's surfaces and the way the measurement systems are connected and synchronized. Three subsystems constitute the complete experimental apparatus, namely, the loading, the PSC, and the AE subsystems. Each one is supported by its own acquisition device to collect the corresponding data. In order to be able to synchronize the collected data, the level of applied load is shared with all the acquisition subsystems. The loading subsystem comprises the load frame and the control (i.e., Bluehill\({}^{\text{\textregistered}}\) software). The INSTRON 300 DX loading frame is equipped with the appropriate extension card, to achieve both load and displacement control and output the level of the applied load to be shared with other acquisition devices. The PSC subsystem is equipped with the Keithley (6517A), a strain measurement resistor bridge (Mikrolink 770), and the Keithley A/D (KUSB 3108) unit that is used to guide the strain level and the shared mechanical load level to the acquisition device (i.e., Agilent Vee\({}^{\text{\textregistered}}\) software). The AE subsystem consists of the pre-amplifier 0/2/4 PAC, the PCI-2 AE acquisition system (Physical Acoustics Corp-PAC), and the parametric unit, that is also used to record the shared mechanical load level. The AE system is handled with the aid of the PAC AE Win\({}^{\text{\textregistered}}\) software. In order to avoid manual or time-marked data synchronization, the values of the shared mechanical load level were used to synchronize the data of all three subsystems. The PCI-2 AE acquisition system was used for the monitoring and recoding of the AEs, aided by one R15a acoustic sensor (manufactured by PAC, resonant at 75 kHz) that was attached at the center of the specimen's back-side. Silicone was used as a coupling medium between the specimens' surface and the sensor. For the detection of the PSC signal, a pair of copper gold-plated electrodes was used. The electrodes were attached at the remaining lateral sides of the specimens perpendicular to the axis of the applied stress (see Figure 1), so as to reduce friction effects with the loading system and minimize the subsequent electric noise. A pair of suitable springs was used in order to ensure good coupling between the electrodes and the specimens. Teflon plates with high electrical resistance were used to electrically isolate the electrodes from the springs. In order to ensure best coupling conditions between the PSC electrodes and specimen, and before the attachment of the electrodes, a thin layer of conductive silver paste was applied on the specimens' surface at the location the electrodes were to be placed.