The Samsung Galaxy S9+ just like its previous phone counterparts enables USB Power Delivery 2.0 rapid charging besides its own proprietary Samsung Adaptive Fast Charging over USB Type-C. The Samsung phone also has no issues charging over Qualcomm Quick Charge as observed here using the Qualcomm Quick Charge 3.0 based Xiaomi 18W QC 3.0 Charger which doesn't support USB Power Delivery. Overall the phone took around 123 minutes to fully charge with the Xiaomi charger that originally ships with the Xiaomi Mi 8 phone.
With a completely drained battery on the Samsung Galaxy S9+, we plugged in the phone to the Xiaomi 18W QC 3.0 Charger using a USB Type-C to Type-A cable. The voltage initially started off at 5 volts where the phone briefly began pulling quick changing current. Once the voltage quickly rose to a 9 volts rail, the current settled out for the phone to charge at around 12 watts (9.2 volts @ 1.3 amps). After charging for about 44 minutes, the current started bouncing which later ramped down to eventually drop to almost zero when reaching 2 hours and 3 minutes as the battery filled up. To view the entire charging cycle, download the trace captured by the GRL-A1 power analyzer here.
The Samsung Galaxy S9+ is versatile enough to be able to support various charging technologies including Qualcomm Quick Charge allowing the Xiaomi 18W QC 3.0 Charger to act as a replacement charger just in case the phone's own Samsung Adaptive Fast Charger is missing.