This study comprehensively investigates the effect of volcanic trass (tuff) on the fundamental properties of Portland cement–based dry construction mixes. X-ray phase analysis revealed a high content (81%) of the active glassy phase in its composition, confirming its high pozzolanic potential. The research demonstrated that replacing 10–15% of the quartz sand by weight in the mix with finely ground trass significantly improves the technological properties of the material. In particular, the workable life (setting time) of the mortar increased from 8 to 11 hours, and its plasticity was enhanced.

The results of mechanical tests indicate that although the addition of trass leads to a certain reduction in the strength of the hardened mortar at the early stage (7 days), these values are restored after 28 days of curing or even exceeded when an optimal content (15%) is used. This behavior is associated with the gradual development of pozzolanic reactions and the formation of additional calcium silicate hydrate (C–S–H) phases due to the binding of free calcium hydroxide (Ca(OH)₂). One of the most significant positive effects of trass is a 22–26% increase in adhesion strength to the surfaces of various construction materials (concrete, ceramics, and natural stone).

In addition, the moisture-regulating capacity of the material increases substantially: at a relative humidity of 90%, the sorption moisture content rises from 1.5% to 3.2% (approximately twofold). This enables the effective use of the developed mortar in high-humidity environments, such as bathrooms and basements.

Consequently, the use of Kemerli trass at a content of 10–15% as a pozzolanic additive replacing quartz sand in dry construction mixes is recommended. This not only provides improved workability and high adhesion properties but also makes the mortar a promising functional material for applications under humid conditions.