This study examines the effect of volcanic ash (VA) on the physical, mechanical, rheological, and functional properties of cement-based dry construction mixtures (DCM). As a result of X-ray phase analysis (XPA) and chemical analysis (XRF), the composition of VA was found to contain 78% amorphous glass phase, 11% quartz and 9% feldspar, and its pozzolan activity was 79 mg CaO/g. As a result of replacing quartz sand with VA in proportions of 5%, 10%, 15% and 20%, the optimal range was found to be 10–15%, which allowed the amount of free calcium oxide to be reduced from 5.2% to 1.4%. This confirms the intense reaction of pozzolan. The presence of VA in the composition increases the setting time of the mortar from 8 to 12 hours, its plasticity and adhesive strength, but reduces the density from 1800 kg/m³ to 1760 kg/m³. After 28 days of hardening, the compressive strength of compositions with 15% VA approaches the control sample (16.5 MPa) and is 15.6 MPa. The most important result was that VA increased the sorption moisture content from 1.8% to 4.2%. This indicates the material's potential for regulating moisture balance in high-humidity environments (basements, bathrooms). The results confirm the promise of Mollada VA for the production of a new generation of high-moisture, environmentally friendly and high-performance DCM.