<p>This study investigates the relationship between alteration processes, mineralogy, and mechanical performance of andesitic rocks from two intrusions in northern Hungary: Tállya and Recsk. While both sets of samples share similar primary mineralogy, they have undergone different alteration histories, influencing their mechanical behaviour. Recsk samples experienced hydrothermal deferrization, forming secondary minerals such as siderite, Mg-rich siderite, goethite, and smectite. In contrast, Tállya samples were affected by autometasomatic alteration, marked by feldspar dissolution–recrystallization and limited smectite formation. These mineralogical changes correlate with variations in mechanical properties, particularly in resistance to fragmentation (Los Angeles test) and wear (micro-Deval test). Smectite content, in particular, was found to significantly reduce wear resistance. Potassium feldspars had contrasting effects depending on their origin: primary K-feldspars in Tállya samples weakened the rock texture, while fine-grained interstitial K-feldspars in Recsk reinforced it. The results underscore the importance of alteration intensity and secondary mineral formation in controlling rock strength. Rather than bulk mineral content alone, the type and microstructural context of alteration products play a decisive role in engineering performance. These findings highlight the need for integrated petrographic and mechanical evaluation in the assessment of volcanic rocks for construction and aggregate use.</p>