Volcanic additives are increasingly considered as low-cost modifiers for lightweight aggregate (LWA) production, yet their effectiveness is strongly deposit-dependent. The unresolved question is not whether perlite and rhyolite tuff can improve bloating behavior, but which raw-clay characteristics determine when those improvements occur simultaneously in expansion, density reduction, and strength development. This study re-examines a ten-clay experimental matrix from Hungary and Egypt in which 10 wt % perlite or rhyolite tuff was incorporated into expandable clays and the resulting maximum height expansion (MHE), bulk density (BD), uniaxial compressive strength (UCS), and fired amorphous content were measured. A response-oriented analytical framework was used, combining outcome-specific effect sizes, a transparent composite benefit score, Spearman rank correlations, paired non-parametric tests, threshold-based classification, and robustness checks based on additive-specific subsets and clay-level aggregation. Quartz-rich, low-kaolinite clays were consistently more responsive than kaolinite-rich clays. Across 20 additive-treated observations, raw quartz correlated positively with the composite benefit score (ρ = 0.773, p < 0.001), whereas kaolinite correlated negatively (ρ = −0.655, p = 0.002). Density improvement was the most mineralogically sensitive outcome, tracking raw quartz (ρ = −0.728, p < 0.001) and kaolinite (ρ = 0.498, p = 0.025). All eight additive-treated observations with a quartz/kaolinite ratio greater than 1 achieved concurrent improvement in MHE, BD, and UCS, compared with only 2 of 12 observations at or below that threshold (Fisher exact p = 0.0007). The same ranking pattern remained directionally consistent when responses were examined separately for perlite and tuff and when they were averaged to one observation per clay. Perlite and tuff did not differ significantly in paired comparisons, indicating that raw clay mineralogy exerts a stronger control than additive identity at the selected dosage. The results support a practical mineralogical screening heuristic for additive selection before pilot firing and clarify the process–structure–property pathway linking raw mineralogy, glass-phase development, and final LWA performance, while also indicating the need for broader validation across additional deposits and firing schedules.