Applying the Total Alkali–Silica Diagram to Classify and Predict Pozzolanic Reactivity of Volcanic Materials for use as Supplementary Cementitious Materials in Concrete

Abstract This study introduces the total alkali–silica (TAS) diagram as a geochemical prescreening tool to guide the processing of tested volcanic materials for use as pozzolanic supplementary cementitious materials (SCMs) in concrete. The diagram classifies volcanic materials into six compositional classes based on SiO 2 content and further distinguishes them as alkaline or subalkaline based on Na 2 O + K 2 O content. In this study, 19 volcanic samples from the western United States, including pumices, perlites, rhyolites, volcanic ash, and tuffs, were classified as alkaline or subalkaline rhyolitic, dacitic, andesitic, basaltic, and basaltic–andesitic materials using the TAS diagram. Each sample was evaluated as an SCM based on pozzolanic reactivity using rapid, relevant, and reliable ( R 3 ) testing (ASTM C1897), strength activity index (SAI) (ASTM C311), water requirement (ASTM C1437), and alkali–silica reaction (ASR) (ASTM C1567). All the samples were premilled by the supplier except andesite (AND-1) and basaltic andesite (BAND-1), which were milled after receiving. Alkaline rhyolites showed the highest performance (7-day heat release 265 J / g SCM; SAI 115/128/123% at 7/28/90 days; calcium hydroxide (CH) 92 g/100 g SCM; ASR mitigation 93%). Subalkaline rhyolites were also highly reactive, though slower in early strength gain ( 228 J / g SCM; SAI 95/111/104%; CH 85 g/100 g SCM; ASR 95%). Dacites exhibited moderate reactivity ( 153 J / g SCM; SAI 83/107/97%; CH 60 g/100 g SCM; ASR 76%) and a higher water requirement (107%). Andesitic materials exhibited limited reactivity even after extra milling (7-day heat release rising from 31 to 107 J / g SCM), whereas the basaltic sample was strongly reactive ( 316 J / g SCM; CH 82 g/100 g SCM) and achieved SAI > 100 % . Basaltic–andesitic samples averaged in lower heat release (104 J/g SCM) but showed higher SAI at all ages (70%, 96%, 96%) and more favorable water requirement (100%) than andesitic. This supports that TAS positioning is a practical and predictive screening framework for selecting and processing volcanic materials as pozzolans for concrete applications.