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This paper presents a study of the characteristics and pozzolanic activity of three volcanic tuffs to assess the quality and availability of the tuffs for use as supplementary cementitious materials in cement-based composites. The main parameters of the tuffs (chemical composition, proportion of vitreous phase, mineral species and rock structure) were assessed by chemical composition analysis and x-ray diffraction. The pozzolanic activity component (PAC) of tuff, calculated from chemical composition and amorphous phase content, is proposed as an intrinsic index of pozzolanic activity without consideration of particle size and was used to evaluate the tuffs. The pozzolanic reaction degree (PRD) of the tuff/cement pastes was determined by hydrochloric acid dissolution. The pozzolanic activity strength index (PASI) was evaluated as the ratio of the compressive strength of the blended tuff/cement mortar to that of a control cement mortar. The results showed that the tuffs possess great potential for pozzolanic activity in terms of PAC, PRD and PASI because they have high contents of silicon dioxide and aluminium oxide and plentiful amorphous phases. A correlation between PAC and pozzolanic activity index was revealed and a consistent relationship between PRD and PASI was also found.

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The pozzolanic activity of volcanic rocks from southern Jiangxi Province, China, was studied in terms of material properties, pozzolanic reaction degree (PRD), and pozzolanic activity index. The pozzolanic activity component of the volcanic rocks calculated from chemical composition and amorphous phase content was appraised as a characteristic index of potential pozzolanic activity. The PRD of the volcanic rock was determined by hydrochloric acid dissolution. The pozzolanic activity index of the volcanic rocks was evaluated as the ratio of the compressive strength of the cement mortar blended with volcanic rock to that of a control cement mortar. The results showed that the volcanic rocks demonstrated great potential for pozzolanic activity in terms of pozzolanic activity component, PRD, and pozzolanic activity index, because they have high SiO 2 + Al 2 O 3 contents and plentiful amorphous phases. Finally, a consistent relationship between PRD and pozzolanic activity index was found.