Constitution and pozzolanic activity of volcanic rocks used as mineral admixture in concrete

Representative specimens of siliceous volcanic rocks were firstly collected from main volcanic areas in Jiangxi Province, and then their chemical composition, mineral type, structure, texture and vitreous proportion were analyzed by chemical composition analysis, petrographic analysis and X-ray diffraction. The pozzolanic activity index for these volcanic rocks was determined via compression strength ratio analysis of cement mortar and cement mixing mortar with powder of volcanic rock. The relations between rock performance and pozzolanic activity were further discussed based on the statistical significance. The results indicate that the volcanic rocks possess a pozzolanic activity index of ≥65% so to satisfy a demand of natural pozzolanic materials used as cement mortar and concrete according to the relevant standards. Perlite has the highest pozzolanic activity among these volcanic rocks due to the rich vitrescence on texture and silicon aluminum minerals generated at high temperature. However, the tuff with some big brecciated gravel fragments and minerals generated at low temperature appears a lower pozzolanic activity. The matrix content of volcanic rocks by petrographic analysis is consistent with the amorphous phase content of volcanic rocks on the rock properties and pozzolanic activity. There is a positive multiple correlation between the pozzolanic activity index and the amorphous content, silicon content or silicon aluminum contents, predicting the pozzolanic activity index by the fitting equations and further to analyze the potential of volcanic rocks used for mineral admixture in concrete.