Significance of fineness of pozzolans in determining pozzolanic reactivity

Abstract

This paper presents the quantification of the pozzolanic reactivity of pozzolans examined in terms of compressive strength, bound water and electrical resistivity. The pozzolans studied included natural pozzolans, glass pozzolans and fly ash that were ground to four fineness levels or median particle sizes (d₅₀) of approximately 3, 5, 10 and 15 µm. Quantitative X-ray diffraction was employed to determine the amorphous content of pozzolans. The UNB lime-reactivity test and a modified ASTM C311 activity with portland cement test were performed in mortars. In these two tests, bulk electrical resistivity measurements were conducted before measuring compressive strength. Additionally, pastes were prepared for bound water in accordance with the R³ test or ASTM C1897. While the pozzolanic reactivity for all materials tested generally improves with the fineness, one pozzolan could demonstrate a very different rate of pozzolanicity improvements compared to that of others. Bulk electrical resistivity provides a reliable assessment of pozzolanic reactivity and can help differentiate pozzolanic and pozzolanic-hydraulic materials when used in conjunction with compressive strength. The modified ASTM C311 test is also found to be suitable and effective in rapidly distinguishing pozzolans, especially slow reactive ones, from inert materials at 7 days. A novel amorphous-fineness index derived by combining the amorphous content and fineness of pozzolans to reasonably predict the pozzolanic reactivity and limitations of the index are discussed.