Evaluating the Pozzolanic Reactivity of Natural Pozzolans

Abstract

The cement industry significantly contributes to global CO₂ emissions, necessitating the development of alternative supplementary cementitious materials (SCMs). Conventional pozzolans such as Fly ash, GGBFS, etc, are limited and scarce nowadays. To ensure the continued production of durable and low CO₂ concrete, high-quality pozzolans are needed. Natural pozzolans (NP) such as calcined clay, pumice and volcanic ash, etc, impede their use in concrete. It is imperative to evaluate the pozzolanic reactivity of the NP. This study investigates the pozzolanic reactivity of quaternary blended cementitious binder of pumice and calcined clay in different proportions to the cement. The evolution of the reaction products was monitored for various curing days using different standardized tests like Strength Activity Index (SAI), Lime Reactivity Test (LRT), Frattini Test (FT), Lime Saturation Test (LST), and Electrical Conductivity Method (ECM). The results demonstrated that calcined clay exhibited superior pozzolanic performance compared to pumice, with significant calcium hydroxide consumption and strength gains across all tests. Thermodynamic modelling further validated the formation of secondary phases like straetlingite and OH-hydrotalcite contributing to microstructural densification. A strong correlation between SAI and FT confirmed the reliability of these methods for reactivity assessment. The findings highlight the potential of calcined clay and pumice as effective SCMs, with calcined clay showing dominant reactivity and enhanced properties.