Towards performance-based specifications for fly ash and natural pozzolans – Insights from a Monte-Carlo based thermodynamic modeling framework

Monte-Carlo-based thermodynamic modeling framework is used to assess the chemical limits set by ASTM C618-23. Realistic chemistries and reactivities for supplementary cementitious materials (SCMs) and chemistries for cement are generated using statistical data from the literature. 200,000 thermodynamic simulations are performed for each cement+SCM system to evaluate performance through Ca(OH)2 content, capillary porosity, and C-S-H volume. The results indicate that the reactivity of SCMs is directly related to its performance in paste, and its inclusion in ASTM C618 would be beneficial for optimized utilization of SCMs in performance-based mixtures. When providing chemical limits for SCMs, separating the reactive pozzolanic oxides (RPO) from bulk oxide composition is critical for paste performance. Fe2O3 content did not significantly impact the performance. If RPO is specified, fly ashes and natural pozzolans could have similar chemical limits. SCMs can be classified based on CaO and SO3 contents to affect Ca(OH)2 and AFt in paste, respectively.