Investigating The Pozzolanic Reactivity of Natural Zeolite as a Supplementary Cementitious Material: Correlations to Mechanical Performance

Cement, the binding material in concrete, produces a significant amount of atmospheric CO₂ during its production. Therefore, methods to reduce the amount of cement used in concrete production, while maintaining adequate strength and durability, has substantial environmental implications. Natural zeolites are being considered to replace a portion of cement content as a supplementary cementitious material (SCM) due to its high pozzolanic activity, which is attributed to their porous aluminosilicate framework. However, the detailed reactions between the zeolite and cement, namely the effect of the hydration reactions, as well as zeolite’s contribution from physical characteristics, are not fully understood. Previous studies have characterized pozzolanic activity, assessed particle size and preprocessing contributions, mechanical properties, and performed chemical component analyses separately. However, there are inconsistencies among reported results and the materials science of the structure/processing/performance was not elucidated. Here, the pozzolanic activity of natural zeolite is characterized with respect to the degree of hydration of mortar mixes and correlated to the mechanical performance of concrete composites. The natural zeolite is first analyzed to understand its microstructure, elemental, and mineralogical composition using scanning electron microscopy (SEM), energy dispersive x-ray spectroscopy (EDS), and x-ray diffraction (XRD). Bound water measurements are utilized to quantitatively assess the pozzolanic reactivity of the zeolite, and over a range of particle sizes (achieved via milling). We then varied the percentage of natural zeolite in mortar composites at different particle sizes and concentration with respect to cement content. The effects of particle size on the micromorphology, degree of hydration, and development of bonding environments of the composites is observed via SEM, thermogravimetric analysis (TGA), and Fourier transform infrared spectroscopy (FTIR). The experimental results confirm that zeolite replacement of cement content in mortar composites enhances the mechanical performance at a discrete particle size and concentration. There are optimum parametric conditions to maximize strength that appear based on the increased degree of hydration stemming from increased pozzolanic activity of zeolite. Overall, this experiment demonstrates that natural zeolites are a viable SCM to reduce cement content in concrete composites and that further work could support the development of a truly superior sustainable concrete.