Reactivity assessment of calcined illite, kaolinite, and montmorillonite in R3 mixtures

Abstract

This study evaluates the pozzolanic reactivity of calcined illite, kaolinite, and montmorillonite using the more recently standardized rapid, relevant, and reliable (R³) test methods as specified in ASTM C1897. The reactions involving calcite and the calcined clay minerals were examined by comparing the cumulative heat release in the R³ mixtures with and without calcite, and by analyzing the hydrated R³ mixtures using X-ray diffraction (XRD), thermogravimetric analysis (TGA), and scanning electron microscopy (SEM). The results indicate that the R³ reactivity of calcined clay minerals exhibits a positive linear relationship with the number of hydroxyl groups present in their original mineral structures. Strong correlations were observed among cumulative heat release, bound water content, and dehydroxylation mass loss of clay minerals. This suggests that the dehydroxylation mass loss can be used as an indicator for the initial screening of potential clay sources for use as supplementary cementitious materials (SCMs). However, the optimal calcination temperatures for illite and montmorillonite do not fall within their dehydroxylation temperature ranges. While calcite has a limited impact on the cumulative heat release, it facilitates the formation of monocarbonate (C₄A $\overline{C}$ H₁₁) and enhances its uniform distribution in the calcined kaolinite R³ mixture. In comparison, the reactions involving calcite with calcined illite and montmorillonite in their R³ mixtures are limited.