Reactivity of hollow glass microspheres and cenospheres in cement mortar under heat curing conditions

Abstract

Hollow glass microspheres (HGM) and cenospheres (CS) are lightweight, hollow particles commonly used as sand or cement replacements in high-strength mortars. However, their pozzolanic reactivity under steam curing conditions was not adequately characterized. This study investigates the intrinsic reactivity of four HGMs and one CS using the R3 reactivity test (ASTM C1897), which isolates and quantifies the latent pozzolanic potential of supplementary cementitious materials based on heat evolution and chemically bound water. To explore how their reactivity is influenced by the presence of other highly reactive pozzolans, silica fume (SF) was incorporated into selected mixes. Compressive strength, X-ray diffraction, thermogravimetric analysis, mercury intrusion porosimetry, scanning electron microscopy, nitrogen physisorption, and isothermal calorimetry were employed to assess hydration behavior and microstructural development. Results showed that finer HGMs released up to 23 % more heat than coarser types, indicating higher reactivity, while CS remained largely inert. Sand replacement with HGMs improved compressive strength by up to 19 % due to enhanced particle packing. These findings demonstrate that finer HGMs can promote secondary hydration and improve performance, even when used alongside strongly pozzolanic additives like SF.