Impact of etched cenospheres size on internal curing of mortar for reducing autogenous shrinkage

Fly ash cenospheres (FACs) have gained prominence as effective internal curing (IC) materials for mortar and concrete, and chemically etched fly ash cenospheres (eFACs) have the potential to further enhance the effectiveness. This study explores the influence of FAC and eFAC with three distinct particle size ranges—small (125–250 μm), medium (250–550 μm), and large (550–1750 μm)—on autogenous shrinkage and compressive strength. The autogenous shrinkage of mortar containing small FACs was approximately 37 % lower than that of control sample (-370.98 με), and the IC efficacy decreases as particle size increases. Different from FACs, small eFACs led to mortar expansion (94.55 μm) and enhanced strength by up to 12.3 %, exhibiting the highest hydration degree among all samples due to additional C-S-H gel formation. This expansion is attributed to growth of Ca(OH)₂ crystal sizes, due to high hydration degrees. Medium-sized eFACs demonstrated the highest water desorption rates, contributing to a higher saturation degree of C-S-H gels, thereby preventing gel swelling. Although the hydration degree in mortar containing medium-sized eFACs was lower than that of smaller variants, they still effectively reduced shrinkage by 55.7 %. Overall, the study suggests small-sized particles of FACs and eFACs as optimal IC agents.