Effects of Expanded Vermiculite on the Autogenous and Drying Shrinkage of Alkali-Activated Slag-Based Mortars

Abstract The growing need to reduce CO 2 emissions has encouraged the use of alkali-activated materials as sustainable alternatives to Portland cement. However, these binders are often susceptible to significant autogenous and drying shrinkage, which can compromise dimensional stability and durability. This study evaluates the influence of expanded vermiculite (EV) on the autogenous and drying shrinkage behavior of alkali-activated slag mortars. Mixtures containing 5%, 10%, and 15% EV were produced under dry, 100%, and 200% pre-saturation conditions, with a constant binder-to-aggregate ratio of 1:2 and a (w/b) ratio of 0.48. Physical and mechanical performance was analyzed through consistency, air content, density, compressive strength, capillary absorption, and microtomography tests. The results indicated that increasing the vermiculite content reduced the bulk density by up to 9.1% and increased the incorporated air content, which in turn decreased compressive strength by up to 31.4%. Capillary water absorption rose markedly in mixtures with 15% EV at 200% pre-saturation after 90 min. Autogenous shrinkage decreased by 66.4% in mixtures with dry EV, and 200% pre-saturation produced an additional 36.1% reduction. These results demonstrate that the moisture state of EV strongly governs shrinkage and pore connectivity in alkali-activated mortars. Controlled internal curing enhances dimensional stability while limiting strength losses when EV saturation and w/b are properly balanced. A forthcoming life-cycle assessment will quantify the environmental trade-offs of incorporating EV as an internal-curing agent.